Although N-heterocyclic carbenes (NHCs) have been known as ligands for organometallic complexes since the 1960s, these carbenes did not attract considerable attention until Arduengo et al. reported the isolation of a metal-free imidazol-2-ylidene in 1991. In 2001 Crabtree et al. reported a few complexes featuring an NHC isomer, namely an imidazol-5-ylidene, also termed abnormal NHC (aNHCs). In 2009, it was shown that providing to protect the C-2 position of an imidazolium salt, the deprotonation occurred at the C-5 position, affording imidazol-5-ylidenes that could be isolated. Over the last ten years, stable aNHCs have been used for designing a range of catalysts employing Pd(II), Cu(I), Ni(II), Fe(0), Zn(II), Ag(I), and Au(I/III) metal based precursors. These catalysts were utilized for different organic transformations such as the Suzuki–Miyaura cross-coupling reaction, C–H bond activation, dehydrogenative coupling, Huisgen 1,3-dipolar cycloaddition (click reaction), hydroheteroarylation, hydrosilylation reaction and migratory insertion of carbenes. Main-group metal complexes were also synthesized, including K(I), Al(III), Zn(II), Sn(II), Ge(II), and Si(II/IV). Among them, K(I), Al(III), and Zn(II) complexes were used for the polymerization of caprolactone and rac-lactide at room temperature. In addition, based on the superior nucleophilicity of aNHCs, relative to that of their nNHCs isomers, they were used for small molecules activation, such as carbon dioxide (CO₂), nitrous oxide (N₂O), tetrahydrofuran (THF), tetrahydrothiophene and 9-borabicyclo[3.3.1]nonane (9BBN). aNHCs have also been shown to be efficient metal-free catalysts for ring opening polymerization of different cyclic esters at room temperature; they are among the most active metal-free catalysts for ε-caprolactone polymerization. Recently, aNHCs successfully accomplished the metal-free catalytic formylation of amides using CO₂ and the catalytic reduction of carbon dioxide, including atmospheric CO₂, into methanol, under ambient conditions. Although other transition metal complexes featuring aNHCs as ligand have been prepared and used in catalysis, this review article summarize the results obtained with the isolated aNHCs.

中文翻译：

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稳定的异常N-杂环卡宾及其应用

尽管自1960年代以来，N-杂环卡宾（NHC）被认为是有机金属配合物的配体，但直到Arduengo等人才对这些卡宾引起了广泛的关注。Crabtree等人在1991年报道分离出无金属的咪唑-2-亚基。2001年，Crabtree等人。报道了一些以NHC异构体为特征的复合物，即咪唑5亚基，也称为异常NHC（aNHC）。在2009年，显示出提供保护咪唑鎓盐的C-2位的去质子化发生在C-5位，从而提供了可以被分离的咪唑-5基。在过去的十年中，稳定的aNHC已被用于设计一系列使用Pd（II），Cu（I），Ni（II），Fe（0），Zn（II），Ag（I）和Au（I / III）金属基前驱体。这些催化剂被用于不同的有机转化，例如铃木-宫浦交叉偶联反应，C-H键活化，脱氢偶联，Huusgen 1,3-偶极环加成（点击反应），加氢杂芳基化，加氢硅烷化反应和羧甲基苯的迁移。还合成了主要金属配合物，包括K（I），Al（III），Zn（II），Sn（II），Ge（II）和Si（II / IV）。其中，K（I），Al（III）和Zn（II）配合物用于在室温下聚合己内酯和外消旋丙交酯。此外，基于aNHC相对于其nNHCs异构体的优越亲核性，它们可用于小分子活化，例如二氧化碳（CO ₂），一氧化二氮（N ₂ O），四氢呋喃（THF），四氢噻吩和9-borabicyclo [3.3.1] nonane（9BBN）。在室温下，aNHCs也被证明是有效的无金属催化剂，可用于不同环酯的开环聚合。它们是ε-己内酯聚合反应中活性最强的无金属催化剂之一。最近，aNHC使用CO ₂成功完成了酰胺的无金属催化甲酰化反应在环境条件下将包括大气中的CO ₂在内的二氧化碳催化还原为甲醇。尽管已经制备了其他以aNHCs为配体的过渡金属配合物并将其用于催化，但本文综述了分离出的aNHCs的结果。