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.