Chiral phosphinoylalcohol complexes of monooxobis(peroxo)molybdenum(VI) and their use as asymmetric oxidants

Complexes of the type [MoO(O-2)(2)(L*)(ROH)], where L* = chiral beta-phosphinoylalcohol, have been synthesised and used as stoichiometric oxidants for a number of unfunctionalised alkenes. In all of the complexes the chiral auxiliary is bound through the phosphinoyl oxygen as a monodentate ligand. The coordination about the metal atom in these pseudo-pentagonal bipyramidal molecules is completed by a solvent molecule (ethanol/water) lying opposite the axial Mo=O bond. Oxidation of small-chain non-functionalised alkenes occurs in variable yield to give epoxides with an enantiomeric excess of up to 40%. These compounds also behave as catalysts for the (BuOOH)-O-t oxidations of alkenes, but with similar modest enantioselectivities. The modest enantioselectivities are explained on the basis of the mode of coordination of the chiral ligand, and it is argued that there may be inherent limits in the use of these systems in asymmetric oxidations. (C) 1999 Elsevier Science B.V. All rights reserved.