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In order to explore the contribution of the cation−π interaction in the photocyclodimerization of styrylpyridines, the effects of the acid concentration and the substituent on the aryl ring on the product distribution were investigated. The structures of the product dimers were clarified by X-ray crystallographic analysis. In addition, the X-ray packing structures of trans -4-styrylpyridine ( 1 ) and its HCl salt were compared. On increasing the acid amount, the yield of the syn- HT dimer significantly increased, whereas that of the cis -isomer decreased. The substituent on the aromatic ring had a significant effect on the product distribution. Irradiation of the substrate styrylpyridine bearing a CF3 group resulted in much lower selectivity, whereas the photolysis of the substrate having a MeO group resulted in a syn- HT dimer in 95% selectivity. Comparison of the X-ray packing structures of 1a and 1a ·HCl clarified the significant differences between them. The molecules of 1a ·HCl are packed alternately in antiparallel alignment in a face-to-face manner, the distance of which is 3.295 Å, strongly suggesting the existence of an intermolecular cation−π interaction. On the other hand, no such interaction was observed in 1a . These results lead to a conclusion that pyridinium−π interactions govern the orientation of the trans -styrylpyridinium in solution, the irradiation of which would result in the selective formation of the syn- HT dimer.
Yamada et al. (Wed,) studied this question.