2+2] Photodimerization of Stilbazoles Promoted by Oxalic Acid in Suspension

Abstract

In this study we reported on a very simple technique to perform efficiently photodimerization of some vinylpyridines. By irradiating a stirred mixture of several stilbazoles with solid oxalic acid dihydrate dispersed in a non-polar ( e.g. cyclohexane) or moderately polar (benzene, dichloromethane, dioxane) solvent, the corresponding dimeric cyclobutane adducts were obtained in high yields with excellent of regio- and stereo- selectivities. The strategy could also be applied successfully to oily, waxy or even insoluble stilbazoles. Moreover, the oxalic acid loading could be lowered to sub-stoichiometric amounts. When further optimizations were needed, our strategy was found to be highly flexible to identify other oligocarboxylic acids as alternative additive to improve or even overturn regioselectivity. Oxalic acid and other oligocarboxylic acids were found to be capable of orienting more than fifty stilbazoles toward photodimerization under these conditions.