Carboxylate-Probase Catalyzed Favorskii Reaction.

An O-silylative carboxylate-catalyzed Favorskii reaction for terminal alkynes with aldehydes, ketones or α-ketoesters is described, using a quaternary ammonium carboxylate catalyst and silylating agent, bis(trimethylsilyl)acetamide as the probase system. A wide range of alkynes and carbonyl electrophiles are tolerated as substrates, providing the propargylic alcohol products in up to 96% yield under metal-free, catalytic conditions. Chiral cinchona alkaloid-derived quaternary ammonium catalysts afforded moderate enantioselectivity (up to 73:27 er). Mechanistic studies with p-substituted aryl alkynes and α-keto ester electrophiles provided additional evidence for a mechanism that involves the turnover-determining deprotonation to give quaternary ammonium ion - acetylide ion pairs, which then react with the electrophiles in the chemo- and enantioselectivity-determining step.