Methylrhenium Trioxide Revisited: Mechanisms for Nonredox Oxygen Insertion in an M−CH_3 Bond

Abstract

Methylrhenium trioxide (MTO) has the rare ability to stoichiometrically generate methanol at room temperature with an external oxidant (H_2O_2) under basic conditions. In order to use this transformation as a model for nonredox oxidative C−O coupling, the mechanisms have been elucidated using density functional theory (DFT). Our studies show several possible reaction pathways to form methanol, with the lowest net barrier (ΔH‡) being 23.3 kcal mol^(-1). The rate-determining step is a direct "Baeyer−Villiger" type concerted oxygen insertion into MTO, forming methoxyrhenium trioxide. The key to the low-energy transition state is the donation of electron density, first, from HOO(−) to the –CH_3 group (making –CH_3 more nucleophilic and HOO− more electrophilic) and, second, from the Re−C bond to both the forming Re−O and breaking O−O bonds, simultaneously (thus forming the Re−O bond as the Re−C bond is broken). In turn, the ability of MTO to undergo these transfers can be traced to the electrophilic nature of the metal center and to the absence of accessible d-orbitals. If accessible d-orbitals are present, they would most likely donate the required electron density instead of the M−CH_3 moiety, and this bond would thus not be broken. It is possible that other metal centers with similar qualities, such as Pt^(IV) or Ir^V, could be competent for the same type of chemistry.

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