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 singleelectron 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.

Automated summary

The newly discovered triazolylidenes mesoionic carbene have strong reducing ability, allowing them to undergo single-electron transfer with iodoarenes and efficiently catalyze arylacylation reactions on alkenes.