Optical enrichment of a racemic chiral crystal by X-ray irradiation
GENERATION of optical activity in a racemic mixture of enantiomers, a process relevant to the origin of life and of naturally chiral compounds1, requires chirality in the interaction responsible for the transformation or the environment in which it occurs. Without violating this strict requirement, we have found a way in which optical enrichment of a racemic mixture can occur without any chiral specificity in the mechanism. A racemic mixture of (1-cyanoethyl)(piperidine)cobaloxime, which contains the chiral 1-cyanoethyl group, crystallizes in a chiral space group (P212121) in which R and S enantiomers occupy crystallographically distinct (diastereoisomeric) positions in the asymmetric unit cell. We have prepared single crystals of the D configuration. Irradiating the crystal with X-rays at 343 K causes racemization of the S en-antiomer while the R enantiomer remains unaltered. This racemization is thought to be due solely to the different volume constraints on the two enantiomers in their different crystal environments. The result is that the number of R molecules is increased at the expense of S, and so the overall composition of the crystal changes from racemic to enriched in the R enantiomer.
X-ray analysis of Co-C bond cleavage in the crystalline state
DURING serial structure analysis of cobaloxime derivatives1, aimed at interpreting the catalytic capability of the asymmetric hydrogenation2–5, we have found that the crystal of R-α-cyanoethyl (S-α-methylbenzylamine)-cobaloxime changes its unit cell dimensions by X-ray exposure without degradation of a single crystal form. The rate of the change was so slow that it was possible to collect the intensity data for several intermediate stages. We have proved, by calculating electron density in each stage, that the change reflects the racemisation of the cyanoethyl group.
ASYMMETRIC HYDROGENATION OF OLEFINIC COMPOUNDS CATALYZED BY BIS(DIMETHYLGLYOXIMATO)COBALT(II)-OPTICALLY ACTIVE AMINE COMPLEX
Bis(dimethylglyoximato)cobalt(II)-quinine complex catalyzed asymmetric hydrogenation of oleflnic compounds to afford optically active carboxylic acid and amino acid esters. α-Phenylacrylophenone gave higher optical yield (ca. 50%) than α,β-unsaturated esters (7–20%).