Faculty of Chemistry and Biochemistry, Chair of Inorganic Chemistry II, Ruhr University Bochum, Universitätsstr. 150, 44801 Bochum, Germany.
Molecules. 2020 Feb 12;25(4):796. doi: 10.3390/molecules25040796.
Cyclic amino(ylide)carbenes (CAYCs) are the ylide-substituted analogues of -heterocyclic Carbenes (NHCs). Due to the stronger π donation of the ylide compared to an amino moiety they are stronger donors and thus are desirable ligands for catalysis. However, no stable CAYC has been reported until today. Here, we describe experimental and computational studies on the synthesis and stability of CAYCs based on pyrroles with trialkyl onium groups. Attempts to isolate two CAYCs with trialkyl phosphonium and sulfonium ylides resulted in the deprotonation of the alkyl groups instead of the formation of the desired CAYCs. In case of the PCy-substituted system, the corresponding ylide was isolated, while deprotonation of the SMe-functionalized compound led to the formation of ethene and the thioether. Detailed computational studies on various trialkyl onium groups showed that both the α- and β-deprotonated compounds were energetically favored over the free carbene. The most stable candidates were revealed to be α-hydrogen-free adamantyl-substituted onium groups, for which β-deprotonation is less favorable at the bridgehead position. Overall, the calculations showed that the isolation of CAYCs should be possible, but careful design is required to exclude decomposition pathways such as deprotonations at the onium group.
环氨基(基)卡宾（CAYCs）是 -杂环卡宾（NHCs）的亚胺取代类似物。由于与氨基部分相比，亚胺的π供电子能力更强，因此它们是更强的供体，是催化中理想的配体。然而，直到今天，还没有报道过稳定的 CAYC。在这里，我们描述了基于带有三烷基鏻基团的吡咯的 CAYC 的合成和稳定性的实验和计算研究。试图分离带有三烷基磷和硫叶立德的两种 CAYC，结果是烷基的去质子化而不是形成所需的 CAYC。在 PCy 取代体系中，分离出相应的叶立德，而 SMe 功能化化合物的去质子化导致乙烯和硫醚的形成。对各种三烷基鏻基团的详细计算研究表明，α-和β-去质子化化合物在能量上都优于游离卡宾。最稳定的候选物被揭示为α-无氢金刚烷基取代的鏻基团，对于后者，在桥头位置β-去质子化不太有利。总体而言，计算表明可以分离 CAYC，但需要仔细设计以排除诸如在鏻基团上的去质子化等分解途径。
