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

This study thoroughly examines the potential energy surfaces (PESs) of two possible mechanisms for reduction of imines by B(C6F5)(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(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 H-2 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 H-2 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 H-2 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 H-2 activation barriers is within the expected error of the computational method. Two catalytic routes are compared based on B3LYP-computed PESs in solvent (toluene). ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)