Oxymercuration works particularly well with alkynes. Here are the conditions, and the product, following the analogy of alkene hydration, should be the compound shown at the right-hand end of the scheme below.

But the product isolated from an alkyne oxymercuration is in fact a ketone. You can see why if you just allow a proton on this initial product to shift from oxygen to carbon—first protonate at C then deprotonate at O. C=O bonds are stronger than C=C bonds, and this simple reaction is very fast.

We now have a ketone, but we also still have the mercury. That is no problem when there is a carbonyl group adjacent because any weak nucleophile can remove mercury in the presence of acid, as shown below. Finally, another proton transfer (from O to C again) gives the real product of the reaction: a ketone.

This is a very useful way of making methyl ketones because terminal alkynes can be made using the methods  (addition of metallated alkynes to electrophiles).

Anticancer compounds

The anthracyclinone class of anticancer compounds (which includes daunomycin and adriamycin) can be made using a mercury (II)-promoted alkyne hydration. You saw the synthesis of alkynes , where we discussed additions of metallated alkynes to ketones. Here is the final step in a synthesis of the anticancer compound deoxydauno mycinone: the alkyne is hydrated using Hg2+ in dilute sulfuric acid to give the final product.

مواضيع ذات صلة

The Mannich reaction

Base-promoted halogenation

Halogenation

Racemization

Thermodynamically stable enols: 1,3-dicarbonyl compounds

Kinetically stable enols

Summary of types of enol and enolate

The intermediate in the base-catalysed reaction is an enolate ion

Enolization is catalysed by acids and bases

Evidence for the equilibration of carbonyl compounds with enols

Tautomerism: formation of enols by proton transfer

Hydration of alkynes