Preparation of phosphines through C-P bond formation.

Phosphines are an important class of ligands in the field of metal-catalysis. This has spurred the development of new routes toward functionalized phosphines. Some of the most important C-P bond formation strategies were reviewed and organized according to the hybridization of carbon in the newly formed C-P bond.

Introduction

class="Chemical">Phosphines are an important class of <class="Chemical">span class="Chemical">organophosphorus compounds. They are often used as ligands in metal complex catalysis and they have become a popular reagent for organocatalysis [1]. The methods most widely used for the synthesis of phosphines include the reaction of organometallic compounds with halophosphines, the reaction of metal phosphides with alkyl halides, the reduction of other phosphorus compounds and the hydrophosphination [2]. Research in the past years has focused on the catalytic synthesis of phosphines [3-4]. The asymmetric catalytic synthesis of chiral phosphines has only recently emerged and is under full development. Chiral phosphines are interesting ligands for the preparation of transition metal complex catalysts for asymmetric synthesis [5-6]. Only a minor part of the chiral phosphines are chiral at the phosphorus atom (P-stereogenic) [7-9].

A major drawback of class="Chemical">phosphines is their highly oxidizable nature. They are easily converted to the correclass="Chemical">sponding <class="Chemical">span class="Chemical">phosphine oxide which makes the isolation difficult. To prevent losses during purification, the phosphines are sometimes deliberately transformed into the corresponding oxides (or sulfides). However, this requires an additional reduction step afterwards to get the phosphine back [10-15]. Therefore phosphines are sometimes protected by generation of the corresponding phosphine–borane complex [16-17]. The phosphine–borane complex is a stable intermediate toward the free phosphine. If necessary the boranato group can be removed by treatment with an excess of amine [18]. However, not all phosphines are prone to oxidation and show good air-stability [19].

This review will provide a general overview on class="Chemical">phosphine synthesis over the last 10 to 15 years. Only reactions establishing a C–P bond will be discussed. The synthesis of <class="Chemical">span class="Chemical">phosphine-based polymers was not included [20]. Reactions involving pentavalent phosphorus derivatives (phosphine oxides, phosphonates, phosphinates and phosphate derivatives, etc.) are out of the scope of this review.

Review

Preparation of alkylphosphines via formation of a C(sp3)–P bond

Reaction of organometallic reagents with halophosphines

One of the main approaches to synthesize a class="Chemical">carbon–<class="Chemical">span class="Chemical">phosphorus bond involves the displacement of a halogen atom from phosphorus by an organometallic reagent. This method has proven its usefulness for many years. A variety of organometallic compounds have been described. Most frequently used are the Grignard [21-22] and lithium species. But also organozinc [23-24], organolead [25], organomercury [26] or aluminum-based [27] reagents have been used. However, nowadays it is recommended to avoid the use of certain reagents such as organomercury or organolead compounds as they pose a serious toxicological hazard [28-29].

Despite the fact that the methodology is historically useful it also has major drawbacks. The presence of an anionic class="Chemical">carbon reagent in the reaction restricts the scope of the methodology. The aclass="Chemical">spired <class="Chemical">span class="Chemical">phosphines cannot contain certain functional groups that are able to react with the organometallic compound. Further, stoichiometric amounts of reagents are required. Also, attention should be paid to the handling of halophosphines as some of the simple alkyldichlorophosphines are extremely corrosive and flammable in air.

Asymmetric class="Chemical">phosphines are difficult to access via a nucleophilic substitution at a <class="Chemical">span class="Chemical">halophosphine due to the limited availability of unsymmetrical halophosphines and their weak configurational stability. P-stereogenic chlorophosphines racemize easily even at room temperature [30].

Enantiopure P-stereogenic compounds can be synthesized via a diastereoselective nucleophilic substitution at class="Chemical">phosphorus utilizing chiral auxiliaries. Diastereomeric intermediates are formed that are separable by chromatography or recrystallization. The protocol has proven to be effective and has become the preferred approach for the synthesis of chiral <class="Chemical">span class="Chemical">phosphines. Commonly used chiral auxiliaries are chiral secondary alcohols (for example (−)-menthol (3), endo-borneol, etc.) or thiols that are reacted with halophosphines [31-34].

The diastereoisomers of class="Chemical">menthylphosphinite boranes are popular synthetic intermediates for this approach (Scheme 1) [35]. The diastereomeric phoclass="Chemical">sphinites 2, that were prepared from an <class="Chemical">span class="Chemical">alkyldichlorophosphine 1, were separated by preparative HPLC or recrystallization. Nucleophilic substitution of pure diastereomer (RP)-2a with methyllithium afforded the phosphine–borane (S)-4 with 94% enantiomeric excess. The substitution resulted in inversion of the configuration at the phosphorus center. Deboranation of the air stable borane adduct (S)-4 to obtain 5, was achieved by treatment with N-methylpyrrolidine.

Scheme 1

Synthesis of P-stereogenic phosphines 5 using menthylphosphinite borane diastereomers 2.

Synthesis of P-stereogenic <span class="Chemical">phosphines 5 using <class="Chemical">span class="Chemical">menthylphosphinite borane diastereomers 2.

An alternative method is based on class="Chemical">ephedrine as a chiral auxiliary and was developed by Genêt and Jugé [36-37]. The key synthetic intermediates in this approach are <class="Chemical">span class="Chemical">1,3,2-oxazaphospholidine boranes 7. These compounds are the result of the reaction between bis(diethylamino)alkylphosphine 6 and ephedrine, followed by protection with borane. The subsequent stereoselective ring opening of compound 7 with an organolithium reagent gives way to acyclic products 8 with retention of configuration at the phosphorus center. These phosphamide boranes 8 undergo methanolysis with inversion of configuration to produce intermediate phosphinite boranes 9 that are subsequently substituted with a second nucleophile. A following deprotection of the boranato group gives the chiral phosphines 10. Both enantiomers can be obtained by preparation of different starting oxazaphospholidine borane complexes 7 from (−)-ephedrine or (+)-ephedrine [38] or by starting from the same oxazaphospholidine borane adduct 7 and then changing the order of addition of the organolithium reagents (Scheme 2).

Scheme 2

Enantioselective synthesis of chiral phosphines 10 with ephedrine as a chiral auxiliary.

Enantioselective synthesis of chiral <span class="Chemical">phosphines 10 with <class="Chemical">span class="Chemical">ephedrine as a chiral auxiliary.

Acidolysis with class="Chemical">HCl of compounds 8a results in the stereoselective synthesis of chiral <class="Chemical">span class="Chemical">chlorophosphine boranes 11a [39]. The borane complex has a good configurational stability with borane as a protecting group, in contrast to chlorophosphines that can undergo inversion at the phosphorus center [30]. They allow the synthesis of a variety of P-chiral tertiary phosphine boranes 12a via substitution of the chlorine atom with organometallic nucleophiles. This substitution causes an inversion of configuration at the phosphorus center (Scheme 3). Schuman et al. have prepared several dialkenylphosphines using this methodology [40].

Scheme 3

Chlorophosphine boranes 11a as P-chirogenic electrophilic building blocks.

<span class="Chemical">Chlorophosphine boranes 11a as P-chirogenic electrophilic building blocks.

Nucleophilic substitution with metallated organophosphines

Another classical method for the preparation of class="Chemical">phosphines is the nucleophilic substitution of <class="Chemical">span class="Chemical">alkyl halides with phosphide anions derived from secondary phosphines or phosphine–borane complexes [41]. This approach requires stoichiometric amounts of base. Numerous examples of this approach are available [22,42-48].

In recent years methodologies were developed for the asymmetric alkylation. Livinghouse and Wolfe have reported an enantioselective method for the preparation of chiral tertiary class="Chemical">phosphine–<class="Chemical">span class="Chemical">boranes starting from a racemic secondary phosphine borane precursor such as 13a (Table 1) [49]. A nucleophilic phosphide reagent was prepared by deprotonation of 13a in the presence of (−)-sparteine. The subsequent alkylation of the lithium phosphide with an electrophile proceeded with good enantiocontrol via dynamic resolution. One enantiomer is thermodynamically favored by the spartein auxiliary. The enantioselectivity was found to be time and temperature dependent. Simple stirring of the intermediate (−)-sparteine–lithium complex of 13a for 1 h at 25 °C prior to alkylation resulted in an increase in enantiomeric excess of 14a.

Table 1

Alkylations of dynamically resolved tert-butylphenylphosphine borane 13a.

Entry	Electrophile	Yields of 14a (%)	ee of 14a (%)
1		90	>82
2		85	95
3		90	92

Alkylations of dynamically resolved <span class="Chemical">tert-butylphenylphosphine borane 13a.

The organocatalyst 16 has also been used to carry out an asymmetric alkylation reaction (Scheme 4). The monoalkylation of class="Chemical">phosphine–<class="Chemical">span class="Chemical">borane complex 15 was performed in the presence of the Cinchona alkaloid ammonium salt 16 [50]. However, the enantioselectivity of the reaction was low.

Scheme 4

Monoalkylation of phenylphosphine borane 15 with methyl iodide in the presence of Cinchona alkaloid-derived catalyst 16.

Monoalkylation of <span class="Chemical">phenylphosphine borane 15 with <class="Chemical">span class="Chemical">methyl iodide in the presence of Cinchona alkaloid-derived catalyst 16.

Imamoto et al. prepared a new class="Chemical">tetraphosphine ligand 19 by deprotonation of enantiopure secondary <class="Chemical">span class="Chemical">diphosphine borane 17 at low temperature (Scheme 5) [51]. The configuration was retained during the nucleophilic attack at 18. This approach provides a very straightforward access to P-stereogenic tertiary phosphines but requires the availability of P-chiral substrates.

Scheme 5

Preparation of tetraphosphine borane 19.

Preparation of <span class="Chemical">tetraphosphine borane 19.

