Highly Selective Hydroboration of Carbonyls by a Manganese Catalyst: Insight into the Reaction Mechanism

Hydroboration of carbonyl compounds is an important transformation in organic chemistry, and a growing interest in catalysis has focused on abundant and nontoxic base metals. Herein we describe an efficient salen manganese catalyst for the hydroboration of a broad range of carbonyl compounds with pinacolborane. The catalytic reactions proceeded rapidly (>99% conversion in <5 min) at room temperature with very low catalyst loadings. High turnover frequency (up to 5700 h(-1)) was observed under these conditions. Several synthetically important functional groups were tolerated, and chemoselective hydroboration of aldehydes over ketones was achieved. The H/D kinetic isotopic effect of borane was determined to be 2.3. The Hammett correlation plot of a series of para-substituted acetophenone substrates, p-X-C6H6COCH3 (X = H, Me, OMe, NO2, Cl, Br, and CF3) yielded a positive slope of rho = +0.99. The crossover products were detected in the competition reaction of catecholborane and deuterated pinacolborane with acetophenone. The findings indicated a potential borane-mediated pathway that could account for the observation of the crossover products.