A comprehensive overview in the field of homogeneous catalysis by gold is given and the basic principles are discussed. It is also highlighted where homogeneous gold catalysts are already superior to other catalysts and where future possibilities for advantageous use of homogeneous gold catalysts exist.For a long time, gold was considered to possess only low catalytic activity ("catalytically dead") and therefore only its stoichiometric co-ordination and organo-metallic chemistry was investigated intensively. Even in the 19th century it was quite popular to characterize precious alkaloid bases after their troublesome isolation from natural sources as tetrachloroaurates (1,2).Since then, a change of paradigm has taken place (3). Among the numerous applications for gold, its use as a catalyst, albeit not yet consuming large amounts of this metal, has attracted much attention. Overall, in the chemical databases ca. 230,000 papers on gold can be found, of which about 9,000 deal with gold catalysis (from the SciFinder database in October 2003). The field of gold catalysis is dominated by heterogeneous catalysts, while homogeneous catalysts still represent the much smaller part (about 100 hits). Still, a significant amount of different homogeneous gold-catalysed reactions is known and basic principles can be deduced from them.In this article only publications in which gold is unambiguously the site of the catalysis reaction will be covered. This, for example, excludes the use of some mixed metal clusters and mixed metal colloids.
BackgroundHomogeneous catalysis includes all reactions in which the substrate(s) and the catalyst are in the same state. As there exists only a limited number of highly volatile gold compounds, most of these reactions are conducted in the liquid phase with either liquid or dissolved substrate(s) and a dissolved catalyst. Homogeneous catalysis in the solid state, a part of the solvent-free reactions (4) that are nowadays quite popular in the field of green chemistry, has not been explored with gold yet (with the exception shown in Figure 31)! In this context, the non-toxicity of gold, which helps to avoid any environmental problems, is also of importance.The catalyst itself promotes a chemical reaction, a process in which chemical compounds (the substrates) are converted to other chemical compounds (the products), to proceed under milder reaction conditions (lower temperatures, lower pressure, lower concentrations of the substrates, higher or different selectivity). As the catalyst is not consumed, a catalytic amount is sufficient -but this is often a matter of debate: for some scientists 20 mol% of 'catalyst' are still catalytic, while others may consider this to be a substoichiometric reaction and accept only 0.001 mol% or less as catalytic.Two motives drive the desire to produce a chemical reaction: The use of gold as a catalyst is desirable when it has a similar activity as for a more expensive catalyst, when it shows a higher activity or a higher selectivity than less expensive catalysts...
