β-( Z )-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- t BuImCH 2 PyCH 2 OMe)] and [Rh(cod)(κ 2 C,N- t BuImCH 2 PyCH 2 OMe)]PF 6 , have been prepared. Carbonylation of the cationic compound yields the dicarbonyl complex [Rh(CO) 2 (κ 2 C,N- t BuImCH 2 PyCH 2 OMe)]PF 6 whereas carbonylation of the neutral compound affords a mixture of di- and monocarbonyl neutral complexes [RhBr(CO) 2 (κC- t BuImCH 2 PyCH 2 OMe)] and [RhBr(CO)(κ 2 C,N- t BuImCH 2 PyCH 2 OMe)]. These complexes efficiently catalyze the hydrosilylation of 1-hexyne with HSiMe 2 Ph with a marked selectivity towards the β-( Z )-vinylsilane product. Catalyst [RhBr(CO)(κ 2 C,N- t BuImCH 2 PyCH 2 OMe)] 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 HSiMe 2 Ph, HSiMePh 2 , HSiPh 3 and HSiEt 3 , showing excellent β-( Z ) selectivity for the hydrosilylation of linear aliphatic 1-alkynes. Hydrosilylation of internal alkynes, such as diphenylacetylene and 1-phenyl-1-propyne, selectively affords the syn -addition vinylsilane products. The β-( Z ) 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-( E ) selective. An energy barrier Δ G ‡ of 19.8 ± 2.0 kcal mol -1 (298 K) has been determined from kinetic studies on the hydrosilylation of 1-hexyne with HSiMe 2 Ph. 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 via a classical Si-H oxidative addition.