Jugé and co-workers synthesized chiral tertiary class="Chemical">phosphine–<class="Chemical">span class="Chemical">borane complexes 12b starting from P-stereogenic chlorophosphine–borane complexes 11b (Scheme 6) [52]. These complexes are accessible with the ephedrine methodology (vide supra). Treatment of 11b with t-butyllithium leads to metal–halogen exchange. After reaction of the phosphide anion 20 with an electrophile, the chiral tertiary phosphine boranes 12b are formed with retention of configuration at the phosphorus atom.

Scheme 6

Using chiral chlorophosphine-boranes 11b as phosphide borane 20 precursors.

Using chiral <span class="Chemical">chlorophosphine-boranes 11b as <class="Chemical">span class="Chemical">phosphide borane 20 precursors.

Catalytic C(sp3)–P bond formation

Only a few examples of a <span class="Chemical">metal catalyzed C(class="Chemical">sp3)–P cross-coupling exist and they are mostly restricted to <class="Chemical">span class="Chemical">benzylic and allylic coupling partners.

Ager and Laneman have synthesized tertiary class="Chemical">phosphine oxide 23 through the <class="Chemical">span class="Chemical">nickel-catalyzed coupling of benzyl bromide (21a) with diphenylphosphine chloride (22a) (Scheme 7) [53]. However oxidation occurred during work-up.

Scheme 7

Nickel-catalyzed cross-coupling (dppe = 1,2-bis(diphenylphosphino)ethane).

class="Chemical">Nickel-catalyzed cross-coupling (<class="Chemical">span class="Chemical">dppe = 1,2-bis(diphenylphosphino)ethane).

The group of class="Chemical">Togni has investigated a <class="Chemical">span class="Chemical">palladium-catalyzed enantioselective coupling reaction between allylic substrates 24 and several secondary phosphines 25a as nucleophiles [54]. The scope of the reaction was limited to 1,3-diphenylallyl acetate 24. The reaction produced not only 26, but gave several side products 27–29 (Table 2).

Table 2

Palladium-catalyzed asymmetric allylic phosphination (dba = dibenzylideneacetone).

Entry	R	26:27:28:29 (%)	Yield of 26 (%)	ee of 26(%)
1	Ph	89:11:0:0	79	96
2	Cy	65:28:6:1	44	45
3	2-naphthyl	91:6:1:2	85	83
4	o-Tol	88:8:2:2	82	42

class="Chemical">Palladium-catalyzed asymmetric allylic phoclass="Chemical">sphination (<class="Chemical">span class="Chemical">dba = dibenzylideneacetone).

Another example of a C(sp3)–P cross-coupling was reported by Lanteri et al. [55]. A class="Chemical">palladium catalyst effectuated the coupling of <class="Chemical">span class="Chemical">n-Bu3SnPPh2 (30) with several perfluoroalkyl iodides 31 (Scheme 8). The stannane 30 was in situ generated by the reaction of the diphenylphosphide anion with n-Bu3SnCl. After oxidation, the perfluoroalkyl-substituted phosphine oxides 32 were obtained in low to moderate yields (15–51%) although full conversion was observed. The byproduct formed was reduced perfluoroalkane HCF2+1.

Scheme 8

Pd-catalyzed cross-coupling reaction with organophosphorus stannanes 30.

<span class="Chemical">Pd-catalyzed cross-coupling reaction with <class="Chemical">span class="Chemical">organophosphorus stannanes 30.

class="Chemical">Ethyl diazoacetate (33) was reacted with the secondary <class="Chemical">span class="Chemical">phosphine borane 13a in the presence of a copper catalyst [56]. The product 14b was obtained in good yield with retention of configuration at the phosphorus center (Scheme 9). Other chiral phosphine boranes 13 were reacted similarly. This protocol is limited to the availability of these chiral substrates.

Scheme 9

Copper iodide catalyzed carbon–phosphorus bond formation.

class="Chemical">Copper iodide catalyzed <class="Chemical">span class="Chemical">carbon–phosphorus bond formation.

Protocols for the enantioselective cross-coupling of class="Chemical">benzyl or alkyl halides with racemic secondary <class="Chemical">span class="Chemical">phosphines have been developed. These reactions were catalyzed by chiral platinum or ruthenium complexes. The enantioselectivity is based on a dynamic kinetic resolution. Upon reaction with the catalyst precursor containing a chiral ligand (L*), a diastereomeric metal–phosphido complex 34 is formed. Rapid pyramidal inversion of this key catalytic intermediate 34 occurs. This complex performs a nucleophilic attack on the electrophile resulting in tertiary phosphines 10, in which the substituent ‘E’ comes from the electrophile. If the inversion of the diastereomers 34 is much faster than their reactions with an electrophile, P-stereogenic phosphines 10 are formed enantioselectively. The ratio of phosphine end products 10 is determined by the equilibrium (Keq) between the complexes 34 and the rate of nucleophilic attack (kS and kR) on the electrophile. The enantioselectivity of the end products 10 is related to the ratio of the diastereomeric phosphido complexes 34. The major phosphine product is derived from the major diastereomeric phosphido complex. The dynamic kinetic resolution approach has been reviewed in more detail by Glueck [57-58]. Scheme 10 relates to reactions of secondary phosphines with several electrophiles, including alkyl halides (alkylation), alkenes (hydrophosphination) and aryl iodides (arylation).

Scheme 10

Thermodynamic kinetic resolution as the origin of enantioselectivity in metal-catalyzed asymmetric synthesis of P-stereogenic phosphines.

Thermodynamic kinetic resolution as the origin of enantioselectivity in <span class="Chemical">metal-catalyzed asymmetric synthesis of P-stereogenic <class="Chemical">span class="Chemical">phosphines.

Chan et al. synthesized P-stereogenic class="Chemical">phosphine boranes using a <class="Chemical">span class="Chemical">ruthenium catalyst. The secondary phosphine 36a underwent an enantioselective alkylation to 12c (Scheme 11). The mechanism of the reaction is based on the formation of an electron-rich ruthenium–phosphido complex that enhances the nucleophilicity at the phosphorus atom. This permitted the reaction to proceed with the less electrophilic benzylic chlorides 35 instead of bromides. The metal-catalyzed reaction was faster than the achiral base-mediated alkylation of 36a. Bisphosphines 37 were also reported with high enantiomeric excesses. The procedure is mainly restricted to benzylic halides but also allowed for the asymmetric alkylation with ethyl bromide. All the phosphines were isolated as their air-stable phosphine–borane complexes 12c, 37 [59-60].

Scheme 11

Ru-catalyzed asymmetric phosphination of benzyl and alkyl chlorides 35 with HPPhMe (36a, PHOX = phosphinooxazoline).

class="Chemical">Ru-catalyzed asymmetric phoclass="Chemical">sphination of <class="Chemical">span class="Chemical">benzyl and alkyl chlorides 35 with HPPhMe (36a, PHOX = phosphinooxazoline).

The group of Glueck has reported a method for the asymmetric alkylation of racemic secondary class="Chemical">phosphines 36b by means of a chiral <class="Chemical">span class="Chemical">platinum-based catalyst 39 (Scheme 12) [61]. The enhanced nucleophilicity at phosphorus of the platinum–phosphido intermediate was beneficial for the alkylation. The scope of the reaction was investigated using diverse benzylic bromides 22b and secondary phosphines 36b. Bidentate ligands 40 and 41 were also synthesized [61-62]. This procedure was also restricted to benzylic halides. High enantiomeric excesses were reported. As expected, a mechanistic study suggested that the major enantiomer of product was formed from the major diastereomer of the platinum–phosphido intermediate [63]. Glueck and co-workers also developed an analogous method for the tandem alkylation/arylation of primary phosphines on the basis of a platinum catalyst resulting in several enantio-enriched phosphaacenaphtalenes [64].

Scheme 12

Pt-catalyzed asymmetric alkylation of secondary phosphines 36b.

Pt-catalyzed asymmetric alkylation of secondary <span class="Chemical">phosphines 36b.

Hydrophosphination

Hydrophosphination involves the addition of P–H to an unsaturated C–C bond. In this reaction class="Chemical">phosphines, <class="Chemical">span class="Chemical">silylphosphines [65-66] or phosphine–borane complexes are used as phosphinating agents to react with unactivated or activated alkenes, dienes and alkynes. Hydrophosphination has gained much interest as an alternative to the classical phosphine syntheses involving a substitution that is incompatible with certain functional groups. Moreover the addition of P–H to an unsaturated C–C bond is more efficient than substitution reactions when considering atom efficiency, what makes it not only greener but also more economical. Other phosphination reactions of unsaturated bonds, such as diphosphination, thiophosphination or selenophosphination, were not included [67].

Depending on the regioselectivity of the reaction, the addition of P–H to the unsaturated bond results in the formation of different products 43 (Scheme 13). The product that results from the Markovnikov addition of P–H corresponds to the α-adduct and the anti-Markovnikov addition is referred to as the β-adduct. The stereoselectivity of the method determines the conformation at the newly formed chiral centers.

Scheme 13

Different adducts 43 can result from hydrophosphination.

The hydrophosphination typically proceeds via thermal [68-69], radical, acidic [70-72] or basic [73-74] initiation. Radical addition of secondary class="Chemical">phosphines to <class="Chemical">span class="Chemical">alkenes can be accomplished by thermal activation [75-76], through the use of radical initiators (AIBN) [77-82] or photochemically by irradiation with UV or visible light [22,83-85]. Most of these reactions give anti-Markovnikov products. The hydrophosphination of activated alkenes (e.g., Michael acceptors) has also been shown to take place at room temperature in the absence of a catalyst [86-87] and even under solvent-free conditions [88]. More recently also metal complex-assisted or organocatalyzed hydrophosphinations have been reported. Several reviews focusing on hydrophosphination have been pusblished [89-91].

