Mesoionic carbene-Breslow intermediates as super electron donors: Application to the metal-free arylacylation of alkenes

Classical N-heterocyclic carbenes (NHCs), such as thiazolylidenes, 1,2,4-triazolylidenes, and imidazol(in)-2-ylidenes, are powerful organocatalysts for aldehyde transformations through the so-called Breslow intermediates (BIs). The reactions usually occur via electron-pair-transfer processes. In contrast, the use of BIs in single-electron transfer (SET) pathways is still in its infancy, and the scope is limited by the moderate reduction potential of BIs derived from classical NHCs (ca. −1.0 V versus standard calomel electrode [SCE]). Here, we report that BIs from 1,2,3-triazolylidenes, a type of mesoionic carbene (MIC), have a reduction potential as negative as −1.93 V versus SCE and thus are among the most potent organic reducing agents reported to date. They are reductive enough to undergo SET with iodoarenes, which allows the highly efficient inter- and intramolecular MIC-catalyzed arylacylation of styrenes and alkenes, respectively.