Generation of Alkyl Radicals: From the Tyranny of Tin to the Photon Democracy.

Alkyl radicals are key intermediates in organic synthesis. Their classic generation from alkyl halides has a severe drawback due to the employment of toxic tin hydrides to the point that "flight from the tyranny of tin" in radical processes was considered for a long time an unavoidable issue. This review summarizes the main alternative approaches for the generation of unstabilized alkyl radicals, using photons as traceless promoters. The recent development in photochemical and photocatalyzed processes enabled the discovery of a plethora of new alkyl radical precursors, opening the world of radical chemistry to a broader community, thus allowing a new era of photon democracy.

Introduction

Among all t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> open-s<class="Chemical">spn>an class="Disease">hell class="Chemical">species, <class="Chemical">span class="Chemical">carbon-centered radicals are intriguing neutral intermediates that find extensive use in organic synthesis, despite the initial distrust about their possible application.[1−5] In particular, the generation of unstabilized alkyl radicals under mild conditions granting the controlled and selective outcome of the ensuing reactions has been a challenge for many years. The first and more obvious way to form such species is the homolytic cleavage of a labile C–X bond; alkyl halides appeared as the ideal choice in this respect. The real breakthrough in radical chemistry was the discovery of Bu3SnH to promote radical chain reactions as reported about 60 years ago in the reduction of bromocyclohexane.[6] Reduction of an organotin halide by lithium aluminum hydride formed the reactive tin hydride in solution. In subsequent modifications of the protocol, both sodium borohydride[7] and sodium cyanoborohydride[8] acted as effective reducing agents. Alkyl radicals generated via tin chemistry were then used for C–C bond formation mainly via the addition to (electron-poor) olefins, the well-known Giese reaction,[9−12] an evolution of the original process which made use of organomercury compounds.[12,13]

As illustrated in Figure a, <n class="Chemical">spaclass="Chemical">n class="Chemical">tributylticlass="Chemical">n hydrideclass="Chemical">n> has t<class="Chemical">spn>an class="Disease">he double role of allowing t<class="Chemical">span class="Disease">he formation of Bu3Sn• as the radical chain carrier and as a hydrogen donor to close the catalytic cycle. The unique features of this catalytic cycle are attributed to the forging of stronger Sn–X and C–H bonds at the expense of the cleavage of the more labile Sn–H and C–X ones. A more quantitative aspect of this reaction can be appreciated comparing the different bond dissociation energies (BDE) associated with the steps mentioned above (see Figure a).[14,15] More recent applications showcase the crucial role of tin intermediates in controlling the outcome of different reactions. Sn–O interactions direct the regioselective addition of the radical in the radical stannylation of the triple bond in propargyloxy derivatives,[16] whereas tin radicals induced the synthesis of stannylated polyarenes via double radical peri-annulations, increasing the solubility of the products.[17]

Figure 1

(A) Thermal generation of radicals from alkyl halides in the Giese reaction. (B) LD50 values for selected organotin compounds. (C) Thermal generation of radicals from alcohols via xanthates (I). (D) Thermal and photochemical generation of radicals from carboxylic acids via Barton esters (II).

(A) T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>rmal generation of <class="Chemical">spn>an class="Chemical">radicals from <class="Chemical">span class="Chemical">alkyl halides in the Giese reaction. (B) LD50 values for selected organotin compounds. (C) Thermal generation of radicals from alcohols via xanthates (I). (D) Thermal and photochemical generation of radicals from carboxylic acids via Barton esters (II).

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> performance of <class="Chemical">spn>an class="Chemical">Bu3SnH was so <class="Chemical">span class="Chemical">competitive[9,18−20] that more than 20 years ago it was claimed that it was improbable to have “flight from the tyranny of tin” in radical processes,[21,22] a hard statement that subtly introduces the problem of the substantial toxicity and high biological activity of triorganotin compounds.[23] The LD50 of 0.7 mmol/kg in murine species (see Figure b) combined with the long half-lives in aquatic environment represent the biggest concerns for the application of these otherwise extremely versatile species, especially in the absence of viable alternatives.[24] Indeed, O-thiocarbonyl derivatives like xanthates (I, obtained from alcohols) were considered an alternative to the alkyl halides, albeit the radical generation required in most cases the use of tin hydrides (Figure c).[3,22,25,26]

Efforts in substitu<n class="Chemical">spaclass="Chemical">n class="Chemical">ticlass="Chemical">nclass="Chemical">n>g toxic <class="Chemical">spn>an class="Chemical">tin derivatives with ot<class="Chemical">span class="Disease">her hydrides such as (TMS)3SiH[27] or lauroyl peroxides and xanthates met some success, however, only in limited cases.[28−30] Other initiators to promote tin-free radical chain reactions were organoboranes,[31] thiols,[32] P–H-based reagents,[32] and 1-functionalized cyclohexa-2,5-dienes,[32−34] but nowadays they are not commonly used in synthetic planning.

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> use of <class="Chemical">spn>an class="Chemical">metal oxidants (<class="Chemical">span class="Chemical">MnIII acetate)[35] or metal reductants (TiIII catalyst[36] or SmII iodide[37]) were sparsely used, but only in the latter case unstabilized alkyl radicals were formed from alkyl iodides.

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> introduction of t<class="Chemical">spn>an class="Disease">he <class="Chemical">span class="Chemical">Barton esters II in 1985 represented a step forward in solving the conundrum of the tyranny of tin: the conversion of the strong O–H bond of an acid into the (photo)labile O–N bond of the corresponding thiohydroxamate ester (Figure d).[38,39] Barton esters have the advantage of being slightly colored, allowing the use of visible light irradiation to induce the cleavage of the O–N bond. The last point is significant, demonstrating the formation of alkyl radicals in a very mild way under tin-free conditions with no need of further additives, albeit Barton esters have currently a limited application. On this occasion, photochemistry showed an attractive potential for the development of novel synthetic strategies based on radical chemistry. However, Barton esters remained for several years an isolated niche. In most cases, the photochemical generation of radicals required harmful UV radiation and dedicated equipment.[40] Since the milestone represented by the development of the chemistry of Barton esters, new photochemical ways were sought toward more efficient ways to generate radicals. The photon appears to be the ideal component for a chemical reaction, assuming the form of a traceless reagent, catalyst, or promoter that leaves no toxic residues in the final mixture.[41−44] The breakthrough that would allow moving forward from the “tyranny of tin” to a greener “photon democracy” can be associated with the use of solar or visible photons, freely available from the sun that shines throughout the scientific world. The renaissance of the photocatalyzed processes that we have witnessed in the last years represents a significant step toward this direction.[45−58]

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> multifaceted use of photoredox catalysis and photocatalyzed <class="Chemical">spn>an class="Chemical">hydrogen transfer reactions expanded t<class="Chemical">span class="Disease">he range of possible radical precursors and unconventional routes for the generation of several carbon (or heteroatom based) radicals, including the challenging formation of unstabilized alkyl radicals.[59−65] Consequently, in this review, we aim to present a summary of the novel ways to generate alkyl radicals by photochemical means that, in the last years[66] have revolutionized the way to carry out radical chemistry. This work will focus exclusively on the reactions promoting the formation of unstabilized alkyl radicals, and not the stabilized ones, e.g., α-oxy, α-amino, benzylic, or allylic.

Figure <n class="Chemical">spaclass="Chemical">n class="Chemical">coclass="Chemical">n>llects t<class="Chemical">spn>an class="Disease">he main para<class="Chemical">span class="Chemical">digmatic approaches to the photogeneration of alkyl radicals (either photochemically or photocatalyzed). The more classical, although the less employed, path to generate alkyl radicals consists of the introduction of a photoauxiliary group which renders a bond labile to a direct photochemical cleavage (Figure A).[67] The Barton esters are the archetypal moiety belonging to this class.[39] A conspicuous body of literature have been focusing on the development of suitable alkyl substituents able to facilitate redox reactions making the derivatives more oxidizable or reducible. The strategy that is followed in Figure B consists in the conversion of a common functional group (e.g., OH or COOH), which in most cases is tethered to the alkyl group, into a different electroauxiliary group[68] (Figure B). As a result, the interaction of the activated species with an excited photoredox catalyst (PCSET) able to induce a single electron transfer (SET) process generates the corresponding radical ions, either by an oxidative pathway or a reductive pathway. The desired alkyl radical is then formed by fragmentation of these radical ion intermediates. The oxidative pathway is efficient when the radical precursor is negatively charged (see further Figure ) as in the case of alkyl carboxylates and alkyl sulfinates causing the CO2 or SO2 loss, respectively, despite the fact that the exothermicity of the process is verified only in the C–C cleavage rather than the C–S cleavage.[69] On the contrary, positively charged Katritzky salts were ideal candidates for the releasing of radicals via the reductive pathway (Figure ).[70]

Figure 2

Different approaches for the photogeneration of alkyl radicals (A) by photochemical means through the introduction of a photoauxiliary group (B) via fragmentation of a radical cation (oxidative pathway) or anion (reductive pathway) formed by photoredox catalysis (C) via a halogen atom transfer reaction (XAT) with a photogenerated radical (D) through the photocatalyzed cleavage of a C–H bond via direct (d-HAT) or indirect (i-HAT) hydrogen atom transfer (E) by the remote-controlled C–H activation via a photogenerated heteroatom based radical (F) by a ring-opening via a photogenerated heteroatom (nitrogen) based radical.

Figure 3

On the left, substrates used to promote the photochemical formation of alkyl radicals divided according to the C–Y bond cleaved. The oxidation potentials (Eox, in orange) or the reduction potentials (Ered, in green) of the precursors as well as the BDE values of the bond that is broken (highlighted in gray) by direct photocleavage are reported. On the right, a selection of common photoredox catalysts with their main redox features are collected.

Different approac<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>s for t<class="Chemical">spn>an class="Disease">he photogeneration of <class="Chemical">span class="Chemical">alkyl radicals (A) by photochemical means through the introduction of a photoauxiliary group (B) via fragmentation of a radical cation (oxidative pathway) or anion (reductive pathway) formed by photoredox catalysis (C) via a halogen atom transfer reaction (XAT) with a photogenerated radical (D) through the photocatalyzed cleavage of a C–H bond via direct (d-HAT) or indirect (i-HAT) hydrogen atom transfer (E) by the remote-controlled C–H activation via a photogenerated heteroatom based radical (F) by a ring-opening via a photogenerated heteroatom (nitrogen) based radical.

On t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> left, substrates used to promote t<class="Chemical">spn>an class="Disease">he photoc<class="Chemical">span class="Disease">hemical formation of alkyl radicals divided according to the C–Y bond cleaved. The oxidation potentials (Eox, in orange) or the reduction potentials (Ered, in green) of the precursors as well as the BDE values of the bond that is broken (highlighted in gray) by direct photocleavage are reported. On the right, a selection of common photoredox catalysts with their main redox features are collected.

A viable alternative is t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> photogeneration (often from a photoredox process) of a reactive <class="Chemical">spn>an class="Chemical">radical on a <class="Chemical">span class="Disease">heteroatom like a silyl radical, which can exploit a halogen atom transfer reaction to afford an alkyl radical through the smooth Si-X bond formation (XAT, Figure C).[71,72] This strategy provides an elegant way to overcome the Giese conditions in the tin mediated activation of alkyl halides. Recently, an α-amino radical was used in the same strategy, promoting the formation of alkyl radicals via C–X bond cleavage.[73]

A more challenging approach requn class="Chemical">ires t<class="Chemical">n class="Chemical">span class="Disease">he photocatalyzed selective cleavage of a strong <spclass="Chemical">n>an class="Chemical">alkyl-H bond, via a direct hydrogen atom transfer reaction (d-HAT, Figure D) operated by an excited photocatalyst (PCHAT).[72,74−76] The indirect version of the latter path exploits the photogeneration of a stable heteroatom based radical (i-HAT, Figure D) that will become the competent intermediate in the abstraction of the H atom from the alkyl moiety.[76] An indirect HAT (i-HAT) may also take place by intramolecular hydrogen transfer thus releasing an alkyl radical (Figure E).[76−81] As an alternative, the photochemical radical generation may induce a ring-opening in strained structures like cyclobutanes, to form a substituted alkyl radical (Figure F).[82]

Figure showcases a <n class="Chemical">spaclass="Chemical">n class="Chemical">coclass="Chemical">n>llection of t<class="Chemical">spn>an class="Disease">he main <class="Chemical">span class="Chemical">alkyl radical precursors devised for the generation under photochemical conditions of unstabilized alkyl radicals. In this figure, the radical precursors were collected depending on the C(sp3)-Y bond cleaved during the radical release. As apparent, the photochemically triggered cleavage of several C-heteroatom bonds like C–X,[71,83−88] C–O,[89−98] C–B,[99−102] C–S,[103−106] or C–N[70] (Figure ) affords carbon-centered radicals. The alkyl radical generation is granted by the very versatile photochemical tool. This feature includes particular cases such as C–Se (in alkyl selenides),[107,108] C–Te [in (aryltelluro)formates,[109,110] for a previous thermal generation of alkyl radical from diorganyl tellurides, see ref (111)], and C–Si (in tetra alkyl silanes and bis-chatecolates)[112−114] to be added to C–Sn (in alkyl stannanes).[112,115] Interestingly, even the more resilient C–H[15,76] or C–C[38,83,94,116−130] bonds may be cleaved for alkyl radical generation, opening up new exciting possibilities for the synthetic (photo)chemist (Figure ).

For t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> clarity of t<class="Chemical">spn>an class="Disease">he reader, each <class="Chemical">span class="Chemical">radical precursor is accompanied by its oxidation potential (EOX, in orange) or its reduction potential (ERED, in green) to guide the feasibility on the generation of the radical via the oxidative or reductive pathway (type B, Figure B), respectively. Since the redox potentials may vary with the nature of the alkyl group, the values reported are referred to known structures. In alternative, the BDE values of the bond that is broken by direct photocleavage (type A, Figure A) or by photocatalyzed hydrogen abstraction (type D, Figure D) are reported. Figure (right part) likewise collects the redox properties of commonly used photoredox catalysts including metal-free photoorganocatalysts (POC) to be used in the oxidative/reductive pathways.[131−137]

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> reactions <class="Chemical">spn>an class="Chemical">collected and <class="Chemical">span class="Chemical">commented on in this review are primarily divided according to the type of the bond formed, namely the forging of C–C or C–heteroatom bonds, along with the construction of rings of different sizes. When possible, in each section, we will further categorize the reactions depending on the mechanism of the radical generation, ascribing them to the six types (A–F) described in Figure .

Formation of a C(sp3)-C Bond

Photoc<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>mically generated <class="Chemical">spn>an class="Chemical">alkyl radicals have been employed to forge <class="Chemical">span class="Species">C(sp3)-C(sp) bonds (n = 1–3) in an intermolecular fashion following different strategies. In most cases, a conjugate addition onto a Michael acceptor or a Minisci-like reaction occurred, albeit alkenylations, acylations, or oxyalkylations are likewise used.

Formation of a C(sp3)-C(sp3) Bond

Addition to C–C Double Bonds: Hydroalkylations

Many reactions belonging to this class involve t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> nucleophilic <class="Chemical">spn>an class="Chemical">alkyl radical addition onto an electrophilic <class="Chemical">span class="Chemical">Michael acceptor, resulting in a formal hydroalkylation of the double bond viz. the incorporation of an alkyl group (in position β with respect to the EWG group in the starting unsaturated compound) and a hydrogen atom (in position α). This reaction is usually one of the first that many authors would test during the discovery process of a new radical precursor, as testified by the plethora of reagents that are used in this transformation. Photoredox catalysis is by far the preferred approach here, especially by using the oxidation of a negatively charged precursor (oxidative pathway in Figure B).

