Homogeneous catalysis by gold

Abstract

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...

Catalysis by Gold: Isolated Surface Au3+ Ions are Active Sites for Selective Hydrogenation of 1,3‐Butadiene over Au/ZrO2 Catalysts

“…[14,27] Due to their ability to activate carbon-carbon double and triple bonds (e.g., alkynes and allenes), Au 3+ ions (i.e., gold salts) in solution are finding applications as efficient homogeneous catalysts in many organic reactions. [20,21] If the Au 3+ ions can be properly heterogenized onto a solid surface while still maintaining their catalytic efficiency in the homogeneous systems, the "golden gate" [21] of catalysis by homogeneous gold would be even more useful for advanced heterogeneous catalysis. By reducing the loading of gold to very low levels (< 0.1 % Au), the present work is the first to demonstrate that high loadings of Au nanoparticles are actually not required for the hydrogenation reaction; the very low concentration of Au 3+ ions (0.02 Au 3+ /nm 2 ) heterogenized on zirconia surface from a cheap inorganic gold source (HAuCl 4 ) is enough to grant much higher catalytic activity.…”

“…[12][13][14][15][16][17] This is surprising, since catalytic hydrogenation is one category of the best studied and industrially most important reactions on conventional transition metal catalysts, [18,19] and Au 3+ ions in solution can activate carboncarbon double and triple bonds and are finding applications as efficient homogeneous catalysts in many organic reactions. [20,21] Hydrogenation of 1,3-butadiene has long been a model reaction for the study of active sites on conventional metal catalysts. [18,19] A few preliminary investigations [12,13] on the hydrogenation of 1,3-butadiene with supported gold catalysts showed that small gold nanoparticles were active in the reaction, although theoretical calculations seem to suggest that the surface of metallic gold is chemically inert for hydrogen activation.…”

Catalysis by Gold: Isolated Surface Au3+ Ions are Active Sites for Selective Hydrogenation of 1,3‐Butadiene over Au/ZrO2 Catalysts

“…Little attention has been paid to the nature of active gold, especially cationic gold species,5, 11 in the other reactions such as hydrogenation of unsaturated compounds 12–17. This is surprising, since catalytic hydrogenation is one category of the best studied and industrially most important reactions on conventional transition metal catalysts,18, 19 and Au 3+ ions in solution can activate carbon–carbon double and triple bonds and are finding applications as efficient homogeneous catalysts in many organic reactions 20. 21 Hydrogenation of 1,3‐butadiene has long been a model reaction for the study of active sites on conventional metal catalysts 18.…”

“…Due to their ability to activate carbon–carbon double and triple bonds (e.g., alkynes and allenes), Au 3+ ions (i.e., gold salts) in solution are finding applications as efficient homogeneous catalysts in many organic reactions 20. 21 If the Au 3+ ions can be properly heterogenized onto a solid surface while still maintaining their catalytic efficiency in the homogeneous systems, the “golden gate”21 of catalysis by homogeneous gold would be even more useful for advanced heterogeneous catalysis.…”

Construction of cyclic enones via gold-catalyzed oxygen transfer reactions

“…During the early years of this century, organic chemists became aware that gold salts or complexes were highly active catalysts in homogeneous catalysis because of the strong π- and σ-electrophilicity of gold [28–33]. Since then, the number of new gold-catalyzed reactions reported in the literature has increased substantially and gold catalysis has become one of the hottest research fields in synthetic organic chemistry [34–42].…”