Mechanistic Investigation of Molybdate-Catalysed Transfer Hydrodeoxygenation

The molybdate-catalysed transfer hydrodeoxygenation (HDO) of benzyl alcohol to toluene driven by oxidation of the solvent isopropyl alcohol to acetone has been investigated by using a combination of experimental and computational methods. A Hammett study that compared the relative rates for the transfer HDO of five para-substituted benzylic alcohols was carried out. Density-functional theory (DFT) calculations suggest a transition state with significant loss of aromaticity contributes to the lack of linearity observed in the Hammett study. The transfer HDO could also be carried out in neat PhCH₂ OH at 175 °C. Under these conditions, PhCH₂ OH underwent disproportionation to yield benzaldehyde, toluene, and significant amounts of bibenzyl. Isotopic-labelling experiments (using PhCH₂ OD and PhCD₂ OH) showed that incorporation of deuterium into the resultant toluene originated from the α position of benzyl alcohol, which is in line with the mechanism suggested by the DFT study.