Unsymmetrically N,N′-substituted saturated carbenes: synthesis, reactivity and preparation of a rhodium(i) carbene complex

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Abstract
A versatile synthesis of unsymmetrically N,N′-substituted saturated carbenes is described. The novel racemic imidazolidin-2-ylidenes rac-5 have been synthesized by reductive desulfurization of the corresponding imidazolidin-2-thiones rac-4. The thiones were prepared in two reaction steps from aldimines and secondary amines. Three different substituents at N1, N3 and C4 of the five-membered N-heterocyclic ring can be introduced by choice of suitable aldimines and secondary amines. The dimerization behaviour (diaminocarbene/enetetramine equilibrium) for the unsymmetrically N,N′-substituted imidazolidin-2-ylidenes has been investigated by NMR spectroscopy. Unsymmetrically N–iPr and N–iBu substituted N-heterocyclic carbenes undergo a slow dimerization, whereas N–tBu substituted derivatives are stable as monomeric carbenes indefinitely. The carbene ligand rac-5d has been coordinated to rhodium(I) to give the square-planar rhodium carbene complex [Cl(cod)Rh(rac-5d)] rac-6d which has been characterized by an X-ray diffraction analysis.