A typical example is t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> oxidation of <class="Chemical">spn>an class="Chemical">carboxylates[138,139] that releases an <class="Chemical">span class="Chemical">alkyl radical via CO2 loss from the carboxyl radical intermediate (Scheme ). Adamantylation of both acrylonitrile (Scheme a)[140] and dimethyl 2-ethylidenemalonate starting from adamantane carboxylic acid 1-1 (Scheme b)[141] were carried out following this strategy. In the former case, the authors employed 1,4-dicyanonaphthalene (DCN) as the POC under UV light irradiation, while visible light and an IrIII complex in the latter case. The approach used in Scheme b was also useful for the three steps preparation of the medicinal agent (±)-pregabalin.[141] Also, the Fukuzumi catalyst (9-mesitylene-10-methylacridinium perchlorate, [Acr+Mes]ClO4) can promote this Giese-type reaction,[142] allowing the alkylation of α-aryl ethenylphosphonates for the synthesis of fosmidomycin analogues.[143]

Scheme 1

Different Strategies for the Decarboxylative Adamantylation of Electron-Poor Alkenes

A vn class="Chemical">ariatioclass="Chemical">n of this procedure is t<class="Chemical">n class="Chemical">span class="Disease">he decarboxylative-de<spclass="Chemical">n>an class="Chemical">carbonylative process occurring on an α-keto acid 2-1 under sunlight-driven photoredox catalyzed reaction conditions (Scheme ).[125]

Scheme 2

Decarboxylative-Decarbonylation of an α-Keto Acid

<n class="Chemical">spaclass="Chemical">n class="Chemical">Oxalateclass="Chemical">n>s are anot<class="Chemical">spn>an class="Disease">her class of electron-donors having two <class="Chemical">span class="Chemical">carboxylate moieties that can be lost upon photocatalyzed oxidation. These species may be introduced in situ by reaction of the alcohol with oxalyl chloride. The process induced the cleavage of a C–O bond, and the resulting radical could be trapped by butenolide 3–1 to form the menthyl derivative 3–2 used for the enantioselective preparation of cheloviolene A (3–3, Scheme ).[144] An IrIII-based photocatalyst efficiently promoted the reaction also in this case, allowing the synthesis of quaternary centers[89] and the total synthesis of trans-clerodane diterpenoids.[145]

Scheme 3

Enantioselective Preparation of Cheloviolene A

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alkyl trifluoroboratesclass="Chemical">n> stand out as anot<class="Chemical">spn>an class="Disease">her important class of easily oxidizable moieties.[146] T<class="Chemical">span class="Disease">he photocatalyzed oxidation of these salts (e.g., 4–1) causes the smooth release of BF3 and the formation of the reactive alkyl radical. Such a reaction was employed to functionalize Michael acceptors under sunlight irradiation (Scheme a) exploiting Acr+Mes as the POC.[147] Complexation of 4–4 by a chiral rhodium complex (Λ-RhS, Scheme b) delivered 4–5 in good yields with 97% ee.[148]

Scheme 4

Visible and Solar Light Photocatalyzed Functionalization of Michael Acceptors with Alkyl Trifluoroborate Salts

This synt<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>tic strategy can be extended to neutral <class="Chemical">spn>an class="Chemical">boronic acids or <class="Chemical">span class="Chemical">esters, upon in situ activation by a Lewis base (LB). The so formed negatively charged species is consequently more prone to oxidation, which eventually will provide the formation of the alkyl radicals. A typical example is illustrated in Scheme where the boronic acid 5–1 was activated by 4-dimethylaminopyridine (DMAP) and then oxidized by an IrIII complex. The resulting cyclobutyl radical was trapped by methyl vinyl ketone to access the substituted ketone 5–2 in a good yield.[100] This reaction was later scaled up under flow conditions by using the Photosyn reactor. In such a way, the authors could synthesize gram amounts per hour of the analogues of some drugs belonging to the GABA family.[149]

Scheme 5

Activation of Boronic Acids with a Lewis Base

Following t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> examples of t<class="Chemical">spn>an class="Disease">he <class="Chemical">span class="Chemical">carboxylate derivatives, the electron-donating species may be generated in situ by deprotonation, as in the case of sulfonamides, employed in the desulfurative conjugate addition of alkyl radicals onto Michael acceptors (Scheme ). Again, the process is based on a photocatalyzed oxidation pathway. The starting sulfonamide (6–1) was first deprotonated by a mild base (K2HPO4), and the resulting anion was easily oxidized to a N-centered radical. Loss of N-sulfinylbenzamide generates the desired radical that gave the adduct 6–3 upon reaction with 6–2 in 75% yield.[103]

Scheme 6

Desulfurative Strategy for the Conjugate Addition of Alkyl Radicals onto Michael Acceptors

In some instances, t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> <class="Chemical">spn>an class="Chemical">radical precursor is a neutral <class="Chemical">span class="Chemical">compound. This situation is possible only when the derivative contains a highly oxidizable or reducible moiety. 4-Alkyl-1,4-dihydropyridines (alkyl-DHPs) under PC-free conditions act as radical precursors when combined with photoexcited iminium ion catalysis (Scheme ). Here, enal 7–1 formed a chiral iminium ion 7–4 by reaction with amine 7–3. Cation 7–4 upon visible light excitation oxidized the alkyl-DHP 7–2 that in turn released the radical 7–5 upon fragmentation, along with radical 7–4. Radical recombination followed by hydrolysis gave the desired alkylated dihydrocinnamaldehyde 7–6 in a satisfactory yield with a good enantiomeric excess (Scheme ).[150] A similar Giese reaction was later proposed, where the alkyl-DHP was excited and a SET reaction with Ni(bpy)32+, acting as an electron mediator, took place. The alkyl radical derived from the radical cation of alkyl-DHP readily attacked a series of Michael acceptors.[151]

Scheme 7

Alkyl-DHPs as Radical Precursors in Combination with Iminium Catalysis

Looking at t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> ot<class="Chemical">spn>an class="Disease">her edge of t<class="Chemical">span class="Disease">he redox spectrum, easily reducible compounds were devised as radical precursors via a photocatalyzed process. As an example, the incorporation of a N-phthalimidoyl moiety in an organic compound helps its photocatalyzed reduction, ultimately leading to the release of the alkyl radical. A typical case is represented by N-(acyloxy)phthalimides.[126] A stereoselective variant of this reaction was applied to the synthesis of (−)-solidagolactone (8–4, Scheme ). Thus, the photocatalyzed reduction of phthalimide 8–1 by a RuII complex released a tertiary carbon radical. Attack to the terminal carbon of the unsaturated core present in β-vinylbutenolide 8–2 yields 8–3 with a very high diastereomeric excess. Further elaboration of compound 8–3 gave 8–4 in a single step.[152] This reaction emerges as a very interesting tool to construct quaternary carbons[153] and to synthesize biologically active derivatives, e.g., (−)-aplyviolene.[154]

Scheme 8

Synthesis of (−)-Solidagolactone via N-(Acyloxy)phthalimides

Interes<n class="Chemical">spaclass="Chemical">n class="Chemical">ticlass="Chemical">nclass="Chemical">n>g results were also obtained using <class="Chemical">spn>an class="Chemical">N-phthalimidoyl oxalates such as 9–1 in place of t<class="Chemical">span class="Disease">he N-(acyloxy)phthalimides for the generation of alkyl radicals starting from tertiary alcohols (Scheme ).[92,97] The similarity of this reaction to the one presented in Scheme is striking, despite a less atom economical radical generation.

Scheme 9

Generation of Alkyl Radicals from N-Phthalimidoyl Oxalates

Reduction of an organic <n class="Chemical">spaclass="Chemical">n class="Chemical">coclass="Chemical">n>mpound may be carried out even on organic <class="Chemical">spn>an class="Chemical">iodides by using <class="Chemical">span class="Chemical">cyanoborohydride anion as the reducing agent. The reaction is chemoselective, since no alkyl bromides or chlorides could be activated following this way. Giese adducts were formed by irradiation with a Xe lamp of the reaction mixture in good yields as illustrated by the formation of 10–2 from 10–1 in Scheme .[155] This is another interesting example to circumvent the use of tin hydrides in the activation of alkyl halides.

Scheme 10

Giese-Type Reaction of Iodides in the Presence of BH3CN–

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alkyl chloridesclass="Chemical">n> can be activated using Ir(dtbby)(ppy)2PF6 in t<class="Chemical">spn>an class="Disease">he presence of micelles. T<class="Chemical">span class="Disease">he micellar environment stabilizes the photogenerated [Ir(dtbby)•–(ppy)2] species (−1.51 V vs SCE), unable to directly reduce the alkyl chlorides (ca. −2.8 V vs SCE). A second excitation of this long-lived intermediate allows the electron transfer to the halide, which could react with different electron-poor olefins, forging a novel C–C bond. The micellar system allowed intramolecular cyclizations to form five-membered cycles.[156]

Reduction of t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> <class="Chemical">spn>an class="Chemical">alkyl halide 11–1 <class="Chemical">span class="Chemical">could be avoided applying a halogen transfer reaction. In fact, due to the strong BDE of the Si-halogen bond, an alkyl radical is formed thanks to the action of a purposely generated silyl radical (from (Me3Si)3SiH, TTMSS) by a photoredox catalytic step. Radical addition onto an unsaturated amide (11–2) gave the 1,8-difunctionalized derivative 11–3, a key compound in the preparation of Vorinostat 11–4, a histone deacetylase (HDAC) inhibitor active against HIV and cancer (Scheme ).[157] This is the typical case where the radical is formed by the cleavage of an Alk-Br bond without the assistance of tin derivatives. It is interesting that the reaction requires a substoichiometric amount of silane to proceed. Indeed, with higher loadings the product yield decreases, possibly due to the presence of competing nonproductive pathways. A chain reaction mechanism could be envisaged; however, the quantum yield for this reaction (Φ = 0.45) does not fully clarify the mechanistic details of the transformation.

Scheme 11

Synthesis of Vorinostat via XAT Strategy

In many instances t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> formation of t<class="Chemical">spn>an class="Disease">he <class="Chemical">span class="Chemical">alkyl radical arose from a direct or indirect photocatalyzed C–H homolytic cleavage. The excited state of the decatungstate anion in its tetrabutylammonium salt form (TBADT) promoted in several cases the direct chemoselective cleavage of a C–H bond.[75,158]Scheme depicts two examples involving the hydroalkylation of acrylonitrile. Unsubstituted cycloalkanes were suitable hydrogen donors under flow conditions (yielding 12–1Scheme a).[159] Moreover, the chemoselective cleavage of the methine hydrogen in isovaleronitrile allowed the preparation of dinitrile 12–2 in 73% yield (Scheme b).[160]

Scheme 12

TBADT-Photocatalyzed Hydroalkylation of Acrylonitrile

n class="Chemical">Similclass="Chemical">n class="Chemical">arly, the presence of a tertiary hydrogen was the driving force of the chemoselective TBADT-photocatalyzed C–H cleavage in several derivatives, as depicted in Scheme . As an example, alkylpyridine 13–1 was selectively functionalized and gave derivative 13–2 as the exclusive product in the reaction with a vinyl sulfone (Scheme a).[161] Interestingly, the labile benzylic hydrogens present in 13–1 remained untouched under these reaction conditions. Noteworthy, steric and polar effects cooperatively operated in the derivatization of lactone 13–3. As a result only the methine hydrogen of the isopropyl group was selectively abstracted and afforded 13–4 in very high yields by reaction with fumaronitrile (Scheme b), albeit the seven different types of hydrogens present in 13–3.[162] The C–H cleavage may also take place in branched alkanes as witnessed by the derivatization of 13–5 to form the succinate derivative 13–6 (Scheme c).[163]

Scheme 13

TBADT-Driven Functionalization of Tertiary Carbons

In rn class="Chemical">are iclass="Chemical">nstaclass="Chemical">nces, t<class="Chemical">n class="Chemical">span class="Disease">he hydro<spclass="Chemical">n>an class="Chemical">alkylation reaction may be applied to olefins different to the usual Michael acceptors. Thus, substituted vinylpyridines were functionalized by TBADT-photocatalyzed addition of cycloalkanes. Scheme showed the smooth synthesis of 14–2 starting from 14–1 simply by irradiation of the reaction mixture containing a slight excess of cyclohexane in the presence of a catalytic amount of the decatungstate salt.[164]

Scheme 14

Functionalization of a Vinylpyridine with a Cycloalkane

Recently, alternative n class="Chemical">PCs have beeclass="Chemical">n developed for t<class="Chemical">n class="Chemical">span class="Disease">he direct photocatalyzed activation of C–H bonds in <spclass="Chemical">n>an class="Chemical">cycloalkanes, namely uranyl cation[165] and Eosin Y,[166] both having the advantage of absorbing in the visible light region. The alkyl radical formation may be induced by a photogenerated stable radical which acts as a radical mediator. An IrIII based photoredox catalyst oxidized the chloride anion (being the counterion of the Ir complex) to the corresponding chlorine atom, which abstracted a hydrogen atom from cyclopentane, thus forming adduct 15–1 in 69% yield upon addition onto a maleate ester (Scheme ).[167]

Scheme 15

Indirect HAT Mediated by a Cl• Radical

Anot<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>r intriguing way to induce t<class="Chemical">spn>an class="Disease">he cleavage of unactivated <class="Chemical">span class="Species">C(sp3)-H bonds is by a photocatalyzed intramolecular hydrogen abstraction. Usually a photoredox or a proton-coupled electron transfer (PCET)[47] step induced the formation of a heteroatom centered radical that abstracts a tertiary C–H bond intramolecularly in a selective fashion, following a 1,5-HAT process mimicking the Hoffmann-Löffler-Freytag reaction (Scheme ).[168−170]

Scheme 16

Intramolecular 1,5-HAT Forming Tertiary Alkyl Radicals

W<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>n t<class="Chemical">spn>an class="Disease">he reaction was applied to <class="Chemical">span class="Chemical">compound 16–1, an oxidative PCET generated a neutral amidine radical that promotes the 1,5-hydrogen atom abstraction forming a tertiary radical which is able to functionalize olefin 16–2 in a complete regioselective fashion affording 16–3 (Scheme a).[171] The reaction was also applied to medicinally relevant molecules such as the steroid-derived trifluoroacetamide 16–4 (Scheme b). Despite the fact that this compound has several labile C–H bonds including tertiary C–H bonds and C–H bonds adjacent to heteroatoms, the intramolecular hydrogen abstraction followed by conjugate addition onto 16–5 gave 16–6 as the sole product.[172]

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> remote activation of t<class="Chemical">spn>an class="Disease">he C–H bond in t<class="Chemical">span class="Disease">he δ-position following this approach is a general reaction as demonstrated in related systems applied to amides protected with a carbamate group[173] or in simple benzamide derivatives.[174] In the latter case, the reaction was carried out in the presence of a chiral Rh-based Lewis acid catalyst that allowed the asymmetric alkylation of α,β-unsaturated 2-acyl imidazoles.[174]

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> abstrac<class="Chemical">spn>an class="Chemical">ting class="Chemical">species <class="Chemical">span class="Chemical">could be likewise a photogenerated iminyl radical as illustrated in Scheme . Here a carbonyl group is converted in an oxime derivative (e.g., 17–1) by reaction with an α-aminoxy acid. Photocatalyzed oxidation followed by fragmentation of the resulting carbonyloxy radical gave an iminyl radical prone to a 1,5-HAT to afford a tertiary radical that upon addition to acrylate 17–2 gave compound 17–3 in 77% yield.[175]

Scheme 17

1,5-HAT Promoted by an Iminyl Radical

Heteroalkylation of C–C Double Bonds

An interes<n class="Chemical">spaclass="Chemical">n class="Chemical">ticlass="Chemical">nclass="Chemical">n>g variation of t<class="Chemical">spn>an class="Disease">he functionalization of a double bond is t<class="Chemical">span class="Disease">he formation of a C–C bond (upon an alkylation step) followed by the formation of another C–Y bond (Y ≠ H) on the adjacent carbon. As an example, alkyl diacyl peroxides were reduced photocatalytically and the fragmentation released an alkyl radical and a carboxylate anion both incorporated in the structure of the product. Thus, 2-vinylnaphthalene 18–2 was converted into compound 18–4 in a very good yield upon reaction with lauroyl peroxide 18–1 upon an oxidative quenching process by consecutive C–C and C–O formation (Scheme ).[176] The reaction was made possible by the oxidation of the resulting radical adduct 18–3 (by RuIII, the oxidized form of the PC) that generated the cation 18–3 that was easily trapped by the carboxylate anion previously released.

Scheme 18

Oxyalkylation via Alkyl Diacyl Peroxides

<n class="Chemical">spaclass="Chemical">n class="Chemical">class="Chemical">n class="Chemical">N-(acyloxy)phthalimide 19–1 as <spclass="Chemical">n>an class="Chemical">radical precursor found use in a similar multicomponent oxyalkylation of styrenes. The addition of the alkyl radical onto the vinylarene followed by the incorporation of water present in the reaction mixture afforded derivative 19–2 in 72% yield (Scheme ).[177] Noteworthy, the labile C–Br bond in 19–1 remained untouched in the process.

