Hemiboronic Acid-Catalyzed Reduction of ,-Unsaturated Ketones with Reagent-Controlled Chemoselectivity

The use of an air-stable cationic hemiboronic acid catalyst for the chemoselective reduction of enones is described. By changing the identity and stoichiometry of the silane reducing agent, either the conjugate reduction products or the fully reduced products can be obtained in high selectivity. In contrast to analogous reactions catalyzed by air-and moisture-sensitive borane catalysts, the hemiboronic acid catalyzed protocol can be performed under ambient conditions. Profiling studies revealed that global reduction proceeds via a rapid initial 1,4-addition, followed by ketone deoxygenation with a rate that is highly silane-dependent.