Alkynyl phenyliodonium salts

Because of the multitude and diversity of iodonium salts and zwitterions, some of which are labile or are prepared for specific purposes, their preparative methods are discussed in Chapter 8, for diaryl iodonium salts and related compounds, Chapter 9 for perfluoroalkyl, alkenyl and alkynyl phenyliodonium salts and Chapter 10 for zwitterionic iodonium compounds. In addition, the preparation of some lesser known reagents, including 2-iodosylbenzoic acid, is given in Chapter 12. 

In contrast to (diacetoxyiodo)benzene, [bis(trifluoroacetoxy)iodo]benzene, (BTI) reacts in aqueous solvents with both terminal and non-terminal alkynes affording eventually a-hydroxyketones and 1,2-diketones, respectively. The primary reaction of terminal alkynes leads to the formation of alkynyl phenyliodonium salts, which are not isolable under the experimental conditions but have been prepared by other routes (Section 9.1.3) these are hydrolysed in situ to a-hydroxymethyl ketones, through the intermediacy of their O-tri fluoroacetates, which sometimes may be isolated as by-products. 

Sulfides are readily formed from thiols on reaction with halides (including glyco-syl halides) or epoxides in the presence of a phase-transfer catalyst. In aqueous micelles of cetyltrimethylammonium bromide the reaction of Na2S with haloalkanes generates symmetrical sulfides. Alkynyl phenyliodonium salts are suitable electrophiles for S-alkylation of O.O -dialkyl phosphorodithiolates. ... 

The reaction was of general applicability not only internal but also terminal alkynes reacted in the same way, e.g. 1-hexyne was converted into hexanoic acid (58%) and 2-ethynylthiophene to 2-thienylacetic acid (54%). By contrast, heating terminal alkynes in chloroform with HTI afforded alkynyl or alkenyl phenyliodonium salts the former in refluxing methanol rearranged to methyl carboxylates [11],... 

Instead of silyl-, stannylalkenes are also suitable precursors [4] cyano phenyliodonium triflate (Section 9.1.4) was here the reagent of choice. This variation enabled the preparation of the parent ethenyl and several trisubstituted alkenyl phenyliodonium triflates [5], More elaborate members were obtained through additions to the triple bond of alkynyl iodonium salts, notably Diels-Alder adducts. 

Other alkynyl phenyliodonium arylsulphonates were prepared by modified procedures, notably from iodosylbenzene and alkynylsilanes, in good yields (62-89%). The decomposition stage proceeded also in satisfactory yields, up to 88% [61]. Bis alkynyl dibenzoates and ditosylates were prepared from the corresponding bis iodonium salts of the general formula PhI+C=C(CH2) C=CI + Ph, where n = 5,6,8 [62]. 

Sodium thiophenoxide and bis phenyliodonium acetylene triflate afforded cleanly l,2-bis(phenylthio)acetylene [6]. Alkynyl iodonium salts have alkynylated several arene sulphonates which were converted into alkynyl aryl sulphones. The process is probably the best among other methods, as far as yield, availability of starting materials, non-toxicity and ease of handling are concerned. 

Variations of this method permit the preparation of diverse salts, most of which are stable crystalline compounds. The group R in the alkynyl moiety can be of virtually any kind, for example aryl, aroyl, alkenyl, alkynyl, cyano, trimethylsilyl, etc. For some of these compounds only alkynylstannanes were suitable precursors, in combination with cyano phenyliodonium triflate double salts were also obtained in this way [10,11],... 

Using this method, several mesogen diacetylenes were obtained [49]. Palladium-catalysed coupling of an allylic cyclic carbonate with 1-pentynyl phenyliodonium tetrafluoroborate to give an enyne was highly successful [50]. Alkynyl iodonium triflates and lithium salts of diethyl 2-[(diphenylmethylene)amino]malonate were used for the preparation of alkynyl-a-amino acid derivatives, e.g. [51] ... 

The propensity of the thiocyanate ion for alkynylation with alkynyliodonium ions has also been demonstrated with a series of bis(phenyliodonium)diyne triflates (equations 61 and 62)43. The efficient production of diynediyl dithiocyanates in these reactions may be contrasted with the favored formation of mono- and bis-cyclopentenyl sulfones from bisiodonium diyne salts and sodium/ -toluenesulfinate (see equation 57)86. 

Because the hydrogen atom and phenyl group migrate so readily, the reactions of / -dicarbonyl enolates with ethynyl- and (phenylethynyl)iodonium salts can be expected to result in alkynylation. It has already been noted that the 2- -hexyl-l,3-indandionate ion undergoes alkynylation with (phenylethynyl)phenyliodonium tetrafluoroborate (equation 43), despite the availability of the -hexyl group for [2 + 3] annulation. Ethynylations of six / -dicarbonyl enolates and the anion of 2-nitrocyclohexane with ethynyl(phenyl)-iodonium tetrafluoroborate in THF have also been reported27. For example, admixture of the ethynyliodonium salt and the anion of ethyl 2-cyclopentanone-l-carboxylate in THF affords the 1-ethynyl derivative in 71% isolated yield (equation 124)27. 

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. 

Diels-Alder reactions of alkynyl(phenyl)iodonium triflates (i.e. containing electron-withdrawing groups in the alkynyl moiety) and [bis(phenyliodonium)] ethyne ditrifiate have been employed for the synthesis of cyclic vinyliodonium salts (equations 143 and 144)17,41. The availability of such compounds offers considerable potential for the elaboration of densely functionalized cyclic molecules. 

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