Chiral oxoperoxomolybdenum(VI) complexes for enantioselective olefin epoxidation: Some mechanistic and stereochemical reflections

Ever since the discovery of oxidation catalysts of the type MoO(O-2)(2)L-n, (n = 1-2, L = H2O, DMF, HMPA, Py) concerted efforts have been made by various groups to prepare or generate in situ chiral versions that lead to high asymmetric induction in either stoichiometric or catalytic olefin epoxidation reactions. To-date the highest enantioselectivity obtained was 81% obtained using a pre-prepared chiral Mo(VI)-oxodiperoxo containing chiral alpha-hydroxyamide ligand. Reports on using chiral Mo(VI)-oxodiperoxo complexes prepared in situ are exceedingly rare and what has been reported mentions no enantioselectivity. In this review we look at these efforts, highlighting their inherent limitations and suggest some new approaches which might enhance the asymmetric inductive powers of these catalysts in olefin epoxidation. (C) 2007 Elsevier B.V. All rights reserved.