In recent years a lot of progress has been made in the class="Chemical">metal complex-catalyzed hydrophoclass="Chemical">sphination. It was shown that several <class="Chemical">span class="Chemical">metals can function as catalysts for the inter- and intramolecular addition of PH3 and R2PH to alkenes. Most research has focused on the use of platinum [92-96], palladium [97-99] or nickel [100-104] complexes. Other catalysts that have been less investigated are iron [105-107], rhodium [108-110], lanthanides [111-114], copper [115] and alkaline-earth metals [114,116]. The catalyst activates either the P-nucleophile or the C-electrophile.

Chiral class="Chemical">phosphines have attracted more and more interest since they are employed as ligands in transition <class="Chemical">span class="Chemical">metal complexes to perform asymmetric catalysis [117]. Enantiopure phosphines have mostly been prepared by starting from enantiopure products or by resolution. The methodologies for catalytic asymmetric hydrophosphination of olefins are limited. Chiral metal complexes have been used to promote and control the asymmetric P–H addition reaction. Recent reviews covering the asymmetric hydrophosphination reaction catalyzed by metal catalysts have been published by Glueck [118-119] and Pullarkat and Leung [120]. Some recent developments in the asymmetric catalytic hydrophosphination will be discussed.

The group of Glueck reported on an approach to chiral class="Chemical">phosphines by the addition of secondary <class="Chemical">span class="Chemical">phosphines 36c to Michael acceptor alkenes (acrylonitrile or derivatives and acrylate esters 44) in the presence of Pt((R,R)-Me-DuPhos) complexes (Scheme 14). However, the products 45 suffered from low enantioselectivities [121]. The mode of action is based on the activation of the P-nucleophile. The proposed mechanism includes the P–H oxidative addition to platinum giving a platinum–phosphido complex. Subsequent nucleophilic attack on a Michael acceptor alkene was suggested to lead to a zwitterion intermediate. Addition of a protic additive was beneficial for the selectivity and reaction rate [95].

Scheme 14

Pt-catalyzed asymmetric hydrophosphination.

Several chiral class="Chemical">cyclic phosphines were acquired via the <class="Chemical">span class="Chemical">lanthanide catalyzed intramolecular hydrophosphination of phosphinoalkenes. Scheme 15 shows the diastereoselective synthesis of 2,5-dimethylphospholanes 49 from 47 with a lanthanide catalyst 48 [122]. The common mechanism when using lanthanide [113] or alkaline earth metal [123] catalysts is based on the formation of a phosphido–metal complex that undergoes insertion of the olefin. Protonolysis of the metal–alkyl complex via σ-bond metathesis with the phosphine reagent completes the catalytic cycle giving the product and regenerating the phosphido intermediate.

Scheme 15

Intramolecular hydrophosphination of phosphinoalkene 47.

Intramolecular hydrophosphination of <span class="Chemical">phosphinoalkene 47.

The group of class="Chemical">Togni has developed an enantioenriched hydrophoclass="Chemical">sphination of <class="Chemical">span class="Chemical">vinyl nitriles catalyzed by a dicationic nickel complex (Table 3). The method is based on the activation of the electrophile. It was suggested that complexation of the nitrile 50 to the chiral nickel Lewis acid activates the double bond towards 1,4-addition of the phosphine 25b. A final proton transfer yields the phosphine product 51 [124-125].

Table 3

Ni-catalyzed asymmetric hydrophosphination of methacrylonitrile 50.

Entry	R	Yield of 51 (%)	ee of 51 (%)
1	Ph	10	32
2	Cy	71	70
3	iPr	not isolated	78
4	Ad	95	94
5	t-Bu	97	89
6	EtMe2C-	86	90

Ni-catalyzed asymmetric hydrophosphination of <span class="Chemical">methacrylonitrile 50.

A chiral Pincer-class="Chemical">palladium complex 55 has been used for the addition of <class="Chemical">span class="Chemical">diarylphosphines 25c to enones 53 (Table 4) [126]. Several enones 53, having electron-donating or -withdrawing groups on the aromatic ring, reacted with a variety of electron-rich and -poor diarylphosphines 25c. The chiral phosphine oxides 54 were obtained in high yield with excellent stereoselectivities. In the proposed mechanism the catalyst 55 acts as a base toward the diarylphosphine 25c. Some other examples of palladium-catalyzed asymmetric hydrophosphination are the addition of diphenylphosphine to α,β-unsaturated ketones [127-128], esters [129], sulfonic esters [130] or to dienones [131]. The proposed mechanism is ubiquitous in metal-catalyzed hydrophosphination involving a P–H oxidative addition, insertion of the olefin into the Pd–H bond and reductive elimination.

Table 4

Palladium-catalyzed asymmetric addition of diarylphosphines 25c to enones 53.

Entry	R1	R2	Ar	Yield of 54 (%)	ee of 54 (%)
1	H	H	Ph	93	99
2	p-Br-	H	Ph	89	99
3	p-MeO-	H	Ph	75	98
4	m-Br-	H	Ph	93	97
5	p-NO2-	H	Ph	78	95
6	H	p-Br-	Ph	90	98
7	H	p-NO2-	Ph	88	99
8	H	m-Br-	Ph	90	99
9	H	o-MeO-	Ph	69	90
10	H	p-Me-	Ph	63	90
11	H	H	p-MeO-C6H4	86	94
12	H	H	p-Cl-C6H4	92	96

class="Chemical">Palladium-catalyzed asymmetric addition of <class="Chemical">span class="Chemical">diarylphosphines 25c to enones 53.

In 2007 several papers appeared reporting on organocatalyzed asymmetric hydrophosphinations. The organocatalytic process has the advantage that in contrast to a class="Chemical">metal-catalyzed method, it cannot undergo product inhibition as a result of the coordination ability of <class="Chemical">span class="Chemical">phosphorus to a metal catalyst.

The addition of class="Chemical">diphenylphosphine to a range of <class="Chemical">span class="Chemical">nitroalkenes 56 has been described using a bifuntional Cinchona alkoid/thiourea catalyst 58 [132]. The catalyst 58 is able to simultaneously activate both the electrophilic and nucleophilic reagents. On one hand the thiourea presumably binds the nitro group while on the other hand the tertiary amine enables proton transfer from phosphorus to carbon (Table 5).

Table 5

Organocatalytic asymmetric hydrophosphination of nitroalkenes 56.

				After crystallizaton
Entry	R	Yield of 57 (%)	ee of 57 (%)	Yield of 57 (%)	ee of 57 (%)
1	Ph-	86	67	36	99
2	p-Me-C6H4-	67	52
3	o-F-C6H4-	83	45	24	99
4	o-BnO-C6H4-	90	60	37	99

Organocatalytic asymmetric hydrophosphination of <span class="Chemical">nitroalkenes 56.

The organocatalyzed hydrophosphination of α,β-unsaturated class="Chemical">aldehydes has been described by Carlone et al. [133] and Ibrahem et al. [134]. The method is based on activation of the <class="Chemical">span class="Chemical">aldehyde 59 via iminium-ion formation by reaction with chiral pyrrolidine 62 derivatives and acid (Scheme 16). Subsequent treatment with sodium borohydride forms the air-stable phosphine–borane product and also reduces the aldehyde. The method gives compounds 61 in high yields and enantioselectivities (ee up to 99%) for α,β-unsaturated aldehydes containing either aliphatic or aromatic groups.

Scheme 16

Organocatalytic asymmetric hydrophosphination of α,β-unsaturated aldehydes 59.

Organocatalytic asymmetric hydrophosphination of α,β-unsaturated <span class="Chemical">aldehydes 59.

Preparation of alkenylphosphines via formation of a C(sp2)–P bond

The C(sp2)–P bond formation is reviewed for arylic and vinylic <span class="Chemical">phosphines. The group of Gaumont has provided a recent review (2010) on the main synthetic methods to obtain <class="Chemical">span class="Chemical">alkenylphosphines [135].

The reaction of an organoclass="Chemical">metallic reagent with the P-atom of <class="Chemical">span class="Chemical">halophosphines is a classical method used for the synthesis of both alkenyl- and arylphosphines. The organometallic reagents are mostly Grignard reagents [136-138] or organolithium [139-142] derivatives. Other organometallic reagents such as aluminum [143] or organomercury [26,144] reagents have been used less frequently.

Grignard or class="Chemical">organolithium compounds are highly reactive nucleophiles and do not tolerate the presence of various functional groups. As a consequence, new approaches were developed including zinc, <class="Chemical">span class="Chemical">zirconium and copper reagents.

Polyfunctional class="Chemical">alkenylphosphine 65 was accessible via the reaction of organozinc derivative 64 with <class="Chemical">span class="Chemical">chlorophosphine 22a. The organozinc bromide 64 was prepared from the corresponding alkenyl iodide 63. To prevent oxidation, the phosphines were protected as the corresponding borane adducts 65. The methodology is also applicable for aryl bromide 66 (Scheme 17) [23-24].

Scheme 17

Preparation of phosphines using zinc organometallics.

Preparation of <span class="Chemical">phosphines using zinc organo<class="Chemical">span class="Chemical">metallics.

class="Chemical">Alkenylphosphines were also synthesized by reacting <class="Chemical">span class="Chemical">alkenylzirconocenes 69 with a chlorophosphine 22b. Alkenylzirconocene compounds 69 displaying different substitution patterns were used, giving access to a variety of alkenylphosphines 71a via this method. If a more sterically hindered substrate ((α-substituted alkenyl)zirconocene) or reagent (iPr2PCl) is used, a transmetallation of Zr(IV) to Cu(I) is necessary for the reaction in order to proceed (Scheme 18). An intermediate phosphorus-copper complex 70 is formed. The phosphines 71a were liberated by treatment with Na2(dtc) or Na4(edta) [145].

