Direct observation of deuterium migration in crystalline-state reaction bysingle-crystal neutron diffraction. III. Photoracemization of 1-cyanoethylcobaloxime complexes

The H atoms bonded to the chiral C atoms (stereogenic center) of the 1-cyan oethyl groups in two cobalt complexes, [(R)-1-cyanoethyl]bis(dimethylglyoxi mato)(pyridine)cobalt(III) (2) and [(R,S)-1-cyanoethyl]bis(dimethylglyoxima to)(piperidine)cobalt(III) (3), were replaced with D atoms, such as Co-C*D( CH3)CN. The crystals of the two cobalt complexes were irradiated with a xen on lamp for 72 h and 27 d, respectively. The unit-cell dimensions were grad ually changed with retention of the single-crystal form. The crystal struct ures after irradiation were determined by neutron diffraction. In each crys tal the chiral 1-cyanoethyl group of one of the two crystallographically in dependent molecules was partly inverted to the opposite configuration, wher eas that of the other molecule kept the original configuration. The C*-D bo nd in the inverted group was completely conserved in the process of the inv ersion of the chiral alkyl group. This suggests that the inversion of the c hiral 1-cyanoethyl group proceeds with the rotation of the cyanoethyl radic al after the Co-C bond cleavage by photo-irradiation so that the opposite s ide of the radical faces the Co atom. This is followed by recombination of the Co-C bond to form the inverted 1-cyanoethyl group.