Excited-state proton transfer induced [4+2] and [4+4] photocycloaddition reactions of an oxazoline: Mechanism and selectivity

摘要

[4+4] and [4+2] photocycloadditions have not computationally attracted the same level of attention as [2 + 2] photocycloadditions. Herein we have employed density-functional theory (DFT), complete active space self-consistent field (CASSCF), and its multi-state second-order perturbation theory (MS-CA.SPT2) methods to systematically explore excited-state intramolecular proton transfer (ESIPT) induced [4+4] and [4+2] photocycloaddition mechanisms of an oxazoline. On the basis of the results, we have found that the ESIPTs in the S-1 state are not as efficient as those in the T-1 state concerning experimentally used irradiation wavelength. Instead, the T-1 ESIPTs generate two sets of endo and exo precursors of [4+4] and [4+2] photocycloadditions. In these reactions, the C atom of the furan group and the N atom of the aminophenyl group are first bonded leading to a diradical intermediate. This reaction step has a small barrier and is thermodynamically favorable. However, the remaining C-C bond formation is blocked in the T-1 state due to its huge reaction barrier. Therefore, the system hops to the So state via T-1 - S-0 intersystem crossings at T-1/S-0 crossing points. These nonadiabatic processes are efficient because of structural and energetic similarities of T-1/S-0 crossing points with their nearby T-1 intermediates. In the So state, the C C bond formation becomes a barrierless process. Finally, we have discussed possible origins for high regioselectivity and enantioselectivity of these photocycloaddition reactions. (C) 2017 Elsevier B.V. All rights reserved.