Solid-State [2+2] Photodimerization of 1-Aryl-4-pyridylbutadienes in Cation-π-Controlled Crystals

Irradiation of HX (X=CF₃ SO₃ or CF₃ CO₂ ) salts of 1-aryl-4-pyridylbutadienes 1 a-1 c in the solid-state afforded syn head-to-tail dimers in good yields among a number of possible dimers, whereas irradiation of the neutral substrates gave a complex mixture or no products. A comparison of the X-ray crystal structures of the neutral compounds and the HX salts clarified that their orientation modes are head-to-head and head-to-tail, respectively. Moreover, while the distances between the two neighboring double bonds of the neutral compounds are relatively far apart from each other, those of HX salts are close together, satisfying Schmidt's requirement. These findings suggested that cation-π interactions between the pyridinium and aromatic rings are effective for the preorientation of the HX salts of substrates, leading to photodimers in high regio- and stereoselectivities.