Scheme 18

Preparation of alkenylphosphines 71a from alkenylzirconocenes 69 (dtc = N,N-diethyldithiocarbamate, edta = ethylenediaminetetraacetate).

Preparation of class="Chemical">alkenylphosphines 71a from <class="Chemical">span class="Chemical">alkenylzirconocenes 69 (dtc = N,N-diethyldithiocarbamate, edta = ethylenediaminetetraacetate).

The method is based on the reaction of class="Chemical">phosphorus nucleophiles, derived from secondary <class="Chemical">span class="Chemical">phosphines or phosphine–borane complexes, and carbon electrophiles. Nucleophilic substitution with metallated organophosphines is less frequently used for the synthesis of vinylphosphines [42,146] due to possible isomerization to phospha-alkenes under basic conditions [147]. The method is mainly applied for the synthesis of arylphosphines. However, the nucleophilic reagents are incompatible with functional groups susceptible to nucleophilic attack. These sensitive groups have to be protected first to avoid undesired reactions. Despite these limitations this approach is still generally used for the synthesis of simple phosphines [137-138148-149].

The group of Imamoto reported the SNAr reaction of P-chiral secondary class="Chemical">phosphine boranes <class="Chemical">span class="Chemical">13c with halobenzenechromium complexes 72 in the presence of sec-butyllithium [150]. The stereochemistry at the phosphorus atom was retained during the substitution when it was performed in THF at low temperature (Scheme 19). When fluorobenzenechromium complex 72 was used as a substrate, the yields of 73 were high (81–93%), in contrast to the reaction with chloro- and bromobenzenechromium complexes. The former reacted in low yield (7%), the latter did not react. The highly electronegative fluorine atom is needed for the SNAr reaction to take place, even though the arenechromium complexes are already very electron-deficient aromatic compounds.

Scheme 19

SNAr with P-chiral alkylmethylphosphine boranes 13c.

SNAr with P-chiral <span class="Chemical">alkylmethylphosphine boranes <class="Chemical">span class="Chemical">13c.

The same group also developed a P-chiral ligand, class="Chemical">QuinoxP 74, via deprotonation of chiral secondary <class="Chemical">span class="Chemical">phosphine borane 13d with n-butyllithium and subsequent nucleophilic substitution with 2,3-dichloroquinoxaline at low temperature (Scheme 20) [151]. After removal of the boranato group, the ligand was obtained in a good yield (80%).

Scheme 20

Synthesis of QuinoxP 74 (TMEDA = tetramethylethylenediamine).

Synthesis of class="Chemical">QuinoxP 74 (<class="Chemical">span class="Chemical">TMEDA = tetramethylethylenediamine).

Catalytic C(sp2)–P bond formation

The transition class="Chemical">metal typically used for catalytic C–P bond formation is <class="Chemical">span class="Chemical">palladium [152] and, in some cases, nickel or copper. The phosphinating agents may comprise primary and secondary phosphines, silylphosphines [153] or phosphine–borane complexes.

The vinylic coupling partner mostly consists of class="Chemical">alkenylhalides or <class="Chemical">span class="Chemical">alkenyltriflates. Vinyl triflates are used more since they can easily be derived from the corresponding ketone and they are more reactive then the vinyl chloride or bromide during the oxidative addition. More recently also vinyl tosylates and enol phosphates have proven to be suitable reagents.

The catalytic arylic C–P cross-coupling reaction can be a greener approach towards the widely used class="Chemical">arylphosphines that are inaccessible by hydrophoclass="Chemical">sphination. Recent advances in this area concern the synthesis of P-stereogenic <class="Chemical">span class="Chemical">phosphines through a dynamic kinetic resolution of racemic secondary phosphines in a metal-catalyzed P–H/aryl halide coupling.

C(sp2)–P bond formation of vinylphosphines

class="Chemical">Palladium: Beletskaya and co-workers have described the synthesis of secondary and tertiary <class="Chemical">span class="Chemical">vinylphosphines by means of palladium catalyzed cross-coupling of vinylhalides and (silyl)phosphines [154-156]. Table 6 shows the protocols (A or B) generally used [157]. The vinylhalide substrates 75a were cross-coupled with diphenylphosphine or diphenyltrimethylsilylphosphine. When diphenylphosphine was used, triethylamine was added for the basic activation of the phosphinating agent. All the tested substrates 75a contained an alkoxy or amino group and depending on their position relative to the halogen, it was necessary to adjust the reaction temperature. The substrates bearing the halogen in the α-position to the alkoxy or amino group proved to be more reactive. With the halogen in β-position the substrate was less activated and the temperature had to be raised. Method B gave lower yields and longer reaction times were required to compensate for the use of the less reactive diphenyltrimethylsilylphosphine.

Table 6

Pd-catalyzed cross-coupling reactions of diphenylphosphine with alkenylhalides 75a.

Entry	R1	R2	R3	X	Method	Temp (°C)	Time (h)	Yield of 71b (%)
1	H	H	OEt	Br	AB	2020	11.5	9792
2	Me	Me	NEt2	Cl	AB	2020	612	8480
3	H	OBu	Br	Br	AB	120120	3640	9490
4	Ph	N-morpholine	H	Br	AB	7070	2450	9260
5	Ph	N-piperidine	H	Br	AB	7070	2445	9055

class="Chemical">Pd-catalyzed cross-coupling reactions of <class="Chemical">span class="Chemical">diphenylphosphine with alkenylhalides 75a.

Lipshutz et al. used a class="Chemical">Pd(0) catalyst to synthesize <class="Chemical">span class="Chemical">triarylphosphine boranes by coupling secondary diphenylphosphine borane 13e with aryl nonaflates or triflates [158]. The article included one example with vinyl triflate 76 as a substrate (Scheme 21). The vinyl electrophile 76 was activated by the presence of the carbonyl group so the reaction also took place without a palladium catalyst albeit in lower yield (60%) and with formation of byproducts.

Scheme 21

Pd-Mediated couplings of a vinyl triflate 76 with diphenylphosphine borane 13e.

class="Chemical">Pd-Mediated couplings of a <class="Chemical">span class="Chemical">vinyl triflate 76 with diphenylphosphine borane 13e.

class="Chemical">Julienne et al. have reported the coupling of secondary <class="Chemical">span class="Chemical">phosphine boranes with unactivated vinyl triflates (Table 7 and Table 8) [159]. Cyclic and acyclic vinyl triflates (78 and 80a) were reacted with diaryl-, dialkyl- and alkylarylphosphine–borane complexes, 13f and 13g respectively. The reactions were performed with a palladium catalyst in the presence of a weak base. Sometimes microwave irradiation was used to shorten the reaction time.

Table 7

Palladium-catalyzed C–P coupling between acyclic vinyl triflates and phosphine boranes (dppp = 1,3-bis(diphenylphosphino)propane).

Entry	R1	R2	R3	Yield of 79a (%)
1	H	t-Bu	Ph	71
2	H	t-Bu	Me	72
3	Ph	Me	Ph	82
4	Ph	Me	Me	87

Table 8

Palladium-catalyzed C–P coupling between cyclic vinyl triflates and phosphine boranes (dppp = 1,3-bis(diphenylphosphino)propane).

Entry	R1	R2	R3	R4	Heating	Yield of 81a (%)
1	H	H	Ph	Ph	Oil bath	68
2	H	H	Ph	Ph	MWI	71
3	H	H	Me	Ph	Oil bath	71
4	H	H	t-Bu	Ph	Oil bath	70
5	H	H	t-Bu	Ph	MWI	77
6	H	H	Et	Et	Oil bath	50
7	H	H	Cy	Cy	MWI	67
8	Me	H	Ph	Ph	Oil bath	70
9	H	Me	Ph	Ph	Oil bath	65

class="Chemical">Palladium-catalyzed C–P coupling between <class="Chemical">span class="Chemical">acyclic vinyl triflates and phosphine boranes (dppp = 1,3-bis(diphenylphosphino)propane).

class="Chemical">Palladium-catalyzed C–P coupling between <class="Chemical">span class="Chemical">cyclic vinyl triflates and phosphine boranes (dppp = 1,3-bis(diphenylphosphino)propane).

Gilbertson et al. have converted a series of class="Chemical">vinyl triflates 80b into the correclass="Chemical">sponding <class="Chemical">span class="Chemical">vinyl phosphine boranes 81b through palladium catalysis with HPPh2 (Table 9) [160]. The reaction proceeded under mild conditions (40 °C). These vinyltriflates 80b were obtained from the corresponding ketone 82 opening access to a range of other structures. The chiral phosphines 83 and 84 were prepared from the natural products menthone and camphor in the same manner (Figure 1). All products were converted to the corresponding borane complex to facilitate further handling. However, when the same conditions were applied with diphenylphosphine borane and cyclohexenyltriflate no reaction was observed. A similar methodology has been applied for the synthesis of several ligands [161-163].

Table 9

Palladium-catalyzed synthesis of vinylphosphines 81b from ketones 82 (dppb = 1,4-bis(diphenylphosphino)butane).

Entry	R1	R2	Yield of 81b (%)
1	H	H	96
2	Me	H	89
3	H	Me	89
4	t-Bu	H	88

Figure 1

Menthone (83) and camphor (84) derived chiral phosphines.

class="Chemical">Palladium-catalyzed synthesis of <class="Chemical">span class="Chemical">vinylphosphines 81b from ketones 82 (dppb = 1,4-bis(diphenylphosphino)butane).

class="Chemical">Menthone (83) and <class="Chemical">span class="Chemical">camphor (84) derived chiral phosphines.

class="Chemical">Julienne et al. succeeded in coupling vinyl tosylates 85 and 87 with <class="Chemical">span class="Chemical">diphenylphosphine borane 13e despite the fact that alkenyl tosylates are poor reagents for cross-coupling [164]. The products 86 and 79b were formed in the presence of a palladium catalyst. The reaction proceeded at lower temperature when the vinyl tosylate was substituted with an electron-withdrawing group like in 85 (Scheme 22).

