[2+2] photodimerization of 1-aryl-4-pyridylbutadienes through cation–π interactions

Irradiation of 1-aryl-4-pyridylbutadienes in the presence of 1 equiv of HCl produced syn and anti head-to-tail dimers, among a number of possible dimers, whereas irradiation in the absence of HCl gave a complex mixture. This indicated that the acid serves as a catalyst for the regio- and stereoselective [2+2] photodimerization of 1-aryl-4-pyridylbutadienes through cation–π interactions between the pyridinium and aromatic rings. The produced synHT dimers underwent Cope rearrangement to produce cyclooctadienes, and they were in equilibrium at a ratio of 85:15 in CDCl3.