Influence of intramolecular vs. intermolecular phosphonium-borohydrides in catalytic hydrogen, hydride, and proton transfer reactions.In this computational study, the thermodynamics of hydrogen, hydride, and proton transfer from 22 phosphonium-borohydride intramolecular and intermolecular frustrated Lewis pairs (FLPs) to eight probe substrates was investigated.  The purpose of this study was to gain insight into the thermodynamics of H2 transfer with intramolecular phosphonium-borohydrides; to determine whether intramolecular or intermolecular FLPs are preferred in FLP-catalyzed hydrogenation reactions.  Comparison of the computed thermodynamic values showed that by connecting a borohydride and phosphonium center through a linker, H2 loss from the respective intramolecular phosphonium-borohydride became less favorable by about five and seven kcal mol-1 in acetonitrile and toluene, respectively.  From the thermodynamic analysis of proton and hydride transfer, an intramolecular phosphonium-borohydride was the desired catalyst for the reduction of imines and enamines, while an intermolecular phosphonium-borohydride was the favored catalyst for the reduction of ketones and aldehydes.
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