Castro-Stephens reaction copper-catalyzed Sonogashira

Pd/Cu-catalyzed cross-coupling of organohalides with terminal alkynes. Cf. Cadiot-Chodkiewicz coupling and Castro-Stephens reaction. The Castro-Stephens coupling uses stoichiometric copper, whereas the Sonogashira variant uses catalytic palladium and copper. 

Substituted alkynes play a pivotal role in modem organic synthesis. Although many methods are viable for their constmction, metal-catalyzed processes belong to the most efficient ones. The copper-mediated (Palladium free) alkynylation is known under the name Castro-Stephens reaction, j e palladium-catalyzed counterpart was discovered by Heck and Cassar (see Scheme 5-122), which is subject of the next paragraph. Nowadays the Sonogashira reaction, j ich uses copper co-catalysts, can be found as the method of choice. 

Terminal alkynes can be alkenylated by alkenyl triflates (bromides, iodides) and aryl-ated by aryl triflates (bromides, iodides). These reactions are called Cacchi coupling reactions if the reaction is catalyzed by Cu(I) and Pd(0) and if triflate reagents are employed, Sonogashira-Hagihara coupling reactions if the reaction is catalyzed by Cu(I) and Pd(0) and halides are employed as substrates, and Stephens-Castro coupling reactions for the more specialized case of the noncatalyzed coupling of copper acetylides with aryl halides. 

In this variation of the Pd-catalyzed cross-coupling reaction, which is closely related to the Stephen-Castro reaction, copper acetylides are reacted with (hetero)aryl halides or triflates to produce (hetero)aryl alkynes [111]. The Sonogashira reaction is comparable to the Suzuki or Stille reactions in its scope and functional group tolerability. 

Under certain conditions, alkynes can be coupled to aryl halides.280 When aryl halides react with copper acetylides to give 1-aryl alkynes such as 431, the reaction is known as Stephens-Castro coupling.28l Both aliphatic and aromatic substituents can be attached to the alkyne unit, and a variety of aryl iodides has been used. A palladium-catalyzed variation is also known in which an aryl halide reacts with a terminal alkyne to give derivatives such as 434. In this variation, called the Sonogashira coupling,2 2 jhe reaction is catalyzed by... 

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