Molybdenum catalysts based on salan ligands for the deoxydehydration reaction

Abstract

Dioxomolybdenum complexes based on salan ligands have been evaluated for their potential in catalyzing the deoxydehydration (DODH) reaction. The DODH reaction is a formal reduction that converts vicinal diols into olefins using an oxometal catalyst and a sacrificial reductant. The reaction holds enormous potential in transforming biomass-derived molecules into platform chemicals. This study evaluated twenty (20) molybdenum complexes supported by salan ligands in the DODH reaction with the goal of establishing structure–activity relationships. Catalyst screenings were performed using styrene glycol as a model substrate and 1–10 mol% loading of the molybdenum complexes at 170 °C producing styrene in up to 54% yield. Aliphatic diols and meso-/R,R-hydrobenzoin were also converted to the corresponding alkenes in moderate to good yields (60–71%) that are comparable to previously reported molybdenum catalysts. A bio-derived glycol, (+)-diethyltartrate, could be converted to the alkene product (diethyl fumarate) in >98% yield using 10 mol% catalyst. A high yield of diethyl fumarate (78%) was also obtained with Na₂SO₃ (cheap, readily available, and benign) as a reductant. Quite significantly, diethyl fumarate was produced in a 42% yield at a 1 mol% catalyst loading which represents a turnover number (TON) of 42; this is one of highest activity in a DODH reaction observed with molybdenum catalysts. The catalytic studies along with preliminary kinetic investigations reveal significant ligand effects: sterically bulky ortho-substituents and electron-withdrawing para-substituents on the phenol arms resulted in enhanced catalytic activity while a rigid phenyl as well as an ethylene backbone featuring a tertiary amine were found to impede catalysis.