Alkynes silver fluoride

Interestingly, (/i)-alkenylpentafluorosilicates, readily available from alkynes by hydrosilylation, could also be dimerized to the corresponding symmetric (E,E)- 1,3-dienes in good yields on treatment with silver fluoride in acetonitrile or with silver nitrate in water/ether (Scheme 10.16).30 This reaction suggested that pentafluorosi-licates could be transmetallated to silver, leading to a vinylsilver that dimerized. 

Azomethine ylides can also be generated from a-cyano- or a-phenylthiomethylamines <85JOC4006, 86CB813). Silver fluoride promotes the elimination. The adducts from alkynes are 2,5-dihydropyrroles which can be oxidized to pyrroles by ddq (Scheme 82). 

Terminal alkynes can be converted readily into alkynylsilanes by reaction of the corresponding alkyne anion or its metalloid equivalent with a suitable chlorosilane (/). The reverse reaction, that of liberation of the alkyne, is quite facile, being effected by several reagent combinations, including hydroxide ion, methanolysis, fluoride anion, silver(i) followed by cyanide anion, and methyl lithium-lithium bromide (2). 

All the reagents listed can be used to cleave trimethylsilyl groups from acetylenes fluoride, potassium carbonate under basic conditions in methanol, or silver nitrate/potassium cyanide.9 With the third method advantage can be taken of the fact that the later transition metals (e.g., copper or silvei) complex readily with acety-lides. Workup with concentrated potassium cyanide solution causes compound 32 to be cleaved tu alkyne 33. In this way silylated a -kynes can be deprotected in the presence of 0-silyl groups. 

The non-silylated triple bond in derivatives such as 211 is reduced preferentially , and because of the ease with which the alkynyl-silicon bond is cleaved, a route is made available for the selective reduction of the internal triple bond of polyynes which also contain terminal alkyne linkages. Desilylation can be accomplished with dilute base , silver nitrate or fluoride salts . An example which illustrates the sequence is the conversion of 211 to 212 in 53% yield . 

Hydride transfers are implicated in several acylations of alkynes by saturated acyl chlorides leading to cyclopentenes. Intramolecular [1,5] hydride shifts have been shown to be a common feature in silver-assisted reactions of cyclohexanylcarbonyl chloride with alkynes.The nature of the final product depends on the structure of the resulting cation. Capture of fluoride ion, ring contraction and acyl or alkyl migrations of axial substituents have been observed (Scheme 24). 

The hydrosilylation of terminal alkynes disclosed by Trost can be applied to internal alkynes as well. i Remarkably, the (Z)-isomer is generated in this process, resulting from trans addition during hydrosilylation. The protodesilylation of these sily-lated products in the presence of copper(I) iodide and tetrabuty-lammonium fluoride (TBAF) or silver(I) fluoride (eq 15) leads to internal fraws-olefins. This two-step method is a useful synthetic transformation to access ( j-alkenes from internal alkynes. In contrast, the chemoselective reduction of alkynes to the corresponding ( -alkenes is conventionally accomplished readily with Lindlar s catalyst. The complementary process to afford ( )-olefins has proven much more difficult. Methods involving metal hydrides, dissolving metal reductions, low-valent chromium salts provide the desired chemical conversion, albeit with certain limitations. For example, functional substitution at the propargylic position (alcohols, amines, and carbonyl units) is often necessary to achieve selectivity in these transformations. Conversely, the hydrosilylation/protodesilyla-tion protocol is a mild method for the reduction of alkynes to ( )-alkenes. 

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