Role of Cation π-Interactions in the Photodimerization of trans-4-Styrylpyridines

摘要

Controlling the regio- and stereoselectivities during the photo-cyclodimerization of alkenes is one of the key subjects in synthetic organic photochemistry. To this end, much effort has been directed toward the use of various organized media and supramolecular environments, such as crystals, clays, cyclodextrines, zeolites, biomolecules, micelles, hydrogen-bonding templates, and self-assembled cages and hosts. Another approach is the employment of a molecule having a self-assembling property; however, there are few successful examples in solution except for the case using supramolecular assistance.rnWe were interested in a reported [2 + 2]-photodimerization reaction of trans-styrylpyridines, in which a syn head-to-tail (HT) dimer is obtained as the major product only in an acidic solution. However, this remarkable effect of the acid on the product distribution still remains unexplained. On the basis of our recent studies regarding the application of a pyridinium—π interaction toward organic synthesis, we envisioned that the pyridinium—π interactions could be significantly responsible for the remarkable acid effect on the stereoselectivity during the photodimerization of the trans-styrylpyridines.