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 (KOBu)-Bu-t 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 C-13 NMR spectroscopy and X-ray crystallography.