Iodonium compounds Vinyliodonium salts

Vinyliodonium ions, 35 and 36, are hypervalent iodine species in which one or two alkenyl ligands are bound to a positively charged iodine(III) atom. Although they are reactive with nucleophilic reagents, they are less labile than alkynyliodonium ions, and stable halide salts of vinyliodonium ions can be prepared. The first vinyliodonium compounds [i.e. (a, / -dichlorovinyl)iodonium salts] were synthesized by the treatment of silver acetylide-silver chloride complexes with (dichloroiodo)arenes or l-(dichloroiodo)-2-chloroethene in the presence of water (equation 152). The early work was summarized by Willgerodt in 1914115. This is, of course, a limited and rather impractical synthetic method, and some time elapsed before the chemistry of vinyliodonium salts was developed. Contemporary synthetic approaches to vinyliodonium compounds include the treatment of (1) vinylsilanes and vinylstannanes with 23-iodanes, (2) terminal alkynes with x3-iodanes, (3) alkynyliodonium salts with nucleophilic reagents and (4) alkynyliodonium salts with dienes. 

Diels-Alder reactions of alkynyl(phenyl)iodonium triflates (i.e. containing electron-withdrawing groups in the alkynyl moiety) and [bis(phenyliodonium)] ethyne ditrifiate have been employed for the synthesis of cyclic vinyliodonium salts (equations 143 and 144)17,41. The availability of such compounds offers considerable potential for the elaboration of densely functionalized cyclic molecules. 

Two examples of Pd(II)-catalyzed carbomethoxylations of vinyl(phenyl)iodonium salts have been reported (equations 251 and 252)125,126. The mild reaction conditions and stereospecificity of carbonylation recommend further applications of vinyliodonium compounds for the synthesis of a,/ -unsaturated carboxylate esters. By way of comparison, similar carbobutoxylations of vinyl halides (Br, I) typically require higher temperatures (60-100 °C) and longer reaction times, and they sometimes proceed with low stereospecificity151. 

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