Highly Selective Hydroboration of Carbonyls by a Manganese Catalyst: Insight into the Reaction Mechanism
Srikanth Vijjamarri, Timothy M. O’Denius, Bin Yao, Alena Kubátová, Guodong Du
Abstract
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 ρ = +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.