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 and T states as well as relevant photophysical processes. It is found that in the S 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 state [2 + 2] photocycloaddition reaction is much more favorable than [5 + 2] photocyclo-addition reaction. Mechanistically, both S [5 + 2] and T [2 + 2] photocycloaddition reactions occur in a stepwise, nonadiabatic means. In the S [5 + 2] reaction, the secondary C atom of the ethenyl moiety first attacks the N atom of the maleimide moiety forming an S intermediate, which then decays to the S state as a result of an S → S internal conversion. In the T [2 + 2] reaction, the terminal C atom of the ethenyl moiety first attacks the C atom of the maleimide moiety, followed by a T → S intersystem crossing process to the S state. In the S state, the second CC bond is formed. Our present computational results not only rationalize available experiments but also provide new mechanistic insights. © 2017 Wiley Periodicals, Inc.