LATTICE-CONTROLLED SOLID STATE-SPECIFIC AND ENANTIOSELECTIVE REACTIONS

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Previously, we found solia state-specific phenomenon in the photoracemization of optically active alkyl cobaloxime complexes. We also found solid state-specific and unidirectional (betha to alpha) photoisomerization of 2-substituted ethyl cobaloxime complexes.This study was carried out to clarify the mechanism for the above mentioned solid state-specific phenomenon and to find chiral lattice-controlled asymmetric (betha to alpha) photoisomerization, and the following results [ (1)- (3)] were obtained.(1) The solid state-specific phenomenon in the photoracemization of optically active alkyl cobalocime complexes was attributed (ascribed) to an unsymmetrical restriction of the reactive group by an intermolecular interaction such as hydrogen bonding.(2) Solid-state asymmetric photoisomerization occurred to afford optically active 1-substituted ethyl cobaloxime complexes with moderate to high enantioselectivities (up to 88%ee) upon visible light-irradiation on 2-cyanoethyl, 2-methoxycarbonylethyl, and N-substituted 2-carbamoylethyl cobaloximes coordinated with chiral axial ligand which has no asymmetric effect on the reactive group at the trans position in the same molecule but serves as the chiral handle for forming the chiral lattice.(3) The X-ray crystallographic analyzes of a series of 2-cyanoethyl and 2-methoxycarbonylethyl cobaloximes revealed that the 2-substituted ethyl group in a complex bringing about high enantioselectivity is apt to take a conformation parallel to the inplane ligand and is also forced to take a chiral conformation by chiral lattice, while the reactive group in a complex bringing about low enantioselectivity take a conformation perpendicular to the inplane ligand.