Iodonium salts vinyliodonium

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. 

The nitrite ion behaves as a nitrogen nucleophile with vinyliodonium ions. Thus, admixture of the vinyliodonium tetrafluoroborates shown in equations 188 and 189 with sodium nitrite results in the production of nitroalkenes119,125,126. Cupric sulfate is apparently necessary for the success of the latter reaction, although its role has not been clarified. The (cyclohexenyl)iodonium salt also reacts with sodium thiophenoxide to give the corresponding vinyl phenyl sulfide125,126. 

Reported a-elimination studies of vinyliodonium salts are limited to the substrates shown in Table 10. While aryl migrations might be expected, a-elimination reactions of (/2-arylvinyl)iodonium salts have not been described. Thus far, migrations of /2-hydrogen atoms (equations 230,233 and 234)128, /2-halogen atoms (Cl, Br)104, /2-ArS(0) groups (n = 0,1,2)32 and the methyl group128 have been reported. 

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. 

Palladium(II)-catalyzed couplings of vinyl(phenyl)iodonium salts with mono-substituted alkenes to give conjugated dienes have also been explored132. Such reactions proceed readily at room temperature and generally occur with retention of configuration in the vinyliodonium component and rnam-stereoselectivity in the olefinic substrate (equations 253-256). 

Alder reactions with a variety of dienes as illustrated in equations 37-40, and the resultant cycloadducts 84-87 are stable, microcrystalline vinyliodonium salts. These cycloadducts are set up for further synthetic elaboration by virtue of the two functionalities Y and the iodonium moiety. Vinyliodonium salts, like alkynyliodonium species, are known to react with a wide variety of nucleophiles. 

In order to strengthen evidence in favour of the proposition that concerted inplane 5n2 displacement reactions can occur at vinylic carbon the kinetics of reactions of some /3-alkyl-substituted vinyliodonium salts (17) with chloride ion have been studied. Substitution and elimination reactions with formation of (21) and (22), respectively, compete following initial formation of a chloro-A, -iodane reaction intermediate (18). Both (17) and (18) undergo bimolecular substitution by chloride ion while (18) also undergoes a unimolecular (intramolecular) jS-elimination of iodoben-zene and HCl. The [21]/[22] ratios for reactions of (18a-b) increase with halide ion concentration, and there is no evidence for formation of the -isomer of (Z)-alkene (21) iodonium ion (17d) forms only the products of elimination, (22d) and (23). 

The treatment of alkynyliodonium salts not amenable to cyclopentene formation with sodium azide in methanol affords vinyliodonium salts and/or enol ethers (equation 107)". Enol ether formation also occurs when glyme is employed as the solvent (equation 108)". Finally, regeneration of the vinylidene-iodonium ylide, PhC (N3)=C—IPh, from (Z)-(/ -azido-/ -phenylvinyl)phenyliodonium tosylate with potassium J-butoxide in glyme likewise affords an enol ether (equation 109). 

While vinylsilanes and -stannanes have been used primarily for the synthesis of vinyliodonium salts with one or two / -alkyl substituents in the vinyl moiety, the treatment of alkynes with oxyiodanes permits the introduction of oxygen functionality at jft-carbon. The conversion of terminal alkynes with [hydroxy(tosyloxy) iodo]benzene (HTIB) to alkynyliodonium tosylates (equation 8) and/or (j5-tosyloxyvinyl)iodonium tosylates [TsOC (R)=CHiPh, "OTs R = n-Pr, n-Bu, n-C5Hn, i-Pr, i-Bu] (equation 9), depending on the size of R, has already been discussed8,11. In at least three cases, E Z mixtures were... 

As discussed in Section II.D, the ability of alkynyliodonium salts to undergo Michael additions with nucleophilic reagents provides access to / -functionalized vinyliodonium salts (equation 177). However, this approach will not succeed unless the intermediate vinylidene-iodonium ylides can be captured by protonation. Thus, the best results are obtained when the nucleophile bears an acidic hydrogen or when the reactions are conducted in an acidic medium. 

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. 