Scheme 19

Multicomponent Oxyalkylation of Styrenes

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> use of <class="Chemical">spn>an class="Chemical">water as t<class="Chemical">span class="Disease">he oxygen source was likewise used in the difunctionalization of aryl alkenes where the carbon-centered radical was formed by an intramolecular 1,5-HAT of a photogenerated iminyl radical.[178]

Performing t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> reaction in <class="Chemical">spn>an class="Chemical">DMSO, allows for t<class="Chemical">span class="Disease">he use of the solvent as an oxygen donor adopting the Kornblum oxidation. The intermediate benzyl radical formed after the alkylation step reacts with the solvent and eventually forming a carbonyl group in place of a simple C–O bond. An elegant example is shown in (Scheme ) for the synthesis of ketonitrile 20–4.[179] A cycloketone oxime ester (20–1) was photocatalytically reduced, inducing a ring opening on the resulting iminyl radical. The resulting cyano-substituted alkyl radical reacted with styrene 20–2, and the addition with DMSO formed the intermediate 20–3, that, upon Me2S loss, afforded the product.

Scheme 20

Photocatalyzed Oxyalkylation of Styrenes Based on the Kornblum Oxidation

A related oxy<n class="Chemical">spaclass="Chemical">n class="Chemical">alkylclass="Chemical">n>ation of <class="Chemical">spn>an class="Chemical">styrenes made again t<class="Chemical">span class="Disease">he use of the Kornblum oxidation as the last step in the synthesis of substituted acetophenones. Indeed, N-hydroxyphthalimides (e.g., 21–1) were employed as the radical source, and an IrIII complex was used as the PC, obtaining good yields even on a 7 mmol scale (64% of 21–3, Scheme a).[180,181] Ester 21–1 was also adopted for the preparation of the aryl alkyl ketone 21–5 in 61% yield (Scheme b). In this case, however, the decarboxylative alkylation was applied to silyl enol ethers having the carbonyl oxygen already incorporated in the initial structure such as 21–4.[182] The same process described in Scheme b can be carried out under uncatalyzed conditions under blue LED irradiation in the presence of an excess of NaI (150 mol %) and PPh3 (20 mol %). The reaction was based on the photoactivation of a complex formed by N-(acyloxy)phthalimide with NaI and PPh3 through Coulombic and cation-π interactions. In this case, the excitation caused the reduction of the phthalimide by a SET reaction within the complex.[183]

Scheme 21

Oxyalkylation by Using N-(Acyloxy)phthalimide Derivatives as Radicals Source

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alkylated ketoclass="Chemical">nesclass="Chemical">n> 22–3a–d were likewise obtained by t<class="Chemical">spn>an class="Disease">he IrIII-photocatalyzed reaction between a <class="Chemical">span class="Chemical">2-mercaptothiazolinium salt (22–1, as alkyl radical precursor) and silyl enol ethers 22–2a–d (Scheme ).[106]

Scheme 22

Synthesis of Alkylated Ketones from Mercaptothiazolinium Salts

<n class="Chemical">spaclass="Chemical">n class="Chemical">Lauryl peroxideclass="Chemical">n> (<class="Chemical">spn>an class="Chemical">LPO, see Sc<class="Chemical">span class="Disease">heme ) was adopted for the Ru-catalyzed three-component carbofluorination of styrenes as illustrated in Scheme a. The vinylic double bond of compound 23–1 derived from estrone was functionalized twice by using triethylamine trihydrofluoride Et3N·HF as the fluoride anion source to deliver the desired alkyl-fluorinated olefin 23–2 in 61% yield. The reduction of LPO is mediated by the presence of a copper salt in the role of a cocatalyst in a dual catalytic process.[184] The carbofluorination was later applied to dehydroalanine derivative 23–4 by using alkyltrifluoroborates and an excess of Selectfluor as an electrophilic fluorine source (Scheme b). The use of a visible light POC ([Acr+Mes]ClO4) allowed for the synthesis of a wide range of unnatural α-fluoro-α-amino acids including F-Leu (23–5).[185]

Scheme 23

Carbofluorination of (a) Styrenes and (b) Dehidroalanine Derivatives

In rn class="Chemical">are iclass="Chemical">nstaclass="Chemical">nces, two C–C boclass="Chemical">nds <class="Chemical">n class="Chemical">span class="Chemical">could be formed in t<spclass="Chemical">n>an class="Disease">he adjacent position of the double bond as in cyanoalkylations. The enantioselectivity of the reaction was controlled exploiting the capability of a copper catalyst to form complexes with chiral Box ligands. Thus, a methyl radical was obtained by IrIII-photocatalyzed reduction of phthalimide 24–1 that readily attacked styrene (Scheme ). Meanwhile, the CuI salt incorporated Box 24–2 as the ligand, and the resulting complex reacted with the adduct radical in the presence of TMSCN. As a result, cyanoalkylated 24–3 was obtained in a good yield and in good ee.[186]

Scheme 24

Enantioselective Cyanoalkylation of Styrenes

A pn class="Chemical">articlass="Chemical">n class="Chemical">cular case of cyanoalkylation was later reported in the photocatalyzed reaction between cyclopropanols and cyanohydrins having a pendant C=C bond. Oxidative ring opening of the three-membered ring followed by addition onto the double bond and cyano migration gave a series of multiply functionalized 1,8-diketones incorporating the cyano group.[187]

Allylation

Allylation reactions can be ean class="Chemical">sily performed by reactioclass="Chemical">n of aclass="Chemical">n <class="Chemical">n class="Chemical">span class="Chemical">alkyl radical with substituted <spclass="Chemical">n>an class="Chemical">allyl sulfones (mainly with 1,2-bis(phenylsulfonyl)-2-propene 25–1, Scheme a). The alkyl radical was generated under visible light irradiation by hydrogen abstraction from cycloalkanes by an aromatic ketone, e.g., 5,7,12,14-pentacenetetrone 25–2. Addition of a cycloalkyl radical onto 25–1 followed by sulfonyl radical elimination gave access to vinyl sulfones 25–3a–b in good yields (Scheme a).[188]

Scheme 25

Allylation through Alkyl Radicals Generated (a) via HAT and (b) from Si bis-Catecholates

Ot<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>r related reactions were designed to forge <class="Chemical">spn>an class="Species">C(sp3)–allyl bonds following this simple sc<class="Chemical">span class="Disease">heme. The alkyl radical was formed by photocatalytic oxidation of hypervalent bis-catecholato silicon compounds as shown in Scheme b. Thus, compound 25–4 upon oxidation released the desired substituted alkyl radical, and addition onto allyl sulfone 25–5 gave the corresponding allylated derivative 25–6 in 70% yield.[113] Olefin 25–5 was also used in a decarboxylative allylation of alkyl N-acyloxyphthalimides under RuII photocatalysis. The great advantage of the process was the reaction time since the allylation was completed in a few minutes at room temperature.[189]

A pn class="Chemical">articlass="Chemical">n class="Chemical">cular class of phthalimides could be employed with no need of a photocatalyst to promote the reaction. N-alkoxyphthalimide (26–1) is able to form a donor–acceptor complex with electron donor compounds, such as the Hantzsch ester HE. Upon excitation, an electron transfer occurred within the complex generating a radical anion, which released an alkoxy radical upon N–O bond cleavage. Loss of formaldehyde formed the desired alkyl radical which reacted with 26–2, to obtain product 26–3 in 60% yield (Scheme ).[127]

Scheme 26

Hantzsch Ester Mediated Photocleavage of N-Alkoxyphthalimides

Photocatalyzed reduction of <n class="Chemical">spaclass="Chemical">n class="Chemical">Katritzky saltsclass="Chemical">n> 27–1a–c obtained from t<class="Chemical">spn>an class="Disease">he <class="Chemical">span class="Chemical">corresponding amines (Scheme ) gave access to the allylated compounds 27–3a–c. Thus, the monoelectronic reduction of pyridinium salts 27–1a–c caused the release of the corresponding pyridines along with the substituted cyclohexyl radicals than upon trapping by 27–2 efficiently afforded acrylates 27–3a–c.[190]

Scheme 27

Photocatalyzed Allylation by Using Katritzky Salts

A remote allylation under vin class="Chemical">sible light class="Chemical">n class="Chemical">irradiation was devised starting from amide 28–1 making use of eosin Y (EY) as the PC (Scheme ). The excited EY is able to reduce 28–1 thanks to the electron-withdrawing capability of the substituted phenoxy group on the nitrogen of the amide. The amidyl radical formed upon fragmentation of 28–1 gave rise to a tertiary radical upon 1,5-HAT, allowing the remote allylation, forming 28–2 in 75% yield.[191]

Scheme 28

Remote Allylation via Amidyl Radicals

A different approach involved t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> use of trifluoromethyl-substituted <class="Chemical">spn>an class="Chemical">alkenes (e.g., 29–1) that upon addition of t<class="Chemical">span class="Disease">he alkyl radical gave access to valuable gem-difluoroalkenes such as 29–2a–b (Scheme ). The oxidation of alkyltrifluoroborates was here assured by the organic photocatalyst 4CzIPN, leading to nonstabilized primary, secondary, and tertiary radicals. The defluorinative alkylation resulted from the reduction of the radical adduct, followed by an E1cB-like fluoride elimination.[192]

Scheme 29

Synthesis of gem-Difluoroalkenes from Alkyl Trifluoroborates

A dual catalytic approach was den class="Chemical">sigclass="Chemical">ned for valuable allylatioclass="Chemical">n uclass="Chemical">n class="Chemical">sing vinyl epoxides as allylating agents (Scheme ). The mechanism was investigated by quantum mechanical calculations [by DFT and DLPNO–CCSD(T)] and supported an initial complexation of Ni0 to 30–2 that quickly underwent a SN2-like ring opening, followed by the incorporation of the alkyl radical formed by DHP-derived compounds 30–1a,b into the metal complex. Allyl alcohols 30–3a,b were then formed by inner sphere C(sp2)-C(sp3) bond formation from the resulting NiIII complex.[193]

Scheme 30

Dual-Catalytic Allylation of Vinyl Epoxides

sp3–sp3 Cross-coupling

Anot<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>r intriguing possibility offered by t<class="Chemical">spn>an class="Disease">he photoc<class="Chemical">span class="Disease">hemical approach to alkyl radicals is the formation of a C–C bond by a sp3–sp3 cross-coupling reaction. The transformation could lead to novel pathways to interesting targets, as represented by the synthesis of the drug tirofiban in only four steps, starting from easily available compounds. The protocol made use of two consecutive photocatalyzed reactions applying a metallaphotoredox strategy (Scheme ). The key step is the coupling between carboxylic acid 31–1 and alkyl halide 31–2. The halide is first complexed by a Ni0 catalyst and the resulting NiI complex trapped the alkyl radical (obtained by photocatalyzed decarboxylation) to yield a NiIII complex that in turn released the sp3–sp3 coupled product 31–3 after desilylation with TBAF. The desired tirofiban was then obtained by elaboration of 31–3 in two subsequent steps.[194]

Scheme 31

Synthesis of a Precursor of Tirofiban by a Metallaphotoredox Strategy

Anot<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>r example of a <class="Chemical">spn>an class="Species">C(sp3)–<class="Chemical">span class="Species">C(sp3) cross-coupling process is the reaction between alkylsilicates and alkyl halides. As in the previous case, a dual catalytic Ir/Ni system was required.[195] The alkyl radical may be likewise generated from an alkyl halide by a halogen atom transfer with a photogenerated silyl radical (from a silanol). The radical that is hence formed could be coupled with another alkyl bromide, e.g., methyl bromide, using a Ni0 catalyst to perform valuable methylation reactions.[196] Aliphatic carboxylic acids were used to form alkyl-CF3 bonds via a photocatalyzed reaction making use of Togni’s reagent as the trifluoromethylating agent. The reaction was promoted under visible light irradiation employing an IrIII photocatalyst coupled with a CuI salt. This process tolerates various functionalities including olefins, alcohols, heterocycles, and even strained ring systems.[197]

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> <class="Chemical">spn>an class="Chemical">alkylation of a benzylic position in N-aryl tetrahydro<class="Chemical">span class="Chemical">isoquinoline 32–1 was reported following two different approaches (Scheme ). The first allowed the reaction of an unactivated alkyl bromide (32–2) by the excitation of a Pd0 complex. Compound 32–1 was oxidized in the catalytic cycle, and the resulting α-amino radical coupled with the isopropyl radical to form 32–4 in 81% yield (Scheme a).[198] An alternative heavy-metal-free route catalyzed by a dye-sensitized semiconductor is depicted in Scheme b. Excitation of an inexpensive dye (erythrosine B) caused the reduction of titanium dioxide that in turn was able to reduce phthalimide 32–3 that eventually yielded quinoline 32–4.[199]

Scheme 32

Different Strategies in the Alkylation of N-Aryl Tetrahydroisoquinolines

Other Reactions

In pn class="Chemical">articlass="Chemical">n class="Chemical">cular cases, a C=N bond can be made sufficiently electrophilic to undergo alkyl radical addition as in the case of N-sulfinimines, exploited for the preparation of protected amines. A high degree of diastereoselectivity can be obtained when starting from chiral N-sulfinimines (33–2a–c, Scheme ). Thus, the asymmetric addition of an isopropyl radical (formed from derivative 33–1) onto 33–2a–c allowed the isolation of sulfinamides 33–3a–c in good yields.[200]

Scheme 33

Addition of an Alkyl Radical to Chiral Sulfinimines

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> <class="Chemical">spn>an class="Chemical">alkylation of related <class="Chemical">span class="Chemical">imines can be carried out by using ammonium alkyl bis(catecholato)silicates as the radical precursors under metal-free conditions adopting 4CzIPN as the POC[201] or by using potassium alkyltrifluoroborates in the alkylation of N-phenylimines.[202]

Anot<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>r particular case is t<class="Chemical">spn>an class="Disease">he <class="Chemical">span class="Chemical">alkylative semipinacol rearrangement devised for the synthesis of 2-alkyl-substituted cycloalkanones. The reaction involved the photocatalytic reaction between TMS protected α-styrenyl substituted cyclic alcohol 34–2 and the unactivated bromoalkane 34–1 (Scheme a). The reaction was promoted by the dimeric gold complex [Au2(dppm)2]Cl2. This complex is able to reduce 34–1 (ca. −2.5 V vs SCE) despite having an oxidation potential in the excited state considerably lower for the reaction to occur (ca. −1.63 V vs SCE). This can be explained by the formation an inner sphere exciplex between the excited dimeric catalyst and 34–1 that promotes the otherwise thermodynamically unfeasible redox process, generating an AuI–AuII dimer and 34–4. The combination of the latter species formed an AuIII complex that induces a semipinacol rearrangement coupled with C(sp3)–C(sp3) reductive elimination, which furnished 34–3 in 84% yield (Scheme b).[203]

Scheme 34

Gold Catalyzed Activation of Bromoalkanes

A n class="Chemical">similclass="Chemical">n class="Chemical">ar reaction was later developed starting from cycloalkanol-substituted styrenes and N-acyloxyphthalimides under IrIII photocatalysis.[204]

Formation of a C(sp3)-C(sp2) Bond

Alkenylation

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> reaction between an <class="Chemical">spn>an class="Chemical">alkyl radical with a <class="Chemical">span class="Chemical">cinnamic acid followed by loss of the COOH group is one of the more common approaches to promote an alkenylation reaction. Thus, the radical formed from salt 35–3 attacked the benziodoxole adduct 35–2, synthesized from acrylic acid 35–1. The reaction yielded 83% of the diphenylethylene derivative 35–4 upon a deboronation/decarboxylation sequence (Scheme ).[205] The benziodoxole moiety gave efficient results in promoting the radical elimination step, while other noncyclic IIII reagents were ineffective.

Scheme 35

Alkenylations Mediated by Benziodoxole

Different decn class="Chemical">arboxylative <class="Chemical">n class="Chemical">span class="Chemical">alkenylations have been reported by changing t<spclass="Chemical">n>an class="Disease">he radical source and the photocatalyst (Scheme ). The homolytic cleavage of an alkyl-I bond has been promoted by a CuI complex and the resulting cyclohexyl radical afforded styrene 36–3 in 68% yield upon addition onto cinnamic acid 36–1 (path a).[206] The same product may be formed as well starting from the same substrate by using phthalimide 36–2 under visible light irradiation with the help of an IrIII[207] (path b) or a RuII photocatalyst.[208] As an additional bonus, the formation of adduct 36–3 was obtained with a preferred E configuration.

