Chemoselective alkane oxidation by superoxo-vanadium(V) in vanadosilicate molecular sieves

Abstract

Electron paramagnetic resonance (EPR) spectroscopy of reactive superoxo-vanadium(V) species in vanadosilicate molecular sieves (microporous VS-1 and mesoporous V-MCM-41) generated on contact with H₂O₂, tert-butyl hydroperoxide (TBHP), or (H₂ + O₂) is reported for the first time. By suitable choice of the silicate structure, solvent, and oxidant, we could control the vanadium-(O₂^−•) bond (i.e., the V-O bond) covalency, the mode of O-O cleavage (in the superoxo species), and, therefore, chemoselectivity in the oxidation of n-hexane: Oxidation by TBHP over V-MCM-41, for example, yielded 27.2% of (n-hexanol + n-hexanal + n-hexanoic acid), among the highest chemoselectivities for oxidation of the terminal -CH₃ in a linear paraffin reported to date. Over these vanadosilicates, oxidation of the primary C-H bond occurs only via a homolytic O-O bond cleavage; the secondary C-H bond oxidations may proceed via both the homo- and heterolytic O-O cleavage mechanisms.