Mercury reactions with alkynes

Direct mercuration of alkynes has also been achieved. " The interest in mercury alkynyls stems from their documented applications in the rapid separation and detection of toxic organomercurials (particularly, methylmercury derivatives). Upon reaction with alkynes, samples containing methylmercury salts yield Hg(C=CR)2 or Hg(Me)(C=CR) species that can then be analyzed using chromatographic techniques. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Radical intermediates are also trapped by intramolecular reaction with an alkene or alkyne bond. At a mercury cathode this process competes with formation of the dialkylmercury [51], At a reticulated vitreous carbon cathode, this intramolecular cyclization of radicals generated by reduction of iodo compounds is an important process. Reduction of l-iododec-5-yne 5 at vitreous carbon gives the cyclopentane... 

The hydration reaction of alkynes leading to carbonyl compounds is generally carried out in dilute acidic conditions with mercuric 1on salts (often the sulfate) as catalysts (ref. 5). Only very reactive alkynes (phenylacety-lene and derivatives) can be hydrated in strong acidic conditions (HgSO ) without mercury salts (ref. 6). Mercury exchanged or impregnated sulfonic resins have also been used in such reactions (ref. 7). Nevertheless, the loss of the catalyst during the reaction and environmental problems due to the use of mercury make this reaction method not as convenient as it should be for the preparation of carbonyl compounds. 

Several cyclofunctionalization reactions of alkynic alcohols are synthetically useful. Metal ion-promoted cyclofunctionalization of ris-2-propargylcyclopentanol systems proceeds by the 5-exo mode (equation 77 and Table 23).197 Protiodemetallation or reductive demetallation provides the cyclic enol ether in high yields. This method has been used by Noyori in the synthesis of prostacyclin (PGh).197b,197c Reactions with catalytic amounts of mercury(II) or palladium(II) salts gave the endocyclic enol ether as the major product.197 -198 A related cyclization with Ag2C03 has been reported by Chuche.191 Schwartz... 

Mercury(II) salts react with alkynes in two different ways. As discnssed in Section 2.2, terminal alkynes react with mercnry nnder basic conditions to yield species with general formula Hg(C=CR)2 or Hg(X)(C=CR). Alternatively, under neutral or acidic conditions, alkynes undergo solvomercuration reactions to yield anti addition products (for example, see equation 17). ... 

As showm in Figure 8.3, the mechanism of the mercury ID-catalyzed alkyne hydration reaction is analogous to the oxymercuration reaction of alkenes (Section 7.4). Electrophilic addition of mercury(Il) ion to the alkyne gives a vinylic cation, which reacts with water and loses a proton to yield a mercury-containing enol intermediate. In contrast with alkene oxymercuration, however, no treatment with NaBl-14 is necessary to remove the mercury. The acidic reaction conditions alone are sufficient to effect replacement of mercury by hydrogen. 

Vinyltin derivatives react with lead tetraacetate to yield usually the alkynes, and vinyl-mercurials react with lead tetraacetate to yield the corresponding enol acetates. However, addition of mercury(II) salts to the vinyltin reactions draw the reaction towards formation of the enol acetate. The involvement of an alkylidenecarbene intermediate as an alternative decomposition pathway has been excluded. ... 

Oxymercuration occurs with an alkyne as with an alkene, but differences in reactivity lead to a modification in the procedure. For reasons that will not be discussed, a mixture of mercuric sulfate (HgS04) and mercuric acetate [Hg(OAc)2] is used. When 1-heptyne is treated with this mixture in aqueous solvent, the initially formed enol (107) tautomerizes to 2-heptanone (108), which is isolated in 80% yield. Note that the ketone product mentioned in connection with vinyl chloride 92 in Section 10.4.5 results from formation of an enol. There is an important difference in the oxymercuration of alkynes and alkenes that is notable in this transformation. The mercury reacts with the alkyne, but the mercury is lost when the enol is formed and there is no need to add NaBH in a second step. This observation is general for oxymercuration of alkynes under these conditions. The more stable secondary vinyl carbocation is an intermediate, but the vinyl-mercury compound formed by reaction with the carbocation is unstable in the presence of water, so the enol is the product. 

As a center of high electron density, the triple bond is readily attacked by electrophiles. This section describes the resnlts of three such processes addition of hydrogen halides, reaction with halogens, and hydration. The hydration is catalyzed by mercury(II) ions. As is the case in electrophilic additions to unsymmetrical alkenes (Section 12-3), the Markovnikov rule is followed in transformations of terminal alkynes The electrophile adds to the terminal (less snbstituted) carbon atom. 

Deslongchamps et al. [109] have also applied their mercury(II) protocol to mono (benzaimulated) spiroacetals (Scheme 50). Thus, treatment of alkyne diol 201 with catalytic Hg(OTf)2 afforded the 6,6-spiroacetal 205 in excellent yield. The THP-protected alcohol could also be used directly in the reaction with only a small drop in yield. 

There also exists an acidregioselective condensation of the aldol type, namely the Mannich reaction (B. Reichert, 1959 H. Hellmann, 1960 see also p. 291f.). The condensation of secondary amines with aldehydes yields Immonium salts, which react with ketones to give 3-amino ketones (=Mannich bases). Ketones with two enolizable CHj-groupings may form 1,5-diamino-3-pentanones, but monosubstitution products can always be obtained in high yield. Unsymmetrical ketones react preferentially at the most highly substituted carbon atom. Sterical hindrance can reverse this regioselectivity. Thermal elimination of amines leads to the a,)3-unsaturated ketone. Another efficient pathway to vinyl ketones starts with the addition of terminal alkynes to immonium salts. On mercury(ll) catalyzed hydration the product is converted to the Mannich base (H. Smith, 1964). 

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