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(TH F)L* and MoO2L2* [L**=chiral bis(oxazoline) and L* = chiral 2'-pyridyl alco holate] have been prepared in good yields by reaction of the solvent substi tuted complex MoO2Cl2(THF)(2) with one or two equivalents of chiral ligand. Optically active aminoalcohols (L*) were obtained by reaction of the appro priate organolithium compound with (-)-menthone, (+)-8-phenylisomenthone, ( -)-8-phenylmenthone. (+)-camphor and (-)-thujone. The molybdenum complexes were characterized by multinuclear NMR (H-1, C-13, O-17, Mo-95) spectroscop y, IR spectroscopy and elemental analysis. Mo-95-NMR data reflected the don or capability of the organic ligands, whereas H-1-NMR and IR data were comp aratively indifferent to the changes in the Lewis base ligand. The complexe s were evaluated as catalysts for the asymmetric epoxidation of trans-beta -methylstyrene by tert-butylhydroperoxide. The bis(oxazoline) complexes sho wed good catalytic activity but had low optical yields. Complexes of the ty pe MoO2Cl(THF)L* (L* = chiral 2'-pyridyl alcoholate) also exhibited high ca talytic activity and enantiomeric excesses of up to 23%. The corresponding MoO2L2* alcoholate complexes were considerably less active with comparable optical yields. (C) 2001 Elsevier Science B.V. All rights reserved.