Chiral dioxomolybdenum(VI) complexes for enantioselective alkene epoxidation

Chiral dioxomolybdenum(VI) complexes of the type MoO2Cl2(L*) (L*=oxime), MoO2(THF)2L* (L*=cis-p-menthane-3,8-diol) and MoO2Cl(THF)L* (L*=8-phenylthioneomenthol and 8-phenylthioisoneomenthol) have been prepared in good yields by reacting MoO2Cl2(THF)2 with the appropriate chiral organic bidentate O,O-, O,N- and O,S-ligands. The complexes were characterised by solution NMR (1H, 13C, 95Mo) and IR spectroscopy as well as elementary analysis, and were evaluated as catalysts in solution for the asymmetric epoxidation of cis-β-methylstyrene by tert-butylhydroperoxide (TBHP). The cis-diol complex shows high catalytic activity and enantiomeric excesses of up to 25%. An attempt was made to immobilise the complex MoO2(THF)Cl[(−)-8-phenylthioneomenthol] within the channels of MCM-41 mesoporous silica by using a tethering ligand [L=NC(CH2)3Si(OEt)3]. The material was characterised by powder X-ray diffraction (XRD), IR spectroscopy and magic-angle-spinning (MAS) NMR (13C, 29Si). Catalytic examinations demonstrated that it was active in the epoxidation of cyclooctene by TBHP.