Scheme 22

Palladium-catalyzed cross-coupling reaction of vinyl tosylates 85 and 87 with diphenylphosphine borane 13e (dppp = 1,3-bis(diphenylphosphino)propane).

class="Chemical">Palladium-catalyzed cross-coupling reaction of vinyl tosylates 85 and 87 with <class="Chemical">span class="Chemical">diphenylphosphine borane 13e (dppp = 1,3-bis(diphenylphosphino)propane).

The group of Gaumont has also reported their preliminary results for the enantioselective class="Chemical">palladium-catalyzed C–P cross-coupling reaction between an achiral <class="Chemical">span class="Chemical">vinyl triflate 80c and a racemic secondary phosphine–borane complex 13b (Scheme 23) [165]. Chiral phosphines with a C-stereogenic center have been studied but this was the first attempt for the asymmetric synthesis of a P-stereogenic compound. After evaluating several conditions the best catalyst was (S,S)-Me-DuPhos (46). An enantioenriched alkenylphosphine 81c was formed. The highest enantiomeric excess measured by chiral HPLC was 56%. No reaction was observed without the palladium catalyst [165].

Scheme 23

Attempt for the enantioselective palladium-catalyzed C–P cross-coupling reaction between an alkenyltriflate 80c and a phosphine borane 13b.

Attempt for the enantioselective class="Chemical">palladium-catalyzed C–P cross-coupling reaction between an <class="Chemical">span class="Chemical">alkenyltriflate 80c and a phosphine borane 13b.

Gillaizeau and co-workers have demonstrated the use of α-amido class="Chemical">enol phosphates 88 as vinylic coupling partners in the <class="Chemical">span class="Chemical">palladium-catalyzed C–P cross-coupling reaction (Scheme 24) [166]. The enol phosphates 88 were prepared from the corresponding amides. The phosphane function was introduced in the α-position of the nitrogen. Several chiral and achiral secondary phosphine borane complexes 13 were used. The coupling was achieved under mild conditions. Most reactions gave 89 in low to good yields but in some cases the product could not be isolated, probably due to instability of the product. During the coupling reaction with 13h partial inversion of the phosphorus atom occurred, resulting in racemization.

Scheme 24

Enol phosphates 88 as vinylic coupling partners in the palladium-catalyzed C–P cross-coupling reaction (dppf = 1,1'-bis(diphenylphosphino)ferrocene).

class="Chemical">Enol phosphates 88 as vinylic coupling partners in the <class="Chemical">span class="Chemical">palladium-catalyzed C–P cross-coupling reaction (dppf = 1,1'-bis(diphenylphosphino)ferrocene).

class="Chemical">Nickel: Most research has focused on the use of a <class="Chemical">span class="Chemical">palladium catalyst to perform the C–P cross-coupling between secondary phosphines and vinylic electrophiles. A few reports are available concerning the nickel-catalyzed cross-coupling. Ager and Laneman have prepared phosphines 91 and 93 from vinyl triflate 90 and vinyl bromide 92, respectively, under similar conditions (Scheme 25) [53]. The reaction was catalyzed by NiCl2(dppe) in the presence of zinc. The role of zinc was to reduce Ni(II) to Ni(0) and to form Ph2PZnCl for the transmetallation step.

Scheme 25

Nickel-catalyzed cross-coupling in the presence of zinc (dppe = 1,2-bis(diphenylphosphino)ethane).

class="Chemical">Nickel-catalyzed cross-coupling in the presence of zinc (<class="Chemical">span class="Chemical">dppe = 1,2-bis(diphenylphosphino)ethane).

Kazankova and co-workers have explored the catalysts class="Chemical">(Ph3P)2NiCl2 and <class="Chemical">span class="Chemical">Ni(acac)2 for the coupling of several vinyl bromides 75b and chlorides with 25d (Table 10). These reactions proceeded without the addition of zinc [167].

Table 10

Alternative nickel-catalysed cross-coupling without zinc (acac = acetylacetone).

Entry	R1	R2	R3	Yield of 71c (%)
1	H	OEt	H	90
2	Me	H	Me	90
3	TMS	H	H	93
4	TES	H	H	96
5	H	Ph	H	90

Alternative class="Chemical">nickel-catalysed cross-coupling without zinc (<class="Chemical">span class="Chemical">acac = acetylacetone).

class="Chemical">Copper: The group of Buchwald has reported one example of a <class="Chemical">span class="Chemical">copper catalyst to accomplish the phosphination of the vinyl halide 94 (Scheme 26) [168]. The protocol uses CuI as catalyst in combination with N,N’-dimethylethylenediamine (96) as ligand and a weak base Cs2CO3.The desired phosphine 95 is isolated in good yield.

Scheme 26

Copper-catalyzed coupling of secondary phosphines with vinyl halide 94.

class="Chemical">Copper-catalyzed coupling of secondary <class="Chemical">span class="Chemical">phosphines with vinyl halide 94.

C(sp2)–P bond formation of arylphosphines

The C–P bond formation of class="Chemical">aryl phosphines is typically catalyzed by <class="Chemical">span class="Chemical">palladium, nickel and less frequently copper. The phosphorus coupling partners used are primary, secondary and tertiary phosphines, secondary phosphine–borane complexes, silyl- and stannylphosphines and phosphine chlorides. These phosphinating agents are coupled with aryl halides and triflates. Several general protocols are available.

class="Chemical">Palladium: In 1987, Tunney and Stille reported on the <class="Chemical">span class="Chemical">palladium-catalyzed synthesis of several aryldiphenylphosphines by cross-coupling aryl halides with (trimethylsilyl)diphenylphosphine or (trimethylstannyl)diphenylphosphine [169]. No base is required for this method. Trimethylsilyl compounds are preferred over tristannyl derivatives since they are less toxic. However, in recent years the group of Rossi has reported a one-pot procedure for the palladium-catalyzed coupling of aryl iodides 97 with in situ generated Ph2SnBu3 (30, Scheme 27) [170]. When naphthyl triflate was used as a substrate, CuI was added as a co-catalyst [171].

Scheme 27

Palladium-catalyzed cross-coupling of aryl iodides 97 with organoheteroatom stannanes 30.

class="Chemical">Palladium-catalyzed cross-coupling of <class="Chemical">span class="Chemical">aryl iodides 97 with organoheteroatom stannanes 30.

Imamoto et al. have developed a method for the class="Chemical">palladium-catalyzed C–P bond formation using secondary <class="Chemical">span class="Chemical">phosphine boranes [41]. The authors also discovered how the choice of the solvent influences the stereochemistry of 100. When the coupling between aryl iodide 99 and asymmetric secondary phosphine borane 13b was performed in acetonitrile or DMF, the stereochemistry at the phosphorus atom was almost completely retained while the reaction performed in THF or toluene resulted mainly in inversion (Scheme 28) [172-173]. The stereochemistry also depended on the base used. The presence of K2CO3 or KOAc favored a good stereoselectivity in contrast to K3PO4 or DBU. Sodium hydride or Ag2CO3 promoted retention of configuration. The mechanism of the reaction was studied by Gaumont et al. through isolation of the reactive intermediate [174]. Lipshutz et al. reported the palladium-catalyzed phosphination of aryl triflates and nonaflates instead of aryl iodides with phosphine boranes [158]. The first examination towards an enantioselective C–P cross-coupling starting from racemic secondary phosphine boranes was performed by Gaumont and Pican [175]. The highest enantiomeric excess obtained was 45%. The same group has shown that imidazolium based ionic liquids can be used as a medium to perform the C–P cross-coupling reactions. This method allows an easy separation of the product from the catalyst and the recycling of the palladium catalyst [176].

Scheme 28

Synthesis of optically active phosphine boranes 100 by cross-coupling with a chiral phosphine borane 13b.

Synthesis of optically active <span class="Chemical">phosphine boranes 100 by cross-coupling with a chiral <class="Chemical">span class="Chemical">phosphine borane 13b.

Stelzer and co-workers have developed a general method for the coupling of primary or secondary class="Chemical">phosphines instead of their silyl derivatives or <class="Chemical">span class="Chemical">borane complexes with functional aryliodides 101 [177-179]. It should be noted, however, that the reactions were again limited to (di)phenylphosphine (Scheme 29). The protocols use palladium as a catalyst in the presence of tertiary amines as base. A variety of hydrophilic phosphines (102, 103) was synthesized. Since no protective groups were introduced, the method proves to be compatible with several functionalities. This methodology or in a slightly modified form has been used by several authors for the phosphination of a large variety of compounds [180-188]. Microwave-assisted procedures have also been developed [189-191].

Scheme 29

Palladium-catalyzed P–C cross-coupling reactions between primary or secondary phosphines and functional aryliodides 101 (dba = dibenzylideneacetone, dppp = 1,3-bis(diphenylphosphino)propane).

class="Chemical">Palladium-catalyzed P–C cross-coupling reactions between primary or secondary <class="Chemical">span class="Chemical">phosphines and functional aryliodides 101 (dba = dibenzylideneacetone, dppp = 1,3-bis(diphenylphosphino)propane).

Kwong et al. implemented a class="Chemical">palladium-catalyzed phoclass="Chemical">sphination of <class="Chemical">span class="Chemical">aryl bromides and triflates 104 with triarylphosphines 105a as phosphinating agents. This aryl–aryl exchange reaction was compatible with several functional groups such as ketones, aldehydes, esters, nitriles, ethers (Table 11) [192-195]. Products 106a were isolated in only moderate yields. Several P,N-biaryl ligands were prepared from the corresponding triflate under similar conditions [196-197]. The reaction also proceeded under solvent-free conditions with slightly higher yields [198]. A heterogeneous Pd/C catalyst has been applied as well [199-200].