Scheme 36

Different Strategies Toward Decarboxylative Alkenylations

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> <class="Chemical">spn>an class="Chemical">alkenylation may mimic a <class="Chemical">span class="Disease">Heck reaction as in the visible light-induced Pd-catalyzed reaction between a vinyl (hetero)arene and an α-heteroatom-substituted alkyl iodide or bromide (see Scheme ). Here, the generation of the radical is induced by the reduction of the TMS-derivative 37–1 by the excited Pd0 species. Radical addition onto 37–2 followed by β-H-elimination from the adduct radical delivered allyl silane 37–3 in 81% yield.[209] Noteworthy, the same reaction did not take place under usual thermal Pd-catalysis.

Scheme 37

Heck-Like Alkenylation of an Alkyl Iodide

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alkylclass="Chemical">n>ation of <class="Chemical">spn>an class="Chemical">styrenes <class="Chemical">span class="Chemical">could be carried out using an inexpensive palladium source (Pd(PPh3)4) with no need of any base or classical photocatalyst. The reaction was promoted by visible light, adopting N-hydroxyphthalimides as radical sources.[210] Other visible light Pd promoted alkenylations include the reaction of vinyl arenes with carboxylic acids[211] or tertiary alkyl halides[212] as radical precursors. Other metal catalysts, however, were helpful for the substitution of a vinylic hydrogen atom with an alkyl group. In this respect, a dinuclear gold complex was employed for the activation of an alkyl bromide to promote a photocatalyzed Heck-like reaction.[213] The synergistic combination of a POC and a cobaloxime catalyst promoted the photocatalyzed decarboxylative coupling between 38–1 and styrene 38–2 to give the alkenylated product 38–3 in 82% yield and with a complete E/Z selectivity as illustrated in Scheme .[214]

Scheme 38

Cobaloxime-Mediated Decarboxylative Coupling of Carboxylic Acids with Styrenes

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> addition of t<class="Chemical">spn>an class="Disease">he <class="Chemical">span class="Chemical">alkyl radical may take place even on substituted alkenes via an ipso-substitution reaction. An example is shown in Scheme where a vinyl iodide (39–2) is used for an alkenylation by the reaction with a radical generated from silicate 39–1, obtaining compound 39–3. The RuII photocatalyst in the dual catalytic system has the role of generating the radical, while the Ni0 catalyst activates the C(sp2)-I bond.[215]

Scheme 39

Alkenylation via Alkenyl Iodides

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alkeclass="Chemical">nylclass="Chemical">n>ation of <class="Chemical">spn>an class="Chemical">alkyl <class="Chemical">span class="Chemical">iodide 40–1 can also take place starting from an alkenyl sulfone (40–2). Also in this case, an ipso-substitution is central to the novel bond formation and the Pd0 catalyst formed the radical by a SET reaction with 40–1. After the addition of the radical onto 40–2, the sequence is completed by the elimination of a sulfonyl radical affording 53% yield of 40–3 (Scheme ).[216]

Scheme 40

Alkenylation of an Alkyl Iodide with Alkenyl Sulfones

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alkyl bromidesclass="Chemical">n> were used in <class="Chemical">spn>an class="Chemical">alkenylations by reaction with vinyl sulfones made possible by t<class="Chemical">span class="Disease">he photocatalytic generation of silicon centered radical that in turn formed the alkyl radical by a halogen atom transfer reaction.[217]

Acylation

Acylation owes its importance to t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> possibility to <class="Chemical">spn>an class="Chemical">convert an <class="Chemical">span class="Chemical">alkyl radical into a ketone, a reaction that proceeds in most cases with the intermediacy of an acyl radical.[218,219] A classical approach is based on the homolytic cleavage of an alkyl-I bond followed by carbonylation with CO and reaction with electrophiles of the resulting nucleophilic acyl radical. Scheme illustrates the concept. Irradiation of iodide 41–1 with a Xe lamp in the presence of CO (45 atm) and a Pd0 complex led to an electron transfer reaction which formed an alkyl radical that, upon carbonylation and addition onto phenyl acetylene, gave ynone 41–2 in 63% yield.[220] The reaction is supposed to proceed via a photoinduced electron transfer from the Pd0 catalyst to the iodoalkane, furnishing a PdII species and the alkyl radical. The carbon-centered radical promptly reacts with CO to generate an acyl radical. The PdI catalyst intervenes here again to couple the acyl derivative with the alkyne, preserving the triple bond in the final product. This reaction was later applied to the acylation of styrenes to give the corresponding enones.[221] The electrophilic nature of the alkyne could be exploited if the moiety is placed in the same reagent bearing the iodide. In this case, the first reaction observed was an intramolecular cyclization forming an alkenyl radical which eventually reacted with CO, furnishing an α,β-unsaturated ketone.[222]

Scheme 41

Photocatalyzed Synthesis of Ynones

A reductive step induced t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> generation of t<class="Chemical">spn>an class="Disease">he <class="Chemical">span class="Chemical">alkyl radical through an IrIII-photocatalyzed C–N bond activation in pyridinium salt 42–1 (Scheme ). Trapping of the alkyl radical with CO followed by addition onto 1,1-diphenylethylene gave access to the Heck-type product 42–2 with no interference by the 2,4-dioxo-3,4-dihydropyrimidin-1-yl ring.[223]

Scheme 42

Synthesis of Enones via Photocatalyzed C–N Bond Activation

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> <class="Chemical">spn>an class="Chemical">alkyl radical to be <class="Chemical">span class="Chemical">carbonylated was likewise formed starting from a cycloalkane for the preparation of unsymmetrical ketones via radical addition onto Michael acceptors. The reaction proceeded via a photocatalyzed decatungstate hydrogen atom transfer reaction[224] When cyclopentanones were subjected to the photocatalyzed C–H activation, a regioselective β-functionalization occurred. Thus, 1,4-diketones 43–3a–c were smoothly formed by reaction of the photogenerated acyl radical 43–1 onto Michael acceptors 43–2a–c (Scheme ).[225]

Scheme 43

Photocatalyzed Synthesis of Unsymmetrical Ketones

Unsymmetrical <n class="Chemical">spaclass="Chemical">n class="Chemical">ketoclass="Chemical">nesclass="Chemical">n> have been likewise formed by <class="Chemical">spn>an class="Chemical">carbonylation of <class="Chemical">span class="Chemical">alkyl radicals generated from organosilicates by using 4CzIPN as POC under visible-light irradiation.[226]

<n class="Chemical">spaclass="Chemical">n class="Chemical">Potasclass="Chemical">n class="Chemical">sium alkyltrifluoroborates were extensively used for acylation reactions having re<spclass="Chemical">n>an class="Chemical">course to a dual photocatalytic system. The unstabilized alkyl radical was generated from trifluoroborate 44–1 with the help of an IrIII PC (Scheme ). Meanwhile, the acid 44–2 was converted in situ into a mixed anhydride (by reaction with dimethyl dicarbonate, DMDC) that was activated by a Ni0 complex. Addition of the alkyl radical onto the resulting complex led to the acylated product 44–3.[227] In a similar vein, Ir-photoredox/nickel catalytic cross-coupling reactions were devised by using acyl chlorides[228] and N-acylpyrrolidine-2,5-diones[229] as acylating reagents.

Scheme 44

Dual Catalytic Acylation of Alkyl Trifluoroborates

A n class="Chemical">Ni/<class="Chemical">n class="Chemical">span class="Chemical">Ru, dual-catalyzed amidation proto<spclass="Chemical">n>an class="Chemical">col was possible thanks to the coupling between an alkylsilicate and an isocyanate. Even in the latter case, the alkyl radical attacked the complex formed between the isocyanate and a Ni0 species and, as a result, the mild formation of substituted amides took place.[230]

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> acylation of t<class="Chemical">spn>an class="Disease">he <class="Chemical">span class="Chemical">radical was also exploited for the synthesis of esters. This elegant approach involves the generation of radicals from unactivated C(sp3)–H bonds (e.g., in cycloalkanes). The hydrogen abstraction on cycloalkanes was induced by a chlorine atom released from the photocleavage of the complex formed between chloroformate 45–1 and a Ni0 complex, allowing one to synthesize scaffolds with different ring sizes (45–2a–d in Scheme ).[231]

Scheme 45

Dual Catalytic Acylation of Cycloalkanes

Minisci-Like Reactions

A fundamental transformation for t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> <class="Chemical">spn>an class="Chemical">const<class="Chemical">span class="Chemical">ruction of C(sp)-C(sp) bond is the Minisci reaction, where the functionalization of heteroaromatics took place by substituting a H atom with an alkyl group. The reaction was widely investigated in the last years and mainly involves the functionalization of a nitrogen-containing heterocycle.[232] An interesting example is the methylation reported in Scheme .[94] A methyl radical was formed by using a peracetate such as 46–1. The protonation of 46–1 by acetic acid facilitates a PCET reduction of the peracetate by the IrIII PC. A double fragmentation ensued, and the resulting methyl radical may attack the protonated form of biologically active heterocycles (e.g., fasudil 46–2) in a mild selective manner to afford 46–3 in 43% yield.[94]

Scheme 46

Alkylation of Fasudil

Anot<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>r approach made use of an <class="Chemical">spn>an class="Chemical">alkyl boronic acid as t<class="Chemical">span class="Disease">he radical precursor. The process is initiated by the RuII-photocatalyzed reduction of acetoxybenziodoxole (BI-OAc) that liberated the key species ArCOO• (Scheme ). Upon addition onto an alkyl boronic acid, this ortho-iodobenzoyloxy radical made available the alkyl radical that in turn functionalized pyridine 47–1 in position 2 in 52% yield (47–2, Scheme ).[233]

Scheme 47

Minisci Reaction by Using Alkyl Boronic Acids

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> generation of t<class="Chemical">spn>an class="Disease">he <class="Chemical">span class="Chemical">alkyl radical from boron-containing derivatives was made easier starting from alkyltrifluoroborates. A POC (Acr+Mes) is, however, required, but in all cases, the regioselective functionalization of various nitrogen-containing heterocycles was achieved.[234] A related chemical oxidant-free approach process was later developed where alkyl radicals were formed by merging electro and photoredox catalysis.[235]

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alkyl halidesclass="Chemical">n> are versatile substrates for t<class="Chemical">spn>an class="Disease">he photoinduced functionalization (e.g., butylation) of <class="Chemical">span class="Chemical">lepidine 48–1 (Scheme ). An uncatalyzed redox process is a rare occurrence here, since alkyl halides reduction is more demanding. This drawback can be overcome by the adoption of a dimeric AuI complex (see Scheme ) that upon excitation coordinates an unactivated haloalkane promoting an inner sphere PET. This interaction pushes the activation of R-Br despite its larger Ered with respect to the PC (Scheme a).[236] A different approach promoting the homolytic cleavage of the R-I bond is shown in Scheme b. Decacarbonyldimanganese Mn2(CO)10 was cleaved upon visible light irradiation, and the resulting Mn-based radical was able to abstract the iodine atom from an alkyl iodide thus generating the desired butyl radical. This route was smoothly applied to the late-stage functionalization of complex nitrogen-containing substrates.[237] Moreover, the activation of alkyl halides may be obtained by the photogeneration of a silyl based radical derived by TTMSS (Scheme c, see also Scheme ). The robustness and the mildness of this approach was witnessed by the broad substrate scope and the compatibility of several functional groups present in the radical.[238] The use of acidic conditions (required to make the nitrogen heterocycle more electrophilic) may however be avoided. Excited [Ir(ppy)2(dtbbpy)]PF6 was sufficiently reducing to convert alkyl iodides to alkyl radicals under basic conditions by combining conjugate and halogen ortho-directing effects.[239]

Scheme 48

Photocatalyzed Butylation of Lepidine

In general, <n class="Chemical">spaclass="Chemical">n class="Chemical">lepidiclass="Chemical">neclass="Chemical">n> is t<class="Chemical">spn>an class="Disease">he preferred substrate to test new ways for t<class="Chemical">span class="Disease">he C–H alkylation of heteroarenes. Accordingly, adamantane carboxylic acid 49–1 served for the visible light induced synthesis of 49–3 starting from lepidine 49–2 (Scheme ). An IrIII PC was adopted to alkylate various nitrogen heterocycles, making use of a large excess of persulfate anion as the terminal oxidant (path a).[240] The presence of a PC is not mandatory for the adamantylation reaction with (bis(trifluoroacetoxy)-iodo)benzene as starting material. This compound in the presence of a carboxylic acid gave the corresponding hypervalent iodineIII reagent that upon irradiation generates the alkyl radical. The TFA liberated in the process was crucial for the activation of the nitrogen heterocycle and adduct 49–3 was isolated in 95% yield (path b).[241]

Scheme 49

Decarboxylative Minisci Alkylation

Very recently, an interes<n class="Chemical">spaclass="Chemical">n class="Chemical">ticlass="Chemical">nclass="Chemical">n>g approach for t<class="Chemical">spn>an class="Disease">he generation of <class="Chemical">span class="Chemical">alkyl radicals from the C–C cleavage in alcohols was reported making use of a CFL lamp as irradiation source. The combination of 2,2-dimethylpropan-1-ol (50–1) with benziodoxole acetate (BI-OAc) gave adduct 50–3. Photocatalytic reduction of compound 50–3 released and alkoxy radical that upon fragmentation formed a tbutyl radical that reacted with N-heteroarene 50–2 to form 50–4 in 57% yield (Scheme ).[130]

Scheme 50

Aliphatic Alcohols as Radical Precursors in Minisci Reaction

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> use of hypervalent <class="Chemical">spn>an class="Chemical">iodineIII in promo<class="Chemical">span class="Chemical">ting the decarboxylation of R-COOH was effective in the derivatization of drugs or drug-like molecules. As a result, the quinine analogue 51–2 was formed in a 76% yield from quinine 51–1, utilizing Acr+Mes as the POC (Scheme ).[242]

Scheme 51

Alkylation of Quinine

<n class="Chemical">spaclass="Chemical">n class="Chemical">Azolesclass="Chemical">n> can be adamantylated star<class="Chemical">spn>an class="Chemical">ting from <class="Chemical">span class="Chemical">adamantane carboxylic acid by a dual catalytic approach (Acr+Mes as the POC and [Co(dmgH)(dmgH2)Cl2] as the cocatalyst)[243] or simply C2-alkylated under photoorganocatalyzed conditions.[244]

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> photocatalyzed reduction of <class="Chemical">spn>an class="Chemical">N-(acyloxy)phthalimide 52–1 induced by an IrIII* <class="Chemical">span class="Chemical">complex is an alternative approach for the functionalization of N-heterocycles such as 2-chloroquinoxaline 52–2 to form the cyclopentenyl derivative 52–3 (Scheme ).[245]

Scheme 52

Cyclopentenylation of 2-Chloroquinoxaline

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> reductive pathway is feasible even w<class="Chemical">spn>an class="Disease">hen t<class="Chemical">span class="Disease">he generation of the alkyl radical was carried out starting from the redox-active pyridinium salt 53–1. In this case, the obtained cycloalkyl radical gave a regioselective addition onto 6-chloroimidazo[1,2-b]pyridazine 53–2 to yield 53–3 under mild conditions (Scheme ).[70]

Scheme 53

Functionalization of 6-Chloroimidazo[1,2-b]pyridazine

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> <class="Chemical">spn>an class="Chemical">alkyl radical <class="Chemical">span class="Chemical">could be formed even from simple hydrocarbons via hydrogen atom transfer reaction. A valuable example is reported in Scheme . The hypervalent iodine oxidant PFBI–OH is reduced by an excited RuII complex generating a carbonyloxy radical that acted as hydrogen atom abstracting agent. Functionalization of isoquinoline 54–2 by the resulting radical (derived from 54–1) afforded adduct 54–3 in 65% yield (>15:1 dr).[246] The high selectivity observed in the functionalization of 54–1 was ascribed to the slow addition of the tertiary alkyl radical possibly formed onto 54–2.[246] The direct (rather than indirect) C–H cleavage in cycloalkane was possible by using decatungstate anion as PC. Various nitrogen-containing heterocycles were then easily derivatized even under simulated solar light irradiation.[247]

Scheme 54

PFBI–OH Mediated Minisci Reaction

PFBI–OH was likewise used for t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> remote <class="Chemical">spn>an class="Species">C(sp3)–H <class="Chemical">span class="Disease">heteroarylation of alcohols (Scheme ). As an example, the reaction of pentanol with PFBI–OH gave adduct 55–1 that was reduced by the photocatalyst releasing the alkoxy radical 55–2. 1,5-HAT and addition onto protonated phthalazine 55–3 afforded adduct 55–4 and the functionalized heterocycle 55–5 from it in 72% yield after sequential oxidation and deprotonation.[248]

Scheme 55

Remote C(sp3)–H Heteroarylation of Alcohols

As previously stressed, an acid is often requn class="Chemical">ired for aclass="Chemical">n efficieclass="Chemical">nt Miclass="Chemical">nisci-like reactioclass="Chemical">n. To over<class="Chemical">n class="Chemical">span class="Chemical">come this problem t<spclass="Chemical">n>an class="Disease">he alkylation may be carried out on the corresponding N-oxide derivatives as it is the case of pyridine N-oxides (56–2, Scheme ). The radical is generated from a trifluoroborate salt (56–1) and the alkylation is regioselective in position 2 (forming compound 56–3).[249] The process is efficient thanks to the photocatalytic degradation of BI-OAc that promoted a hydrogen abstraction, operated by the resulting carbonyloxy radical, on the Minisci radical cation adduct.

