Gold catalysis has proven to be an important breakthrough for organic synthesis. The tunable nature of gold catalysts, the unique properties of gold, and the mild reaction conditions required in many gold-catalyzed reactions have all contributed substantially to this metal’s popularity in catalysis. However, gold-catalyzed reactions still suffer from limitations such as low turnover numbers (TON). Optimization of the catalysts and reaction conditions may significantly improve the efficiency of gold-catalyzed reactions. In this review, we will present leading examples of counterion or additive-regulated gold catalysis from a mechanistic perspective. We will pay special attention to the physical properties of counterion/additive, such as gold affinity and hydrogen bond basicity, and discuss their effects on the reactivity of gold catalysts.

Conspectus Homogeneous gold catalysis is regarded as a landmark addition to the field of organic synthesis. It is the most effective way to activate alkynes for the addition of a diverse host of nucleophiles. However, the literature reveals that a relatively high catalyst loading is needed in many gold-catalyzed applications (1–10 mol %), which is impractical in large-scale synthesis or multistep synthesis because of the high price and recyclization difficulty of the gold. A more thorough understanding of the factors that operate on homogeneous gold catalysis can provide better guidelines for the future design of more efficient gold-catalyzed reactions. In this Account, we will summarize our group’s extensive investigation of factors impacting cationic gold catalysis, namely, the effects of ligands, counterions, additives, and catalyst decay and deactivation, using a mechanism-based approach with the aim of improving the efficiency of homogeneous gold catalysis. Through NMR-assisted kinetic studies, we investigated the above factors. Our systematic ligand effect investigation provided a clearer understanding of how ligands influence each of the three stages in the gold catalytic cycle. On the basis of this study, we synthesized a novel phosphine ligand and achieved parts per million-level gold catalysis by manipulating the electron density of the substituents and the steric strain around phosphorus. Our investigation of counterion effects led to the design of a gold affinity index and hydrogen-bonding basicity index for counterions, which can forecast the reactivity of counterions in cationic gold catalysis. We studied the adverse silver effects in cationic gold catalyst activation and proposed a more efficient practical guide. Our additive effect investigation revealed that additives that are good hydrogen-bond acceptors increase the efficiency of gold-catalyzed reactions in those occurrences where protodeauration is the rate-determining step. The first detailed experimental analysis of gold catalyst decay and the influence of each component in the reaction system (substrate, counterion, solvent) on the decay process was also conducted. We found that high-gold-affinity impurities (halides, bases) in solvents, starting materials, filtration, or drying agents decrease the reactivity of a gold catalyst but that a suitable acid activator can reactivate the gold catalyst and enable the reaction to proceed smoothly at competitively low gold catalyst loadings. The effects of acid additives were also systematically investigated using typical reactions. We are convinced that better mechanistic understandings will offer clearer guidelines for the search for more efficient gold-catalyzed reactions.

The different types and advantages of nanoparticles in sonodynamic therapy.

We have developed a gold affinity index and hydrogen bonding basicity index for counterions and have used these indexes to forecast their reactivity in cationic gold catalysis.

An excess amount of silver salt to generate cationic gold from a gold catalyst precursor such as L-Au-Cl almost always has adverse effects on the reactivity of the cationic gold catalyst. A preformed L-Au(+)X(-) complex, generated by sonication followed by centrifugation, increases the reactivity in a gold catalyzed reaction. The adverse silver effect might be caused by the interaction of silver salts with gold intermediates.