Arene hydrogenation by homogeneous catalysts: fact or fiction?

A few years ago the concept of homogeneous arene hydrogenation catalysis was called into question, since many catalysts originally assigned as homogeneous have subsequently been shown to be precursors to heterogeneous ( colloidal or nanoparticulate) catalysts. In this Perspective, the mechanism for arene hydrogenation is described and extended to include polynuclear ( cluster, colloidal and nanoparticulate) catalysts. The various experiments used to determine whether catalysis is by homogeneous or heterogeneous species are then assessed. The current catalysts in the literature are categorised, including those able to bring about partial reduction of aromatic compounds. Finally, the role of the solvent, including ionic liquids, is described, as the solvent could prove to be crucial in stabilising one form of catalyst over the other and in providing regioselective hydrogenation.