Ligand effects in the copper catalyzed aerobic oxidative carbonylation of methanol to dimethyl carbonate (DMC)

Abstract

The influence of the type and amount of added N-donor ligand, of the anion and the oxidation state on the copper catalyzed aerobic oxidative carbonylation of methanol to dimethyl carbonate (DMC) is systematically studied. A surprising increase in activity and selectivity compared to the plain copper halides CuX n (X=Cl, Br, I; n=1, 2) is found for catalyst complexes with three or four N-methylimidazole (NMI) ligands at Cu +/2+, a ligand regime reminiscent of the oxygen activating copper enzymes in nature. However, a large excess of NMI inhibits the catalysis. The NMI complexes turned out to be more active and selective as redox catalysts and less active in the competing undesired hydrolytic cleavage of DMC into methanol and carbon dioxide by the unavoidable byproduct water. Furthermore, corrosion of stainless steel autoclaves is efficiently inhibited in the presence of ≥2 eq. of NMI per copper halide.