Mechanism of Sulfite-Driven, MeReO3-Catalyzed Deoxydehydration of Glycols

The mechanism of the MeReO3-catalyzed deoxydehydration of glycols to olefins by sulfite salts has been investigated with density functional theory (DFT) calculations. Potential intermediates and transition states were evaluated for the three stages of the reaction: (a) dehydration of the glycol by an oxo-rhenium complex to form a Re-(O,O-glycolate), (b) sulfite-induced O transfer (to sulfate and a reduced oxo-Re), and (c) fragmentation of the Re-V-glycolate to give the olefin and to regenerate MeReO3. Various sulfite, sulfate, and Na-sulfite/sulfate species have been evaluated as reactants/products and as ligands. Alternative pathways have been analyzed differing in the order of steps a and b and whether and which sulfite/sulfate species are coordinated; the mechanism of sulfite association/sulfate loss has also been evaluated. Transition states and activation energies have been calculated for several of the key transformations, including H-transfer glycol dehydration, the (LMeReO)-O-V(glycolate) fragmentations (L = H2O, NaSO3-, NaSO4-), and NaSO3- attack on oxo-Re-VII species. The lowest energy catalytic pathway identified involves NaSO3- attack on an oxo oxygen of MeReO3 to produce (MeReO2)-O-V(OSO3Na)(-) (22), glycol coordination by 22, followed by a series of H-transfer steps to Re=O and/or Re-OSO3Na- to give (MeReO)-O-V(glycolate)(OSO3Na)(H2O)(-) (26), concerted fragmentation of the Re-V-glycolate 26 to olefin and MeReO3(OSO3Na)(-) (21), and dissociation of NaSO4- from 21 to regenerate MeReO3 (1). Fragmentation of the Re-glycolate 26 is turnover-limiting. The computational results are compared with available experimental observations for the MeReO3/sulfite and other DODH reactions as well as related oxo-metal-mediated O-transfer reactions and sulfite and phosphine oxidation.