Triplet phenacylimidazoliums-catalyzed photocycloaddition of 1,4-dihydropyridines: An experimental and theoretical study

The photocycloadditions of 1,4-dihydropyridines (DHPs) were achieved by using phenacylimidazoliums (PIms) as photosensitizers. Irradiation of DHPs 3a–g in the presence of PIms 1a–e and 2 performed an efficient formation of 3,9-diazatetraasteranes in shorter times under a lower power lamp. The mechanism of photocycloaddition catalyzed by PIm was studied by laser flash photolysis and theoretical DFT computation. These time-resolved results showed that the triplet excited states of PIms were generated with high efficiency and detected by their characteristic ultraviolet absorptions, which were quenched by DHP at almost diffusion controlled rate. Theoretical studies suggest that PIm is involved in the photocycloaddition process through the 3(DHP⋯PIm)* triplet complexes and assists the stabilization of intermediates. All subsequent steps are predicted to be favorable and exothermic, leading to the cage dimers.