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

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 (ΔS‡ = −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 trans-stilbene from (R,R)-(+)-hydrobenzoin to regenerate MDO and complete
the catalytic cycle.