Scheme 56

Minisci Alkylation of Pyridine Oxides

On t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> ot<class="Chemical">spn>an class="Disease">her hand, t<class="Chemical">span class="Disease">he pyridine N-oxide 57–1 can be acylated in situ with suitable acyl chlorides to furnish the electron-poor 57–2a–c derivatives. Photocatalytic reduction of these intermediates leads to the generation of alkyl radicals prone to attack the pyridine nucleus itself in the ortho position resulting in a decarboxylative alkylation (57–3a–c, Scheme ).[122]

Scheme 57

Decarboxylative Alkylation of Heterocycles

Ipso-Substitution Reactions

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> forging of an <class="Chemical">spn>an class="Chemical">alkyl-class="Chemical">sp2 bond (e.g., an <class="Chemical">span class="Chemical">alkyl-Ar bond) is undoubtfully one of the most crucial goals pursued by a synthetic organic chemist. Alkyl radicals generated via different mild routes can be successfully employed for the arene ipso functionalization, given the presence of a suitable group X on the (hetero)aromatic ring that directs the selective formation of a new Ar–C bond at the expense of an Ar-X bond. Dual catalysis (with the help a Ni-based complex) is one of the preferred approaches.

In a recent example, t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> <class="Chemical">spn>an class="Chemical">hydrogen atom transfer ability of t<class="Chemical">span class="Disease">he excited TBADT catalyst (see also Scheme ) is used to form an alkyl radical starting from different aliphatic moieties (see Scheme ).[250] The combined action of the tungstate anion and the nickel catalyst (Ni(dtbbpy)Br2) allowed the coupling of (hetero)aromatic bromides with unactivated alkanes, overcoming their high bond dissociation energies (ca. 90–100 kcal/mol) and low acidities. Both linear (41–56% yield) and cyclic (57–70% yield) alkanes could be functionalized with a vast range of competent partners. Interestingly, the radicals are generated preferentially on the less sterically demanding secondary carbons in alkanes, affording a remarkable selectivity.

Scheme 58

Dual TBADT-Ni Catalysis for the Synthesis of Pyridyl-Functionalized Bicycles

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> s<class="Chemical">spn>an class="Chemical">cope of this method <class="Chemical">span class="Chemical">could be proved by the functionalization of natural products and drugs, such as in the preparation of the bicyclic derivative 58–3a (61% yield) and the N-Boc protected epibatidine alkaloid 58–3b (28%, Scheme ).[250] A very similar approach was later reported for the dual photocatalytic formation of an Ar–C bond starting from aryl bromides and cycloalkanes.[251]

Anot<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>r dual-catalytic approach allowed t<class="Chemical">spn>an class="Disease">he <class="Chemical">span class="Chemical">coupling reaction of aryl bromides (59–2, Scheme ) and alkyl sulfinates (59–1), in the presence of Ni(COD)2 and tetramethylheptanedione (TMHD, Scheme a) to give 59–3 in 84% yield under air.[104] The photogenerated radical was trapped by the Ni complex that mediated the coupling with the aryl halide 59–2. The method was then applied to the synthesis of 59–5, selective ATP-competitive inhibitors of the casein kinase 1δ, an enzyme related to the regulation of the circadian rhythm (Scheme b).[104]

Scheme 59

Dual Catalytic Cross-Coupling of Aryl Bromides with Alkyl Sulfinates

A very n class="Chemical">similclass="Chemical">n class="Chemical">ar strategy to access C(sp3) radicals involves the photoredox induced cleavage of alkyl oxalate 60–1, starting from the corresponding alcohols (see Scheme , see also Scheme ).[252] The rapid in situ formation of the oxalate (without purification) was followed by the metallaphotoredox sequence based on Ni catalysis, allowing to obtain the C(sp2)-C(sp3) coupling to give derivatives 60–3a–e in good yields.

Scheme 60

Coupling of Alkyl Oxalates with Aryl Bromides

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> advantage of t<class="Chemical">spn>an class="Disease">he use of <class="Chemical">span class="Chemical">potassium and ammonium bis-catecholato silicates relies in the smooth generation of unstabilized primary and secondary alkyl radicals to be engaged in dual catalysis.[253,254] An example is the consecutive functionalization of bromo(iodo)arene 61–2 (Scheme , see also Scheme ) for the preparation of 61–4 where the radical (from 61–1) is trapped by Ni0 (stabilized by a phenanthroline ligand). The synthesis of 61–3 can be achieved in high yields on 10 mmol scale with reduced effect on yield (75%) and selectivity (98%). The crude bromide 61–3 was further functionalized by a second Ni/photoredox cross-coupling of the alkylsilicate 61–5, affording product 61–4 in 66% yield.[255] The procedure was extended successfully to alkyl triflates, tosylates and mesylates,[256] and to brominated borazaronaphthalene cores.[257] The latter approach was crucial to access previously unknown isosteres of azaborines.

Scheme 61

Consecutive Functionalization of Bromo-Iodo Arenes with bis-Catecholato Silicates

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> action of a <class="Chemical">spn>an class="Chemical">silyl radical on an <class="Chemical">span class="Chemical">alkyl bromide 62–1 forms an alkyl radical that, again with the help of a Ni based catalyst, reacted with aryl bromides 62–2 (Scheme , see also Scheme ). The scope of products 62–3 that can be obtained is varied and includes both aromatic and heteroaromatic substrates, along with cycloalkanes of different size.[71]

Scheme 62

Ir/Ni Complex Mediated Coupling Between Alkyl and Aryl Bromides

A pen class="Chemical">culiclass="Chemical">n class="Chemical">ar case is when the ipso-substitution took place via a radical rearrangement such as shown in Scheme .[258] Thus, the heteroaromatic sulfonamide 63–1 was subjected to the Finkelstein reaction, obtaining the corresponding iodide 63–2 that acted as the source of radical able to induce a Smiles rearrangement via 63–5. Intermediate 63–5 has lost its aromaticity; however, the radical has become tertiary, gaining further stabilization from the ester group nearby. Restoration of the aromaticity is followed by a presumable hydrogen atom transfer to obtain compound 63–6 in 95% yield.[258]

Scheme 63

Photocatalyzed Smiles Rearrangement

Dual photoredox/<n class="Chemical">spaclass="Chemical">n class="Chemical">class="Chemical">nickelclass="Chemical">n> catalysis was successfully applied to <class="Chemical">spn>an class="Chemical">couple β-trifluoroborato<class="Chemical">span class="Chemical">ketones 64–1 with aryl bromides 64–2a–f (Scheme , see also Scheme ). Arylated compounds 64–3a–f were efficiently prepared with substituents of different electronic nature on the aryl ring.[259]

Scheme 64

Photoredox/Nickel Dual Catalytic Coupling of β-Trifluoroboratoketones with Aryl Bromides

<n class="Chemical">spaclass="Chemical">n class="Chemical">Potasclass="Chemical">n class="Chemical">sium tetrafluoroborate salts have been applied to generate se<spclass="Chemical">n>an class="Chemical">condary alkyl radicals via Ir photocatalysis coupled with Ni.[260] However, they were found to be likewise suitable for cross-coupling reactions devoted to the forging of quaternary carbon centers (Scheme ) without the need of using reactive organometallic species.[261] In the adamantylation of bromides 65–1a–d better yields were obtained when the aryl ring was substituted with electron-withdrawing groups.

Scheme 65

Adamantylation of Aryl Bromides

An interes<n class="Chemical">spaclass="Chemical">n class="Chemical">ticlass="Chemical">nclass="Chemical">n>g application of this synt<class="Chemical">spn>an class="Disease">hetic strategy is t<class="Chemical">span class="Disease">he functionalization of 7-azaindole pharmacophores with cycloalkyl scaffolds to improve the drug likeness of the azaindole core structure. Different potential drug candidates (66–3a–c, Scheme ) were prepared via dual photocatalysis in a flow setup varying the dimension and substitution of the ring.[262]

Scheme 66

Functionalization of 7-Azaindole Pharmacophores in Flow

In a n class="Chemical">similclass="Chemical">n class="Chemical">ar way, a DHP-functionalized cyclohexene 67–1 was used to generate a secondary alkyl radical. In this case, the authors promoted the oxidation of 67–1 by using the strongly oxidizing 4CzIPN photocatalyst. Coupling with bromopyridine 67–2 gave substituted pyridine 67–3 in moderate yields (Scheme , see also Scheme ).[118]

Scheme 67

Coupling of DHP-Cyclohexene with Cyanobromopyridine

<n class="Chemical">spaclass="Chemical">n class="Chemical">DHPclass="Chemical">n>-derivatives (68–1) may be used in ipso-substitution reaction even in t<class="Chemical">spn>an class="Disease">he absence of a photocatalyst (Sc<class="Chemical">span class="Disease">heme ). Violet light LED illumination directly excited didehydropyridine 68–1, fueling electrons to the NiII species which formed the catalytic competent Ni0 along with the desired radical by the fragmentation of 68–1. Noteworthy, the alkyl aromatic 68–3 was then formed where the use of electron-withdrawing groups on the ring contributes to the good yields of the process.[263]

Scheme 68

Photocatalyst-Free Activation of DHPs

Formation of a C(sp3)-C(sp) Bond

Cyanation

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alkyl radicalsclass="Chemical">n> have been used for t<class="Chemical">spn>an class="Disease">he synt<class="Chemical">span class="Disease">hesis of alkyl nitriles by using different cyanide sources. The cyanation may be carried out in the presence of cyanide anion as tetrabutyl ammonium salt (TBACN). The C–C bond formation here may be carried out under very mild conditions by using the inexpensive precatalyst CuI, starting from unactivated alkyl chlorides (e.g., 69–1a–c, Scheme ). Probably, a CuI-cyanide adduct is the species that was excited and engaged an electron transfer reaction with the alkyl halide to form a CuII-cyanide adduct. This intermediate combines with the alkyl radical formed to release nitriles 69–2a–c. The CuI-halide complex formed in the reaction restores the initial photocatalyst by exchange with the cyanide anion.[264]

Scheme 69

CopperI-Mediated Synthesis of Nitriles

<n class="Chemical">spaclass="Chemical">n class="Chemical">TMSCclass="Chemical">n class="Chemical">N was instead used for t<spclass="Chemical">n>an class="Disease">he remote δ-C(sp3)-H cyanation of alcohols under Ir/Cu-photocatalyzed conditions. The reduction of an N-alkoxypyridinium salt generated an alkoxy radical that upon intramolecular 1,5-HAT formed an alkyl radical that is cyanated with the help of the copper catalyst.[265]

A typical cyanation procedure, however, makes use of <n class="Chemical">spaclass="Chemical">n class="Chemical">tosyl cyaclass="Chemical">nideclass="Chemical">n> as cyana<class="Chemical">spn>an class="Chemical">ting agent. Thus, t<class="Chemical">span class="Disease">he radical obtained by oxidation of trifluoroborate 70–1a (by excited Acr+Mes)[266] or acid 70–1b (by riboflavin tetraacetate RFTA)[267] was trapped by tosyl cyanide to afford nitrile 70–2 by a substitution reaction (Scheme ).

Scheme 70

Cyanation of (a) Trifluoroborates and (b) Carboxylic Acids

A related <n class="Chemical">spaclass="Chemical">n class="Chemical">Ruclass="Chemical">n>II-photocatalyzed cyanation employing Ts-CN star<class="Chemical">spn>an class="Chemical">ting from <class="Chemical">span class="Chemical">alkyl trifluoroborates but requiring BI-OAc as a mild oxidant has been likewise reported.[268]

An elegant way to forge an <n class="Chemical">spaclass="Chemical">n class="Chemical">alkylclass="Chemical">n>-CN bond required t<class="Chemical">spn>an class="Disease">he photocatalyzed elaboration of <class="Chemical">span class="Chemical">cyanohydrines 71–1a–d. At first, the interaction of the OH group with the sulfate anion (generated by the decomposition of persulfate anion) allowed its oxidation by a proton-coupled electron transfer (PCET) process promoted by an in situ formed IrIV species. Alkoxy radicals 71–2a–d were then formed and promoted a regioselective cyanation of remote C(sp3)–H bonds by a 1,5-HAT followed by cyano migration to form cyanoketones 71–3a–d (Scheme ).[269]

Scheme 71

Photocatalyzed Cyano Migration in Cyanohydrines

Alkynylation

Dn class="Chemical">irect alkyclass="Chemical">nylatioclass="Chemical">n of photogeclass="Chemical">nerated <class="Chemical">n class="Chemical">span class="Chemical">alkyl radicals <spclass="Chemical">n>an class="Chemical">could be accomplished utilizing a reagent or catalyst that activates the alkyne moiety, making it more prone to the forging of a novel C(sp3)–C(sp) bond. One of the first strategies that were employed made use of benziodoxole-functionalized alkynes to promote the reaction with the alkyl radical.[270] A representative case is illustrated in Scheme . [Ru(bpy)3](PF6)2 promoted the alkyl radical formation from trifluoroborate salt 72–1 that upon addition onto the alkynyl derivatives 72–2a–d induced the alkynylation via the intermediacy of vinyl radicals 72–3a–d. This deboronative alkynylation strategy could be performed in neutral DCM:water (1:1) at room temperature, giving access to the alkynylation of primary, secondary, and tertiary derivatives. To further prove the mildness of the conditions used, the authors carried out the reaction in PBS at pH 7.4 in the presence of biomolecules such as amino acids, but also single-stranded DNA and proteins (e.g., bovine serum albumin), obtaining satisfactory yields ranging from 68 to 86% of selectively alkynylated product.[270]

Scheme 72

Alkynylation of Alkyl Trifluoroborates

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> nature of t<class="Chemical">spn>an class="Disease">he substituents on t<class="Chemical">span class="Disease">he alkynylbenziodoxole reagent were proved to determine the outcome of the alkynylation process. The electron-rich compounds performed better in the photocatalyzed transformation, both as radical acceptor and oxidative quencher of the RuII* photocatalyst.[271]

A n class="Chemical">similclass="Chemical">n class="Chemical">ar strategy to the one mentioned before consists in the IrIII-photoredox-catalyzed alkynylation of carboxylic acids 73–2 (see Scheme a, path a).[272,273] In this case benziodoxole derivatives 73–1 were again used to activate the sp carbon of the alkyne to the radical attack, affording good yields of products 73–3. Following these results, they developed a reaction to synthesize ynones 73–4 utilizing the same reaction conditions in the presence of gaseous CO (see Scheme a, path b and Section ). Gram-scale reactions and late-stage functionalization of natural terpenoids such as ursolic acid (73–5, Scheme b) were likewise reported.[273]

Scheme 73

IrIII-Catalyzed Alkynylation of Carboxylic Acids

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alkyclass="Chemical">nyl sulfoclass="Chemical">nesclass="Chemical">n> were extensively employed as alkynyla<class="Chemical">spn>an class="Chemical">ting reagents, with a mechanism like t<class="Chemical">span class="Disease">he one described in Scheme . Alkynyl phenyl sulfone was used in combination with N-acyloxyphthalimide derivatives as radical precursors in a RuII-photocatalyzed reaction that gave direct access to TIPS-substituted alkynes.[274]N-Phthalimidoyl oxalates and tolyl alkynyl sulfones were found to be competent for the reaction (even for the preparation of internal alkynes having quaternary carbons),[275,276] the latter even in combination with pyridinium salts as radical precursors.[277] The consecutive photoredox decarboxylative coupling of doubly functionalized adipic acid derivatives with alkynyl phenyl sulfones induced the cascade formation of interesting cyclic derivatives with an exo double bond (Scheme ). In this case, compound 74–1 underwent two efficient consecutive photoredox decarboxylative couplings leading first to alkyne 74–3 that it was subjected to radical cyclization to form radical 74–4 and styrene 74–5 from it.[278] The authors reported the formation of five-membered rings via the consecutive formation of two C–C bonds, along with one example showing the application to the synthesis of six-membered derivatives (31% yield).[278]

