Homogeneous Catalytic Hydrogenation of Aromatic Hydrocarbons and Heteroaromatic Nitrogen Compounds: Synthetic and Mechanistic Aspects

Abstract

The synthetic and mechanistic aspects of the homogeneous catalytic hydrogenation of mono and polynuclear aromatic hydrocarbons and the corresponding heteroaromatic nitrogen compounds will be reviewed. A comparison of the regioselectivities under various hydrogenation conditions for both classes of compounds will be discussed for a wide variety of transition-metal complexes with regard to substrate binding at the metal center and the role of free radical intermediates in metal carbonyl hydride reactions, where substrate does not bind to the metal center prior to hydrogen transfer. The polynuclear heteroaromatic nitrogen compounds appear to hydrogenate more readily, under similar reaction conditions, than the polynuclear aromatic hydrocarbons. The relative rates of hydrogenation of a variety of heteroaromatic nitrogen compounds as well as compounds that inhibit and enhance selective hydrogenation of the nitrogen-containing ring will be addressed. A relatively new spectroscopic technique, high pressure nuclear magnetic resonance spectroscopy, will be shown to be a powerful tool to elucidate the mechanisms of the regioselective hydrogenation of polynuclear heteroaromatic nitrogen compounds.