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

Abstract

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. • The triplet state of deprotonated MIC-Breslow intermediates is thermally accessible • Catalytic distal difunctionalization of aryl aldehydes via biradical pathways • Redox-neutral reactions with simultaneous oxidative esterification and reduction Over the past decades, N-heterocyclic carbene (NHC)-based organocatalysis, which involves the umpolung of aldehydes via Breslow intermediates (BIs), has been developed as an important approach in organic synthesis. Among them, NHC-catalyzed electron-pair reactions and radical reactions have been established via BIs and BI − s, respectively. In this article, we disclose that the triplet state of deprotonated mesoionic carbene BIs is thermally accessible. Taking advantage of this property, we describe a mesoionic-carbene-catalyzed distal difunctionalization reaction of aryl aldehydes via biradical pathways. These reactions feature redox neutrality with simultaneous oxidative esterification and reduction. Yan and co-workers report that deprotonated mesoionic carbene Breslow intermediates (BIs) 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.