Controlling Selectivity in Carbon Dioxide Hydroboration to Formic Acid and Methanol Levels Regulated by Lewis Acid: A Computational Mechanistic Study

Abstract

The regulation of CO₂ hydroboration selectivity by Lewis acid (LA) additives was investigated using density functional theory (DFT) calculations. This study elucidates the mechanism by which a [Ni]H catalyst mediates the reduction of CO₂ to methanol derivatives. The entire transformation involves three hydride (H^δ-) transfer steps, with the [Ni]H complex actively participating in each step. The catalyst promotes the transfer of H^δ- from pinacolborane (HBPin) to CO₂, formoxyborane (HCOOBPin), and formaldehyde (CH₂O). Direct reaction of HBPin with CO₂ was found to be highly unfavorable. In the absence of the LA, the reaction was reduced only to the level of formic acid; however, in the presence of the LA, the reaction progresses to the methanol. The LA additive facilitates the formation of methoxyborane, a six-electron reduction product. Compared to the unassisted transition state (TS), the LA-assisted transition state (TS-LA) benefits from donor-acceptor interaction between the electron-deficient boron center of Trimethylborate [B(OMe)₃] and the carbonyl oxygen atom. This interaction renders the carbonyl carbon more electron-deficient and thus more electrophilic, promoting H^δ- transfer from [Ni]. The computational results align well with the experimental observations. Overall, the inclusion of LA additives represents a promising strategy to modulate selectivity in CO₂ hydroboration. This approach may be extended to CO₂ hydroboration systems catalyzed by frustrated Lewis pairs or other transition metal catalysts, as well as to CO₂ hydrosilylation reactions.