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 MoO₂L^*₂ [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 MoO₂L^*₂ alcoholate complexes were considerably less active with comparable optical yields.