Vinyl iodonium salts

Nucleophilic vinylic substitution and vinyl cation intermediates in the reactions of vinyl iodonium salts. 37, 1... 

The focus of Chapters 3 and 4 is on vinyl cations. In Chapter 3, T. Muller et al. discuss the preparation, isolation, and characterization of unusually stable vinyl cations whereas Chapter 4 by T. Okuyama and M. Fujita describes the generation and reactions of vinyl cations formed via solvolysis of vinyl iodonium salts. 

Vinyl Iodonium Salts as Precursors to Vinyl Cations... 

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. 

Vinyl iodonium salts 1 are reactive due to the high leaving ability of the positive iodine group, and the stability is inversely dependent on the stability of the carbocation 2 formed upon departure of the iodonio group (eq 1). ... 

In conclusion, vinyl iodonium salts are excellent pregenitors for vinylic cations due to high nucleofugality of the iodonio group (about 106-fold of triflate), although they are in equilibrium with the less reactive hypervalent... 

The SyN2 reactions of vinylic compounds have been reviewed.23 An. S NVo- (an in-plane attack at the cr orbital giving inversion of configuration) and an S Nn mechanism (an out-of-plane attack on the tt orbital giving retention of configuration) have been found with vinyl iodonium salts. 

The reaction of vinylic phenyliodium salts (57) with cyanide anions could be mistaken for a simple substitution reaction.59 However, the presence of both allylic (58) and vinylic (59) nitrile products suggests a more complex picture. Deuterium labelling experiments show that the allylic product is formed via the Michael addition of cyanide to the vinylic iodonium salt, followed by elimination of iodobenzene and a 1,2-hydrogen shift in the 2-cyanocycloalkylidene intermediate (60). H-shift occurs from the methylene carbon in preference to the methine carbon. The effects of substitution and different nucleophiles were examined. 

Finally, the participating alkyl chain for such insertion reactions need not be attached directly to vinyl carbon. Thus, exposure of the [/2-(f2-butylsulfonyl)vinyl]iodonium salt shown in equation 236 to triethylamine gives a cyclic sulfone generated via carbenic insertion into the -butyl group32. 

As evidenced by the treatment of three (Z)-(/l-arylsulfonyl-l-decenyl)phenyliodonium fluoroborates with triethylamine, the presence of electronically diverse substituents (H,p-N02, / -MeO) in the arylsulfonyl groups appears to have little impact on their migratory aptitudes (equation 239)32. Indeed, among six [(/1-alkyl-/l-arylsulfonyl)vinyl]iodonium salts that have been investigated, the cyclopentene/alkyne ratios show little variability (i.e. 70 30 to 80 20)32. Finally, the presence of an appropriately placed alcohol function in the side chain of a (/l-sulfonylvinyl)iodonium salt has been shown to completely suppress migration in favor of insertion (equation 240)32. 

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