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

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 (Hδ−) transfer steps, with the [Ni]H complex actively
participating in each step. The catalyst promotes the transfer of
Hδ− 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 Hδ− 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.