A close look at the wavelength-dependent [2+2] and [3+2] photocycloaddition of 1,4-dihydropyridines

Abstract

To investigate the wavelength-dependent [2 + 2] and [3 + 2] photocycloaddition of 1,4-dihydropyridines (1,4-DHPs), the combined experimental and theoretical studies were conducted on the ethyl 1,4-diaryl-1,4-dihydropyridine-3-carboxylic acid under different wavelength lights. When the irradiation wavelength is shorter than 365 nm, the main products are [2 + 2] photocycloaddition dimers (3,9-diazatetraasteranes and 3,6-diazatetraasteranes), and when the irradiation wavelength is longer than 395 nm, the main products are [3 + 2] photocycloaddition dimers (6,12-diazatetrakishomocubanes). The density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were performed to gain a deeper understanding of the photocycloaddition of 1,4-DHPs. The results showed that both the [2 + 2] and [3 + 2] photocycloadditions are two-step mechanisms, including an intermolecular and an intramolecular photocycloaddition, and the chemoselectivity is mainly caused by the intramolecular reaction. The intramolecular [2 + 2] photocycloaddition was proven to occur in a short wavelength that gives the higher energy to overcome the reaction energy barriers. The intramolecular [3 + 2] photocycloaddition was proven to be a heat-induced reaction, which can occur in both short and long wavelengths. Therefore, depending on the irradiation wavelength, the dimers of [2 + 2] and [3 + 2] photocycloaddition can be selectively obtained. All the results would provide a theoretical and experimental basis for the synthesis and wide applications of photocycloaddition products of 1,4-DHPs.