Mercury alkynes: acidic hydrogen

The mechanism of the mercurydD-catalyzed alkyne hydration reactioi is analogous to the oxymercuration reaction of alkenes (Section 7.4). Elec trophilic addition of mercury(II) ion to the alkyne gives a vinylic cation which reacts with water and loses a proton to yield a mercury-containii enol intermediate. In contrast to alkene oxymercuration, no treatment widi NaBH4 is necessary to remove the mercury the acidic reaction conditions alone are sufficient to effect replacement of mercury by hydrogen (Figure 8.3). 

The electrochemical generation of hydrogen in aqueous acid or alkaline solutions reduces unactivated alkynes 46 a b). This process is similar to catalytic hydrogenation, however, and does not involve electron transfer to the substrate. The electrochemical generation of solvated electrons in amine solvents or HMPA has also been effective in reducing these compounds 29). The focus of this section, however, is the electrolysis of these difficult to reduce alkynes and alkenes at mercury cathodes with tetraalkyl-ammonium salts as electrolytes. Specific attention is also given to competitive reductions of benzenoid aromatics and alkynes or alkenes. 

The addition of water to alkynes is also aided by the presence of mercury (II) salts. The reaction is usually conducted in water, with the presence of a strong acid, such as sulfuric acid, and a mercury salt, such as HgS04 oi HgO. In this case the mercury is spontaneously replaced by hydrogen under the reaction conditions, so a second step is not necessary. The addition occurs with a Markovnikov orientation stereochemistry is not an issue. 

The phase-transfer-assisted permanganate oxidation of alkynes and alkenes has been reviewed. Terminal and internal alkynes are oxidized to 1,2-dicarbonyl compounds by the combined action of diphenyl disulphide, ammonium peroxidisulphate and water or by sodium periodate in the presence of ruthenium dioxide (equation 34). Other reagents for the conversion of acetylenes into 1,2-dicarbonyl compounds are hydrogen peroxide in the presence of (2,6-dicarboxylatopyridine)iron(II), the complex oxo(A, A -ethylenebissalicylideneiminato)chromium(V) trifluoromethanesulphonate (216)and ruthenium tetroxide as a mediator in electrooxidation. l-Acetoxyalkan-2-ones 217 are obtained by the oxidation of terminal acetylenes with sodium perborate and mercury(II) acetate in acetic acid ". Terminal alkynes give a-ketoaldehydes 218 on treatment with dilute hydrogen peroxide, combined with mercury(II) acetate and sodium molybdate or sodium tungstate under phase-transfer conditions. ... 

Oxidation of organoboranes to alcohols is usually effected with alkaline hydrogen peroxide. The reaction is of wide applicability and many functional groups are unaffected by the reaction conditions, so that a variety of substituted alkenes can be converted into alcohols by this procedure. Several examples have been given above. A valuable feature of the reaction is that it results in the overall addition of water to the double (or triple) bond, with a regioselectivity opposite to that from acid-catalysed hydration. This follows from the fact that, in the hydroboration step, the boron atom adds to the less-substituted carbon atom of the multiple bond. Terminal alkynes, for example, give aldehydes in contrast to the methyl ketones obtained by mercury-assisted hydration. 

---