Comparison of Intramolecular and Intermolecular Ammonium and Phosphonium Borohydrides in Hydrogen‐, Proton‐, and Hydride‐Transfer Reactions

Abstract

In this computational study, the thermodynamics of H2, proton, and hydride transfer from 13 ammonium borohydride and six phosphonium borohydride intramolecular and intermolecular frustrated Lewis pairs (FLPs) was probed to gain insight into whether intramolecular or intermolecular amine‐containing FLPs are preferred to phosphine‐containing FLPs in FLP‐catalyzed hydrogenation reactions. H2, hydride, and proton transfer from intramolecular ammonium borohydrides was, on average, 2.6 kcal/mol, 0.7 kcal/mol, and about 3.8 pKa units, respectively, less likely than from intramolecular phosphonium borohydrides. Using the thermodynamic quantities for H2, hydride, and proton transfer to eight probe substrates, intramolecular phosphonium borohydrides were found to be thermodynamically preferred to intramolecular ammonium borohydrides in catalytic hydrogenation reactions.