Oxymercuration of alkynes

Only scattered information is available on oxymercuration of alkynes. Alkylphe-nylacetylenes were found to undergo stereoselective anti addition.500 Diphenylace-tylene, in contrast, gives only syn adducts.501... 

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. 

FIGURE 10.71 The oxymercuration of alkynes resembles the oxymercuration of alkenes. In the alkyne case, the product is an enol, which can react further. 

To circumvent some of the above-mentioned drawbacks of sulfur-based mercury chemodosimeters, a system based on the alkyne oxymercuration of 58 has been developed (Fig. 22) [146]. 58 shows high selectivity, a limit of detection of ca. 8 ppm, resistance against strong oxidants, and a positive reaction even in the presence of cysteine, which is known to form stable mercury complexes and is used for the extraction of mercury from tissue samples. Another metal that is well-known for its catalytic ability is palladium, catalyzing different reactions depending on its oxidation state. Since this metal is toxic, assessment of the maximum allowable concentration of Pd in consumer products such as pharmaceuticals requires highly sensitive and selective detection schemes. For this purpose, indicator 60 was conceived to undergo allylic oxidative insertion to the fluorescein... 

Hydration of alkynes (via oxymercuration) gives good yields of single compounds only with symmetrical or terminal alkynes. Show what the products would be from hydration of each compound. 

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. ° ... 

Mercury. A short account of the discovery of metal-catalyzed hydration of alkynes by Kucherov (1881) appeared on the occasion of its 125th anniversary [116]. Mercury-catalyzed hydration of alkynes has been used as mechanistic principle for devising fluorogenic probes for mercuric ions by two research teams. In one system, a 3-butyn-l-yl group at the phenolic oxygen of a fluorescein dye was cleaved via catalytic oxymercuration and elimination to releases a fluorescent dye (Scheme 20) [117]. In another system the mercury-catalyzed hydration of an ethynyl to an acetyl group provoked the quenching of fluorescence in a coumarine-based dye [118]. 

Keto-enol tautomerism was discussed in Chapter 10 (Section 10.6) in connection with the hydration reaction of alkynes, including oxymercuration or hydroboration. The carbonyl form is more stable, and for most aldehydes and ketones only a tiny amount of enol is present. Acetone (2), for example, exists primarily as the ketone, and experiments show only 1.5 x 10 % enol. This experiment titrated acetone with diatomic bromine and measured the extent of reaction, which converted 2 to l-bromo-2-propanone (a-bromoacetone, 4). The second product in this reaction is HBr. By this experiment, the enol content of acetone—and presumably of other simple ketones—is remarkably small. 

In the presence of concentrated sulfuric acid and Hg(II) salts as catalysts, alkynes undergo the addition of water in a reaction analogous to the oxymercuration of al-kenes (Section 6.3F). The Hg(II) salts most often used for this purpose are HgO, HgSO, or Hg(OAc)2- For terminal allies, addition of water follows Markovnikov s rule hydrogen adds to the carbon atom of the triple bond bearing the hydrogen. The resulting enol is in equilibrium with the more stable keto form, so the product isolated is a ketone (an aldehyde in the case of acetylene itself). 

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. 

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. 

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). 

Oxymercuration reactions are usually carried out in a three-component system involving an alkene, alkyne, or cyclopropane, a mercury compound HgX2, and a nucleophile HY, where HY may be HOH, ROH, HOOH, HOOR, or HOCOR and is frequently the solvent . Reactivity follows the sequence of Lewis acid strength in HgX2, with Hg(OCOMe)2 < Hg(OCOCF3)2, Hg(C104)2, and a steric sequence in alkene, with CH2=CH2 > RCH=CH2 > cis-RCH=CHR > trans-RCH=CHR > R2C=CHR > R2C=CR2. 

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