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 MO2I2*(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) a nd Na-2[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 ana lyses. 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 comple xes exhibit good catalytic activity and substantial optical induction. By w ay of contrast, the analogous tungsten complexes have low activities at com parable optical yields. (C) 2000 Elsevier Science S.A. All rights reserved.