β-()-Selective alkyne hydrosilylation by a N,O-functionalized NHC-based rhodium(I) catalyst.

Neutral and cationic cyclooctadiene rhodium(I) complexes with a lutidine-derived polydentate ligand having NHC and methoxy side-donor functions, [RhBr(cod)(κC-BuImCHPyCHOMe)] and [Rh(cod)(κC,N-BuImCHPyCHOMe)]PF, have been prepared. Carbonylation of the cationic compound yields the dicarbonyl complex [Rh(CO)(κC,N-BuImCHPyCHOMe)]PF whereas carbonylation of the neutral compound affords a mixture of di- and monocarbonyl neutral complexes [RhBr(CO)(κC-BuImCHPyCHOMe)] and [RhBr(CO)(κC,N-BuImCHPyCHOMe)]. These complexes efficiently catalyze the hydrosilylation of 1-hexyne with HSiMePh with a marked selectivity towards the β-()-vinylsilane product. Catalyst [RhBr(CO)(κC,N-BuImCHPyCHOMe)] showed a superior catalytic performance, in terms of both activity and selectivity, and has been applied to the hydrosilylation of a range of 1-alkynes and phenylacetylene derivatives with diverse hydrosilanes, including HSiMePh, HSiMePh, HSiPh and HSiEt, showing excellent β-() selectivity for the hydrosilylation of linear aliphatic 1-alkynes. Hydrosilylation of internal alkynes, such as diphenylacetylene and 1-phenyl-1-propyne, selectively affords the -addition vinylsilane products. The β-() selectivity of these catalysts contrasts with that of related rhodium(I) catalysts based on 2-picolyl-functionalised NHC ligands, which were reported to be β-() selective. An energy barrier Δ of 19.8 ± 2.0 kcal mol (298 K) has been determined from kinetic studies on the hydrosilylation of 1-hexyne with HSiMePh. DFT studies suggest that the methoxy-methyl group is unlikely to be involved in the activation of hydrosilane, and then hydrosilane activation is likely to proceed a classical Si-H oxidative addition.