CATALYTIC AND ASYMMETRIC EPOXIDATION OF OLEFINS WITH IRON COMPLEXES OF TWIN-CORONET PORPHYRINS - A MECHANISTIC INSIGHT INTO THE CHIRAL INDUCTION OF STYRENE DERIVATIVES

As a cytochrome-P-450-modeling asymmetric-oxidation system we designed and synthesized novel chiral C2-symmetric ''twin-coronet'' porphyrins, which have chiral biaryl auxiliaries (i.e. binaphthyl and bitetralin derivatives) linked by ethereal bonds on both faces. Each face of these porphyrins forms a chiral substrate-binding site and is sterically protected from oxidative catalyst deterioration. In the catalytic oxidation of styrene derivatives with the iron porphyrins and iodosobenzene, we observed the highest ee (89%) for 2-nitrostyrene. We also found that the substituent sigma+ values correlated well with both the obtained log k(rel) and the optical yields of the epoxides produced. The existence of a possible intermediate and the mechanism of chiral induction are discussed. Charge-transfer (CT) interactions between the substrates and the binaphthalene moieties over the catalysts at the oxo-transfer stage seem to function cooperatively with steric interactions in the fixation of the substrates and the recognition of their prochiral faces. We propose a new guideline for achieving high enantioselectivity through appropriate CT interactions in the catalytic asymmetric oxidation of simple substrates.