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

A group of chiral molybdenum(VI) complexes comprising MoO2Cl2L**, MoO2Cl(THF)L* and MoO2L2* [L**=chiral bis(oxazoline) and L*=chiral 2′-pyridyl alcoholate] have been prepared in good yields by reaction of the solvent substituted complex MoO2Cl2(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 MoO2Cl(THF)L* (L*=chiral 2′-pyridyl alcoholate) also exhibited high catalytic activity and enantiomeric excesses of up to 23%. The corresponding MoO2L2* alcoholate complexes were considerably less active with comparable optical yields.