Preparation of a series of Ru(p-cymene) complexes with different N-heterocyclic carbene ligands for the catalytic β-alkylation of secondary alcohols and dimerization of phenylacetylene

A series of five different (p-cymene)Ru(NHC) complexes (NHC = imidazolin-2-ylidene, imidazolin-4-ylidene, and pyrazolin-3-ylidene) have been obtained and fully characterized. The crystal structure of two of the new complexes has been determined by X-ray diffraction methods. All five complexes have been tested in the catalytic beta-alkylation of secondary alcohols with primary alcohols and the dimerization of phenyl acetylene, showing an excellent activity in both processes. A clear improvement on the catalytic activity of the complexes is observed when the more basic NHC ligands are used. The pyrazolylidene-Ru complex lies among the best catalysts for the beta-alkylation of secondary alcohols reported to date.