ASYMMETRIC EPOXIDATION OF SIMPLE OLEFINS BY CHIRAL BITETRALIN-LINKED TWIN-CORONET PORPHYRIN CATALYSTS

Catalytic and asymmetric epoxidation of styrenes and related aryl substituted olefins with the iron complexes of chiral bitetralin (Bitet)-linked ''twin-coronet'' porphyrins was performed with iodosylbenzene as an oxidant. Among two topological isomers of the catalyst, the eclipsed one (5b) showed higher enantioselectivity than the staggered (6b). With 5b, the resulting epoxides, except for the olefins bearing an electron-donating substituent, were obtained in good to excellent enantioselectivity (54-96% ee), especially for the styrenes with electron-withdrawing substituent(s). Being different from other porphyrin-based chiral catalysts, the catalyst 5b is robust enough under the applied oxidation conditions to exhibit chiral epoxidation with the same ee and the same rate as those of the initial period of the reaction even after about 500 turnovers. The Bitet catalyst is superior in the epoxide enantioselectivity than the corresponding chiral binaphthalene (Binap)-linked catalyst (3b). In the reactions with the catalysts 3b and 5b, good correlation in epoxide ees was observed. Increase of the epoxide ee in the reaction with the Bitet catalyst was elucidated by the shape and size of the reaction cavities of the Bitet were tighter than those of the latter. The observed ees of the substituted styrene oxides showed good correlation with SIGMAsigma+ values of their substituent(s). In the reaction with the electron-deficient olefins, pi-pi* interaction between the HOMO of the electron-rich Bitet auxiliary ring and the LUMO of the electron-deficient aryl ring of the substrate are pointed out as the key for the realization of high ees. Some nitrostyrenes, however, gave rather lower ees in spite of rather higher degree of their electron deficiency. This deviation was elucidated by the mismatching of their frontier orbitals.