Boron Lewis acid‐catalyzed and catalyst‐free hydroboration reactions of imines are attractive due to the mild reaction conditions. In this work, the mechanistic details of the hydroboration reactions of two different kinds of imines with pinacolborane (HBpin) are investigated by combining density functional theory calculations and some experimental studies. For the hydroboration reaction of N‐(α‐methylbenzylidene)aniline catalyzed by tris[3,5‐bis(trifluoromethyl)phenyl]borane (BArF3), our calculations show that the reaction proceeds through a boron Lewis acid‐promoted hydride transfer mechanism rather than the classical Lewis acid activation mechanism. For the catalyst‐ and solvent‐free hydroboration reaction of imine, N‐benzylideneaniline, our calculations and experimental studies indicate that this reaction is difficult to occur under the reaction conditions reported previously. With a combination of computational and experimental studies, we have established that the commercially available BH3 ⋅ SMe2 can serve as an efficient catalyst for the hydroboration reactions of N‐benzylideneaniline and similar imines. The hydroboration reactions catalyzed by BH3 ⋅ SMe2 are most likely to proceed through a hydroboration/B−H/B−N σ‐bond metathesis pathway, which is very different from that of the reaction catalyzed by BArF3.
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.
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