Département de chimie moléculaire

The oxidn. of the Breslow intermediate resulting from the addn. of an N-heterocyclic carbene (NHC) to benzaldehyde triggers a fast deprotonation, followed by a second electron transfer, directly affording the corresponding acylium at E {\textgreater} -0.8 V (vs. Fc/Fc+). Similarly, the oxidn. of the cinnamaldehyde analog occurs at an even higher potential and is not a reversible electrochem. 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 {\textless} -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 electrochem. surrogates of these paramagnetic species have been isolated. [on SciFinder(R)]