Table 11

The phosphination of aryl bromides 104 with tertiary arylphosphines 105a.

Entry	R	Ar	Yield of 106a (%)
1	-CHO	Ph	32
2	-C(O)Me	Ph	40
3	-CO2Me	Ph	30
4	-CN	Ph	36
5	-OMe	Ph	27
6	-C(O)Me	p-Tol	39
7	-C(O)Me	3,5-Me2-C6H3	34
8	-C(O)Me	p-MeO-C6H4	33

The phosphination of <span class="Chemical">aryl bromides 104 with tertiary <class="Chemical">span class="Chemical">arylphosphines 105a.

The group of Glueck has reported the first asymmetric class="Chemical">palladium-catalyzed C–P bond formation for the synthesis of P-stereogenic <class="Chemical">span class="Chemical">phosphines by adding a catalytic amount of a chiral auxiliary. The enantioenriched phosphine 108 was obtained through coupling of racemic bulky secondary phosphine 107 with PhI in the presence of the base NaOSiMe3 and the Pd-catalyst (Scheme 30) [201]. In the following years, the scope and mechanism were elaborated [202-204]. In accordance with the mechanism given in Scheme 10, it was concluded that the major enantiomer of the product 108 was derived from the major diastereomer of the Pd-phosphido intermediate. Korff and Helmchen have prepared several triarylphosphines with this methodology. However, a modified catalyst system [Pd(Et-FerroTANE)] containing a ferrocene-based ligand was used [205]. This catalyst had the advantage that it was easily prepared in situ while the unstable catalyst used by Glueck et al., required storage at −25 °C in the dark.

Scheme 30

Enantioselective synthesis of a P-chirogenic phosphine 108.

Enantioselective synthesis of a P-chirogenic <span class="Chemical">phosphine 108.

The protocol of Tunney and Stille starting from class="Chemical">silylphosphines has been modified by Chan, Bergman and Toste to be enantioselective by using a [<class="Chemical">span class="Chemical">Pd(Et-FerroTANE)] catalyst. P-stereogenic phosphine boranes 111 and 112 were synthesized by arylation of racemic silylphosphines 110 under dynamic kinetic control (Scheme 31). The best enantiomeric excess was obtained when an ortho-amide substituent was present in the substrate 109 [206].

Scheme 31

Enantioselective arylation of silylphosphine 110 ((R,R)-Et-FerroTANE = 1,1'-bis((2R,4R)-2,4-diethylphosphotano)ferrocene).

Enantioselective arylation of class="Chemical">silylphosphine 110 (<class="Chemical">span class="Chemical">(R,R)-Et-FerroTANE = 1,1'-bis((2R,4R)-2,4-diethylphosphotano)ferrocene).

class="Chemical">Nickel: Cristau et al. were the first which achieved the <class="Chemical">span class="Chemical">nickel-catalyzed arylation of diphenylphosphine [207]. Upon reaction of bromobenzene (113) with 25d in the presence of NiBr2 a mixture of triphenylphosphine 105b and tetraphenylphosphonium bromide salt 114 was obtained (Scheme 32).

Scheme 32

Nickel-catalyzed arylation of diphenylphosphine 25d.

<span class="Chemical">Nickel-catalyzed arylation of <class="Chemical">span class="Chemical">diphenylphosphine 25d.

The first conversion of an class="Chemical">aryltriflate to an <class="Chemical">span class="Chemical">arylphosphine using diphenylphosphine was reported by Cai et al. (Scheme 33) [208-209]. The method was developed for the synthesis of chiral (R)-BINAP 116; a successful chiral ligand. Nickel was chosen as catalyst instead of palladium to minimize catalyst poisoning by binding of the metal with the phosphines present. After optimization, the desired chiral BINAP 116 was obtained in 77% yield. This protocol has been adopted by other research groups for the synthesis of a range of phosphines [138,210-216]. Analogous palladium-catalyzed reactions coupling aryl triflates with diphenylphosphine have been reported [217-218].

Scheme 33

Nickel-catalyzed synthesis of (R)-BINAP 116 (dppe = 1,2-bis(diphenylphosphino)ethane, DABCO = 1,4-diazabicyclo[2.2.2]octane).

class="Chemical">Nickel-catalyzed synthesis of <class="Chemical">span class="Chemical">(R)-BINAP 116 (dppe = 1,2-bis(diphenylphosphino)ethane, DABCO = 1,4-diazabicyclo[2.2.2]octane).

Laneman et al. later developed a modified version of Cai’s method and synthesized several tertiary class="Chemical">phosphines 118 via the cross-coupling of <class="Chemical">span class="Chemical">aryl triflates and halides 117 with chlorodiphenylphosphine (22a) instead of diphenylphosphine (Table 12) [53]. The reaction was catalyzed by NiCl2(dppe) in the presence of zinc. A hydrodehalogenation side reaction resulted in lower yields of aryl halide substrates compared to aryl triflates.

Table 12

Preparation of tertiary phosphines 118 via nickel-catalyzed cross-coupling (dppe = 1,2-bis(diphenylphosphino)ethane).

Entry	R	X	Yield of 118 (%)
1	OMe	OTf	84
2	OMe	Br	46
3	NHBn	OTf	67
4	(S)-NHCHMePh	Br	46

Preparation of tertiary class="Chemical">phosphines 118 via <class="Chemical">span class="Chemical">nickel-catalyzed cross-coupling (dppe = 1,2-bis(diphenylphosphino)ethane).

Zhao and co-workers disclosed a method for the cross coupling of various class="Chemical">aryl bromides 119 with <class="Chemical">span class="Chemical">diphenylphosphine (25d) in the absence of external reductants and supporting ligands [219]. The reaction gave mixtures of phosphines 120 and phosphine oxides 121 (Scheme 34). Several functional groups (ester, ether, ketone and cyano groups) remained intact under the conditions. The reaction was also performed with diphenylphosphine–borane complex but this resulted in only small amounts of products due to decomposition of the phosphinating reagent at 100 °C.

Scheme 34

Nickel-catalyzed cross-coupling between aryl bromides 119 and diphenylphosphine (25d) (dppp = 1,3-bis(diphenylphosphino)propane).

class="Chemical">Nickel-catalyzed cross-coupling between <class="Chemical">span class="Chemical">aryl bromides 119 and diphenylphosphine (25d) (dppp = 1,3-bis(diphenylphosphino)propane).

class="Chemical">Copper: <class="Chemical">span class="Chemical">Copper was first used as a co-catalyst in palladium-catalyzed phosphorylation reactions, Livinghouse et al. demonstrated that the aromatic phosphorylation proceeded even at low temperatures of ≤0 °C when copper was added [220]. The method also allows for the stereocontrolled Pd(0)−Cu(I) co-catalyzed coupling of enantiopure secondary phosphine borane 13b with aryl iodides 122 (Scheme 35) [221].

Scheme 35

Stereocontrolled Pd(0)−Cu(I) cocatalyzed aromatic phosphorylation.

Stereocontrolled <span class="Chemical">Pd(0)−Cu(I) cocatalyzed aromatic phoclass="Chemical">sphorylation.

In 2003, class="Chemical">copper-catalyzed <class="Chemical">span class="Chemical">palladium free phosphorylation methods were developed by Venkataraman and Van Allen [222] and Buchwald et al. [168]. Both methodologies use catalytic amounts of copper(I) salts in the presence of K2CO3 or Cs2CO3 as a base. Buchwald et al. also added N,N’-dimethylethylenediamine 96 as a ligand to enhance the efficiency of the coupling. A secondary phosphine 25e was coupled with a variety of aryl halides 124 with electron-withdrawing or -donating substituents. The method tolerated the presence of functional groups such as esters or amines (Table 13). This approach was also used for the synthesis of phosphinoxazolines [223].

Table 13

Copper-catalyzed synthesis of triarylphosphines 106b.

Entry	R1	X	R2	Yield of 106b (%)
1	2-MeO	I	Ph	91
2	2-NH2	I	Ph	86
3	4-CO2Me	Br	Ph	70
4	2-Ph	I	tol	79
5	4-NH2	I	Cy	72
6	4-CO2Et	I	Cy	85
7	4-CN	I	iBu	65

<span class="Chemical">Copper-catalyzed synthesis of <class="Chemical">span class="Chemical">triarylphosphines 106b.

Hydrophosphination of alkynes

The addition of P–H to a triple bond is a highly desirable method when taking atom economy principles into account. Activated [224-225] or unactivated class="Chemical">alkynes were investigated as substrates. <class="Chemical">span class="Chemical">Phosphines as well as silylphosphines [65-66226-227] or phosphine–borane complexes can be used as phosphinating agents. The addition reaction has been initiated in several ways including base [228-233], radical (thermal radical [234] or AIBN radical [77-7883,235-236]) or transition metal activation.

Depending on the regioselectivity of the procedure, the addition of P–H to the triple bond results in the formation of two regioisomers (Scheme 36). The product that results from the Markovnikov addition of P–H corresponds to the α-adduct 126 and the anti-Markovnikov addition results in the β-adduct 127. The stereoselectivity of the reaction determines the formation of E- or Z-127.

Scheme 36

Preparation of alkenylphosphines by hydrophosphination of alkynes.

Preparation of <span class="Chemical">alkenylphosphines by hydrophoclass="Chemical">sphination of <class="Chemical">span class="Chemical">alkynes.

Despite the great appeal of this method for the preparation of class="Chemical">vinylphosphines it does not allow the syntheses of the widely used <class="Chemical">span class="Chemical">arylphosphines or alkenes bearing no hydrogen on the double bond. Additionally, due to the absence of small rings containing a triple bond, no cyclic alkenylphosphines are accessible. Until now, the protocols lack sufficient control over selectivity and mostly give mixtures. Most addition products (radical, base, metal) are anti-Markovnikov 127, only a few palladium catalyzed reactions give the Markovnikov products 126.

