“Twin-coronet” porphyrins bearing optically active 1,1′-bitetrahydronaphthalene derivatives on the both faces of the porphyrin were prepared as enantioselective oxidation catalysts modeling on cytochrome P-450s. The eclipsed isomer of the corresponding iron(III) porphyrins catalyzed epoxidation of styrenes substituted with electron-withdrawing groups in high e.e. (61–89%) and high product selectivity
在卟啉的两个面上带有光学活性 1,1'-双四氢萘衍生物的“双冠”卟啉被制备为模拟细胞色素 P-450 的对映选择性氧化催化剂。相应的铁 (III) 卟啉的失色异构体在高 ee (61–89%) 和高产物选择性下催化被吸电子基团取代的苯乙烯的环氧化。
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.