Hydrogenation of imines by phosphonium borate zwitterions: a theoretical study

Metal-free hydrogenation of imines by phosphonium borate zwitterion R2PH(C6F4)BH(C6F5)(2) (1, R = CH3, tBu), has been studied by density functional theory (DFT) calculations. The study elucidates a two-step mechanism in which protonation of an imine by the phosphonium part of 1 is followed by hydride transfer to the C center from the borohydride part of 1. According to our calculations, the hydride transfer indeed occurs separately from the protonation; furthermore, both transition states were found to have quite low activation energy. We predict a migration of an iminium ion along the P-B bridging C6F4 group between these two steps; the spatial separation between PH+ and BH- centers makes the concerted hydrogenation highly unlikely. The influence of the steric congestion of the P center of 1 on the transition state for protonation has been studied. Based on the thermodynamics for protonation of an imine and the corresponding amine by 1, the alternative reaction mechanism in which the protonation of an imine is mediated by the corresponding amine is discussed in the context of a complete catalytic cycle. All relevant intermediates and transition states are computationally characterized in this study at the B3LYP/6-31+G* level and relative energies are computed with the 6-31++G** basis; the comparison of relative energies of key reaction steps revealed good agreement between B3LYP, MPW1K and BHandHLYP density functional calculations. Solvent effects are included via the self consistent reaction. field (SCRF/PCM) approach.