ピリジン誘導体と気体の反応による分子配列変化：カチオン−π相互作用による制御
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Exposure of 4-methoxyazachalcone (1) with a head-to-head orientation to HCl gas produced the corresponding HCl salt with a head-to-tail stacked arrangement, which was confirmed by PXRD experiments. A plausible mechanism for the reorientation was proposed, in which cation-π interaction plays a key role during the processes. Irradiation of the produced salt afforded synHT dimer in high regio- and stereoselectivities, thus showing the effectiveness of the cascade process in crystals. Photodimerization of styrylpyridine (5) and 5·HCl·2H2O was also investigated. While 5 was photo-stable, irradiation of 5·HCl·2H2O gave synHT dimer in high selectivity. X-ray structural analysis of 5·HCl·2H2O clarified that water molecules assist the assembly of the molecules in a head-to-tail and face-to-face fashion through N-H⋅⋅⋅O hydrogen bonds in combination with cation-π interactions between the pyridinium and aromatic rings. Dehydration of 5·HCl·2H2O produced anhydrous 5·HCl, which was also photo-stable. This was rehydrated to produce 5·HCl·2H2O by exposure to H2O vapor.