Lewis acid catalyzed stereoselective hydrosilylation of ketones under the control of σ-π chelation

Hydrosilylation of 2-methyl-1-phenyl-pentan-1-one with Et₃SiH in the presence of catalytic amounts of B(C₆F₅)₃ gave the anti-product with slight predominance over syn-product (syn/anti=1:1.5). In contrast, the syn-product was obtained stereoselectively in the reaction of 2-methyl-1-phenyl-pent-4-yn-1-one bearing an ethynyl group at the β-position (syn/anti=7:1). The syn-selectivities were also observed in the B(C₆F₅)₃-catalyzed reactions of other related ketones, such as α-methyl-β-alkynyl aryl ketones and α-methyl-β-alkynyl alkyl ketones. The moderate anti-selectivity observed in the former case can be explained by the ordinary Felkin-Anh model. On the other hand, the unusual syn-selectivity in the latter cases can be accounted for by the σ-π chelation by R₃Si⁺, in which both the lone pair (σ) of the carbonyl group and the π-electrons of the alkyne coordinate to the silylium ion. The σ-π chelation control was also effective for the 1,3-asymmetric induction.