How irradiation wavelengths affect the regioselectivity in photocycloaddition of 1,4-dihydropyridines

Density functional theory calculations have been performed to reveal the regioselective [2 + 2] and [3 + 2] photocycloaddition of ethyl 3-carboxylate-1,4-diphenyl-1,4-dihydropyridine affected by the irradiation wavelengths. It was found that the regioselectivity is affected by irradiation wavelength, resulting in the formation of the parallel cage dimer diazateraasteranes by [2 + 2] cycloaddition and the orthogonal cage dimer diazatetrakishomocubanes by [3 + 2] cycloaddition. The difference in product formation can be attributed to the distinct mechanism revealed by the DFT/TDDFT analysis at the PCM/M06-2X/def-TZVP//B3LYP/6-31g(d) calculation level. The [2 + 2] photocycloaddition has two steps including an intermolecular and an intramolecular [2 + 2] photocycloaddition, and the [3 + 2] photocycloaddition has three steps including the σ bond formation, the σ bond free rotation and intramolecular reaction. Both the intramolecular reactions of [2 + 2] and [3 + 2] photocycloaddition have the second excitation process of the key intermediates which is related to the irradiation wavelengths and the regioselectivity of the photocycloaddition products.