3 years ago

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

Theoretical studies of spin state-specific [2 + 2] and [5 + 2] photocycloaddition reactions of n-(1-penten-5-yl)maleimide
Xiang-Yang Liu, Wei-Hai Fang, Pin Xiao, Ganglong Cui
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 S1 and T1 states as well as relevant photophysical processes. It is found that in the S1 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 T1 state [2 + 2] photocycloaddition reaction is much more favorable than [5 + 2] photocyclo-addition reaction. Mechanistically, both S1 [5 + 2] and T1 [2 + 2] photocycloaddition reactions occur in a stepwise, nonadiabatic means. In the S1 [5 + 2] reaction, the secondary C atom of the ethenyl moiety first attacks the N atom of the maleimide moiety forming an S1 intermediate, which then decays to the S0 state as a result of an S1 S0 internal conversion. In the T1 [2 + 2] reaction, the terminal C atom of the ethenyl moiety first attacks the C atom of the maleimide moiety, followed by a T1 S0 intersystem crossing process to the S0 state. In the S0 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. High-level electronic structure methods (DFT, CASSCF, and CASPT2) have been used to explore spin state-specific [2 + 2] and [5 + 2] photocycloaddition reactions of N-(1-penten-5-yl)maleimide. We have found that both S1 [5 + 2] and T1 [2 + 2] photocycloaddition reactions occur in a nonadiabatic and stepwise way.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/jcc.24897

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