Monitoring reaction centers and molecules during an enantioselective photoreaction in a crystal

The single crystal-to-single crystal retro-Claisen photorearrangement of the salt of 7-(4-carboxybenzoyl) norbornene with (S)-(2)-1-cyclohexylethylamine was studied. Although this reaction is an ionic chiral auxiliary-mediated asymmetric synthesis, which normally affords highly enantio-enriched products, the enantiomeric excess in this case is a relatively low 60% (80% of one enantiomer, 20% of the other). X-ray structure analysis was used in order to describe changes in the reaction centers and the behavior of molecules during the photoreaction and to understand the formation of two enantiomers. It was discovered that the conformation of the reactant molecules changes during the photoreaction to one that enables the formation of the minor enantiomer. This was seen clearly at ca. 30% reaction progress. The variations in the cell constants as a function of reaction progress are also presented. The structural changes taking place during the retro-Claisen photorearrangement are compared to the changes observed for other intra- and intermolecular photoreactions in crystals.