Insight for the photochemical reaction of 4-aryl-4H-pyran: Experimental and theoretical studies

Abstract

In this article, we have investigated the photochemical reactions of 4-aryl-4H-pyran and found that the structural symmetry had a significant influence on its photochemical behavior. When symmetric 4-aryl-4H-pyran was used, such as diethyl 2,6-dimethyl-4-aryl-4H-pyran-3,5-dicarboxylate, mainly underwent [2 + 2] photocycloaddition reaction. While asymmetric 4-aryl-4H-pyran, such as 2-amino-3-cyano-4-aryl-4H-pyrans, mainly underwent photocleavage reactions. To speculate the reaction mechanism, density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were carried out at the B3LYP-D3/def2TZVP//B3LYP-D3/6-31G(d) level. The results showed that diethyl 2,6-dimethyl-4-aryl-4H-pyran-3,5-dicarboxylate underwent a two-step [2 + 2] photocycloaddition to give 3,9-dioxatetraasterane derivatives via an intermolecular and an intramolecular reaction. And the 2-amino-3-cyano-4-aryl-4H-pyran underwent a ring cleavage to produce the conjugated diene derivatives. To explain the different photochemical behaviors of 4-aryl-4H-pyrans, electron spin density, frontier molecular orbitals (FMOs) analysis, electron-hole analysis and root-mean-square-deviation (RMSD) analysis were performed.