Vinyliodonium salts, elimination

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). 

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. 

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. 

The production of the (Z)-haloalkenes is thought to proceed via initial exchange of the tetrafluoroborate and halide ions and collapse of the resulting vinyliodonium halides by the addition-elimination (Ad-E) mechanism (equations 203 and 204)84. As with Ad-E reactions of moderately activated vinyl halides (X = Cl, Br), which typically occur with configurational retention (> 95%)143 145, the intermediate carbanions apparently prefer a least motion rotation of 60° prior to the expulsion of iodobenzene. It has been demonstrated by an NMR study that anion exchange between (Z)-(2)-phenylsulfonyl-l-decenyl)-phenyliodonium tetrafluoroborate and tetrabutylammonium chloride occurs instantaneously in deuteriochloroform84. Furthermore, when authentic halide salts of the... 

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