What Are the Radical Intermediates in Oxidative N -Heterocyclic Carbene Organocatalysis?

International audienceThe oxidation of the Breslow intermediate resulting from the addition of an N-heterocyclic carbene (NHC) to ben-zaldehyde triggers a fast deprotonation, followed by a second electron transfer, directly affording the corresponding acylium at E >-0.8 V (versus Fc/Fc+). Similarly, the oxidation of the cinnamaldehyde analogue occurs at an even higher potential and is not a reversible electrochemical process. As a whole, and contrary to previous beliefs, it is demonstrated that Breslow intermediates, which are the key intermediates in NHC-catalyzed transformations of aldehydes, cannot undergo a Single Electron Transfer (SET) with mild oxidants (E <-1.0 V). Moreover, the corresponding enol radical cations are ruled out as relevant intermediates. It is proposed that oxidative NHC-catalyzed radical transformations of enals proceed either through SET from the corresponding electron-rich enolate or through coupled electron-proton transfer from the enol, in any case generating neutral capto-dative radicals. Relevant electrochemical surrogates of these paramagnetic species have been isolated