Scheme 74

Cascade Double Alkynylation of Functionalized Adipic Acids

In rn class="Chemical">are iclass="Chemical">nstaclass="Chemical">nces <class="Chemical">n class="Chemical">span class="Chemical">alkynyl bromides <spclass="Chemical">n>an class="Chemical">could be used as sp counterpart in the radical addition of alkyl derivatives obtained from the oxidative decomposition of various Hantzsch esters under visible light conditions promoted by 4CzIPN.[279]

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> versatility of t<class="Chemical">spn>an class="Disease">he photocatalytic method, however, allowed to obtain functionalized <class="Chemical">span class="Chemical">alkynes starting from terminal alkynes (Scheme ). The first approach involves the UV light induced cleavage of the C–I bond in iodide 75–1 (used in large excess) in basic aqueous media. Addition of the cyclohexyl radical onto alkyne 75–2 followed by the incorporation of the iodine atom gave vinyl iodide 75–3. The strong basic conditions used (NaOtBu) coupled with heating (up to 50 °C) favored an elimination of HI to yield the desired alkyne 75–4 under metal-free conditions (Scheme a).[280] Visible-light (450 nm) was used in the copper-catalyzed coupling of an alkyl iodide (75–5) and again a terminal alkyne (75–6, Scheme b). The success of the reaction was ascribed to the use of terpyridine ligand 75–8 that avoided the photoinduced copper-catalyzed polymerization of the starting substrates. Probably, the reaction started by the excitation of the first formed copper acetylide that upon SET with 75–5 promoted the synthesis of alkyne 75–7 in high yields.[281]

Scheme 75

Alkylation of Terminal Alkynes

Formation of a C(sp3)-Y Bond

C–B Bond

Borylation of an <n class="Chemical">spaclass="Chemical">n class="Chemical">alkylclass="Chemical">n> derivative to access differently substituted <class="Chemical">spn>an class="Chemical">boron <class="Chemical">span class="Chemical">containing compounds can be carried out under mild conditions, employing different photochemical approaches. Thus, the alkyl radical formed from an N-hydroxyphthalimide 76–1 (derived from dehydrocholic acid) may be trapped either by bis(pinacolato)diboron (B2pin2) to give the corresponding alkyl pinacol boronates 76–2 (Scheme , path a) or by tetrahydroxydiboron (B2(OH)4) followed by treatment with KHF2 to give alkyl tetrafluoroborates (Scheme , path b).[282]

Scheme 76

Photocatalyzed Borylation of N-Hydroxyphthalimides

A vn class="Chemical">ariatioclass="Chemical">n of t<class="Chemical">n class="Chemical">span class="Disease">he previous methodology involves t<spclass="Chemical">n>an class="Disease">he irradiation of N-hydroxyphthalimide esters 77–2 in the presence of B2cat2 (77–1) with the help of N,N-dimethylacetamide (DMAc) as the solvent under uncatalyzed conditions (Scheme ). These components formed a heteroleptic ternary complex able to be excited by blue light and ultimately leading to the corresponding benzo[1,3,2]dioxaborole 77–4 that upon treatment with pinacol and TEA released the desired pinacol boronic ester 77–3. The functionalization of a series of drugs and natural products, such as pinonic acid and fenbufen were likewise effective, underlying the broad scope and functional group tolerance of the method.[283]

Scheme 77

Photocatalyst-Free Borylation

Two related approac<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>s were later developed and involve t<class="Chemical">spn>an class="Disease">he irradiation of t<class="Chemical">span class="Disease">he ternary complex formed by differently substituted N-alkyl pyridinium salts, B2cat2 and DMAc. The reaction gave again pinacol boronic esters in what is considered a deaminative protocol for the borylation of aliphatic primary amines since the latter compounds were used for the synthesis of the pyridinium salts.[284−286]

Interes<n class="Chemical">spaclass="Chemical">n class="Chemical">ticlass="Chemical">nclass="Chemical">n>gly, <class="Chemical">spn>an class="Chemical">2-iodophenyl thionocarbonates were later adopted as <class="Chemical">span class="Chemical">radical precursor for the preparation of boronic ester via photocatalyzed reaction with B2cat2 (Scheme ).[95] The strategy is based on the photoinduced reduction of compound 78–1 that upon iodide anion elimination formed aryl radical 78–2 that underwent a 5-endo-trig cyclization causing the release of benzo[d][1,3]oxathiol-2-one 78–3 and alkyl radical 78–4. Usual borylation gave boronic ester 78–5 in 85% yield.

Scheme 78

Borylation of 2-Iodophenyl Thionocarbonates

C–N Bond

Diverse st<n class="Chemical">spaclass="Chemical">n class="Chemical">ruclass="Chemical">n>ctural motifs based on t<class="Chemical">spn>an class="Disease">he C–N bond such as <class="Chemical">span class="Chemical">hydrazine and hydrazide cores were accessed by the photochemical addition of alkyl radicals onto the N=N of azodicarboxylates. TBADT-photocatalyzed HAT was applied to synthesize hydrazines by the coupling of cycloalkyl radicals with diisopropyl azodicarboxylate (DIAD). A synthetically challenging three component reaction can be achieved in the presence of CO, allowing the synthesis of the corresponding hydrazides.[287]

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> C–H amination can be smoothly achieved even star<class="Chemical">spn>an class="Chemical">ting from light <class="Chemical">span class="Chemical">hydrocarbons, such as methane (79–1, Scheme ), with ditert-butylazodicarboxylate (DBAD, 79–2) in the presence of CeIII salts. This inexpensive photocatalyst furnished the desired product 79–3 in 63% yield, with a turnover number up to 2900. The authors proposed a ligand-to-metal charge transfer excitation between the cerium salt and trichloroethanol as the source of alkoxy radicals that acted as hydrogen atom transfer agents.[288]

Scheme 79

Photocatalyzed Amination of Methane

<n class="Chemical">spaclass="Chemical">n class="Chemical">Amiclass="Chemical">nated alkaclass="Chemical">nesclass="Chemical">n> can be obtained by reac<class="Chemical">spn>an class="Chemical">ting <class="Chemical">span class="Chemical">aliphatic carboxylates with DIAD making use of Acr+Mes as a photoredox catalyst.[289] A cerium catalyst was adopted for the generation of several alkyl radicals starting from carboxylic acids, under basic conditions, allowing for the functionalization of a broad range of substrates, including natural products such as drugs like gemfibroxil (80–2) and tolmetin (80–1, Scheme ).[290]

Scheme 80

Cerium-Catalyzed Decarboxylative Amination

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> N=N bond of differently substituted <class="Chemical">spn>an class="Chemical">azobenzenes (81–1a–e) can be functionalized on both <class="Chemical">span class="Chemical">nitrogens with a tandem N-methylation and N-sulfonylation, by cleavage of DMSO by UV irradiation of the Fenton reagent (FeSO4/H2O2,Scheme ).[105]

Scheme 81

Tandem N-Methylation and N-Sulfonylation of Azobenzenes

Synt<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>sis of <class="Chemical">spn>an class="Chemical">amides can be achieved recurring to <class="Chemical">span class="Chemical">copper photocatalysis. Secondary alkyl bromide 82–1 could be efficiently coupled with cyclohexane carboxyamide 82–2 in 90% yield using CuI in catalytic amounts (Scheme a). The authors were able to isolate the catalytic species (a copper–amidate complex), formed by the assembly of four copper ions and four amides.[291]

Scheme 82

Photocatalyzed Synthesis of (a) Amides and (b) Carbamates

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> same group reported t<class="Chemical">spn>an class="Disease">he functionalization of <class="Chemical">span class="Chemical">carbamates with secondary alkyl bromides by shifting the wavelength of irradiation in the visible region by developing a tridentate carbazolide/bisphosphine ligand 82–4 for the copper catalyst thus able to prepare Boc-pregnenolone 82–5 in 90% yield (Scheme b).[292] A variation of this protocol was applied to the synthesis of amines, using secondary unactivated alkyl iodides and CuI/BINOL as the catalytic system.[293]

Several reagents can be used as an <n class="Chemical">spaclass="Chemical">n class="Chemical">azideclass="Chemical">n> source to synt<class="Chemical">spn>an class="Disease">hesize synt<class="Chemical">span class="Disease">hetically valuable C–N3 bonds. Tertiary aliphatic C–H bonds can be selectively functionalized via Zhdankin azidoiodane reagent 83–2. Visible light was used to excite Ru(bpy)3Cl2 that cleaves the labile I–N3 bond, triggering the cascade of radical reactions that leads to the product formation (Scheme ). The selectivity and compatibility of this reaction with different groups is underlined by the conversion of the dipeptide 83–1 to 83–3 in 30% yield.[294] A related C–H azidation was performed by using tosyl azide as an alternative azide source with the help of 4-benzoylpyridine to promote the photocatalytic C–H cleavage in various cycloalkanes.[295]

Scheme 83

Azidation of Tertiary Aliphatic C–H Bonds

Anot<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>r example of t<class="Chemical">spn>an class="Disease">he functionalization of unactivated C–H bonds is depicted in Sc<class="Chemical">span class="Disease">heme making use of tosyl azide 84–2. The reaction needs the intermediacy of an oxygen radical center on a phosphate group, previously oxidized by the action of the mesityl acridinium photocatalyst 84–3. This allows the C–H to C–N3 conversion in menthol benzoate 84–1 to give azide 84–4 in a satisfying yield, with a regioselectivity favoring the more electron-rich tertiary position.[296]

Scheme 84

Photocatalyzed C–H to C–N3 Conversion

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> synt<class="Chemical">spn>an class="Disease">hesis of <class="Chemical">span class="Chemical">amines is undoubtedly more challenging to be dealt with, relying on radical chemistry. However, several strategies were developed to effectively forge this fundamental functional group. A classic reaction for the synthesis of amine is the Curtius reaction that has the drawback in handling of potentially dangerous azides. A dual copper/photoredox catalytic approach mimicked this process for the obtainment of N-protected amines from the N-hydroxyphthalimide ester of cholic acid triacetate 85–1 (Scheme , see also Scheme ). The alkyl radical was again formed by CuI-photocatalyzed reduction of 85–1, but this recombine with the CuII–phthalimide complex formed to release 85–2 (52% yield) by a formal decarboxylation process. A great variety of functional groups are compatible with this reaction including steroidal structures.[297]

Scheme 85

Decarboxylative C–N Coupling in Cholic Acid Triacetate

A very interes<n class="Chemical">spaclass="Chemical">n class="Chemical">ticlass="Chemical">nclass="Chemical">n>g approach to synt<class="Chemical">spn>an class="Disease">hesize β-amino<class="Chemical">span class="Chemical">alcohols from the unfunctionalized alcohol 86–1 relies on the introduction of a radical relay chaperone to direct the C–H functionalization of the β position of the OH group (Scheme ). Imidate radicals can be accessed via the photodecomposition of PhI(OAc)2. A transient sp2N-centered radical is generated from 86–2, which allows a 1,5-hydrogen atom transfer. A source of iodine promotes the formal transfer of an iodine radical to the β-position to the imidate, followed by cyclization to obtain 86–3 which can be promptly hydrolyzed to 86–4. The nature of the substituents on acetimidate 86–2 may affect the overall yield.[298]

Scheme 86

Radical Relay Chaperone Strategy Driven by the Photodecomposition of PhI(OAc)2

Dn class="Chemical">irect cross-<class="Chemical">n class="Chemical">span class="Chemical">coupling between <spclass="Chemical">n>an class="Chemical">alkyl carboxylic acids and nitrogen nucleophiles can be achieved by dual copper/photoredox catalysis through iodonium activation. The scope of the transformation is broad and applicable to a diverse array of nitrogen nucleophiles such as heterocycles, amides, sulfonamides, and anilines to give the corresponding C–N coupling product in excellent yields on short time scales (5 min to 1 h). The high regioselectivity obtained in late stage functionalization of complex pharmaceuticals such as Skelaxin 87–2 (to give 87–3 in 90% yield from 87–1, Scheme , see also Scheme ) gave an idea of the importance of the approach.[299]

Scheme 87

Late Stage Functionalization of Skelaxin

n class="Chemical">Similclass="Chemical">n class="Chemical">ar strategies were explored for the synthesis of amines via C(sp3)–N cross-coupling combining a copper catalyst and the action of a photoredox catalyst by using anilines[300] or benzophenone imines[301] as nitrogen source. Hydroxylamines were instead formed under photoorganocatalytic conditions by reaction of carboxylic acids and nitrosoarenes.[302]

C–O Bond

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> C–O bond formation is without doubt a prerogative of polar c<class="Chemical">spn>an class="Disease">hemistry. However, t<class="Chemical">span class="Disease">here are examples of photochemically driven reactions making use of an alkyl radical for the introduction of different oxygen-containing functional groups. In Scheme , the nonenolizable ester 88–1 is transformed into 88–2 via a photochemically promoted decarboxylation of the NPhth-ester (see also Scheme ) in the presence of Hantzsch ester to yield a tertiary radical. The intermediate is promptly quenched by TEMPO, affording 88–2 in 91% yield, in a multigram scale reaction.[303]

Scheme 88

Decarboxylative Oxygenation of Phthalimide Esters

A n class="Chemical">similclass="Chemical">n class="Chemical">ar reaction was employed to synthesize alkyl aryl ethers, given their importance in medicinal and agricultural chemistry. A tandem photoredox and copper catalysis approach allows the decarboxylative coupling of alkyl N-hydroxyphthalimide esters (NHPI) with phenols (89–2Scheme ). Various NHPI esters of different drugs and natural products easily underwent a late-stage decarboxylative etherification. As an example, the chlorambucil derivative 89–1 was converted into the corresponding 2-MeO phenyl ether 89–3 in 49% yield.[304]

Scheme 89

Decarboxylative C(sp3)-O Cross-Coupling

Following a n class="Chemical">similclass="Chemical">n class="Chemical">ar strategy, carboxylates are converted into alcohols via a photocatalytic decarboxylative hydroxylation mediated by the mesityl acridinium salt. In this case, molecular oxygen is used as the oxidant, to promote the formation of the desired C–O bond. Since the reactions gave mainly a mixture of ketones and hydroperoxides, reduction in situ by sodium borohydride allowed the synthesis of alcohols in good yields.[305] A decarboxylative hydroxylation may be carried out with the intermediacy of Barton esters that upon irradiation in oxygen-saturated toluene followed by treatment with P(OEt)3 afforded an alcohol intermediate for the total synthesis of Crotophorbolone.[306] The more challenging oxidation of unactivated alkanes to alcohols or ketones can be achieved through a photoelectrochemical approach, as testified by the C–H bond activation of cyclohexane to prepare a mixture of cyclohexanone and cyclohexanol (the so-called KA oil) with high partial oxidation selectivity (99%) and high current utilization ratio (76%). The highest current ratio was obtained illuminating the solution with 365 nm wavelength.[307] Decatungstate photocatalysis was efficiently applied to oxidize activated and unactivated C–H bonds. Taking advantage of a microflow reactor setup, a late stage regioselective CH2/C=O conversion in several natural compounds, such as artemisinin 90–1 to form artemisitone-9 90–2 was readily pursued even in a 5 mmol scale (Scheme ).[308]

Scheme 90

Late Stage Regioselective Carbonylation of Artemisinin

C-Halogen Bond

<n class="Chemical">spaclass="Chemical">n class="Chemical">Halogeclass="Chemical">nclass="Chemical">n>ation of <class="Chemical">spn>an class="Chemical">alkanes through a <class="Chemical">span class="Chemical">radical reaction under UV irradiation is one of the core pathways to chemically activate a paraffin. Industrially, chlorine gas is used to functionalize methane. A major drawback of the classical chain reaction using either Cl2 or Br2 under direct irradiation is the formation of di or polyhalogenated products. The application of microflow technology in combination with visible light irradiation (with an absorption maximum in the near UV at ca. 350 nm) allowed the monobromination of different alkanes with molecular bromine. High selectivity for the monobrominated compound and excellent overall yields (between 60 and 99%) could be achieved for secondary and tertiary alkanes, along with primary benzylic positions.[309]

Chlorination with molen class="Chemical">culclass="Chemical">n class="Chemical">ar chlorine, on the other hand, suffers from the low yields of the reaction, typically around 50%, from the high concentrations of HCl generated in the process and from the toxicity of the chlorine gas itself. However, when Cl2 was generated by mixing NaClO with HCl and the chlorination took place under flow conditions, efficient C–H to C–Cl conversion resulted.[310,311] A photochemical alternative using NaCl as chlorine source was developed.[312] In the reaction, Cl2 was formed in situ by oxidation of the chloride anion with oxone. The monochlorination of cyclohexane 91–1 to give 91–2 could be obtained in 93% isolated yield thus overcoming the limitation of the classical chlorination process with chlorine gas (Scheme , see also Scheme ).

