Mechanism of Vanadium-Catalyzed Deoxydehydration of Vicinal Diols: Spin-Crossover-Involved Processes

Vanadium-catalyzed deoxydehydration (DODH) reactions provide a cost-effective approach for the conversion of vicinal diols to olefin and polycyclic aromatic hydrocarbons. In this paper, density functional theory (DFT) calculations employing M06 and M06-L methods were conducted to clarify the mechanism of V-catalyzed DODH. Three types of mechanisms generally proposed for transition-metal-catalyzed DODH, associated with the previously omitted spin crossover processes, were considered herein. As a result, a different catalytic cycle including a new olefin-formation mechanism was located, which is in contrast to the findings of a recent study. We found that the favorable mechanism involves the condensation of diols to form vanadium­(V) diolate, reduction of the vanadium­(V) diolate by PPh3, and spin-crossover steps to form a triplet vanadium­(III) diolate. Thereafter, single C–O bond cleavage occurs followed by another spin crossover to form a singlet alkylvanadium­(V) intermediate. The final concerted V–O/C–O bond cleavage generates an olefin and finishes the catalytic cycle. The reduction of vanadium­(V) diolate by PPh3 and the extrusion of olefin have close overall free energy barriers of 34.3 and 33.7 kcal/mol, respectively. These results suggest that both steps influence the reaction rate. On the other hand, the two mechanisms starting by the reduction of the oxovanadium­(V) catalyst with either PPh3 or a secondary alcohol were excluded due to their higher energy demands in the reduction and the olefin-formation stages. The good consistency between the experimental observations and the calculation results verified the proposed mechanism and also enabled us to clarify the reason for the efficiency of different reductants.