Chiral dioxomolybdenum(VI) complexes for enantioselective alkene epoxidation

Chiral dioxomolybdenum(M) complexes of the type MoO2Cl2(L*) (L* = oxime), M oO2(THF)(2)L* (L* = cis-p-menthane-3,8-diol) and MoO2Cl(THF)L* (L* = 8-phen ylthioneomenthol and 8-phenylthioisoneomenthol) have been prepared in good yields by reacting MoO2Cl2(THF)(2) with the appropriate chiral organic bide ntate O,O-, O,N- and O,S-ligands. The complexes were characterised by solut ion NMR (H-1, C-13, Mo-95) and IR spectroscopy as well as elementary analys is, and were evaluated as catalysts in solution for the asymmetric epoxidat ion of cis-beta -methylstyrene by tert-butylhydroperoxide (TBHP). The cis-d iol complex shows high catalytic activity and enantiomeric excesses of up t o 25%. An attempt was made to immobilise the complex MoO2(THF)Cl[(-)-8-phen ylthioneomenthol] within the channels of MCM-41 mesoporous silica by using a tethering ligand [L = NC(CH2)(3)Si(OEt)(3)]. The material was characteris ed by powder X-ray diffraction (XRD), IR spectroscopy and magic-angle-spinn ing (MAS) NMR (C-13, Si-29). Catalytic examinations demonstrated that it wa s active in the epoxidation of cyclooctene by TBHP. (C) 2001 Elsevier Scien ce B.V. All rights reserved.