Univ. Grenoble Alpes , CNRS, DCM , 38000 Grenoble , France.
UMI CNRS 3555, Department of Chemistry and Biochemistry , University of California, San Diego , La Jolla , California 92093-0343 , United States.
J Am Chem Soc. 2019 Jan 16;141(2):1109-1117. doi: 10.1021/jacs.8b11824. Epub 2018 Dec 31.
The oxidation of the Breslow intermediate resulting from the addition 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 > -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.
由于 N-杂环卡宾 (NHC) 加到苯甲醛上，导致 Breslow 中间体发生氧化，引发快速去质子化，然后进行第二次电子转移，直接在 E > -0.8 V（相对于 Fc/Fc）处生成相应的酰基阳离子。类似地，肉桂醛类似物的氧化发生在更高的电位，并且不是可逆的电化学过程。总的来说，与之前的观点相反，证明了 Breslow 中间体是 NHC 催化醛转化的关键中间体，不能与温和氧化剂（E < -1.0 V）发生单电子转移 (SET)。此外，排除了相应的烯醇自由基阳离子作为相关中间体。提出氧化 NHC 催化的烯醛自由基转化可以通过相应富电子烯醇化物的 SET 进行，也可以通过烯醇的偶联电子-质子转移进行，无论哪种情况，都生成中性亲电自由基。已经分离出这些顺磁物种的相关电化学类似物。