Several reviews on hydrophosphination of <span class="Chemical">alkynes have been published [90-91237]. Some recent developments will be discussed. In recent years research has mainly focused on <class="Chemical">span class="Chemical">metal-catalyzed hydrophosphinations.

Metal complex-catalyzed hydrophosphinations

Hydrophosphination catalysts are mainly based on transition <span class="Chemical">metals. However, it has been shown that <class="Chemical">span class="Chemical">lanthanides and alkaline earth metals can offer a valid alternative.

class="Chemical">Palladium and <class="Chemical">span class="Chemical">nickel complexes were used to catalyze the addition of the P–H bond to alkynes 125a (Scheme 37). The regioselectivity was strongly dependent on the catalytic precursor. In the presence of palladium(0) and nickel(0) complexes the β-adduct 127a was formed as the major product. By contrast palladium(II) and nickel(II) complexes mainly gave rise to the α-adduct 126a [98,238]. The nickel based catalyst was more effective than the palladium so the reaction proceeded at lower temperature.

Scheme 37

Palladium and nickel-catalyzed addition of P–H to alkynes 125a.

class="Chemical">Palladium and <class="Chemical">span class="Chemical">nickel-catalyzed addition of P–H to alkynes 125a.

Join et al. had the objective to enantioselectively create P-stereogenic class="Chemical">vinylphosphine boranes [239]. To achieve this goal some asymmetric hydrophoclass="Chemical">sphination reactions were performed using a <class="Chemical">span class="Chemical">palladium catalyst in combination with a chiral ligand. After optimizing the conditions, the addition of methylphenylphosphine borane (13b) to 1-ethynylcyclohexene (128) with the Pd-catalyst afforded tertiary phosphine borane 129 with a conversion of 70% and only 42% ee (Scheme 38).

Scheme 38

Palladium-catalyzed asymmetric hydrophosphination of an alkyne 128.

<span class="Chemical">Palladium-catalyzed asymmetric hydrophoclass="Chemical">sphination of an <class="Chemical">span class="Chemical">alkyne 128.

Nagata et al. performed the class="Chemical">palladium-catalyzed hydrophoclass="Chemical">sphination of <class="Chemical">span class="Chemical">alkynes by using tetraphenyldiphospine (130) (Table 14) [240]. Since there is no P–H bond in this phosphinating agent, a bisphosphination was expected but a hydrophosphination took place. However, an excess (3–5 equiv) of alkyne was used. The reaction proceeded regioselectively and the α-adducts 126b of several terminal alkynes 125b were formed. Air-oxidation during work-up resulted in the formation of the corresponding phosphine oxides 131. The products 131 were isolated in moderate yields with respect to the diphosphine 130 as limiting reagent. It was suggested that the alkynyl hydrogen acts as the hydrogen source for the hydrophosphination. This can also explain why the method was not applicable to internal alkynes. Silanes have also been added as the source for hydrogen [241].

Table 14

Pd-catalyzed hydrophosphination of alkynes 125b using diphosphine 130.

Entry	R	Yield of 131 (%)
1	n-Hex	58
2	Ph	66
3	-(CH2)3CN	50
4	-(CH2)3Cl	75

class="Chemical">Pd-catalyzed hydrophoclass="Chemical">sphination of <class="Chemical">span class="Chemical">alkynes 125b using diphosphine 130.

class="Chemical">Ruthenium complexes are the first catalysts reported for the direct hydrophoclass="Chemical">sphination of <class="Chemical">span class="Chemical">propargyl alcohols [242]. Several catalytic systems were tested and the reaction with 5 mol % RuCl(cod)(C5Me5) in the presence of Na2CO3 provided the best results (Scheme 39). The reaction gave two stereoisomeric adducts (Z)-133 and (E)-133. The hydrophosphination of 132 proceeded with excellent regioselectivity and good stereoselectivity as the Z-isomers, (Z)-133, were preferentially formed with Z/E ratios around 80/20. This method could not be performed on alkynes with an internal triple bond, only terminal alkynes were accessible.

Scheme 39

Ruthenium catalyzed hydrophosphination of propargyl alcohols 132 (cod = 1,5-cyclooctadiene).

class="Chemical">Ruthenium catalyzed hydrophoclass="Chemical">sphination of <class="Chemical">span class="Chemical">propargyl alcohols 132 (cod = 1,5-cyclooctadiene).

A catalytic amount of class="Chemical">Co(acac)2 in combination with <class="Chemical">span class="Chemical">butyllithium can mediate the hydrophosphination of internal alkynes [243]. Various alkynes 134a were subjected to these conditions to provide the corresponding syn-adducts exclusively (Scheme 40). The regioselectivity is mostly influenced by steric hindrance. To avoid loss of product by oxidation, the adducts were isolated as their thiophosphine analogues 135 and 136.

Scheme 40

Cobalt-catalyzed hydrophosphination of alkynes 134a (acac = acetylacetone).

class="Chemical">Cobalt-catalyzed hydrophoclass="Chemical">sphination of <class="Chemical">span class="Chemical">alkynes 134a (acac = acetylacetone).

Hayashi and co-workers have reported a class="Chemical">rhodium-catalyzed phoclass="Chemical">sphination of <class="Chemical">span class="Chemical">alkynes 134b using silylphosphines 137 as phosphinating agents (Table 15) [108]. The cationic rhodium catalyst was generated in situ by adding silver triflate to a chlororhodium complex. The silylgroup was not incorporated in the vinylphosphine product 138a and methanol was added as a proton source for completing the reaction. The adducts 138a were formed with good to high syn-selectivity.

Table 15

Rhodium-catalyzed hydrophosphination of alkynes 134b with a silylphosphine 137 (cod = 1,5-cyclooctadiene).

Entry	R1	R2	Yield of 138a (%)	E/Z
1	Ph	H	89	96/4
2	MeO-C6H4	H	53	92/8
3	n-C5H11	H	78	95/5
4	HOCH2	H	66	80/20
5	Ph	Me	68	92/8
6	Ph	n-Bu	72	95/5
7	n-C5H11	n-C5H11	67	>99/1
8	EtO2C	n-Bu	81	>99/1
9	EtO2C	Ph	76	80/20

class="Chemical">Rhodium-catalyzed hydrophoclass="Chemical">sphination of <class="Chemical">span class="Chemical">alkynes 134b with a silylphosphine 137 (cod = 1,5-cyclooctadiene).

Kondoh et al. demonstrated the P–H addition to class="Chemical">1-alkynylphosphines under <class="Chemical">span class="Chemical">copper catalysis (Table 16) [244]. Besides copper(I) iodide several other copper salts effectuated the reaction albeit in lower yields as did silver(I) iodide, palladium(II) chloride and platinum(II) chloride. Other transition metal catalysts such as gold(I) chloride, nickel(II) chloride and cobalt(II) chloride gave no reaction. In the presence of copper(I) iodide and cesium carbonate diphenylphospine (25d) added to the triple bond in an anti-fashion. A diverse set of alkynylphosphines 139 was subjected to the protocol proving the compatibility of the method with certain functional groups. The Z-adducts were formed exclusively and isolated as the phosphine sulfides 140 to prevent lower yields by oxidation to the corresponding oxides. The phosphines 141 were obtained by radical reduction of 140 with tris(trimethylsilyl)silane (TTMSS).

Table 16

Copper-catalyzed hydrophosphination of 1-alkynylphosphines 139.

Entry	R	Yield of 140 (%)	Yield of 141 (%)
1	n-Hex	88	87
2	iPr	84
3	t-Bu	84	89
4	Ph	72	78
5	4-Ac-C6H4	87	63
6	3-pyridyl	62	44
7	EtOC(O)(CH2)3	79
8	AcS(CH2)9	75
9	PhCH(OH)	84

<span class="Chemical">Copper-catalyzed hydrophoclass="Chemical">sphination of <class="Chemical">span class="Chemical">1-alkynylphosphines 139.

However, when Kumaraswamy et al. explored the class="Chemical">copper-catalyzed hydrophoclass="Chemical">sphination on substituted <class="Chemical">span class="Chemical">phenylacetylenes 125c further oxidation of the double bond led to the corresponding phenacyl tertiary phosphine boranes 142 in moderate to good yields (Scheme 41). The products 142 were obtained when the reactions were performed under inert atmosphere and in open air. Since the latter gave slightly better yields, it was argued that the dissolved air contributed to the product formation. A Cu(II)–TMEDA catalyzed tandem phosphorus–carbon bond formation–oxyfunctionalization was developed [245]. When methyl propiolate was subjected to the same reaction conditions only the β-adducts were isolated.

Scheme 41

Tandem phosphorus–carbon bond formation–oxyfunctionalization of substituted phenylacetylenes 125c (TMEDA = tetramethylethylenediamine).

Tandem class="Chemical">phosphorus–<class="Chemical">span class="Chemical">carbon bond formation–oxyfunctionalization of substituted phenylacetylenes 125c (TMEDA = tetramethylethylenediamine).

The intramolecular hydrophosphination and class="Chemical">cyclization of primary alkynyl <class="Chemical">span class="Chemical">phosphines 143 has been accomplished using organolanthanide precatalysts of the type Cp’2LnCH(SiMe3)2 (Cp’ = η5-C5Me5) and Me2Si(Me4C5)(t-BuN)SmN(SiMe3)2 [111-112]. The reaction succeeded also using homoleptic lanthanocenes of the form Ln[CH(SiMe3)2]3 (Ln = La, Nd, Sm, Y, Lu) or Ln[N(SiMe3)2]3 (Ln = La, Nd, Sm, Y) [246]. The reaction was performed in NMR tubes until full conversion to the phospholane 144 (n = 1) or phosphorinane 144 (n = 2) was obtained (Scheme 42). The reaction is regioselective as only one adduct was obtained. Several butadiene derivatives were synthesized by hydrophosphination of the triple bond in enynes in the presence of yttriumcomplexes [247].

