Alkyne oxymercuration

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... 

Song F, Watanabe S, Floreancig PE et al (2008) Oxidation-resistant fluorogenic probe for mercury based on alkyne oxymercuration. J Am Chem Soc 130 16460-16461... 

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=0 bonds are stronger than C=C bonds, and this simple reaction is very fast. 

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... 

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. 

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). 

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. ° ... 

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. 

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]. 

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