Chiral molybdenum(VI) and tungsten(VI) 2′-pyridinyl alcoholate complexes. Synthesis, structure and catalytic properties in asymmetric olefin epoxidation

A new class of chiral molybdenum(VI) and tungsten(VI) complexes of the type MO2L*2 (M=Mo, W; L*=chiral 2′-pyridinyl alcoholate), available through several synthetic pathways, and their catalytic behavior in the asymmetric epoxidation of unfunctionalized olefins are reported. MO2Cl2, MO2(acac)2, and Na2[MO4] (M=Mo, W) served as starting materials for the synthesis of the chiral molybdenum(VI) or tungsten(VI) complexes, respectively. The new oxo complexes were fully characterized including X-ray crystallographic analyses. The chiral 2′-pyridinyl alcoholate ligands were derived from either (−)-menthone, (−)-fenchone, (−)-camphor or (+)-camphor. For catalytic runs in the enantioselective epoxidation, trans-methylstyrene was used as model substrate and tert-butylhydroperoxide as the oxidant. The molybdenum complexes exhibit good catalytic activity and substantial optical induction. By way of contrast, the analogous tungsten complexes have low activities at comparable optical yields.