Mechanistic Studies of the Deoxydehydration of Polyols Catalyzed by a Mo(VI) Dioxo(pyridine-2,6-dicarboxylato) Complex

The need for renewable resources
to displace petrochemical feedstocks
is of great interest. Deoxydehydration (DODH) reduces biomass-derived
polyols and diols to alkenes and dienes, possibly moving alkene production
closer to carbon-neutral sources. We report the DODH of numerous aromatic,
aliphatic, and biomass model diols by a dioxo-Mo(VI) catalyst, MoO2(pyridine-2,6-dicarboxylato)(HMPA). Optimal reaction conditions
were found using PPh3 and 1-phenyl-1,2-ethanediol yielding
up to 93% styrene. Mechanistic studies using in situ infrared spectroscopy support a biexponential kinetic regime where
the rate constant of the rate-determining step is ∼5 ×
10–5 s–1. DFT calculations support
the kinetic studies and suggest that the largest kinetic barrier is
a proton transfer from the substrate to a metal–oxo bond during
diolate formation.