Highly Regioselective Hydrosilylation of Unsymmetric Alkynes Using a Phenylthio Directing Group

Abstract Cobalt‐assisted hydrosilylation of acetylenes is particularly interesting in organic synthesis since alkynyl group functionalization can give way to more useful substructures. This study aims to answer the general question of how to control hydrosilylation regioselectivity with unsymmetric alkynyl groups. Cobalt‐catalyzed hydrosilylation is highly regioselective with alkyl‐phenylthio‐acetylenes affording corresponding cis ‐α‐silyl‐α‐thioaryl‐substituted olefins. However, this reaction loses regioselectivity in cases involving substrates with relatively small alkyl groups. The roles of the directing group in this reaction are to (i) exert steric effects on the alkyl group, and (ii) express stereoelectronic effects of the sulfur or oxygen atom. The scope and limitation of the hydrosilylation as well as the role of such a directing group are discussed.