Abstract
This study thoroughly examines the potential energy surfaces (PESs) of two possible mechanisms for reduction of imines by B(C~6~F~5~)~3~ and H~2~. The key reaction steps of the first catalytic mechanism, which is the focus of our study, are: (i) the uptake of H~2~ by a thermally activated amine–B(C~6~F~5~)~3~ species; (ii) proton transfer from the NH~2~^+^ moiety of [RNH~2~CH~2~R′]^+^[HB(C~6~F~5~)~3~]^–^ to the imine; (iii) nucleophillic attack of the C‐center of the iminium ion by the BH^–^ group. The potential energy barriers of the latter, as determined by calculating the evolution of the H‐bonded complex of an imine and [RNH~2~CH~2~R′]^+^[HB(C~6~F~5~)~3~]^–^ in toluene, are around 10 kcal mol^–1^ each. In the second mechanism, only imines serve as basic partners of B(C~6~F~5~)~3~ in the H~2~ activation, which affords an [RN(H)CHR′]^+^[HB(C~6~F~5~)~3~]^–^ ion pair; direct reduction then proceeds via nucleophilic attack of the C‐center by the BH^–^ in [RN(H)CHR′]^+^[HB(C~6~F~5~)~3~]^–^. This route becomes catalytic when the product amine is released into the solvent and B(C~6~F~5~)~3~ is re‐used for H~2~ activation. Upon taking into account the association energy of an amine–B(C~6~F~5~)~3~ adduct [–9.5 kcal mol^–1^ for __t__BuN(H)CH~2~Ph and B(C~6~F~5~)~3~ in toluene], the potential energy barrier for H~2~ uptake by an imine and B(C~6~F~5~)~3~ increases to 14.5 kcal mol^–1^. We report a somewhat lower potential energy barrier for H~2~ uptake by thermally activated amine–B(C~6~F~5~)~3~ adducts [12.7 kcal mol^–1^ for the B‐N adduct of __t__BuN(H)CH~2~Ph and B(C~6~F~5~)~3~ in toluene], although the difference between the two H~2~ activationbarriers is within the expected error of the computational method. Two catalytic routes are compared based on B3LYP‐computed PESs in solvent (toluene).(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)