Alkenyl iodonium salts, alkenylation nucleophiles

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. ... 

Alkynyl(phenyl)iodonium salts are transformed into their functionalized alkenyl analogs by reactions involving addition of nucleophiles to their triple bond which is a strong Michael acceptor. In most of them the stereochemistry is normally Z (Scheme 41). The choice of solvent, e.g. methanol, is crucial in some cases for the exclusive and almost quantitative formation of the Z-product... 

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]. 

Recent progress on the use of hypervalent iodine reagents for the construction of carbon-het-eroatom (N, O, P, S, Se, Te, X) bonds is reviewed. Reactions of aryl-A3-iodanes with organic substrates are considered first and are loosely organized by functional group, separate sections being devoted to carbon-azide and carbon-fluorine bond formation. Arylations and alkenyla-tions of nucleophilic species with diaryliodonium and alkenyl(aryl)iodonium salts, and a variety of transformations of alkynyl(aryl)iodonium salts with heteroatom nucleophiles are then detailed. Finally, the use of sulfonyliminoiodanes as aziridination and amidation reagents, and reactions of iodonium enolates formally derived from monoketones are summarized. 

Alkenylations of heteroatom nucleophiles with alkenyl(aryl)iodonium salts occur by a variety of mechanisms, including SN1, SN2, alkylidenecarbene, and addition-elimination pathways [ 126,127]. Reactions that occur with retention of configuration at vinylic carbon are sometimes attributed to a ligand-coupling... 

The exceptional nucleofugality of the phenyliodonio group has been determined in an alkenyl salt and it is about 106 times greater than that of triflate [30]. This remarkable property makes alkenyl iodonium salts excellent vinyl cation equivalents in nucleophilic substitutions. The chemistry of alkenyl iodonium salts is dominated by the transfer of their aliphatic moiety to a variety of nucleophiles other important reactions involve Michael-type addition and alkylidenecarbene generation, along with elimination to alkynes which is actually an undesirable side-reaction. 

Whereas oxygen nucleophiles gave poor yields of alkenylated products with alkenyl iodonium salts, the reactions with sulphur nucleophiles proceeded more efficiently, leading to unsaturated sulphides and sulphones. Thus, 4-t-butylcyclohexenyl phenyliodonium salts afforded with sodium thiophenoxide 4-t-butylcyclohexenyl phenyl sulphide (81%) [3] and with sodium phenylsulphinate the corresponding sulphone (29%) in the presence of 18-crown-6, the yield of the latter rose to 80% [45]. jS-Phenylsulphonylalkenyl iodonium salts with sodium phenylsulphinate at 0°C, without any catalyst, afforded Z-l,2-bis(phenylsulphonyl)alkenes, in high yield with retention of the stereochemistry [45] ... 

These highly reactive yet stable species are strong electrophiles of tetraphilic character, since nucleophiles may attack three different carbon atoms (a,/ ,a ) and iodine. In most reactions the first step is a Michael addition at fi-C with formation of an alkenyl zwitterionic intermediate (ylide) which normally eliminates iodoben-zene, generating an alkylidene carbene then, a 1,2-shift of the nucleophile ensues. The final result is its combination with the alkynyl moiety which behaves formally as an alkynyl cation. The initial adduct may react with an electrophile, notably a proton, in which case alkenyl iodonium salts are obtained also, cyclopentenes may be formed by intramolecular C-H 1,5-insertion from the alkylidenecarbenes ... 

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. 

Alkenyl(aryl)iodonium salts 4 are similarly efficacious for the alkenylation of nucleophiles. Such reactions proceed by various mechanisms, including addition-elimination, -elimination, vinylic S 1 and S 2, and ligandcoupling (LC) pathways (89RHA92, 95MI3, OOJOM494, 00RCR105, 02ACR12). 

Few examples of non-catalytic alkenylations of carbon nucleophiles have been published. For example, enolate anions derived from various 1,3-dicarbonyl compounds can be vinylated with cyclohexenyl or cyclopentenyl iodonium salts 715 to afford products 716 (Scheme 3.286) [964]. 

In a recent series of studies focused on the synthetic utility of alkenyliodo-nium salts, ( )-/J-phenylethenyl(phenyl)- and ( )-l-hexenyl(phenyl)iodonium tetrafluoroborates, 22 and 23, were utilized for alkenylations of a range of soft, anionic nucleophiles (Scheme 45) [128-135]. In all cases but one, alkenylations with 22 occurred with retention of configuration, while alkenylations with 23 occurred with inversion of configuration. Only the dialkyl phosphoroselenolate salts gave mixtures of (Z)- and (fj)-products with 22 [132]. Furthermore, although cuprous iodide was used to catalyze the reactions of 22 and 23 with the phosphorothioate and -dithioate salts, the stereochemical results were the same [131,133]. It was generally assumed that retention was an outcome of the ligandcoupling or addition-elimination pathways, while stereochemical inversion was attributed to the vinylic... 

An unambiguous example is the nucleophilic substitution of l-alkenyl(aryl)io-donium salts with halide ions (Scheme 9). l-Decenyl(phenyl)iodonium tetra-... 

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