Mechanistic Insights into the Metal-Free Deoxygenative Borylation of Ketones and Aldehydes with Bis(catecholato)diborane.

The mechanisms of direct deoxygenative borylation of acetone and benzaldehyde with bis(catecholato)diborane (B 2 cat 2 ) in the solvent N,N-dimethylacetamide (DMA) are investigated through detailed density functional theory calculations. These calculations show that the isomer 1,2-B 2 cat 2 in situ generated from 1,1-B 2 cat 2 induced by DMA is the reactive boron intermediate for the reactions. The addition of the B-B bond of 1,2-B 2 cat 2 to the C=O bond of acetone or benzaldehyde via a concerted [2σ+2π]-cycloaddition-like transition state is the rate-limiting step for both the triboration reaction of acetone and the monoboration reaction of benzaldehyde. DMA not only acts as the solvent but also promotes the structural isomerization of B 2 cat 2 , the deoxygenation of acetone to form the vinyl boronate intermediate and subsequent diboration of vinyl boronate with 1,2-B 2 cat 2 , as well as the protodeboronation of the gem-diboronate intermediate in the deoxygenative borylation of benzaldehyde. The presented computational results can explain the observed experimental facts and provide insight into the roles of the isomeric 1,2-B 2 cat 2 and the solvent DMA in related reactions.