Iodonium salts, rearrangement

General reactivities of vinyl iodonium salts are summarized, and reactions of cyclohexenyl, 1-alkenyl, styryl, and 2,2-disubstituted vinyl iodonium salts are discussed in relation to possible formation of vinyl cation intermediates. Primary vinyl cation cannot be generated thermally but rearrangement via neighboring group participation often occurs. Photosolvolysis to give primary vinyl cation is also discussed. 

Although formation of primary vinyl cation was disproved by the chirality probe approach, a vinyl cationic intermediate can be generated from a primary substrate via participation if a more stable cation could result. Unsymmetrically substituted 2,2-dialkylvinyl iodonium salt 24 gave mainly rearranged products on solvolysis.15 The products involve those of the 1,2-shift of either of the alkyl groups on the p position (Scheme 4). Those formed from migration of the alkyl... 

Scheme 4. Rearrangement in solvolysis of 2,2-dialkylvinyl iodonium salt.

The reactions of the lithium enolate of diethyl 2-[(diphenylmethylene)amino]malonate with several alkynyliodonium triflates are rare examples of enolate alkynylations with iodonium species other than the ethynyl(phenyl)- and (phenylethynyl)phenyliodonium ions (equation 125)16. Two experimental protocols were followed, i.e. addition of the enolates to the iodonium salts and vice versa, the former procedure giving higher yields of alkynylmalonates. As with other enolate alkynylations, these reactions are thought to involve alkylidenecarbene intermediates. It has been proposed, however, that the carbenes rearrange with migration of the diethyl 2-[(diphenyl) amino] malonate anion 16. 

The Af-aryl-Af-acylhydroxylamines 289 and 290 rearrange to aminophenol derivatives in the presence of sulfonyl chlorides (equation 138) as well as of iodonium salts in a reaction similar to the benzidine rearrangement (equation 139). The A-aryl-N,0-diacylhydroxylamine 291 undergoes isomerization on heating to produce dibenzoylated aminophenol 292 (equation 140) . The Wallach rearrangement consists of isomerization of aromatic azoxy compounds 293 to form the hydroxyazobenzenes 294 on heating in... 

Therefore, the reactions of geometrically isomeric iodonium salts of /3,/3-dialkylvinyl derivatives with halide ions have been investigated." However, the reactions are not stereospecific and give significant amounts of rearranged products. The results are discussed in Section 4 as a borderline case. The mechanism is obviously different from that of the monoalkyIvinyl derivatives. 

Ochiai and coworkers have used these tandem Michael-carbene-insertion process in a nice synthesis of cyclopentene derivatives as illustrated by 69 and 70 using alkynyl(phenyl)iodonium salts (equation 31). Similarly, PhS02H addition to alkynyl(phenyl)iodonium salts in methanol results in the formation of e.g., 71 that, upon treatment with Et3N, gives both rearranged sulfone 72 and the cyclopentene 73 via the unsaturated carbene, as illustrated by the example in equation 32. 

Triplet sensitization of sulfonium salts proceeds exclusively by the homolytic pathway, and that the only arene escape product is benzene, not biphenyl or acetanilide. However, it is difficult to differentiate between the homolytic or heterolytic pathways for the cage reaction, formation of the isomeric halobiaryls. Our recent studies on photoinduced electron transfer reactions between naphthalene and sulfonium salts, have shown that no meta- rearrangement product product is obtained from the reaction of phenyl radical with diphenylsulfinyl radical cation. Similarly, it is expected that the 2- and 4-halobiaryl should be the preferred products from the homolytic fragments, the arene radical-haloarene radical cation pair. The heterolytic pathway generates the arene cation-haloarene pair, which should react less selectively and form the 3-halobiaryl, in addition to the other two isomers. The increased selectivity of 2-halobiaryl over 3-halobiaryl formation from photolysis of the diaryliodonium salts versus the bromonium or chloronium salts, suggests that homolytic cleavage is more favored for iodonium salts than bromonium or chloronium salts. This is also consistent with the observation that more of the escape aryl fragment is radical derived for diaryliodonium salts than for the other diarylhalonium salts. 

Scheme 19 Metal-catalyzed arylations involving rearrangements or cyclic iodonium salts...

A nitrogen nucleophile (98) reacts with ethynyl(phenyl)iodonium salt to give an ylide (99), which loses iodobenzene to give vinylidine (100). The vinylidine rearranges by 1,2-hydrogen shift to the corresponding alkyne in the usual way. ... 

Cationic photoinitiators are usually based on iodonium salts [e.g. Phal ) and sulfonium salts (e.g. PhsS" ") which liberate a proton upon exposure to UV light (after subsequent rearrangement of the primary pairs formed, cleavage of a C-I or C-S bond and/or hydrogen abstraction on a H-donating structure). Photoinitiators of radical polymerization were originally classified as Type I PI (cleavable systems e.g. aryl allq l ketones or phosphine oxides mostly through a Norrish I scission, Scheme 2a) and Type II PI (PI and a co-initiator such as an amine AH or electron/proton transfer process, Scheme 2b PI stands e.g. for benzophenones. 

Recent examples of the rearrangement or alkynylation pathway include conversions of arylethynyl- and er -butylethynyl(phenyl)iodonium tosylates 24 and 25 to alkynylphosphonates, -selenides, and -tellurides with the appropriate anion salts in DMF (Scheme 50) [145-147], and a similar synthesis of push-pull selenides and tellurides from alkynyliodonium triflates containing electron-withdrawing groups in the alkynyl moiety [148]. 

Arylethynylations of potassium benzotriazolate with iodonium tosylates 24 in THF/ -BuOH/CH2C12 have also been reported [149,150]. However, when the terf-butylethynyl salt 25 was used instead, insertion of the intermediate alkyli-denecarbene into the O-H bond of tert-butyl alcohol preempted rearrangement. 

Another type of acid catalyzed rearrangement induced by diaryl-iodonium and triarylsulfonium salts was recently reported by Ito and Willson of IBM (12) and is depicted in Equation 19. 

Rearrangement reactions of both iodonium and sulfonium cations proceed out of the lowest excited singlet states of the respective onium salts... 

DialkyIvinyliodonium salts have a better chance to afford primary vinyl cations than their /3-phenyl counterparts due to the stabilizing effect of j8-alkyl substituents discussed above. Solvolyis of (E)- and (Z)-2-methy 1-5-phenyl-1-pentenyl(phenyl)-iodonium tetrafluoroborate (50) gave a variety of extensively rearranged products, as illustrated in Scheme 32 and summarized in Table 12. ... 

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