Hydrogen migration mechanism in crystalline-state photoisomerization by analyzed neutron diffraction

Abstract

The crystal structure analysis with neutron diffraction can clearly make a distinction between the deuterium and hydrogen atoms although they cannot be distinguished by X-ray diffraction analysis. Applying the neutron diffraction analysis for the crystalline-state photoisomerization, in which a deuterium atom is replaced with the other hydrogen atom without degradation of the single crystal form, the mechanisms of the hydrogen atom transfer in the photoisomerization of several organic and organometallic compounds have been made clear. In the first example, the photoinversion of the chiral 1-cyanoethyl group bonded to the cobalt atom in a cobaloxime complex proceeded keeping the deuterium atom in this original position. For the larger alkyl group than the 1-cyanoethyl group in the second example, the chiral 1,2-bis(ethoxycarbonyl)ethyl group was also inverted to the opposite configuration in the same mechanism as that of the smaller 1-cyanoethyl group. The deuterium atom bonded to the chiral carbon atom was completely kept in its original position. For the 2-1 photoisomerization of the 2-cyanoethyl group in the third example, the C1-carbon atom of the produced 2-cyanoethyl radical after Co–C bond cleavage makes a bond with the Co atom, following the transformation of one of the deuterium atoms bonded to the C1-carbon atom to the neighboring C2-carbon atom to become a methyl group. For the 3-1 photoisomerization, the reaction proceeds in the two steps, 3-2 and 2-1 isomerizations. The chirality of the produced 1-cyanopropyl group depends on the asymmetric environment surrounding the 3-cyanopropyl group in the crystal structure before irradiation. For the four-membered ring formation in the fifth example, the hydrogen atom of one of the benzyl groups is extracted by one of the olefin carbon atoms of the cyclohexenyl group and the benzyl carbon atom makes a bond with another olefin carbon to form a thiolactam. The deuterium atom of the benzyl groups is transferred to the intramolecular cyclohexene carbon to occupy the equatorial position of the produced cyclohexyl ring. In the sixth example, the 4-cyanobutyl group of a cobaloxime complex was found to be isomerized to the 1-cyanobutyl group with retention of the single crystal form. The structure analysis of the photo-irradiated crystal by neutron diffraction revealed that the isomerization from 4- to 1-cyanobutyl group proceeded in the ‘alkyl turn’ route. The size and the shape of the reaction cavity for the cyanobutyl group clearly suggest that the alkyl turn mechanism is favorable with retention of the single crystal form. The steric repulsion from the atoms surrounding the reactive cyanobutyl group in the crystal causes such a strange reaction process.