A palladium-catalyzed three-component-coupling strategy for the differential vicinal diarylation of terminal 1,3-dienes.

A palladium-catalyzed intermolecular vicinal diarylation of terminal 1,3-dienes using aryldiazonium tetrafluoroborates and arylboronic acids is reported. Using this technology, two different arenes are regioselectively introduced in a vicinal fashion across the terminal alkene of a variety of terminal 1,3-dienes at ambient temperature. Through the action of a chiral bicyclo[2.2.2]octadienyl ligand at -20 °C, good enantioselectivity has also been achieved.

A number of intermolecular alkene diarylation reactions have been developed wherein 2 equiv of a single arene are added to an alkene in a single step.[1] Consequently, two identical aryl groups are incorporated. In the interest of rapidly generating complexity from simple feedstock starting materials, there is a strong desire to develop single-step alkene diarylation reactions wherein different arenes are added across an alkene selectively in the presence of a palladium catalyst. While norbornadiene,[2] terminal allenes,[3] and more recently simple terminal alkenes[4] have been used as substrates in three-component diarylations, selective three-component diarylations of terminal 1,3-dienes are not yet known. Three-component couplings of terminal 1,3-diene substrates attracted our interest because, unlike the aforementioned technologies, we envisioned that the products could retain an alkene functional group and also incorporate a chiral center.

Our approach to the vicinal difunctionalization of terminal 1,3-dienes is shown in Scheme 1A. Mechanistically, such reactions are initiated upon oxidative addition of an sp2 carbon electrophile to Pd(0). This generates a cationic Pd-alkyl species A that may undergo migratory insertion and defer transmetalation if the leaving group is noncoordinating (Scheme 1B).[5] The resultant Pd-alkyl B is stabilized as a π-allyl C that resists β-hydride elimination.[6] To complete the difunctionalization sequence, transmetalation and reductive elimination of the second sp2 coupling partner, R2, affords the product E. This strategy was implicated in our previous vicinal alkenylarylation of 1,3-dienes, which gave products of the type F (Scheme 1C).[5a] Unfortunately, our attempts to develop an analogous three-component diarylation by using aryl triflates instead of alkenyl triflates consistently failed, which is likely a result of the relative inertia of aryl triflates.[7] We now wish to report the successful development of a three-component vicinal diarylation of 1,3-dienes, which employs aryldiazonium tetrafluoroborates and aryl boronic acids as coupling partners, affording products of type G (Scheme 1D). We have also identified a chiral bicyclo[2.2.2]octadienyl ligand that is capable of delivering diarylation products with good levels of enantiomeric enrichment.

Scheme 1

Three-Component-Couplings of Terminal 1,3-Dienes

We began the study by optimizing the coupling of trans-1-(para-methoxyphenyl)-1,3-butadiene 1a with phenyldiazonium tetrafluoroborate and phenylboronic acid (Table 1). Arenediazonium tetrafluoroborates were chosen not only because they readily oxidize Pd(0) but also because they afford the aforementioned requisite cationic aryl-Pd intermediates required for migratory insertion (see A in Scheme 1B).[4,8] Starting with our previously reported alkenylarylation conditions,[5a] the indicated Heck product predominated over both the desired vicinal diarylation product 2a and the Suzuki product (entry 1). In contrast to N,N-dimethylacetamide (DMA), 1,4-dioxane minimized Heck product formation but afforded an equimolar ratio of 2a and the Suzuki product (entry 2). Interestingly, another ethereal solvent, THF, afforded only trace amounts of 2a and an equimolar ratio of Suzuki and Heck products (entry 3). Next, alcoholic solvents, which have proven valuable in differential geminal diarylations of terminal alkenes with diazonium salts and boronic acids,[4] were examined. While trace amounts of 2a and mostly the Heck product were observed in methanol (entry 4), increasingly bulky alcohols led to improved yields of 2a at the expense of the Heck product (entries 4–8). In particular, the use of tert-amyl alcohol afforded superior selectivity for the desired diarylation product over both Heck and Suzuki products (entry 8). Comparable results were obtained when the reaction was carried out at ambient temperature (entry 9) and with NaHCO3 in place of KF (entry 10). The final breakthrough was the addition of exogenous dibenzylideneacetone (dba), which improved the yield significantly, presumably by preventing the detrimental accrual of unligated Pd(0) (entry 11).[9,10]

