Mercuric-ion-catalyzed hydration

The most synthetically valuable method for converting alkynes to ketones is by mercuric ion-catalyzed hydration. Terminal alkynes give methyl ketones, in accordance with the Markovnikov rule. Internal alkynes give mixtures of ketones unless some structural feature promotes regioselectivity. Reactions with Hg(OAc)2 in other nucleophilic solvents such as acetic acid or methanol proceed to (3-acetoxy- or (3-methoxyalkenylmercury intermediates,152 which can be reduced or solvolyzed to ketones. The regiochemistry is indicative of a mercurinium ion intermediate that is opened by nucleophilic attack at the more positive carbon, that is, the additions follow the Markovnikov rule. Scheme 4.8 gives some examples of alkyne hydration reactions. 

Mercuric Ion-Catalyzed Hydration Alkynes undergo acid-catalyzed addition of water across the triple bond in the presence of mercuric ion as a catalyst. A mixture of mercuric sulfate in aqueous sulfuric acid is commonly used as the reagent. The hydration of alkynes is similar to the hydration of alkenes, and it also goes with Markovnikov orientation. The products are not the alcohols we might expect, however. 

The desired ketone can be formed by mercuric-ion-catalyzed hydration of 1-heptyne. 

Treatment of lithium acetylide with a primary alkyl halide (bromide or iodide) or with aldehydes or ketones produces the corresponding monosubstituted acetylenes or propargylic alcohols. Mercuric ion-catalyzed hydration of these furnishes methyl ketones and methyl a-hydroxy ketones, respectively. 

The mercuric ion-catalyzed hydration of alkynes probably proceeds in a similar manner to the oxymercuration of alkenes (see Section 5.1). Electrophilic addition of Hg to the triple bond leads to a vinylic cation, which is trapped by water to give an vinylic organomercury intermediate. Unlike the alkene oxymercuration, which requires reductive removal of the mercury by NaBH4, the vinylic mercury intermediate is cleaved under the acidic reaction conditions to give the enol, which tautomerizes to the ketone. Hydration of terminal alkynes follows the Mai kovnikov rule to furnish methyl ketones. ° ... 

The first step in the mercuric-ion-catalyzed hydration of an alkyne is formation of a cyclic mercurinium ion. (Two of the electrons in mercury s filled 5d atomic orbital are shown.) This should remind you of the cyclic bromonium and mercurinium ions formed as intermediates in electrophilic addition reactions of alkenes (Sections 4.7 and 4.8). In the second step of the reaction, water attacks the most substituted carbon of the cyclic intermediate (Section 4.8). Oxygen loses a proton to form a mercuric enol, which immediately rearranges to a mercuric ketone. Loss of the mercuric ion forms an enol, which rearranges to a ketone. Notice that the overall addition of water follows both the general rule for electrophilic addition reactions and Markovnikov s rule The electrophile (H in the case of Markovnikov s rule) adds to the sp carbon bonded to the greater number of hydrogens. 

The mercuric-ion-catalyzed hydration of terminal acetylenes has been a significant method for synthesis of methyl ketones. The reaction can also be used with internal acetylenes if they are symmetrical. Unsymmetrical alkynes would be... 

FIGURE 10.76 It is possible to make two different carbonyl compounds from a terminal allyne. Mercuric ion-catalyzed hydration gives the ketone, and hydroboration/oxidation gives the aldehyde. 

The mechanism for the mercuric-ion-catalyzed hydration of an alkyne is shown next. The intermediate formed in the first step should remind you of the cychc hromonium ion formed as an intermediate in the addition of Br2 to an alkene (Section 6.9). 

The most synthetically valuable method for converting acetylenes to ketones is by mercuric-ion-catalyzed hydration. Terminal alkynes give methyl ketones in good yields. Unsymmetrical internal alkynes give a mixture of two possible ketones in the absence of some special stabilizing feature. Scheme 4.6 gives some examples of acetylene hydrations. 

Mercuric ion-catalyzed hydration of alkynes furnishes ketones... 

Give the products of mercuric ion-catalyzed hydration of (a) ethyne (b) propyne (c) 1-butyne (d) 2-butyne (e) 2-methyl-3-hexyne. 

In Summary Alkynes can react with electrophiles such as hydrogen halides and halogens either once or twice. Terminal alkynes transform in accord with the Markovnikov rule. Mercuric ion-catalyzed hydration furnishes enols, which convert into ketones by a process called tautomerism. 

---