Alkyne mercuric acid hydration

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. 

Catalyzed by Acid and Mercuric Salts Hydration of a terminal alkyne is a convenient way of making methyl ketones. This reaction is catalyzed by a combination of sulfuric acid and mercuric ion. The initial product of Markovnikov hydration is an enol, which quickly tautomerizes to its keto form. Internal alkynes can be hydrated, but mixtures of ketones often result. 

Like alkenes, alkynes can be hydrated. The reaction is generally catalyzed hy mercuric ions in an oxymercuration process (Fig. 10.71), although simple acid catalysis is also known. In contrast to the oxymercuration of alkenes, no second, reduction step is required in this alkyne hydration. By strict analogy to the oxymercuration of alkenes, the product should be a hydroxy mercury compound, hut the second double bond exerts its influence and further reaction takes place. The double hond is protonated and mercury is lost to generate a species called an enol. An enol is part alkene and part alcoho/, hence the name. 

Terminal alkyne anions are popular reagents for the acyl anion synthons (RCHjCO"). If this nucleophile is added to aldehydes or ketones, the triple bond remains. This can be con verted to an alkynemercury(II) complex with mercuric salts and is hydrated with water or acids to form ketones (M.M.T. Khan, 1974). The more substituted carbon atom of the al-kynes is converted preferentially into a carbonyl group. Highly substituted a-hydroxyketones are available by this method (J.A. Katzenellenbogen, 1973). Acetylene itself can react with two molecules of an aldehyde or a ketone (V. jager, 1977). Hydration then leads to 1,4-dihydroxy-2-butanones. The 1,4-diols tend to condense to tetrahydrofuran derivatives in the presence of acids. 

The hydration of triple bonds is generally carried out with mercuric ion salts (often the sulfate or acetate) as catalysts. Mercuric oxide in the presence of an acid is also a common reagent. Since the addition follows Markovnikov s rule, only acetylene gives an aldehyde. All other triple-bond compounds give ketones (for a method of reversing the orientation for terminal alkynes, see 15-16). With allqmes of the form RC=CH methyl ketones are formed almost exclusively, but with RC=CR both possible products are usually obtained. The reaction can be conveniently carried out with a catalyst prepared by impregnating mercuric oxide onto Nafion-H (a superacidic perfluorinated resinsulfonic acid). ... 

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. 

This is ordinary electrophilic addition, with rate-determining protonation as the first step.164 Certain other alkynes have also been hydrated to ketones with strong acids in the absence of mercuric salts.165 Simple alkynes can also be converted to ketones by heating with formic acid, without a catalyst.166... 

Mercuric sulfate catalyzed hydration of cyclopropylacetylenes in aqueous sulfuric acid, like other monosubstituted alkynes, gave mainly the corresponding methyl ketone accompanied by small amounts of ring-opened prouducts (equation 168)236. Similar results were obtained using HgO in trichloroacetic acid, with catalytic amounts of BF3-Et20 and methanol. 

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. 

Alkynes, unlike alkenes, are not hydrated readily in aqueous acid unless a mercuric salt is present as a catalyst. Also, the products that are isolated are either aldehydes or ketones instead of alcohols. Even though the addition of one molecule of water to ethyne probably gives ethenol (vinyl alcohol) initially, this compound is unstable relative to its structural isomer (ethanal) and rapidly rearranges ... 

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. ° ... 

In practice, most hydration reactions of alkynes are carried out using mercuric acetate as a Lewis acid catalyst because of the milder conditions required. Under favourable circumstances even this reaction is not immune to the tendency of alkynes to rearrange, and provides a useful route to benzofurans (51). 

This reaction was first reported by Fittig and Schrohe in 1875 and subsequently extended by Kutscheroff in 1881. It is an acid-catalyzed hydration of alkynes into ketones. In this reaction, dilute sulfuric acid and mercuric salt are used as catalysts, and mercuric chloride can form a complex with acetylene in aqueous solution. This reaction has been used to prepare ketones from higher alkynes, such as propyne, and vinylacetylene as well as in commercial production of acetaldehyde from acetylene. ... 

Methyl ketones can be prepared by hydration of terminal alkynes, catalyzed by acid and mercuric ion (review eq. 3.52). For example,... 

Alkynes undergo electrophilic addition reactions with hydrogen halides and bromine, but these reactions have limited synthetic utility. However, one reaction of alkynes that is commonly used in organic chemistry is hydration of the carbon-carbon triple bond to give a ketone, a transformation that is catalyzed by mercuric ion in the presence of sulfuric acid (Eq. 11.10). 

The acid-catalyzed hydration of alkynes (Table 6.7, example 2) is commonly carried out using mercury (11) salts, such as mercuric sulfate (HgS04), as catalysts. The addition (Scheme 6.67) appears to involve a bridged mercurinium ion, which, for unsymmetrical cases such as 1-alkynes other than ethyne (acetylene [HC CH]), is subsequently attacked by water (FI2O) at the carbon that best supports a positive charge. The regiochemistry of Markownikoff addition, seen with alkenes, is followed. 

Scheme 6.68. A potential pathway for the mercuric ethanoate (mercuric acetate [Hg(02CCH3)2]) catalyzed electrophilic addition of ethanoic acid (acetic acid [CH3CO2H]) to an alkyne to form an enol ethanoate (enol acetate) and then, with excess acid, the corresponding diester (an acylal) of ethanal (acetaldehyde, CH3CHO) or ketone hydrate.

Alkynes are also observed to undergo acid-catalyzed hydration, but the reaction is slower than the corresponding reaction with alkenes. As noted earlier in this chapter, the difference in rate is attributed to the high-energy, vinylic carbocation intermediate that is formed when an alkyne is protonated. The rate of alkyne hydration is markedly enhanced in the presence of mercuric sulfate (HgS04), which catalyzes the reaction ... 

Acid-catalyzed hydration of alkynes is catalyzed by mercuric sulfate (HgS04) to produce an enol that cannot be isolated because it is rapidly converted into a ketone. 

The hydration of alkynes to yield ketones is generally useful only for terminal alkynes. For example, ethynylcyclopentane reacts with water in aqueous sulfuric acid and mercuric ion to give an enol, which quickly isomerizes to a methyl ketone. 

Completing the synthesis requires that we first prepare the alkyne above from the starting alkene, shown in the problem statement. This can be accomplished via a two-step procedure. The alkene is treated with molecular bromine (Br2) to give a vicinal dibromide, which is then treated with excess NaNH2 (followed by water work-up) to give an alkyne. And as mentioned earlier, the alkyne can be converted into the desired methyl ketone via acid-catalyzed hydration in the presence of mercuric sulfate ... 

The resulting terminal alkyne can then be converted into the desired product via acid-catalyzed hydration in the presence of mercuric sulfate ... 

This ketone can be made from the starting material in just two steps. First, the dibromide is converted to an alkyne upon treatment with excess sodium amide (via two successive E2 reactions), followed by water work-up (to protonate the resulting alkynide ion). The terminal alkyne is then treated with aqueous acid, in the presence of mercuric sulfate, giving a hydration reaction. The initially formed enol will rapidly tautomerize to give a... 

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