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 2 H -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 KO t Bu 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 13 C NMR spectroscopy and X-ray crystallography.