Department of Chemistry und Pharmacy, Organische Chemie, Henkestrasse 42, 91054 Erlangen, Germany.
Chemistry. 2011 Nov 11;17(46):13078-86. doi: 10.1002/chem.201101999.
The first synthesis of 4,5-bis-(dimethylamino)-substituted imidazolium compounds was developed, which is based on the reaction of a 1,2-diamino-1,2-bis(phosphonio)ethene with lithiated formamidines. This represents the first application of this class of ethene derivatives for the preparation of heterocycles. These N-heterocyclic carbene (NHC) precursors show a remarkably reduced basicity and nucleophilicity of their NMe2 groups, which is due to the strong anomeric interactions of the latter with the imidazolium core. According to DFT calculations, these NHCs are capable of self-umpolung if sufficiently strong acceptor substituents are introduced at the carbene center. To test the self-umpolung capabilities of the NHCs, various substituents were attached to the carbene center and the obtained compounds were characterized by single-crystal X-ray analysis as well as quantum chemical computations. Strong acceptor substituents are required to induce self-umpolung, such as in the phosphonio-substituted derivative, for which partial self-umpolung was found. The N,N′-bis(4-dimethylaminophenyl)-substituted imidazolium compound represents a special case, as it incorporates as much as three two-step redox systems within the NHC framework. This will probably result in a high electronic flexibility of the corresponding nucleophilic carbenes, especially when they serve as ligands in transition metal complexes.
首次开发了 4,5-双-(二甲氨基)-取代的咪唑鎓化合物的合成方法，该方法基于 1,2-二氨基-1,2-双(膦亚基)乙与锂化甲脒的反应。这代表了此类乙稀衍生物首次应用于杂环的制备。这些 N-杂环卡宾 (NHC) 前体的 NMe2 基团的碱性和亲核性显著降低，这是由于后者与咪唑核心的强反式相互作用所致。根据 DFT 计算，如果在卡宾中心引入足够强的受体取代基，这些 NHC 就能够发生自极性反转。为了测试 NHC 的自极性反转能力，在卡宾中心引入了各种取代基，并通过单晶 X 射线分析和量子化学计算对得到的化合物进行了表征。需要强的受体取代基来诱导自极性反转，例如在磷取代衍生物中，发现了部分自极性反转。N,N′-双(4-二甲氨基苯基)-取代的咪唑鎓化合物是一个特殊情况，因为它在 NHC 框架内包含了多达三个两步还原氧化系统。这可能会导致相应亲核卡宾具有很高的电子灵活性，尤其是当它们作为过渡金属配合物的配体时。
