In Situ Spectroscopic Investigation of the Rhenium-Catalyzed Deoxydehydration of Vicinal Diols

The mechanism of the CH₃ReO₃-catalyzed deoxydehydration of a vicinal diol to an alkene driven by oxidation of a secondary alcohol was investigated by time-resolved, in situ IR spectroscopy and was found to occur in three steps: 1) reduction of the catalytically active methyltrioxorhenium(VII) to a rhenium(V) complex (the rate-limiting step), 2) condensation of the diol and the rhenium(V) complex to a rhenium(V) diolate, and 3) extrusion of the alkene accompanied by oxidation of the Re center and thus regeneration of CH₃ReO₃. The reaction follows zero-order kinetics initially but, unexpectedly, accelerates towards the end, which is explained in terms of a deactivating pre-equilibrium, in which the catalytically active CH₃ReO₃ condenses reversibly with the diol to form an inactive rhenium(VII) diolate. This conclusion is supported by the direct observation of a catalytically inactive species as well as DFT calculations of the IR spectra of the relevant compounds.