Scheme 91

Photoinduced Monochlorination of Cyclohexane

Fluorination is essential to modern medicinal c<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>mistry, both as a viable way to insert radiotracers or to deactivate class="Chemical">spn>ecific degradation pathways in drugs. Photocclass="Disease">hemistry is a reliable tool to achieve t<class="Chemical">span class="Disease">he fluorination of C–H bonds, following different strategies. Excited TBADT may formed a radical intermediate (from unactivated alkanes) that abstracts the fluorine atom from the labile N–F bond of the fluorinating agent N-fluorobenzenesulfonimide (NFSI). An N-centered radical resulted which closes the radical cycle oxidizing the reduced photocatalyst. Acetate 92–1 was fluorinated in 40% yield following this procedure to yield 92–2 (Scheme ). The reaction applied to sclareolide, however, was not selective and gave a mixture of fluorinated regioisomers (68% overall yield).[313]

Scheme 92

TBADT-Catalyzed Fluorination of Alkanes

Following a n class="Chemical">similclass="Chemical">n class="Chemical">ar reaction scheme, uranyl acetate was employed in combination with NFSI to promote the fluorination of secondary alkanes but poorly on the benzylic positions. Indeed, in the absence of an aromatic scaffold, the excited U=O abstract a hydrogen atom through HAT, while the presence of an aromatic ring deactivated the excited state of the catalyst via exciplex formation preventing the fluorination to occur.[314] Acetophenone in its excited state promoted the hydrogen abstraction of secondary alkanes, with the advantage that a common CFL housebulb can be used to promote an efficient conversion, using Selectfluor as the fluoride source.[315] In this case, the authors irradiated the tail of the n-π* absorption band of the ketone which can be found in the visible region due to the high concentration of the photocatalyst present in solution. N-Alkyl phthalimides having an alkyl chain linked to the nitrogen was fluorinated by using Selectfluor under photocatalyst-free conditions. An exciplex was supposed to be formed between the reagents and it was proposed that the C–F bond formation took place concomitantly with hydrogen atom abstraction with the nitrogen radical of the fluorinating agent.[316]

A <n class="Chemical">spaclass="Chemical">n class="Chemical">coclass="Chemical">n>nsiderable regioselectivity in t<class="Chemical">spn>an class="Disease">he fluorination reaction can be achieved using <class="Chemical">span class="Chemical">carboxylates as alkyl radical precursors and again Selectfluor as a fluorinating reagent. The reaction is possibly initiated by reduction of Selectfluor 93–2 by means of Ir[dF(CF3)ppy]2(dtbbpy)PF6. Fluorination of different carboxylic acids can be achieved in a very high yields (between 70 and 99%), and 93–1 was readily converted into 93–3 in 90% yield (Scheme ).[317] In case of unactivated primary substrates, a prolonged irradiation (12–15 h) was mandatory to achieve a high conversion of the substrate.

Scheme 93

Regioselective Fluorination of Carboxylates

An interes<n class="Chemical">spaclass="Chemical">n class="Chemical">ticlass="Chemical">nclass="Chemical">n>g case is t<class="Chemical">spn>an class="Disease">he fluorination of <class="Chemical">span class="Chemical">compounds having the MOM group to direct the halogenation event. In this case, the PC oxidized an imidine base (DBN) that acted as hydrogen atom abstractor of the dioxolanyl group in compound 94–1 (Scheme ). The resulting α,α-dioxy radical 94–2 released an alkyl radical (upon formiate loss) that was fluorinated by Selectfluor. This metal-free approach again used visible light and is particularly successful when applied to tertiary alkyl ethers to give sterically hindered alkyl fluorides (e.g., 94–3).[318]

Scheme 94

DBN-Mediated HAT in C–F Bond Formation

Interes<n class="Chemical">spaclass="Chemical">n class="Chemical">ticlass="Chemical">nclass="Chemical">n>gly, fluorination of <class="Chemical">spn>an class="Chemical">carboxylates with <class="Chemical">span class="Chemical">Selectfluor was also reported to occur under heterogeneous photocatalytic conditions, using titania as photocatalyst to promote the oxidation of the carboxylate anion.[319] Fluorination and chlorination of nitriles and ketones could be obtained starting from oximes, using Selectfluor and NCS as halogen sources, respectively. With this methodology, γ-functionalization of ketones and a complex photoinduced ring-opening/halogenation of oximes via the intermediacy of an iminyl radical was pursued. The C=N moiety of the reagent (e.g., 95–1) was preserved in the products (95–2a,b) in its oxidized nitrile form (Scheme ). Several natural products could be functionalized following this methodology, such as androsterone (95–3a,b) and camphor (95–4a,b) derivatives (Scheme , see also Scheme ).[320]

Scheme 95

Ring-Opening Halogenation of Oximes

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alcoholsclass="Chemical">n> were <class="Chemical">spn>an class="Chemical">converted into t<class="Chemical">span class="Disease">heir corresponding pyruvates that upon irradiation in the presence of an IrIII photocatalyst with blue LEDs released an alkyl radical prone to be chlorinated by 2,2,2-trichloroacetate as the chlorine atom source. A series of secondary and tertiary chlorides could be obtained in good to excellent yields.[321]

An n class="Chemical">Ir-based photocatalyst was used to promote bromiclass="Chemical">natioclass="Chemical">n of <class="Chemical">n class="Chemical">span class="Chemical">carboxylic acid (96–1) with bromomalonate as brominating agent (Scheme ).[322] The acids used in this work were likewise converted into the corresponding alkyl chlorides and iodides in the presence of the corresponding N-halosuccinimides.[322]

Scheme 96

Bromomalonate as Brominating Agent

A <n class="Chemical">spaclass="Chemical">n class="Chemical">radicalclass="Chemical">n> relay strategy was employed to synt<class="Chemical">spn>an class="Disease">hesize <class="Chemical">span class="Chemical">gem-diiodides through successive intramolecular 1,5-HAT processes and iodine trapping. Indeed, the excitation with visible light of an N–I imidate group, formed in situ from the reaction of a trichloroacetimidate with PhI(OAc)2 as an iodine source, allowed the synthesis of a small library of gem di-I compounds in good yields. As an example, the cholic acid derivative 97–1 has been converted to its corresponding di-iodo derivative 97–2 in 71% yield (Scheme ). Moreover, the authors could also achieve a dibromination using NaBr and TBABr and visible light, while only monochlorination is reported when NaCl, TBACl, and UV light were adopted.[323]

Scheme 97

Gem di-Iodination of a Cholic Acid Derivative

C–S or C–Se Bonds

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alkylclass="Chemical">n> <class="Chemical">spn>an class="Chemical">radicals were class="Chemical">sparsely used for t<class="Chemical">span class="Disease">he unusual introduction of a SCF2X (X = F, H) or an SAr moiety in an organic compound. The introduction of a SCF2X group has recently sparked attention due to the remarkable hydrogen donor nature of the group when X = H, making it the lipophilic surrogate for OH or NH groups.[324] On the other hand, the trifluoromethylthio group increases the metabolic stability and the lipophilicity of drugs.

One strategy for t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> introduction of a <class="Chemical">spn>an class="Chemical">SCF2X group is t<class="Chemical">span class="Disease">he photocatalyzed (by IrIII PC) oxidation of alkyl carboxylates via visible light irradiation in the presence of PhthN-SCF2H (98–2) as the sulfur donor. Indeed, 98–1 was converted into 98–3 in high yields (Scheme ). The reaction was sustained by the stability of the imidyl radical liberated in the process, that was able to promote a chain reaction oxidizing a further carboxylate group. Indeed, the quantum yield for the reaction was found to be 1.7.[325] Bis-methyltiolation was observed in different cases, possibly due to HAT triggered by an intermediate of the reaction, presumably PhthN• and following transfer of SCF2X from the reactant. To avoid the formation of byproducts either mesitylene or 3-(methyl) toluate were added as sacrificial hydrogen donors.

Scheme 98

Difluorothiomethylation of Carboxylic Acids

An interes<n class="Chemical">spaclass="Chemical">n class="Chemical">ticlass="Chemical">nclass="Chemical">n>g follow-up for this methodology from t<class="Chemical">spn>an class="Disease">he same group made use of t<class="Chemical">span class="Disease">he hydrogen atom transfer process previously reported as detrimental for the reaction yield. In fact, when using an aryl carboxylate instead of an aliphatic one, the carboxyl radical that is formed upon electron transfer with the excited Ir catalyst is now stable enough to act as a hydrogen abstractor, selectively targeting secondary or tertiary H in alkyl chains. Also, in this case, PhthN-SCF2X acted as the sulfur source. The conversion of ambroxide 99–1 to its trifluorothiomethyl derivative 99–2 proceeded with 95% yield (Scheme ).[326]

Scheme 99

Trifluoromethylthiolation of Ambroxide

A photocatalyst-free decn class="Chemical">arboxylative class="Chemical">n class="Chemical">arylthiation took place by mixing an N-acyloxyphthalimide (e.g., 100–2) in the presence of an aryl thiol (100–1) under basic conditions (by Cs2CO3) upon visible light irradiation. In this case, a SET between 100–1 and 100–2 caused the formation of 100–2 along with thiyl radical 100–3 (that easily dimerized to disulfide 100–4). Trapping of the resulting cyclohexyl radical (by loss of PhthN from 100–2) with 100–4 afforded alkylaryl sulfide 100–5 in 89% yield (Scheme ).[327]

Scheme 100

Arylthiation of N-Acyloxyphthalimides

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> most widely used reaction for t<class="Chemical">spn>an class="Disease">he C–S bond synt<class="Chemical">span class="Disease">hesis requires the incorporation of sulfur dioxide by using DABSO (DABCO(SO2)2) as its surrogate as depicted in Scheme .[328] Thus, excited mesityl acridinium salts promoted the oxidation of an alkyl-BF3K salt that generated a nucleophilic radical able to react with DABSO. The sulfonyl radical intermediate formed has been employed in a three-component reaction with electron poor olefins (e.g., a vinyl piridine 101–1, Scheme a)[329] or an alkyne (phenyl acetylene, Scheme b),[330] affording alkyl sulfone (101–2) or (E)-vinyl sulfone (101–3), respectively.

Scheme 101

Sulfonylation of (a) Styrenes and (b) Alkynes

Alternatively, <n class="Chemical">spaclass="Chemical">n class="Chemical">alkyl iodidesclass="Chemical">n> can be used to react with <class="Chemical">spn>an class="Chemical">olefins de<class="Chemical">span class="Chemical">corated with EWGs and DABSO to generate a broad range of alkyl sulfones.[331] A very similar strategy was implemented by the same authors using differently substituted Hantzsch esters as alkyl radical precursors, upon irradiation in the presence of Eosin Y.[332] In the latter case, the sulfonyl radical added onto vinyl azides and, after releasing of molecular nitrogen, an imidyl radical resulted which reacted with the reduced photocatalyst, forming an anion that is easily protonated. After a tautomeric equilibrium, (Z)-2-(alkylsulfonyl)-1-arylethen-1-amines were formed, with good regioselectivity and complete control over the configuration of the double bond.[332]

<n class="Chemical">spaclass="Chemical">n class="Chemical">Cyclobutaclass="Chemical">noclass="Chemical">ne oximesclass="Chemical">n> can be reduced via photocatalytic means in t<class="Chemical">spn>an class="Disease">he presence of Ir<class="Chemical">span class="Chemical">(dtbbpy)(ppy)2PF6 to form γ-cyanoalkyl radicals after radical fragmentation. In this process, a vinyl sulfone was used having the dual role of radical acceptors and SO2 source, allowing the synthesis of β-ketosulfones or allylsulfones through a radical transfer mechanism.[333]

C–H Bond

Clasn class="Chemical">sical <class="Chemical">n class="Chemical">span class="Chemical">radical reductive de<spclass="Chemical">n>an class="Chemical">halogenation is one of the most successful reactions based on tin chemistry.[21] Photocatalysis and photochemistry propose a milder and more environmentally friendly alternative to this process, via different strategies. As an example, fac-Ir(ppy)3 was used to convert alkyl iodides 102–1 into their corresponding alkyl radicals using Hantzsch ester or HCO2H as the hydrogen atom source for the HAT process that drives the reaction to the formation of 102–2 (Scheme ). The authors optimized their procedure by using tributylamine as the sacrificial electron donor to reduce the oxidized form of the catalyst and restore the catalytic cycle.[334] A variation of this protocol using p-toluenethiol, DIPEA, and fac-Ir(mppy)3 was used to synthesize D-albucidin.[335]

Scheme 102

Photocatalyzed Reduction of Alkyl Iodides

Ot<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>r catalytic systems were proved to be <class="Chemical">spn>an class="Chemical">competent in t<class="Chemical">span class="Disease">he reduction of halides. In particular, unactivated aryl and alkyl bromides could be reduced using [Ir(ppy)2(dtbbpy)]PF6 in combination with TTMSS as a reducing agent. The mild conditions typical of the reaction were critical to obtain both the mono and the bis reduction of a gem-dibromocyclopropane in a selective fashion.[336]

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alkyl iodidesclass="Chemical">n> and <class="Chemical">spn>an class="Chemical">bromides were reduced under <class="Chemical">span class="Chemical">metal-free conditions via irradiation of 4-carbazolyl-3-(trifluoromethyl)-benzoic acid as the photocatalyst and 1,4 cyclohexadiene as sacrificial hydrogen donor.[337] The reduction of C–X bonds to C–H bonds can take place under photocatalyst-free conditions by PET reactions between the halide and an amine as sacrificial reductant. In this way, adamantane was obtained in 95% yield by photochemical reduction of 1-bromoadamantane.[338] Borohydride-mediated radical photoreduction of alkyl halides (iodides, bromides, and chlorides) is another valuable tool for the formation of a C–H bond.[339]

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> C–H bond formation <class="Chemical">spn>an class="Chemical">could be achieved via a hydrodecarboxylation of <class="Chemical">span class="Chemical">carboxylic acids. In fact, carboxylic acid 103–1 could be reduced in 97% yield to 103–2 by generating the corresponding carboxyl radical through excitation of an acridinium photocatalyst with 450 nm LEDs, in the presence of 10% mol of (PhS)2 (Scheme ). The authors achieved good yields in the decarboxylation of different carboxylic acids. Most notably they succeeded in the double reduction of doubly substituted malonic acids, although with the necessity of longer irradiation times and higher catalyst loading to compensate for the increased amount of substrate to be reduced.[340]

Scheme 103

Hydrodecarboxylation of Carboxylic Acids

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> challenging reduction of <class="Chemical">spn>an class="Chemical">alcohols to t<class="Chemical">span class="Disease">he corresponding alkane can take place via functionalization of the OH group to form an O-thiocarbamate. This compound is the substrate of a photocatalyzed Barton-McCombie deoxygenation in combination with Ir(ppy)3 and DIPEA under an oxidative quenching. Accordingly, the xylofuranose derivative 104–1 was cleanly reduced to 104–2 in 70% yield by maintaining the benzoyl group in position 5 (Scheme ).[93] The reaction was studied mostly on secondary alcohol derivatives being another interesting alternative to the usual tin-mediated reaction.[22]

Scheme 104

Reduction of a Xylofuranose Derivative

An alternative pathway to reduce t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> hydroxy function required a more sophisticated functionalization of t<class="Chemical">spn>an class="Disease">he <class="Chemical">span class="Chemical">OH group making use of two consecutive photochemical reactions. Conducting the reaction in CBr4 under UVA irradiation, the hydroxy groups of a series of primary alcohols were converted into their bromides and then subjected to a one-pot photoreduction mediated by the dimeric gold complex [Au2(dppm)2]Cl2 in the presence of DIPEA.[341]

Formation of a Ring

Three/Four-Membered Rings

In this last section, selected examples will be given w<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>n a photogenerated <class="Chemical">spn>an class="Chemical">alkyl radical is used for t<class="Chemical">span class="Disease">he construction of a ring. Scheme shows one example of formation of a three-membered ring. 1,1-Disubstituted cyclopropanes 105–3a–d were obtained through the addition of an alkyl radical (from silicate 105–1) onto homoallylic tosylates 105–2a–d. The trick here is a radical/polar crossover process where the reduction of the benzyl radical adducts to benzyl anions (by SET with the reduced form of the photoorganocatalyst 4-CzIPN) followed by intramolecular substitution gave the three-membered ring (Scheme ).[342] The versatility of the method was demonstrated by using alkyl trifluoroborates or 4-alkyldihydropyridines as radical precursors and a good tolerance of various functional groups.

