Dioxomolybdenum(VI) complexes with naphtholate-oxazoline ligands in catalytic epoxidation of olefins

Synthesis, characterization and catalytic epoxidation experiments of two new dioxomolybdenum(VI) complexes [MoO2(L)2] (3a-b) equipped with O,N-bidentate naphtholate-oxazoline ligands L = 2a-b are described. Ligands 2a-b (2a = 2-(4,5-dihydrooxazol-2-yl)naphthalen-1-ol, 2b = 2-(4,4-dimethyl-4,5-dihydrooxazol-2-yl)naphthalen-1-ol) were obtained via a two-step synthesis starting from 1-hydroxy-naphthoic acid. Complexes 3a-b were synthesized starting from [MoO2(acac)2] and obtained in good yields as air and moisture stable solids. The molecular structure of both complexes 3a-b were determined by single crystal X-ray diffraction analysis, showing the expected octahedral coordination of the Mo center by two bidentate ligands of 2a or 2b and two terminal oxo ligands. Interestingly, for complex 3b two different coordination isomers with regards to the orientation of the bidentate ligands (N,N-trans 3b and N,N-cis 3b’), were obtained in the solid state. Both complexes 3a-b show high catalytic activities and selectivities in the epoxidation of various terminal and internal olefins at low catalyst loadings of 0.05 mol% with tert-butylhydroperoxide. With cyclooctene TON of 5000 could be reached, for styrene selectivities of >90% were achieved.