PARAMAGNETIC H-1-NMR RELAXATION PROBES OF STEREOSELECTIVITY IN METALLOPORPHYRIN CATALYZED OLEFIN EPOXIDATION

Enantioselective catalytic epoxidation of olefins is an important prob lem from both practical and mechanistic points of view. The origins of chiral induction by asymmetric porphyrin and salen complexes were inv estigated by FT-NMR T-1 relaxation techniques. A new chiral vaulted po rphyrin (1) that carries (S)-binaphthyl-L-alanine straps across both f aces of the porphyrin macrocycle was synthesized and characterized. (R )-styrene oxide was obtained in >90% ee in the initial stages of styre ne epoxidation with F5PhIO catalyzed by 1-Fe(III)Cl. The transition st ate for olefin epoxidation with high-valent metal-ore species was mode led by coordinating epoxides to paramagnetic copper complexes of the c orresponding ligands. The epoxide enantiomer that better fit the chira l cavity of the catalyst, as revealed by T-1 relaxation measurements, was also the major product of catalytic olefin epoxidation. These resu lts are consistent with the ''lock-and-key'' mechanism of asymmetric c atalysis by metalloporphyrins. The copper complex of a chiral salen li gand showed no differentiation in terms of T-1 relaxation rates betwee n the enantiomers of cis-beta-methylstyrene oxide in contrast to the h igh enantioselectivity observed for catalytic epoxidation. (C) 1998 Wi ley-Liss, Inc.