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

Abstract

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.

Molybdenum Catalysts based on Salan Ligands for the Deoxydehydration Reaction

“…This is in contrast to a recent report from the Kilaynek group where the amount of polystyrene in the reaction mixture increased from 7-52% over a 6-hour period during catalysis (190  C) in the absence of a polymerization inhibitor. 38 The time-dependent data collected using 1b on the other hand exhibits a gradual change in glycol consumption as well as styrene yield reaching 23% over the initial 90 minutes and resulting in 40 % alkene yield overnight. The slightly reduced yield observed after the overnight run during the time-dependence study in comparison to a regular catalytic run (45%; Table 1, entry 1) could be indicative of loss in catalytic activity resulting from the periodic collection of aliquots; a similar effect is also observed when complex 6b was used.…”

“…More recently, a related 6−coordinate complex, an HMPA adduct, resulted in styrene yields up to 69% (in addition to ~20% styrene that polymerized under these conditions) at 190  C using PPh3 as reductant. 38 A dinuclear dioxomolybdenum complex supported by the Cp* ligand, (Cp*MoO2)2O, was reported by the Gebbink group for DODH of aliphatic diols using PPh3 as a reductant in anisole at 200  C over 15 hours. 39 Olefin yield as high as 65% (representing >30 turnovers) was recorded in m-dichlorobenzene as solvent and high olefin selectivity (up to 91%) was observed in trichlorobenzene as solvent for the formation of 1-octene from octane-1,2-diol using this cyclopentadienyl-Mo catalyst.…”

“…The efficiency of 2b in catalyzing the DODH of styrene glycol is comparable to previosuly reported dioxomolybdenum complexes. [36][37][38]41 Aliphatic diols such as decane-1,2-diol and octane-1,2-diol when subjected to the reaction conditions produced the corresponding olefins in 28% and 17% yield respectively using 10 mol% of the catalyst (Table 2, entries 7 & 10). A 23% yield of 1-decene was attained when Na2SO3 was used reductant (comparable to yield obtained using PPh3) while only a 16% yield of decene (at 55% conversion of decane-1,2-diol) was obtained when 1b (10 mol%) was used as the catalyst ( observed alkene yields suggest low reactivity of the system towards aliphatic diols at 170  C in line with previous reports.…”

“…center to generate a -oxo-Mo(V) dimer which is catalytically inefficient. 38 Alternatively, the steric bulk could also facilitate the alkene extrusion step which has been identified as one of the steps with a high activation barrier in the catalytic cycle in previous studies. 17,27,30,31 Overall, these preliminary kinetic studies highlight the importance of various ligand features (steric, electronics, backbone flexibility) in modulating the reactivity of these dioxomolybdenum complexes in the DODH reaction.…”

Reactivity of metal dioxo complexes

“…A less bulky ONO coordination environment was also explored by the same group using 2,6-pyridinedicarboxylate as a ligand. 107 1-Phenyl-1,2-ethanediol was chosen as a model substrate and an extensive scope of reductants was performed. The commercially attractive secondary alcohols 2-propanol and 3-octanol were found to give styrene yields of 30% and 14%, respectively.…”