UCSD-CNRS Joint Research Laboratory (UMI 3555), Department of Chemistry and Biochemistry, University of California, San Diego , La Jolla, California 92093-0343, United States.
J Am Chem Soc. 2014 Apr 2;136(13):5023-30. doi: 10.1021/ja412981x. Epub 2014 Mar 24.
A series of stable carbenes, featuring a broad range of electronic properties, were reacted with simple organic substrates. The N,N-dimesityl imidazolylidene (NHC) does not react with isocyanides, whereas anti-Bredt di(amino)carbene (pyr-NHC), cyclic (alkyl)(amino)carbene (CAAC), acyclic di(amino)carbene (ADAC), and acyclic (alkyl)(amino)carbene (AAAC) give rise to the corresponding ketenimines. NHCs are known to promote the benzoin condensation, and we found that the CAAC, pyr-NHC, and ADAC react with benzaldehyde to give the ketone tautomer of the Breslow intermediate, whereas the AAAC first gives the corresponding epoxide and ultimately the Breslow intermediate, which can be isolated. Addition of excess benzaldehyde to the latter does not lead to benzoin but to a stable 1,3-dioxolane. Depending on the electronic properties of carbenes, different products are also obtained with methyl acrylate as a substrate. The critical role of the carbene electrophilicity on the outcome of reactions is discussed.
一系列具有广泛电子性质的稳定卡宾与简单的有机底物反应。N,N-二甲基咪唑基（NHC）不与异氰化物反应，而反-Bredt 二（氨基）卡宾（吡啶-NHC）、环状（烷基）（氨基）卡宾（CAAC）、非环状二（氨基）卡宾（ADAC）和非环状（烷基）（氨基）卡宾（AAAC）则生成相应的亚胺。NHC 已知可促进安息香缩合，我们发现 CAAC、吡啶-NHC 和 ADAC 与苯甲醛反应生成 Breslow 中间体的酮式互变异构体，而 AAAC 首先生成相应的环氧化物，最终生成 Breslow 中间体，后者可以分离得到。向后者中加入过量的苯甲醛不会生成安息香，而是得到稳定的 1,3-二氧戊环。根据卡宾的电子性质，用甲基丙烯酸甲酯作为底物也得到不同的产物。讨论了卡宾的亲电性对反应结果的关键作用。
