Theoretical studies of spin state‐specific [2 + 2] and [5 + 2] photocycloaddition reactions of n ‐(1‐penten‐5‐yl)maleimide

N ‐alkenyl maleimides are found to exhibit spin state‐specific chemoselectivities for [2 + 2] and [5 + 2] photocycloadditions; but, reaction mechanism is still unclear. In this work, we have used high‐level electronic structure methods (DFT, CASSCF, and CASPT2) to explore [2 + 2] and [5 + 2] photocycloaddition reaction paths of an N ‐alkenyl maleimide in the S 1 and T 1 states as well as relevant photophysical processes. It is found that in the S 1 state [5 + 2] photocycloaddition reaction is barrierless and thus overwhelmingly dominant; [2 + 2] photocycloaddition reaction is unimportant because of its large barrier. On the contrary, in the T 1 state [2 + 2] photocycloaddition reaction is much more favorable than [5 + 2] photocyclo‐addition reaction. Mechanistically, both S 1 [5 + 2] and T 1 [2 + 2] photocycloaddition reactions occur in a stepwise, nonadiabatic means. In the S 1 [5 + 2] reaction, the secondary C atom of the ethenyl moiety first attacks the N atom of the maleimide moiety forming an S 1 intermediate, which then decays to the S 0 state as a result of an S 1 → S 0 internal conversion. In the T 1 [2 + 2] reaction, the terminal C atom of the ethenyl moiety first attacks the C atom of the maleimide moiety, followed by a T 1 → S 0 intersystem crossing process to the S 0 state. In the S 0 state, the second CC bond is formed. Our present computational results not only rationalize available experiments but also provide new mechanistic insights. © 2017 Wiley Periodicals, Inc.