Multicomponent Synthesis of N-Heterocyclic Carbene Complexes

N-Heterocyclic carbenes (NHCs) are valuable ligands in transition metal catalysis, due to their unique electronic properties. However, only a few routes toward unsymmetrically substituted, saturated NHCs are known. We have successfully applied the multicomponent synthesis of 2H-2-imidazolines for the preparation of a range of diversely substituted imidazolidin-2-ylidene complexes under mild conditions. In the desired NHC precursors the substituents at N-1, N-3, C-4, and C-5 were varied easily and independently by choosing the appropriate amine, aldehyde, isocyanide, or halide as starting material. Subsequent formation of the imidazolium salts followed by deprotonation with KOtBu and direct complexation of the in situ-generated NHCs at room temperature affords Rh− and Ir−NHC complexes. Properties of the resulting complexes were studied with 13C NMR spectroscopy and X-ray crystallography.氮杂环卡宾（N-Heterocyclic carbenes, NHCs）凭借其独特的电子性质，是过渡金属催化领域极具应用价值的配体。然而，目前已知的不对称取代饱和氮杂环卡宾的合成路径寥寥无几。本研究成功将2H-2-咪唑啉的多组分合成策略应用于温和条件下多种不同取代模式的咪唑烷-2-亚基配合物的制备。在目标氮杂环卡宾前驱体中，通过选用合适的胺、醛、异氰化物或卤化物作为起始原料，可便捷且独立地调控N-1、N-3、C-4及C-5位的取代基。后续通过生成咪唑鎓盐，再经叔丁醇钾（KOtBu）去质子化，并于室温下将原位生成的氮杂环卡宾直接与金属配位，即可得到铑（Rh）基与铱（Ir）基氮杂环卡宾配合物。研究采用碳-13核磁共振（¹³C NMR）波谱法与X射线晶体学对所得配合物的性质进行了表征分析。