Silver Catalysts

A substance containing silver and having a catalytic function is called a silver catalyst. Silver is a white, shiny metal and is a type of transition metal. The chemical properties of silver are relatively stable. Silver is not abundant in nature, and a small amount of silver exists as a simple substance, but more exists in the form of compounds. The silver catalyst exhibits unique catalytic activity and high functional group compatibility in the reaction, and can catalyze many types of organic chemical reactions, such as cycloaddition reactions, asymmetric synthesis reactions, coupling reactions, cyclization reactions, C-H activation reactions, rearrangement reactions, and the like. The complex formed by the combination of silver and ligand exhibits distinctive catalytic properties and can also be used for the catalysis of various reactions.

Applications:

Silver catalysts have a wide range of applications in the field of organic synthesis due to their many types, good catalytic performance and relatively low price.

• Cycloaddition reaction: A variety of silver catalysts exhibit good catalytic activity in cycloaddition reactions and can catalyze the cycloaddition reactions of different types of compounds. The asymmetric cycloaddition of maleic acid dimethyl vinegar and methylimine can be achieved through AgOAc catalysis and ligand regulation, thereby forming pyrrolidine compounds with various biological activities. The reaction has good regioselectivity and stereoselectivity, and provides an efficient and convenient way for the synthesis of optically active natural products. AgNTf₂ can catalyze the [2+2] cycloaddition reaction of alkynyl acids with electrophilic dilute hydrocarbons. The reaction has the advantages of mild conditions, wide substrate range and high yield. Further, the alkynyl acid can also react with an electrophilic olefin having a specific functional group (e.g., a cyano group, an ester group) to obtain the target product in high yield.

Figure 1. Silver catalyst catalyzed cycloaddition reaction

• Coupling reaction: The coupling reaction can construct a new C-C bond by nucleophilic substitution, which is very important in organic synthesis. Silver often exhibits its unique properties when participating in a coupling reaction. Silver can participate in the coupling reaction as a catalyst or a co-catalyst, and can promote reaction by forming an insoluble silver halide or participate in a coupling reaction as an organic silver complex. For example, silver p-toluenesulfonate (AgOTs) can catalyze the self-coupling reaction of an alkyl Grignard reagent to form a symmetric alkyl compound. Silver iodide (AgI) catalyzes the Sonogashira coupling reaction of iodobenzene with terminal alkynes. In these reactions, silver catalysts are used as independent catalysts. There are also many cases in which a silver catalyst is used as a cocatalyst. For example, in the sp-sp² coupling reaction, silver is often used as a cocatalyst to catalyze its reaction with palladium. Under the combined action of palladium and silver, various alkyne groups (TMS, TIPS, etc.) substituted alkyne, and terminal alkyne can undergo Sonogashira coupling reaction with iodide or trifluoromethanesulfonyloxy (OTf) substituents.

Figure 2. Silver catalyst catalyzed coupling reaction

• Other reactions: In addition to the several reactions described above, the reactions catalyzed by the silver catalyst include carbene reactions (carbene, nitrene, silene), C-C bond cleavage reactions, insertion reactions, and series reactions. For example, AgSbF₆ catalyzes the reaction of arylamine with N-trifluoromethyldiazonium to achieve N-trifluoromethylethylation of aromatic amine, providing a simple and efficient method for the synthesis of trifluoroethylamines. AgCO₃ can catalyze the reaction of a non-activated terminal alkyne with trimethylsilyl azide to form a benzonitrile compound through an alkenyl azide intermediate.

Figure 3. Silver catalyst catalyzed N-trifluoromethylethylation of aromatic amines

References

1. Heinrich, Clement F. (2016), “Silver-Catalyzed 7-exo-dig Cyclization of Silylenolether-ynesulfonamides.” Angewandte Chemie, International Edition 55(17), 5170-5174.
2. Erikson, Heiki. (2019), “Oxygen Reduction Reaction on Silver Catalysts in Alkaline Media: a Minireview.” ChemElectroChem 6(1), 73-86.
3. Paun, Cristina. (2016), “Hydrogenation by silver catalysts.” Hydrogenation with Low-Cost Transition Metals 155-195.