Silylboranes bearing dialkylamino groups on silicon as silylene equivalents: palladium-catalyzed regioselective synthesis of 2,4-disubstituted siloles.

Siloles have received much attention in materials sciences because of their unique electronic properties, arising mainly from their low-lying LUMO.1 Particular interest has been focused on the application of π-conjugated siloles to light-emitting materials.2 As the properties of siloles largely depend upon the substituents on the silole ring, much effort has been devoted to the development of new methods for their efficient and selective synthesis.2,3 Transition-metal-catalyzed [2 + 2 + 1] cyclization of an alkyne (2 equiv) with a silylene equivalent is one of the most attractive routes to substituted siloles. Nickeland palladium-catalyzed reactions have been developed with exploration of organosilicon reagents such as hydrodisilane,4 alkynyldisilane,5 trisilacyclopropane,6 (hydrosilyl)stannane,7 silacyclopropene,8 and silacyclopropane.9 These reaction systems allowed synthesis of 2,3,4,5tetrasubstituted, 2,3,4-trisubstituted, and 3,4-disubstituted siloles, which had been difficult to synthesize. As part of our study of silylborane,10 we recently established synthetic access to silylboranes that are functionalized on the silicon atom.11 By using this method, silylboranes bearing chloro, alkoxy, and dialkylamino groups on silicon are easily prepared on a practical scale. Our interest was then focused on the reactivity of these novel silylboranes in transition-metal-catalyzed silaboration of unsaturated organic molecules. In this paper, we disclose our unexpected finding on the efficient use of amino-substituted silylboronic esters as a silylene equivalent and selective formation of 2,4-disubstituted siloles, for which no efficient synthetic access has so far been established.12 Reactions of 1-octyne (5a) with silylboranes 1-4,11 bearing phenyl, chloro, methoxy, and dialkylamino groups on silicon, were carried out in the presence of 1.0 mol % of CpPd(η-C3H5) and 1.2 mol % of PPh3 (Table 1).13 Addition of Ph-substituted silylborane 1 to 5a took place slowly at room temperature (70 h for full conversion), giving 1-boryl-2-silyl-1-alkene 6 with a quantitative yield (entry 1). Large rate acceleration was observed when the reaction was carried out with Cl-substituted silylborane 2, resulting in efficient formation of alkene 7 within 15 min (entry 2). Moderate rate acceleration was also observed in the addition of MeO-substituted silylborane 3 (entry 3). These results suggest that the reaction rate of the silaboration critically depends upon the electronic nature of the substituents on the silicon. Diethylamino-substituted silylborane 4a11 was then reacted with 5a under the same reaction conditions (entry 4). The starting 4a was completely consumed for 80 min at room temperature, but no silaboration products, such as 9a, were found in the reaction mixture. We found that 2,4-disubstituted silole 10a and 3,4-silole 10a′ were formed in good total yield (79%, 10a:10a′ ) 77:23). The formation of silole was accompanied by the formation of (diethylamino)pinacolborane (11a), which suggests the involvement of Pd-silylene species in the catalytic system.14 This silole formation was found to be general for amino-substituted silylboranes having dimethylamino and pyrrolidino groups on the silicon atom (entries 5 and 6). To improve regioselectivity (10a:10a′), we tested various tertiary phosphine ligands in the reactions of 4a with 5a.15 We found that the highest regioselectivity was attained with sterically hindered P(t-Bu)2(2-biphenyl) (12), which afforded 10a and 10a′ with the ratio of 90:10, although the reaction was slower than the original reaction conditions using PPh3 (entry 1 in Table 2). Various terminal alkynes were subjected to reaction with 4a in the presence of palladium catalysts (Table 2). It was found that easily available Pd(dba)2 could be used for this reaction.16 In the presence of 1.0 mol % of Pd(dba)2 with 1.2 mol % of either 12 or the analogous bulky phosphine P(t-Bu)2[2-(2′-methylbiphenyl)] (13), reactions of functionalized or unfunctionalized aliphatic alkynes 5a-d gave the corresponding siloles 10a-d in good yields with high regioselectivities (10:10′ ) 90:10-96:4, entries 1-4). Reactions of aromatic alkynes were carried out with the Pd/PPh3 catalyst (entries 5-12). Phenylacetylene (5e) and alkynes 5f-h bearing electron-donating groups yielded 10e-h in yields of 80-96% with high regioisomeric ratios (entries 5-8), whereas the reaction of CF3-substituted 5i gave a lower yield with good regioselectivity (94:6, entry 9). Sterically demanding arylacetylenes 5j-l reacted with 4a with no drop in reaction rate to give 10j-l with higher regioselectivities (95:5-99:1, entries 10-12). We then carried out the reaction of 5e with (Et2N)Ph2Si-B(pin) (14)11 in the presence of the Pd/PPh3 catalyst (eq 1). Although the Table 1. Palladium-Catalyzed Reaction of Silylboranes 1-4 with 5aa