Chiral bis(oxazoline) and pyridyl alcoholate dioxo-molybdenum(VI) complexes: synthesis, characterization and catalytic examinations

Abstract

A group of chiral molybdenum(VI) complexes comprising MoO 2Cl 2L **, MoO 2Cl(THF)L * and MoO 2L 2 * [L **=chiral bis(oxazoline) and L *=chiral 2′-pyridyl alcoholate] have been prepared in good yields by reaction of the solvent substituted complex MoO 2Cl 2(THF) 2 with one or two equivalents of chiral ligand. Optically active aminoalcohols (L *) were obtained by reaction of the appropriate organolithium compound with (−)-menthone, (+)-8-phenylisomenthone, (−)-8-phenylmenthone, (+)-camphor and (−)-thujone. The molybdenum complexes were characterized by multinuclear NMR ( 1H, 13C, 17O, 95Mo) spectroscopy, IR spectroscopy and elemental analysis. 95Mo-NMR data reflected the donor capability of the organic ligands, whereas 1H-NMR and IR data were comparatively indifferent to the changes in the Lewis base ligand. The complexes were evaluated as catalysts for the asymmetric epoxidation of trans-β-methylstyrene by tert-butylhydroperoxide. The bis(oxazoline) complexes showed good catalytic activity but had low optical yields. Complexes of the type MoO 2Cl(THF)L * (L *=chiral 2′-pyridyl alcoholate) also exhibited high catalytic activity and enantiomeric excesses of up to 23%. The corresponding MoO 2L 2 * alcoholate complexes were considerably less active with comparable optical yields.