Radicals possess unique electronic characters to promote otherwise inert chemical reactions, and therefore gained tremendous development over the past decades. The generation of open-shelled singlet diradicals via the proton-coupled electron transfer (PCET) pathway followed by radical-radical cross-coupling is an effective approach to achieve the precise construction of C-C bonds with a high atomic economy. However, its application in NHC catalysis has rarely been reported hence requires further investigations. Herein, a comprehensive theoretical model on <i>N</i>-heterocyclic carbene (NHC)-catalyzed radical-radical cross-coupling reactions of aldehydes with various oxidants have been scrutinized using quantum mechanical simulations. We reveal that the single electron transfer (SET) and diradical recombinations proceed via the overlap of <i>p</i> orbitals between two moieties, which is confirmed by frontier molecular orbital (FMO) analysis, and track along with intrinsic reaction coordinates (IRC) and spin densities (SD) to confirm the radical nature of the key transition state. The obtained insights are useful for appreciating the effectiveness of NHC-catalyzed radical-radical cross-coupling reactions, and they should also be valuable for generalizing to other analogous organocatalytic reactions.

NHC-catalyzed radical-radical cross-coupling reactions via PCET

ACS Spring 2022

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