The triplet state of deprotonated mesoionic carbene Breslow intermediates is thermally accessible: Distal difunctionalization of aldehydes

N-Heterocyclic carbene (NHC)-catalyzed transformations through electron-pair-transfer processes have witnessed remarkable developments in the past decades, especially the umpolung of aldehydes via the Breslow intermediates (BIs). The role of BIs in single-electron transfer (SET) pathways has long been established in biosynthesis, and in the last decade, it was extended to NHC organocatalysis. Here, we report that deprotonated mesoionic carbene BIs (BI(-)s) have an astonishing small singlet/triplet (S/T) gap of only 4.0 kcal/mol, resulting in a thermally accessible triplet state. This low-lying excited state allows for a series of distal difunctionalizations of aryl aldehydes via biradical processes. At the formyl side, an oxidative esterification reaction takes place concomitantly with a reduction of the aryl side. Here, three types of chemical transformation are demonstrated: hydro(deutero)defunctionalization, hydrogenation, and reductive alkylation.

Automated summary

N-Heterocyclic carbene-catalyzed transformations via electron-pair-transfer processes have made significant progress, particularly in the umpolung of aldehydes using Breslow intermediates. This study introduces a new type of mesoionic carbene intermediate with a small singlet/triplet gap, allowing for thermally accessible triplet states and various distal difunctionalizations of aryl aldehydes through biradical processes.