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 (B2cat2) in the solvent N,N‐dimethylacetamide (DMA) are investigated through detailed density functional theory calculations. These calculations show that the isomer 1,2‐B2cat2 in situ generated from 1,1‐B2cat2 induced by DMA is the reactive boron intermediate for the reactions. The addition of the B−B bond of 1,2‐B2cat2 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 B2cat2, the deoxygenation of acetone to form the vinyl boronate intermediate and subsequent diboration of vinyl boronate with 1,2‐B2cat2, 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‐B2cat2 and the solvent DMA in related reactions. DFT calculations on the deoxygenative triboration/monoboration of ketones and aldehydes with B2cat2 revealed that the in situ formed 1,2‐B2cat2 rather than 1,1‐B2cat2 is the key borylation intermediate. The solvent N,N‐dimethylacetamide plays an important role in the isomerization from 1,1‐ B2cat2 to 1,2‐B2cat2 and promotes some intermediate processes.