Anion exchange between (E)-1 -decenyl(phenyl)iodonium tetrafluoroborate and TB AC1 occurs instantaneously in CDC13 at room temperature, while addition of the authentic chloride salt to dichloromethane (rt) affords a 45 55 mixture of (Z)-l-chlorodecene and 1-decyne146. Thus, the reactions shown in equations 206-208 are thought to involve the initial formation of vinyliodonium halides which then give rise to the observed products. 

In an effort to demonstrate the synthetic utility of vinyliodonium salts, small-scale reactions of (4-rm-butyl-1 -cyclohexenyl)phenyliodonium tetrafluoroborate (62) with various nucleophilic species, especially copper(I) reagents, have been conducted125,126. The copper(I)-assisted reactions include the conversions of 62 to 1-cyano-, 1-halo-, 1-alkyl- and 1-phenyl-4-ter r-butylcyclohexenes (equation 215). The alkylation and phenylation of the cyclohexenyl ligand in 62 with lithium diorganocuprates is noteworthy, since the treatment of 62 with H-butyllithium leads to fragmentation of the iodonium ion and affords only a 0.2% yield of 1 -w-butyl-4-terr-butylcyclohexene (equation 216)126. 

Apart from copper(I)-mediated reactions, few studies of the treatment of vinyliodonium salts with carbanions have appeared. The vinylations of the 2-phenyl- and 2- -hexyl-l,3-indandionate ions shown in equations 222 and 223 are the only reported examples of vinyliodonium-enolate reactions known to this author26,126. ( ,)-l-Dichloroiodo-2-chloroethene has been employed with aryl- and heteroarvllithium reagents for the synthesis of symmetrical diaryliodonium salts (equation 224)149,150. These transformations are thought to occur via the sequential displacement of both chloride ions with ArLi to give diaryl (/ -chlorovinyl)iodanes which then decompose with loss of acetylene (equation 225). That aryl(/ -chlorovinyl)iodonium chlorides are viable intermediates in such reactions has been shown by the conversion of ( )-(/ chlorovinyl)phenyliodonium chloride to diaryliodonium salts with 2-naphthyl- and 2-thienyllithium (equation 226)149,150. 

Although alkyne formation from / ,/ -disubstituted vinyliodonium salts via the a-elimination-alkylidenecarbene pathway is expected, the generation of 1-decyne from ( )-l-decenyl(phenyl)iodonium fluoroborate (equation 230) might also occur via syn -elimination. That a-elimination is dominant, even in this case, has been demonstrated by treatment of the a- and / -deuterio-l-decenyl isotopomers with triethylamine (equations 233 and 234)128. 

Palladium(II)-copper(I) cocatalyzed couplings of several (/ ,/ -dialkylvinyl)phenyliodo-nium triflates with 4-isopropoxy-3-tri- -butylstannyl-3-cyclobutene-l,2-dione (71) in DMF at room temperature have recently been described (equation 259)157. As observed in earlier studies of Cu(I)- or Pd(II)-promoted reactions of vinyliodonium salts, the vinyl ligands are introduced with retention of configuration. Since iodobenzene is a byproduct of ligand coupling, the production of 3-phenyl-4-isopropoxy-3-cyclobutene-1,2-dione might also be expected. However, the (/ ,/ -dialkylvinyl)iodonium ions are much more reactive... 

Another experimental result, the isolation of vinyliodonium salt 110 in the reaction of alkynyliodonium salt 109 with trimethylsilyl azide in wet CH2CI2 (equation 68), supports the intermediate formation of iodonium ylide 102 (Nu = N3) in the mechanism shown in equation 66. 

A similar approach to aryl- and heteroaryl(phenyl)iodonium triflates 285 involves the ligand-transfer reaction between vinyliodonium salt 284 with aryllithiums (Scheme 2.83) [405]. Likewise, the reaction of ( )-[(3-(trifluoromethanesulfonyloxy)ethenyl](aryl)iodonium triflates 286 with aryllithiums or alkynyllithiums can be used for a selective preparation of the respective diaryl- or alkynyl(aryl)iodonium triflates in high yields [406]. 

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