Silver-catalyzed Csp-H and Csp-Si bond transformations and related processes

As a result of the carbophilic Lewis acid characteristic, silver cations are capable of depriving an alkyne of part of its electron density through coordination with a C-C triple bond. As a result, the acidity of its acetylenic proton is increased, and hence the silver-coordinated alkyne is easily converted into silver acetylide in the presence of a base to abstract the terminal proton. Similarly, trialkylsilylalkynes undergo desilylation more readily under the influence of silver salts. The in situ generated silver acetylides react with various electrophiles to give functionalized alkyne products. Thus, a judicious combination of silver promoters with electrophilic reagents allows the catalytic transformations of terminal or trialkylsilylalkynes. Along this line, various silver-catalyzed processes of practical importance have been developed to date. These include catalytic desilylation and halogenation, which have been extensively utilized in the construction of complex natural products and artificial materials. Catalytic A3-coupling is also realized to provide access to propargylamines. The cocatalysis of palladium and silver operates very successfully in Sonogashira-type cross coupling reactions of alkynes with enol triflates or organohalides. In particular, the selective transformation of polyynes protected by different trialkylsilyl groups via Pd/Ag-catalyzed direct Sonogashira coupling provides a concise approach to highly functionalized polyalkyne materials. The synthetic value of these silver catalyzed processes is further enhanced by the potential utility of the resulting functionalized alkyne products. For instance, haloalkynes are highly versatile precursors for transition-metal-catalyzed coupling reactions such as Cu-catalyzed Cadiot - Chodkiewicz reaction, Pd-catalyzed Sonogashira-type coupling, or Cr/Ni-catalyzed Nozaki - Myaina - Kishi coupling. Haloalkynes can also undergo cyclization and cycloaddition across their C - C triple bonds in the presence or absence of catalysts to give unsaturated organolialides, which are capable of further transformations at the remaining carbon-halogen bonds. In the future, further discoveries of unprecedented silver promoters as well as electrophilic reaction partners will significantly contribute to the progress of alkyne-based synthetic technologies. © 2008 American Chemical Society.