Chirality Inversion Process of a Bulky Alkyl Group in the Crystalline State Photo‐Isomerization of Cobaloxime Complex Crystals

The complex of [(R)‐ tert ‐(butoxycarbonyl)ethyl][(S)‐1‐cyclohexylethylamine]bis(dimethylglyoximato)cobalt(III), ( R ‐bce)( S ‐cha)cobaloxime, 1 , showed a racemization of the R‐bce group on exposure to a xenon lamp with retention of the single crystal form. A part of the R‐bce groups gradually inverted to the opposite configuration in the P2 1 2 1 2 1 cell. However, the inversion of the R ‐bce group converged when the R:S ratio became ca. 75:25. The shape of the reaction cavity for the disordered bce group became symmetric at the ratio. For the related complex crystal with pyridine as an axial ligand, ( R ‐bce)(py)cobaloxime, 2 , there are two crystallographically independent molecules, A and B, which are related by a pseudo inversion center in the P2 1 cell. When the crystal was exposed to the xenon lamp, the R ‐bce group of the A molecule was gradually inverted to the opposite configuration without decomposition of the single crystal form, whereas the R ‐bce group of B remained unchanged. Finally the pseudo inversion center became a crystallographic one and the space group became P2 1 /c. The third complex crystal with water as an axial ligand, ( R ‐bce)(aqua)cobaloxime, 3 , showed no changed on exposure to the xenon lamp. This is well explained by the size and shape of the reaction cavities for the R ‐bce groups in the three crystals.