Rehbein Julia, Ruser Stephanie-M, Phan Jenny
Organische Chemie , Universität Hamburg , Martin-Luther-King-Platz 6 , Germany . Email:
Chem Sci. 2015 Oct 1;6(10):6013-6018. doi: 10.1039/c5sc02186c. Epub 2015 Jul 21.
The Breslow catalytic cycle describing the benzoin condensation promoted by N-heterocyclic carbenes (NHC) as proposed in the late 1950s has since then been tried by generations of physical organic chemists. Emphasis has been laid on proofing the existence of an enaminol like structure (Breslow intermediate) that explains the observed umpolung of an otherwise electrophilic aldehyde. The present study is not focusing on spectroscopic elucidation of a thiazolydene based Breslow intermediate but rather tries to clarify if this key-intermediate is indeed directly linked with the product side of the overall reaction. The here presented EPR-spectroscopic and computational data provide a fundamentally different view on how the benzoin condensation may proceed: a radical pair could be identified as a second key-intermediate that is derived from the Breslow-intermediate an SET process. These results highlight the close relationship to the Cannizarro reaction and oxidative transformations of aldehydes under NHC catalysis.
20世纪50年代末提出的由N-杂环卡宾（NHC）促进苯偶姻缩合的布雷斯洛催化循环，此后几代物理有机化学家都对其进行了尝试。重点在于证明类似烯胺醇结构（布雷斯洛中间体）的存在，该结构解释了原本亲电醛所观察到的极性反转。本研究并非聚焦于基于噻唑啉的布雷斯洛中间体的光谱解析，而是试图阐明这个关键中间体是否确实与整个反应的产物端直接相关。此处呈现的电子顺磁共振光谱和计算数据为苯偶姻缩合的进行方式提供了一种根本不同的观点：可以确定一个自由基对是源自布雷斯洛中间体的第二个关键中间体，这是一个单电子转移过程。这些结果突出了与坎尼扎罗反应以及在NHC催化下醛的氧化转化的密切关系。
