Theoretical studies on the mechanism of molybdenum-catalysed deoxydehydration of diols

Molybdenum-catalysed deoxydehydration (DODH) of vicinal diols to alkenes has been investigated using density functional theory (DFT). In particular, the mechanism of DODH of diols using a phosphane as a reductant and a [Mo(O)₂(Q^R)₂] complex (Q^R = acylpyrazolonate ligand) as a catalyst has been studied. This reaction was the first description of a Mo-catalysed DODH reaction. Two alternative routes, A and B, have been analysed in which the commonly recognised key steps in DODH processes have been considered: (i) activation of the diol by condensation and formation of a Mo-diolate intermediate, (ii) oxygen atom transfer to phosphane with reduction to a Mo(IV) species, and (iii) alkene extrusion from the Mo-diolate with regeneration of the starting dioxidomolybdenum(VI) complex. In pathway A, the activation of the diol by the molybdenum complex occurred before the oxygen atom transfer, while in pathway B, the oxygen atom transfer of the dioxidomolybdenum complex to the phosphane occurred before the diol activation. In both routes, the final step was alkene extrusion from the molybdenum-diolate species. Pathway B, in which the reduction by phosphane preceded the diol condensation, is energetically preferred to pathway A.