Scheme 105

Synthesis of 1,1-Disubstituted Cyclopropanes

A related approach was adopted for t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> <class="Chemical">spn>an class="Chemical">const<class="Chemical">span class="Chemical">ruction of cyclobutanes.[102] Here, the alkyl radical was formed by easily oxidizable electron-rich alkyl arylboronate complexes and added to an iodide-tethered alkene such as methyl 5-iodo-2-methylenepentanoate. However, changing the length of the chain in the haloalkyl alkenes led to the synthesis of three-, five-, six-, and seven-membered rings.[102]

Five-Membered Rings

Five-membered ring is one of t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> privileged st<class="Chemical">spn>an class="Chemical">ructures accessible via photogenerated <class="Chemical">span class="Chemical">alkyl radicals. A common approach is the cyclization onto an alkyne to form an exocyclic double bond as exemplified in Scheme . In most cases, an alkyl halide is reduced by an excited photocatalyst and the resulting radical cyclizes in a 5-exo dig fashion to form the desired alkene. When using a dimeric gold complex 106–5 the reaction of alkyl bromide 106–1 generates diester 106–2 in 93% yield (Scheme a).[343] Cyclopentanes were likewise formed starting from an unactivated alkyl iodide that underwent an intramolecular radical closure by using a strong reductant in the excited state (Ir(ppy)2(dtb-bpy)PF6). The iodine atom, however, was incorporated in the final product forming an alkenyl iodide.[344] The same metal-based photocatalyst was effective to induce a visible light-promoted preparation of five-membered heterocycles (Scheme b). The cyclization step was applied on a Ueno–Stork reaction starting from 2-iodoethyl propargyl ethers (e.g., 106–3a,b) to construct a tetrahydrofuran ring (in 106–4a,b).[345] The examples described in Scheme required an amine as a sacrificial donor. However, amines can be used as efficient reducing agents by a PET reaction with excited alkynyl halides. The resulting photocyclization may then be carried out under metal-free conditions and in a flow photomicroreactor providing the preparation of five-membered rings in a 4 g scale.[346]

Scheme 106

Metal-Photocatalyzed Synthesis of Cyclopentanes from Alkyl Halides

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> de<class="Chemical">spn>an class="Chemical">halogenation/cyclization strategy was explored even under <class="Chemical">span class="Disease">heterogeneous conditions by using platinum nanoparticles on titania (PtNP@TiO2) as the photoredox catalyst. The pyrrolidine scaffold was then obtained by reaction of N-(2-iodoethyl)-4-methyl-N-(prop-2-yn-1-yl)benzenesulfonamide under irradiation (DIPEA as sacrificial donor).[347]

As an alternative, a biphan class="Chemical">sic system may be adopted (Sc<class="Chemical">n class="Chemical">span class="Disease">heme ). In fact, a <spclass="Chemical">n>an class="Chemical">polyisobutylene-tagged fac-Ir(ppy)3 complex (Ir(ppy)2(PIB-ppy)) soluble in heptane was prepared. The substrate 107–1 along with the reagents were soluble in a MeCN phase. However, heating at 85 °C allowed the two phases to mix. Preparation of tetrahydrofuran derivative 107–2 was then accomplished in continuous flow in a satisfying yield with an automatic recovery and reuse of the catalyst (Scheme ).[348]

Scheme 107

Photocatalyzed Synthesis of Tetrahydrofurans in Flow

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alkyl class="Chemical">n class="Chemical">N-hydroxyphthalimide esters were used as <spclass="Chemical">n>an class="Chemical">alkylation reagents in the functionalization of alkenoic acid 108–2 (Scheme ). The alkyl radical added onto the double bond, and the resulting benzyl radical was oxidized to a benzyl cation readily trapped by water and cyclization of the resulting hydroxy acid gave alkyl-substituted lactones 108–3a–e in moderate yields.[349]

Scheme 108

Photocatalyzed Lactonization of Alkenoic Acids

<n class="Chemical">spaclass="Chemical">n class="Chemical">Alkyl class="Chemical">n class="Chemical">N-hydroxyphthalimide esters were exploited for t<spclass="Chemical">n>an class="Disease">he photocatalyzed (by a RuII complex) alkylation of N-arylacrylamides that caused the cyclization of the adduct radical onto the phenyl ring to afford 3,3-dialkyl substituted oxindoles.[350] Moreover, the same radical precursors have been used for the derivatization of alkynylphosphine oxides under metal- and oxidant-free conditions to form benzo[b]phospholes in very good yields.[351]

A five-membered ring may be accessed via late-stage <n class="Chemical">spaclass="Chemical">n class="Species">C(sp3)-Hclass="Chemical">n> functionalization in <class="Chemical">spn>an class="Chemical">N-chlorosulfonamides 109–1 (Sc<class="Chemical">span class="Disease">heme a). The IrIII-photocatalyzed reduction of 109–1 induced the elimination of the chloride anion along the formation of a N-centered radical prone to abstract a hydrogen atom from a remote position to afford an alkyl radical. Oxidation of this radical to the cation followed by incorporation of the chloride anion gave the corresponding chloride 109–2 that upon treatment with solid NaOH formed pyrrolidine 109–3 by an intramolecular nucleophilic substitution.[352] The mildness of the process allowed the application onto biologically important (−)-cis-myrtanylamine and (+)-dehydroabietylamine derivatives.

Scheme 109

Photochemically Induced Synthesis of Pyrrolidines

A related intramolen class="Chemical">culclass="Chemical">n class="Chemical">ar 1,5-HAT transfer was induced in N-tosyl amides 109–4a–e. In this case, the haloamide is formed in situ by iodination by reaction of 109–4a-e with iodine (obtained by oxidation of iodide anion by PhI(OAc)2). The excess of iodine allowed for tuning the amount of iodine released in solution by forming the triiodide anion. Then, visible light irradiation of the mixture induced the cyclization to give N-tosylpyrrolidines 109–5a–e (Scheme b).[353]

Even primn class="Chemical">ary class="Chemical">noclass="Chemical">nactivated class="Chemical">n class="Chemical">sp3-hybridized positions were functionalized again by a remote intramolecular radical 1,5-hydrogen abstraction in γ-bromoamides to produce several γ-lactones in a one-pot fashion.[354] Trifluoroethyl amides were found useful as the directing group increasing the efficiency of the hydrogen abstraction process.

It is also posn class="Chemical">sible to iclass="Chemical">n<class="Chemical">n class="Chemical">span class="Chemical">corporate more than one <spclass="Chemical">n>an class="Disease">heteroatom in the ring starting from benzyl amine 110–2 and unactivated bromides 110–1a–e (Scheme ). Compound 110–2 incorporates CO2 (with the help of the base TBD), and the resulting carbamate underwent attack by an alkyl radical photogenerated by reaction of 110–1a–e and an excited Pd0 photocatalyst (Pd(PPh3)4). Ring closing yielded valuable 2-oxazolidinones 110–3a–e under very mild conditions and easy scalability.[355]

Scheme 110

Three-Component Synthesis of 2-Oxazolidinones

Six-Membered or Larger Rings

Different approac<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n>s were devised to form a six-membered ring even <class="Chemical">spn>an class="Chemical">cointaining <class="Chemical">span class="Disease">heteroatoms. A cyclohexane ring was constructed by ring opening of an iminyl radical by IrIII-photocatalyzed reduction of a 3-phenyl O-acyl oxime (e.g., 111–1) to give radical 111–2 that upon addition onto unsaturated esters 111–3a–e and ensuing cyclization led to cyanoalkylated 1,2,3,4-tetrahydrophenanthrenes (111–4a–e, Scheme ).[356]

Scheme 111

Photocatalyzed Preparation of 1,2,3,4-Tetrahydrophenanthrenes

n class="Chemical">Six-membered riclass="Chemical">ngs have beeclass="Chemical">n likewise obtaiclass="Chemical">ned by riclass="Chemical">ng expaclass="Chemical">nclass="Chemical">n class="Chemical">sion in cycloalkanone derivatives. This expansion was caused by the photocatalyzed decarboxylation of α-(ω-carboxyalkyl) β-keto esters, followed by an exo-trig cyclization of the resulting radical onto the carbonyl group that ultimately led to the one-carbon expanded cycloalkanones by β-cleavage.[357]

Reduction of indoles having an unactivated <n class="Chemical">spaclass="Chemical">n class="Chemical">haloalkaclass="Chemical">neclass="Chemical">n> chain is a useful approach to <class="Chemical">spn>an class="Chemical">const<class="Chemical">span class="Chemical">ruct a ring. As an example, bromo derivatives 112–1a,b were reduced by a AuI photocatalyst and radical cyclization onto the heteroaromatic ring afforded 6,7,8,9-tetrahydropyrido[1,2-a]indoles 112–2a,b in excellent yield (Scheme a).[358] Interestingly, changing the reaction conditions and starting from N-(2-iodoethyl)indoles 112–3a,b in place of 112–1a,b in the presence of Michael acceptors 112–4a–c caused a dearomatizative tandem [4 + 2] cyclization to deliver tri- and tetracyclic benzindolizidines 112–5aa–bc with high diastereoselectivity and yield (Scheme b).[359]

Scheme 112

Intramolecular C–C Bond Formation in Indoles

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> <class="Chemical">spn>an class="Chemical">phenanthridine <class="Chemical">span class="Chemical">core is one of the elective scaffolds to be prepared by using a cyclization step induced by photogenerated alkyl radicals. Scheme illustrated a representative case where an alkyl radical added onto a vinyl azide 113–2, and after nitrogen loss the resulting iminyl radicals 113–3a-c yielded phenanthridines 113–4a-c by ring closure.[360] The method has several advantages including metal-free conditions (a dye as a POC) an excellent functional group tolerance and a broad substrate scope.

Scheme 113

Photocatalyzed Synthesis of Functionalized Phenanthridines

An alternative way to prepn class="Chemical">are <class="Chemical">n class="Chemical">span class="Chemical">phenanthridines is by having re<spclass="Chemical">n>an class="Chemical">course to photoredox gold catalysis employing bromoalkanes as alkyl radical source. In this case, radicals attack a biaryl isonitrile thus forming a sp2-hybridized radical that readily cyclizes upon the pendant arene.[361] Aryl isocyanides (e.g., 114–2a–d) were largely used for the construction of heterocycles such as pyrrolo[1,2-a]quinoxalines 114–4a–d. PhenyliodineIII dicarboxylate 114–1 was used for the incorporation of the cyclohexyl group both in batch and flow under IrIII-photocatalyzed conditions (Scheme , see also Scheme ).[362]

Scheme 114

Photoredox Preparation of Pyrrolo[1,2-a]quinoxalines

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> photocatalyzed insertion of <class="Chemical">spn>an class="Chemical">SO2 into an unactivated <class="Chemical">span class="Species">C(sp3)-H bond was designed to prepare 1,2-thiazine 1,1-dioxide derivatives under uncatalyzed conditions. In fact, visible light irradiation of the complex between an electron-poor O-aryl oxime and DABCO·(SO2)2 releases an iminyl radical that upon 1,5-HAT, SO2 incorporation and cyclization gave the hoped-for heterocycle in a satisfying yield.[363]

In rn class="Chemical">are iclass="Chemical">nstaclass="Chemical">nces a riclass="Chemical">ng lclass="Chemical">n class="Chemical">arger than six may be constructed. By using the approach depicted in Scheme , it was possible to pursue a late stage functionalization on ursolic acid (a compound having excellent pharmaceutical activity). Accordingly, the NHPI ester of ursolic acid acetate (115–2) underwent a radical addition cascade by a photocatalyzed reaction with acrylamide-tethered styrene (115–1) with the intermediacy of radical 115–3. As a result, the benzazepine unit was incorporated in the end compound 115–4 combining two privileged bioactive scaffolds.[364]

Scheme 115

Late Stage Functionalization of Ursolic Acid

Conclusions and Outlook

This review provides a <n class="Chemical">spaclass="Chemical">n class="Chemical">coclass="Chemical">n>ncise and up-to-date selection of modern methods to generate <class="Chemical">spn>an class="Chemical">alkyl radicals via photoc<class="Chemical">span class="Disease">hemistry and photocatalysis. The effort and interest of the chemical community in developing and applying these new methods is witnessed by the rapid increase in the number of articles devoted to this topic that appeared in the literature in the last two decades. Indeed, the rediscovery of photocatalysis and the renaissance of visible light-driven processes have contributed to elevate radical chemistry from the isolated (yet efficient) niche of the tyrannical organotin compounds to a vast plethora of methodologies that relies on more environmental benign compounds. The facile synthesis of the precursors necessary for these transformations, along with the readily available setups (a vast number of reactions can occur by simple irradiation with visible LEDs), made radical chemistry approachable, appointing the photon as the agent of this revolutionary democracy.

Photocatalyn class="Chemical">sis has reac<class="Chemical">n class="Chemical">span class="Disease">hed t<spclass="Chemical">n>an class="Disease">he stage of maturity; however, we are still far from the statement of Ciamician envisioning “industrial colonies without smoke [···] forests of glass tubes [···]; inside of these will take place the photochemical processes that hitherto have been the guarded secret of the plants, but that will have been mastered by human industry which will know how to make them bear even more abundant fruit than nature, for nature is not in a hurry and mankind is”.[365] New practical methods and theoretical assumptions are needed to foster the revolution that has just started. A promising approach makes use of the upconversion of reductants to generate strongly reductive species, but the method was not applied so far to alkyl radicals.[366] This phenomenon can be exploited, for example, if the reaction of a radical anion R to give P is less exoergonic (see the ΔG value in Figure A) than its neutral counterpart (ΔG, referred to R → P conversion). The difference between these two free energies defines the upconversion energy (ΔGup = ΔG – ΔG). The high quantum yields associated with the transformation of R into P in Figure A (Φ = 44) were attributed to the presence of electrocatalytic cycles propagated by P, which is able to transfer an electron to the reactant, closing the catalytic cycle. This phenomenon is attributed to P being a better reductant than R, due to the diminished conjugation (Figure A).

Figure 4

(A) Upconversion of the reducing power of the intermediates in a photocatalytic/photoinitiated cyclization. (B) Two pathways to employ the photoelectrocatalytic strategy: either promoting a single electron transfer with photocatalysis first and a second one with electrocatalysis or vice versa.

(A) Up<n class="Chemical">spaclass="Chemical">n class="Chemical">coclass="Chemical">n>nversion of t<class="Chemical">spn>an class="Disease">he reducing power of t<class="Chemical">span class="Disease">he intermediates in a photocatalytic/photoinitiated cyclization. (B) Two pathways to employ the photoelectrocatalytic strategy: either promoting a single electron transfer with photocatalysis first and a second one with electrocatalysis or vice versa.

T<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> novel approach granted by t<class="Chemical">spn>an class="Disease">he merging of homogeneous photocatalysis with electrocatalysis (see Figure B) is surfacing as t<class="Chemical">span class="Disease">he new challenge in this constantly evolving topic.[367−369]

Joining t<n class="Chemical">spaclass="Chemical">n class="Disease">heclass="Chemical">n> almost unlimited potential of t<class="Chemical">spn>an class="Disease">hese two interchangeable fields of research would open unprecedented scenarios in c<class="Chemical">span class="Disease">hemical synthesis, allowing one to tweak the reactivity of intermediates and excited state species at will, walking on the path carved by the institution of the photon democracy.