A Cp-based Molybdenum Catalyst for the Deoxydehydration of Biomass-derived Diols

Article abstract of DOI:10.1002/cctc.202001115

Dioxo-molybdenum complexes have been reported as catalysts for the deoxydehydration (DODH) of diols and polyols. Here, we report on the DODH of diols using [Cp^*MoO₂]₂O as catalyst (Cp^*=1,2,3,4,5-pentamethylcyclopentadienyl). The DODH reaction was optimized using 2 mol % of [Cp^*MoO₂]₂O, 1.1 equiv. of PPh₃ as reductant, and anisole as solvent. Aliphatic vicinal diols are converted to the corresponding olefins by [Cp^*MoO₂]₂O in up to 65 % yield (representing over 30 turnovers per catalyst) and 91 % olefin selectivity, which rivals the performance of other Mo-based DODH catalysts. Remarkably, cis-1,2-cyclohexanediol, which is known as quite a challenging substrate for DODH catalysis, is converted to 30 % of 1-cyclohexene under optimized reaction conditions. Overall, the mass balances (up to 79 %) and TONs per Mo achievable with [Cp^*MoO₂]₂O are amongst the highest reported for molecular Mo-based DODH catalysts. A number of experiments aimed at providing insight in the reaction mechanism of [Cp^*MoO₂]₂O have led to the proposal of a catalytic pathway in which the [Cp^*MoO₂]₂O catalyst reacts with the diol substrate to form a putative nonsymmetric dimeric diolate species, which is reduced in the next step at only one of its Mo-centers before extrusion of the olefin product.