What is the origin of highly asymmetric induction by a chiral (salen)manganese(III) complex? Design of a conformationally fixed complex and its application to asymmetric epoxidation of 2,2-dimethylchromenes

The optically active (salen)manganese(III) complex 1 having a carboxylate group on the ethylenediamine moiety was found to be an efficient catalyst for the asymmetric epoxidation of several 2,2-dimethylchromene derivatives (up to 99% ee). A high level of turn-over number (up to 9, 200) was also achieved in this epoxidation. The pseudo-axially oriented carboxylate in 1 was postulated to coordinate to the manganese ion and to fix the conformation of the salen ligand in the form enantiomeric to that of normal chiral (salen)manganese(III) complexes. This was supported by the absolute configurations of the produced epoxides. (C) 1998 Elsevier Science Ltd. All rights reserved.