| The asymmetric hydrogenation of prochiral double bonds by rhodium diphosphine catalysts is an important commercial reaction that has been the subject of decades of intensive study. Two mysteries surrounding this catalytic process are the origin of the "anti-lock-and-key" motif, in which the less stable binding orientation of the substrate leads to reaction with dihydrogen, and the reason for the reversal of enantioselectivity, with equally high enantiomeric excess, observed with minor changes in substrates.; Building on previous computational investigations of reaction pathways for [Rh(PH3)2]+, this work uses a hybrid quantum mechanical/molecular mechanical method (ONIOM) to solve the two mysteries for the [Rh(DuPHOS)] system. The "anti-lock-and-key" behavior is reproduced in modeling the hydrogenation of alpha-formamidoacrylonitrile, and the reversal of enantioselectivity is observed in the hydrogenation of N-(1-tert-Butyl-vinyl)formamide, a closely related substrate. A simple model incorporating frontier orbital theory and steric quadrant diagrams is devised to explain both effects.; The importance of reorganization energies in ligand substitution reactions, and the transferability of rhodium-phosphine bond strengths between different molecules, is tested using density functional calculations. Two opposing experimental trends are reproduced computationally, and rationalized through the use of Natural Bond Orbital and Charge Decomposition analyses. Reorganization energies are not responsible for the opposing trends, and rhodium-phosphine bond energies are not transferable.; Extensions are made to Natural Resonance Theory to allow a more consistent and simpler resonance structure interpretation of molecular orbital calculations. A method is devised to determine the statistically significant minimum number of necessary resonance structures. This is a step towards a link between molecular orbital and valence bond computations, although considerable work still needs to be done. |
