Catalytic Asymmetric Carbosilylation of Methyl Propiolate with Bis-silyl Ketene Acetals

We report a catalytic asymmetric silylative α-alkenylation of bis-silyl ketene acetals (bis-SKAs) with methyl propiolate via conjugate addition. In contrast to prior Brønsted-base activation of C-H acidic nucleophiles (β-ketoesters and oxazolones), electrophilic activation of the alkynoate through silylium-asymmetric counteranion-directed catalysis (Si-ACDC) enables this transformation. DFT studies support a pathway in which silylium-mediated addition forms a substituted alkoxy(trimethylsilyloxy)propadiene (silyl propadienone acetal, SPA) that rapidly isomerizes via C-silylation accompanied by an O-desilylation to the α,β-unsaturated α-silyl ester with high stereocontrol. The reaction shows a broad scope with uniformly excellent enantio- and diastereoselectivities, and we demonstrate several downstream modifications of the products.