Impact of d-orbital occupation on metal-carbon bond functionalization.

Abstract

Metal-carbon bond functionalization leading to C-O bond formation is a promising component reaction that can ultimately form the basis for production of methanol from natural gas. Two primary pathways have been considered: (1) an organometallic Baeyer-Villiger (OMBV) pathway, and (2) a two-step, redox oxy-insertion. A series of first-row transition metal-methyl complexes was modeled for these two pathways to elucidate any trend therein. Several important conclusions can be derived from this research. First, the OMBV mechanism for oxy-insertion is only preferred over the redox pathway in those cases when the metal-methyl's d(n) electron count is such that the latter mechanism would render a chemically infeasible formal oxidation state for an oxo-methyl intermediate. Second, moving toward the so-called oxo wall effectively ameliorates one thermodynamic "sink" (i.e., oxo-Me intermediate) in the redox pathway. However, both oxy-insertion mechanisms suffer from the same feature that would thwart catalysis; i.e., the [M(II)]-methoxide product is in a thermodynamic sink relative to the [M(II)]-methyl reactant. Third, future experiments in hydrocarbon partial oxidation catalysis should focus on effecting oxy-insertion with the weakest oxidants and in establishing the linkage between thermodynamic and kinetic oxygen-atom transfer potentials of oxidants.