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) and 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 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. (C) 2000 Elsevier Science S.A. All rights reserved.