Hydrogen bond donor functionalized dioxido-molybdenum(VI) complexes as robust and highly efficient precatalysts for alkene epoxidation

Article abstract of DOI:10.1016/j.mcat.2017.09.036

The synthesis of four novel, tridentate aminophenolate ligands HL1-HL4, bearing amide functionalities is reported. Reaction of these ligands with a dioxido molybdenum(VI) precursor led, depending on the choice of solvent, to mononuclear complexes of the type [MoO₂L(OMe)] (2, 4, 6) or dinuclear complexes [{MoO₂L}₂(μ-O)] (1, 3, 5, 7), containing one facially, tridentate ONO-ligand per metal center. This synthetic discrimination between dinuclear and mononuclear complexes allows for a comparison between structures and reactivity. Complexes 1-7 were found to be highly active catalysts in the epoxidation of several internal and terminal alkenes. With tert-butyl hydroperoxide (TBHP) as oxidant, precatalyst loadings of 0.0005 mol% (5 ppm) could be realized leading to turnover numbers of up to 110000. The precatalysts also allowed for the use of hydrogen peroxide (0.1 mol% precatalyst) as oxidant as well as various alcohols as “green” solvents, such as ethanol or even tert-butanol (usually an inhibitor of epoxidation).