Switching mesoionic carbene-organocatalysis from radical to ionic pathway through base-controlled formation of Breslow intermediates versus Breslow enolates.

N-heterocyclic carbene (NHC) organocatalysis has experienced significant advancements. Two distinct reaction pathways have been developed, ionic and radical, through Breslow intermediates (BIs) and Breslow enolates (BI-s), respectively. The ability to selectively generate these intermediates is crucial for optimizing reaction outcomes. In this paper we show that with mesoionic carbenes (MICs) it is possible to control the formation of BIs versus BI-s, through the use of weak bases and strong bases, respectively. Of particular interest is the coupling of aldehydes and alkyl halides to yield ketones via an ionic pathway.