Table 1

Optimization of the Three-Component-Coupling of 1a with Phenyl Diazonium Tetrafluoroborate and Phenyl Boronic Acid

entry	base	solvent	conv (%)a of 1a	yield (%)a of 2a	selectivitya (2a/Suzuki/Heck)
1	KF	DMA	70	8	04:04:92
2	KF	1,4-dioxane	100	14	50:50:tr
3	KF	THF	100	tr	tr:50:50
4	KF	MeOH	100	tr	tr:08:92
5	KF	EtOH	100	4	05:19:76
6	KF	i-PrOH	100	11	14:14:72
7	KF	t-BuOH	100	33	62:13:25
8	KF	t-AmylOH	93	53	92:02:06
9b	KF	t-AmylOH	90	57	94:03:03
10b	NaHCO3	t-AmylOH	96	58	96:02:02
11b,c	NaHCO3	t-AmylOH	100	80	94:04:02

Determined by GC analysis using dodecane as an internal standard following response factor correction.

This reaction was conducted at room temperature.

An additional 15 mol % of dba was added to this reaction.

Under the optimized conditions, we initially investigated increasingly electron-deficient trans-1-phenyl-1,3-butadienes 1b and 1c in order to directly compare 1-aryl-1,3-diene electronic effects on vicinal diarylations (Figure 1). These substrates afforded diminishing yields of the corresponding 1,3,4-triaryl-1-butene products 2b (65%) and 2c (52%), although the selectivity for the difunctionalization product remained >95% in these cases and all subsequent ones. Next, trans-1-(para-fluorophenyl)-1,3-butadiene 1d was found to react efficiently with para-methoxyphenyldiazonium tetrafluoroborate and phenylboronic acid, affording an 83% yield of 2d. Additionally, the reaction between (E)-1-(1-naphthyl)-1,3-butadiene, 4-nitrophenyldiazonium tetrafluoroborate, and phenyl boronic acid delivered the designated nitrophenyl-containing product 2e in 50% yield.

Figure 1

Exploration of the scope of coupling partners in the three-component diarylation of terminal 1,3-butadienes. The isolated yield of each product is reported.

Having established the broad tolerance of electronically disparate terminal 1,3-dienes and phenyldiazonium salts in 2a–2e, we turned our attention to functional group compatibility. In addition to the aforementioned methylphenyl ether (2a), trifluoromethylphenyl (2c), fluorophenyl (2d), and nitrophenyl (2e) functional groups, a phenylacetamide group was introduced to the substrate via the boronic acid component, giving 2f in high yield. Functionalized phenyldiazonium salts and phenylboronic acids also could be incorporated simultaneously, with installation of the diazonium component in the homallylic position and the boronic acid component in the allylic position of the product, as shown in 2g–2l. In addition to the use of trifluoromethyl (2c and 2h), fluoro (2d and 2h), and nitro (2e) arene substituents, other functional groups that are well-tolerated include a ketone (2g and 2k), an aryl bromide (2i), and even an aryl iodide (2k) in reactions with 1-(para-methoxyphenyl)-1,3-butadiene.

With respect to the phenylboronic acid component, both electron-rich and -poor examples are well-tolerated (compare, for example, para-CF3-containing product 2j and para-OMe-containing product 2l). The boronic acid can also be sterically hindered, as an ortho-tolyl group can be added to the allylic position (see 2i). Conversely, ortho-tolyldiazonium tetrafluoroborate can be used to install the group in the homoallylic position (2j).[11]

Alkyl 1,3-dienes were also tested as substrates under the optimized conditions. To this end, a protected alcohol-containing 1,3-diene coupled efficiently to para-methoxyphenyl and para-(trifluoromethyl)phenyl groups, giving 2m. In contrast, a trisubstituted alkene-containing substrate afforded a diminished yield of 2n when coupled to the indicated arenes. These results are consistent with the requirement of a large substituent at the 1-position of the 1,3-diene to avoid σ–π–σ isomerization of the Pd-allyl intermediate (C, Scheme 1), which would afford 1,4-diarylation products.[12] These examples, combined with those of 2a–2l, make clear that a wide variety of electronically and sterically diverse 1,3-dienes and arenes may be combined under these conditions.

