Mechanism of MTO-Catalyzed Deoxydehydration of Diols to Alkenes Using Sacrificial Alcohols

Abstract

Catalytic deoxydehydration (DODH) of vicinal diols is carried out employing methyltrioxorhenium (MTO) as the catalyst and a sacrificial alcohol as the reducing agent. The reaction kinetics feature an induction period when MTO is added last and show zero-order in [diol] and half-order dependence on [catalyst]. The rate-determining step involves reaction with alcohol, as evidenced by a KIE of 1.4 and a large negative entropy of activation (Δ<i>S</i>^‡ = −154 ± 33 J mol^–1 K^–1). The active form of the catalyst is methyldioxorhenium­(V) (MDO), which is formed by reduction of MTO by alcohol or via a novel C–C bond cleavage of an MTO-diolate complex. The majority of the MDO-diolate complex is present in dinuclear form, giving rise to the [Re]^1/2 dependence. The MDO-diolate complex undergoes further reduction by alcohol in the rate-determining step to give rise to a putative rhenium­(III) diolate. The latter is the active species in DODH extruding stereoselectively <i>trans</i>-stilbene from (<i>R</i>,<i>R</i>)-(+)-hydrobenzoin to regenerate MDO and complete the catalytic cycle