Sonogashira-type coupling

A potentially interesting development is the microwave-assisted transition-metal-free Sonogashira-type coupling reaction (Eq. 4.10). The reactions were performed in water without the use of copper(I) or a transition metal-phosphine complex. A variety of different aryl and hetero-aryl halides were reactive in water.25a The amount of palladium or copper present in the reaction system was determined to be less than 1 ppm by AAS-MS technique. However, in view of the recent reassessment of a similarly claimed transition-metal-free Suzuki-type coupling reaction, the possibility of a sub-ppm level of palladium contaminants found in commercially available sodium carbonate needs to be ruled out by a more sensitive analytical method.25 ... 

Scheme 6.37 (a) Transition metal-free, (b) copper(l)-catalyzed Sonogashira-type couplings. 

Leadbeater, N.E., Marco, M. and Tominack, B.J. (2003) First examples of transition-metal free Sonogashira-type couplings. Org. Lett., 5, 3919. 

The dendrimers 23a-c were used in a copper-free Sonogashira-type coupling reaction between phenylacetylene and iodobenzene or bromobenzene. The catalyst amount was 1 mol % per catalytic group (i.e., 1/4, 1/8 and 1/16 mol% depending on the dendrimer generation generations 1, 2 and 3), and the temperature range was 25-120 °C. 

TABLE 1.6. Silver-Catalyzed Sonogashira-Type Coupling... 

In 2006, Li and Wang reported the palladium-free, silver-catalyzed Sonogashira-type coupling of aryl halides and terminal alkynes. The reaction proceeds in high yield in the presence of catalytic silver iodide, triphenylphosphine, and potassium carbonate. Although the mechanism remains unclear, it is evident that the silver acetylide has a role, as the acetylide is formed on mixing of the reagents (Scheme 1.54).122... 

The versatility of silver-catalyzed desilylation has been further extended by its use in a one-pot catalytic desilylation/Sonogashira-type coupling reaction. In the total... 

This review is divided into four main sections, covering the Heck, Stille, and Suzuki reactions, with miscellaneous reactions being included at the end. Processes featuring alkynes in copper co-catalyzed Sonogashira-type couplings have been included in the section on Heck reactions. This review does not cover carbon-carbon bond formation processes using immobilized catalysts. Similarly, fluorous-phase syntheses " and those on polyethylene glycol " are excluded. 

Alkenyliodonium salts have been used as highly reactive reagents for Heck-type olefination [39, 971], Sonogashira-type coupling with alkynes [965, 972] and similar other palladium-catalyzed cross-coupling reactions [966, 973, 974]. In a specific example, (Z)-p-fiuoro-a,p-unsaturated esters 721 were stereoselec-tively synthesized from (Z)-2-fluoro-l-alkenyliodonium salts 720 by the Pd-catalyzed methoxycarbonylation reaction (Scheme 3.288) [974]. This reaction proceeds at room temperature and is compatible with various functional groups on the substrate. 

Representative for the numerous examples of analogous coupling reactions with alkynes as coupling componentst are the syntheses of multiple functionalized nitrogen hetarenes shown in Scheme 42.In order to avoid Sonogashira-type coupling reactions, the alkynes applied in this type of reaction have to be disubstituted the... 

C.ii.d. Sonogashira-Type Couplings. The coupling of alkynes with diazonium salts has been reported in the context of the T1 linker. In this case the product was isolated in moderate yield and it had to be separated by chromatography from alkyne homodimers and trimers (Scheme 41). ... 

SCHEME 3 Synthesis of a psoralen derivative through a Sonogashira-type coupling reaction. 

Using related synthetic tools as above, Bunz et al. also performed the synthesis of polyphenylene dendrimers containing a cyclobutadiene CoCP at the core of the dendrimers. The necessary ethynyl phenyl starting materials 292 were prepared using a Pd-catalyzed Sonogashira-type coupling reaction in good yields. The Co-initiated [2-1-21-cycloaddition of the ethynyl units with CpCo(CO)2 proceeded exclusively between the desired internal alkynes to afford 293 (Scheme 50). ... 

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