The development of a catalytic enantioselective route to the diarylation products 2 is of interest because these products are precursors to pharmaceutically important enantioenriched α-arylpropionic acids,[13] and their asymmetric synthesis has not yet been achieved.[14] Thus, a catalytic asymmetric route starting from terminal 1,3-dienes, which are prepared in one step, would streamline the access to enantioenriched variants of 2. We therefore sought to identify a suitable chiral ligand that would deliver 2 in high enantiomeric excess (ee) and yield. Unfortunately, the addition of privileged nucleophilic ligands for cross-coupling reactions, such as phosphines (including phosphoramidites), N-heterocyclic carbenes, and bidentate amines, afforded Suzuki cross-coupling as the major reaction outcome; the diene substrates were not consumed (see the Supporting Information for details). This may be due to the fact that Lewis basic ligands change the electrophilic nature of the Pd-alkenyl adduct (intermediate A, Figure 1) so as to favor transmetalation over alkene insertion. This problem is encountered in all of our three-component-coupling methods (sp2–sp2 Suzuki coupling partners are present but must not react with one another) and is the likely reason why each of these optimized reactions perform best under “ligandless” conditions.[5]

Given that dba, an achrial diene, is well-tolerated in each of our three-component-coupling reactions, we looked to commercially available chiral dienes for suitability as non-nucleophilic ligands[15] (although it should be noted that the substrate is itself a diene, which may confound effective asymmetric catalysis). Under the optimized conditions identified in Table 1 above, tetrahydropentalenyl ligand L1 afforded 1,3,4-triphenyl-1-butene 2b in just 3% enantiomeric excess (Table 2, entry 1). A substantial improvement was observed through the use of bicylo[2.2.2]octadienyl ligand L2, which afforded 2b in 29% ee (entry 2). Focusing on the bicyclo[2.2.2]octadienyl core, we found that the pseudo C2-symmetric (R)-(−)-carvone-derived ligand L3 delivered 2b in 51% ee (entry 3), while (R)-(−)-α-phellandrene-derived L4 performed the best under these conditions, affording a promising 65% ee (entry 4).[16] A brief temperature profile revealed that the enantioselectivity could be improved by decreasing the temperature, albeit at the expense of the reaction yield. At −8 °C, just above the freezing point of tert-AmylOH, 83% ee was observed, albeit with a low 10% yield of 2b (entry 5). Conversely, increasing the temperature to 40 °C resulted in an improved yield (51%) but a diminished ee (55%, entry 7).

Table 2

Selected Ligand and Temperature Effects on Asymmetric Induction in the Vicinal Diphenylation of (E)-1-Phenyl-1,3-butadiene

Determined by GC analysis using dodecane as an internal standard following response factor correction.

Determined by SFC analysis.

Our final aim was to improve the yield of the asymmetric diphenylations using L4 while preserving enantiomeric excesses. To this end, 1,2-dichloroethane (DCE) was selected as the solvent, enabling the reaction to be carried out at −20 °C, while a slight excess of 1,3-diene substrates and increases in catalyst, ligand, and base loading were also employed (Figure 2). Under these conditions, terminal dienes 1a–1c could be converted to the corresponding (R) enantiomers[17] in 75–82% ee and in isolated yields of 25–30%. Neither the yield nor the selectivity was found to depend significantly on the electronic nature of the diene, with electron-rich (1a), electron-neutral (1b), and electron-deficient (1c) substrates behaving similarly. This asymmetric variant resisted our best efforts to increase the yield by changing the conditions or the technical procedure.

Figure 2

Reoptimized enantioselective diphenylations of (E)-1-aryl-1,3-dienes at −20 °C. The isolated yield and enantiomeric excess (as determined by SFC) for each product are reported.

In summary, we have developed a highly regioselective vicinal diarylation reaction of terminal 1,3-dienes that incorporates two different aryl groups from diazonium salts and boronic acids. The reaction is conducted under mild conditions, incorporates a variety of functionalized arenes, and can be rendered enantioselective through the use of a chiral bicyclo[2.2.2]octadienyl ligand, albeit in low yield. New approaches to asymmetric three-component-coupling reactions are currently under investigation.