Vinyl iodonium salts reactions

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. 

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. 

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. 

The situation is quite different for the photochemical solvolysis reactions of vinyl iodonium salts. In all cases studied thus far these reactions involve direct, unassisted heterolytic cleavage of the vinylic C-I bond, yielding primary and endocyclic secondary vinyl cations. For example, photosolvolysis of ( )-styryl(phenyl)iodo-nium tetrafluoroborate (37) very efficiently yields the products resulting from heterolytic cleavage of the vinylic C-I bond, depicted in Scheme (Also... 

A dichotomy in the thermal versus photochemical behavior of vinyl iodonium salts has also been observed in the reactions of 37 in chlorinated alkanes (Scheme 56). ... 

Vinyl iodonium salts, prepared by reaction of vinylsilanes with iodosylbenzene and triethyloxonium tetrafluoroborate, have been found to be excellent educts for nucleophilic substitution by a variety of nucleophiles. Reaction with KCu(CN)p in DMF was thus possible and acrylonitrile derivatives prepared accordingly. 

Vinyl iodonium salt 40, which is highly effective as an activated species of vinyl iodide, could be synthesized from vinyl silanes 39 by the reaction with iodosyl benzene and triethyloxonium tetrafluoroborate. Thus, from vinyl silane, a,/3-unsaturated ester 41 could be synthesized by Pd-catalyzed carbonylation (Eq. 12). Alkynylphenyliodo-nium tosylates 42 were easily prepared from 1-alkynes with Phl(OH)OTs or by reaction... 

Previously, we have discussed that the inversion of the stereoehemistry in the solvolysis product of vinyl iodonium salt 2 could be explained by an in-plane S 2 vinylic substitution. The same arguments can be employed now to explain how the minor isomer 6Z is formed (Scheme 35.6). Direct attack of acetic acid to the a-carbon should lead to the observed reaction product in a one-step proeess through transition state 9. The position of the deuterium atom should not change (no scrambling), only the stereochemistry of the product has been reversed during the reaction. 

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

Evidence for a Michael addition of a nucleophile to alkenyl(phenyl)iodonium salts at the Cp atom has now been reported for the first time. Nucleophilic vinylic substitutions of (Z)-(/3-bromoalkenyl)iodonium tetrafiuoroborates (161) and its (Z)-(/3-chloroalkenyl) analogue with sodium benzenesulfinate in THE afforded stereoselectively (Z)-l,2-bis(benzenesulfonyl)alkene (163) with retention of configuration. Intermediate formation of (Z)-[/3-(benzenesulfonyl)alkenyl]iodonium salt (162) in these reactions was established by NMR experiments in CDCI3. The formation of (Z)-(162) involves a hitherto unobserved Michael addition of benzenesulfinate anion to the alkenyliodonium salts at the Cp atom, followed by halogen extrusion. ... 

Numerous reactions of alkenyl(phenyl)iodonium salts leading to the formation of new C-C bond have been reported in the literature. The most important and synthetically useful reactions include the generation and subsequent cyclization of alkylidenecarbenes, alkenylation of carbon substrates via nucleophilic vinylic substitution, and transition metal-mediated coupling reactions. 

Alkenyl(phenyl)iodonium salts are highly reactive in vinylic nucleophilic substitution reactions because of the excellent leaving group ability of the phenyliodonium moiety. Only a few examples of non-catalytic alkenylation of carbon nucleophiles are known [50,51]. In most cases these reactions proceed with predominant retention of configuration via the addition-elimination mechanism or ligand coupling on the iodine [42,50]. 

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