Reaction Pathway for the Aerobic Oxidation of Phosphines Catalyzed by Oxomolybdenum Salen Complexes

Abstract

Catalysis of O‐atom transfer (OAT) reactions is a characteristic of both natural (enzymatic) and synthetic molybdenum‐oxo and ‐peroxo complexes. These reactions can employ a variety of terminal oxidants, e. g. DMSO, N‐oxides, and peroxides, etc., but rarely molecular oxygen. Here we demonstrate the ability of a set of Schiff‐base‐MoO2 complexes (cy‐salen)MoO2 (cy‐salen=N,N’‐cyclohexyl‐1,2‐bis‐salicylimine) to catalyze the aerobic oxidation of PPh3. We also report the results of a DFT computational investigation of the catalytic pathway, including the identification of energetically accessible intermediates and transition states, for the aerobic oxidation of PMe3. Starting from the dioxo species, (cy‐salen)Mo(VI)O2 (1), key reaction steps include: 1) associative addition of PMe3 to an oxo‐O to give LMo(IV)(O)(OPMe3) (2); 2) OPMe3 dissociation from 2 to produce mono‐oxo complex (cy‐salen)Mo(IV)O (3); 3) stepwise O2 association with 3 via superoxo species (cy‐salen)Mo(V)(O)(η1‐O2) (4) to form the oxo‐peroxo intermediate (cy‐salen)Mo(VI)(O)(η2‐O2) (5); 4) the O‐transfer reaction of PMe3 with oxo‐peroxo species 5 at the oxo‐group, rather than the peroxo unit leading, after OPMe3 dissociation, to a monoperoxo species, (cy‐salen)Mo(IV)(η2‐O2) (7); and 5) regeneration of the dioxo complex (cy‐salen)Mo(VI)O2 (1) from the monoperoxo triplet 37 or singlet 17 by a concerted, asynchronous electronic isomerization. An alternative pathway for recycling of the oxo‐peroxo species 5 to the dioxo‐Mo 1 via a bimetallic peroxo complex LMo(O)‐O−O‐Mo(O)L 8 is determined to be energetically viable, but is unlikely to be competitive with the primary pathway for aerobic phosphine oxidation catalyzed by 1.