Scheme 42

Organolanthanide-catalyzed intramolecular hydrophosphination/cyclization of phosphinoalkynes 143.

class="Chemical">Organolanthanide-catalyzed intramolecular hydrophoclass="Chemical">sphination/<class="Chemical">span class="Chemical">cyclization of phosphinoalkynes 143.

An ytterbium–class="Chemical">imine complex 145 [Yb(η2-Ph2CNPh)(<class="Chemical">span class="Chemical">hmpa)3] has also been applied for the synthesis of alkenylphosphines [245,248-251]. The products were isolated as their corresponding phosphine oxides (146 and 147) after oxidative work-up (Scheme 43). The reaction proceeded under mild conditions (rt, 5 min to 4 h), except for the less reactive aliphatic internal alkynes (80 °C, 6 h). The regio- and stereoselectivity was mainly affected by the nature of the substrate and not so much by the reaction conditions. An active ytterbium phosphide species is generated in situ and therefore the imine complex could be categorized as a basic catalyst.

Scheme 43

Hydrophosphination of alkynes 134c catalyzed by ytterbium-imine complexes 145 (hmpa = hexamethylphosphoramide).

Hydrophosphination of class="Chemical">alkynes 134c catalyzed by <class="Chemical">span class="Chemical">ytterbium-imine complexes 145 (hmpa = hexamethylphosphoramide).

The only catalysts based on heavy alkaline earth class="Chemical">metals for the hydrophoclass="Chemical">sphination of <class="Chemical">span class="Chemical">alkynes are derived from calcium [123,252-253]. A similar behavior of calcium(II) and ytterbium(II) compounds seems possible as the oxidation state of Yb(II) does not change during the ytterbium(II)-catalyzed hydrophosphination of alkynes. The reaction of alkyne 134d in the presence of the calcium catalyst resulted in diphenyl-vinylphosphine 138b in good yield (Scheme 44). A set of butadiynes was reacted in a similar way [254]. Mixtures of butadienyldiphosphine isomers were obtained depending on the bulkiness of the end groups at the butadiyne moieties.

Scheme 44

Calcium-mediated hydrophosphanylation of alkyne 134d.

<span class="Chemical">Calcium-mediated hydrophoclass="Chemical">sphanylation of <class="Chemical">span class="Chemical">alkyne 134d.

Other hydrophosphinations

A relatively recent example for the thermal activated hydrophosphination was from Mimeau and Gaumont and described the use of a microwave reactor [254]. This reaction is performed with secondary class="Chemical">phosphine–<class="Chemical">span class="Chemical">borane complexes 13j and terminal alkynes 125d. Mimeau and Gaumont demonstrated that the regioselectivity of the hydrophosphination reaction can be controlled by adjusting the activation method. Thermal activation with the microwave reactor gave the β-adducts 148 (anti-Markovnikov addition) (Table 17). In the same article the α-adducts 149 (Markovnikov addition) were formed by using a palladium catalyst (Table 18). In both cases the regioselectivity was excellent, the stereochemistry in the case of the β-adduct 148 favoured the Z-product. The conditions are compatible with aliphatic and oxygen-functionalized alkynes.

Table 17

Hydrophosphination reactions of terminal alkynes 125d with phosphine boranes 13j under microwave conditions.

Entry	R1	R2	Yield of 148 (%)	Z/E ratio
1	n-Hex	Ph	76	>95/5
2	Ph	Ph	0
3	(CH2)2OH	Ph	49	>95/5
4	CH2OCH3	Ph	33	>95/5
5	n-Hex	Me	82	80/20
6	n-Hex	t-Bu	49	70/30

Table 18

Hydrophosphination reactions of terminal alkynes 125e with phosphine boranes 13f using a Pd catalyst (dba = dibenzylideneacetone, dppp = 1,3-bis(diphenylphosphino)propane).

Entry	R1	R2	Yield of 149 (%)
1	n-Hex	Ph	84
2	Ph	Ph	49
3	-(CH2)2OH	Ph	71
4	-CH2OCH3	Ph	73
5	Cy	Ph	60
6	n-Hex	Me	85
7	Ph	Me	53

Hydrophosphination reactions of terminal <span class="Chemical">alkynes 125d with <class="Chemical">span class="Chemical">phosphine boranes 13j under microwave conditions.

Hydrophosphination reactions of terminal class="Chemical">alkynes 125e with <class="Chemical">span class="Chemical">phosphine boranes 13f using a Pd catalyst (dba = dibenzylideneacetone, dppp = 1,3-bis(diphenylphosphino)propane).

Busacca et al. have described the hydrophosphination of internal class="Chemical">alkynes with <class="Chemical">span class="Chemical">phosphine–borane complexes under basic conditions [255-256]. Several diaryl- and alkylarylalkynes 134e were reacted with a variety of phosphine boranes 25f, some examples are shown in Table 19. Mixtures of E and Z-isomers of 150 were formed, with the E-isomer as the major product.

Table 19

Hydrophosphination of alkynes 134e with phosphine–borane complexes 25f (DMAc = dimethylacetamide).

Entry	R1	R2	R3	Yield of 150 (%)	E/Z ratio
1	Ph	Me	Cy	85	>20/1
2	Ph	Ph	t-Bu	88	>20/1
3	Ph	Me	p-(iPrO)-C6H4	78	>20/1
4	Ph	Ph	iBu	79	4/1
5	p-CF3-C6H4	p-CF3-C6H4	Ph	98	>20/1
6	o-Tol	o-Tol	Cy	99	>20/1

Hydrophosphination of class="Chemical">alkynes 134e with <class="Chemical">span class="Chemical">phosphine–borane complexes 25f (DMAc = dimethylacetamide).

Preparation of alkynylphosphines via formation of a C(sp)–P bond

An extensive review concerning the stoichiometric and catalytic synthesis of <span class="Chemical">alkynylphosphines and their <class="Chemical">span class="Chemical">borane complexes has been published in 2012 by Gaumont et al. [257].

class="Chemical">Alkynylphosphines are commonly synthesized by the nucleophilic diclass="Chemical">splacement of the <class="Chemical">span class="Chemical">halogen at the phosphorus atom of a halophosphine with a metal acetylide. Grignard [258-259] and organolithium [244,260-262] reagents have frequently been used since many years. The main disadvantage is the incompatability of lithium and magnesium reagents with alkynylphosphines having labile functional groups susceptible to nucleophilic attack.

This approach is mainly used for the synthesis of tertiary class="Chemical">phosphines. It is difficult to synthesize secondary <class="Chemical">span class="Chemical">alkynylphosphines since they easily convert into their phosphaallene tautomer. They can only be obtained when they have sterically hindering substituents [263-264].

The asymmetric synthesis of class="Chemical">alkynylphosphines also suffers from limited availability of unsymmetrical <class="Chemical">span class="Chemical">halophosphines and their weak configurational stability. Stereospecific substitution at chiral phosphorus atoms by alkynyl nucleophiles has been reported by Imamoto et al. (Scheme 45) [265]. Firstly, a bromo(tert-butyl)methylphosphanyl borane 151 was formed in situ by treating the enantiomerically pure (S)-(tert-butyl)methylphosphine borane 13d with n-BuLi and 1,2-dibromoethane. An alkynyl lithium reagent was directly added to intermediate 151. The expected substitution products 152 were obtained in high yield and almost exclusively with inversion of configuration, resulting in excellent stereospecificities.

Scheme 45

Formation and substitution of bromophosphine borane 151.

Formation and substitution of <span class="Chemical">bromophosphine borane 151.

Catalytic C(sp)–P bond formation

This type of class="Chemical">carbon–<class="Chemical">span class="Chemical">phosphorus bond formation relies on the cross-coupling reaction in the presence of a catalyst. The cross-coupling reaction is in general performed between a terminal alkyne 125 and an electrophilic phosphorus reagent in the form of a halophoshine 153, mostly chlorophosphine, in the presence of a catalyst such as nickel (Ni(acac)2) [244,266-267] or copper (CuI) [268-270] (Scheme 46). The nickel based catalyst was not suitable for the cross-coupling of alkynes containing a sensitive alkoxy or amino functional group. Therefore, another catalytic method was developed using copper(I) salts.

Scheme 46

General scheme for a nickel or copper catalyzed cross-coupling reaction.

General scheme for a <span class="Chemical">nickel or <class="Chemical">span class="Disease">copper catalyzed cross-coupling reaction.

class="Chemical">Alkynylphosphines were synthesized through the use of a <class="Chemical">span class="Chemical">copper-catalyzed reaction between a secondary phosphine borane 13k and various 1-bromoalkynes 155 in the presence of 1,10-phenanthroline as a ligand and K2CO3 or K3PO4 as a base (Scheme 47). This was the first method involving a nucleophilic phosphorus reagent in the synthesis of alkynylphosphines and was presented by the group of Gaumont [271-272]. The method was applicable for dialkyl, diaryl or alkylaryl phosphine boranes 13k and required only mild conditions.

Scheme 47

Copper-catalyzed synthesis of alkynylphosphines 156.

<span class="Chemical">Copper-catalyzed synthesis of <class="Chemical">span class="Chemical">alkynylphosphines 156.

Conclusion

The developments over the past years in the field were reviewed. The use of class="Chemical">phosphines as ligands in <class="Chemical">span class="Chemical">metal complex catalysis has been a major driving force for the synthesis of functionalized phosphines. In recent years many catalytic procedures have emerged. In general these catalytic protocols proceed under milder conditions that tolerate the presence of functional groups. Gradually a broader variety of phosphines is accessible. Due to the growing importance of asymmetric catalysis, a lot of attention has been paid to the asymmetric synthesis of chiral phosphines. The challenge to find a general protocol that permits simple access to chiral phosphines, is still ongoing and further developments are required.