The Role of Amine–B(C6F5)3 Adducts in the Catalytic Reduction of Imines with H2: A Computational Study

Abstract

AbstractThis study thoroughly examines the potential energy surfaces (PESs) of two possible mechanisms for reduction of imines by B(C6F5)3 and H2. The key reaction steps of the first catalytic mechanism, which is the focus of our study, are: (i) the uptake of H2 by a thermally activated amine–B(C6F5)3 species; (ii) proton transfer from the NH2+ moiety of [RNH2CH2R′]+[HB(C6F5)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 [RNH2CH2R′]+[HB(C6F5)3]– in toluene, are around 10 kcal mol–1 each. In the second mechanism, only imines serve as basic partners of B(C6F5)3 in the H2 activation, which affords an [RN(H)CHR′]+[HB(C6F5)3]– ion pair; direct reduction then proceeds via nucleophilic attack of the C‐center by the BH– in [RN(H)CHR′]+[HB(C6F5)3]–. This route becomes catalytic when the product amine is released into the solvent and B(C6F5)3 is re‐used for H2 activation. Upon taking into account the association energy of an amine–B(C6F5)3 adduct [–9.5 kcal mol–1 for tBuN(H)CH2Ph and B(C6F5)3 in toluene], the potential energy barrier for H2 uptake by an imine and B(C6F5)3 increases to 14.5 kcal mol–1. We report a somewhat lower potential energy barrier for H2 uptake by thermally activated amine–B(C6F5)3 adducts [12.7 kcal mol–1 for the B‐N adduct of tBuN(H)CH2Ph and B(C6F5)3 in toluene], although the difference between the two H2 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)