Mercury alkynes

There also exists an acidregioselective condensation of the aldol type, namely the Mannich reaction (B. Reichert, 1959 H. Hellmann, 1960 see also p. 291f.). The condensation of secondary amines with aldehydes yields Immonium salts, which react with ketones to give 3-amino ketones (=Mannich bases). Ketones with two enolizable CHj-groupings may form 1,5-diamino-3-pentanones, but monosubstitution products can always be obtained in high yield. Unsymmetrical ketones react preferentially at the most highly substituted carbon atom. Sterical hindrance can reverse this regioselectivity. Thermal elimination of amines leads to the a,)3-unsaturated ketone. Another efficient pathway to vinyl ketones starts with the addition of terminal alkynes to immonium salts. On mercury(ll) catalyzed hydration the product is converted to the Mannich base (H. Smith, 1964). 

In general ketones are more stable than their enol precursors and are the products actually isolated when alkynes undergo acid catalyzed hydration The standard method for alkyne hydration employs aqueous sulfuric acid as the reaction medium and mer cury(II) sulfate or mercury(II) oxide as a catalyst... 

The most common synthetic application of mercury-catalyzed addition to alkynes is the conversion of alkynes to ketones. This reaction is carried out under aqueous acidic conditions, where the addition intermediate undergoes protonation to regenerate Hg. ... 

Like alkenes (Sections 7.4 and 7.5), alkynes can be hydrated by either of two methods. Direct addition of water catalyzed by mercury(II) ion yields the Markovnikov product, and indirect addition of water by a hydroboration/ oxidation sequence yields the non-Markovnikov product. 

Alkynes don t react directly with aqueous acid but will undergo hydration readily in the presence of mercury(II) sulfate as a Lewis acid catalyst. The reaction occurs with Markovnikov regiochemistry the -OH group adds to the more highly substituted carbon, and the — H attaches to the less highly substituted one. 

O The alkyne uses a pair of electrons to attack the electrophilic mercury(II) ion, yielding a mercury-containing vinylic carbocation intermediate. 

Figure 8.3 MECHANISM Mechanism of the mercury(II)-catalyzed hydration of an alkyne to yield a ketone. The reaction occurs through initial formation of an intermediate enol, which rapidly tautomerizes to the ketone.

The hydroboration/oxidation sequence is complementary to the direct, mercury(ll)-catalyzed hydration reaction of a terminal alkyne because different products result. Direct hydration with aqueous acid and mercury(IJ) sulfate leads to a methyl ketone, whereas hydroboration/oxidation of the same terminal alkyne leads to an aldehyde. 

The chemistry of alkynes is dominated by electrophilic addition reactions, similar to those of alkenes. Alkynes react with HBr and HC1 to yield vinylic halides and with Br2 and Cl2 to yield 1,2-dihalides (vicinal dihalides). Alkynes can be hydrated by reaction with aqueous sulfuric acid in the presence of mercury(ll) catalyst. The reaction leads to an intermediate enol that immediately isomerizes to yield a ketone tautomer. Since the addition reaction occurs with Markovnikov regiochemistry, a methyl ketone is produced from a terminal alkyne. Alternatively, hydroboration/oxidation of a terminal alkyne yields an aldehyde. 

Barluenga et al. have extensively studied the hydroamination of alkynes catalyzed by mercury compounds, especially mercury(II) chloride. Terminal alkynes and... 

Mercury, Silver perchlorate See Silver perchlorate Alkynes, etc. 

Mercury 7r-complexes with alkene and alkyne ligands 447... 

Organolithium reagents have also been employed for the synthesis of novel bis(alkynyl)mercury derivatives. These include Hg(C=CCF3)263 and 46-48,64 which have been synthesized along with the bis(alkynyl)aurate analogs. Bis(alkynyl)mercury species such as 49 can also be obtained by reaction of monosubstituted alkynes with K2[Hgl4] in basic aqueous solutions.6 ... 

In addition to these examples, several polynuclear mercury alkynyl derivatives have been prepared. Examples of such derivatives include compounds 50-56 which have been obtained by reaction of the MeHgGl with the primary alkyne in basic methanolic conditions.67,68 The phenyl analogs have also been isolated. Most of these compounds have been characterized by FTIR, NMR, and FABMS, and in the case of 51-53 by single crystal X-ray diffraction. 

Organic compounds such as terminal alkynes can undergo direct mercuration using various mercury salts. For instance, alkyne 61 has been shown to react with Hg(OAc)2 to form the symmetrical bis-alkyl-mercury complex 62 (Equation (21)).73... 

Generally, cyclohexyne is an unstable molecule because of its ring strain. However, it can be stabilized by coordination to transition metals.35 The reduction of 1,2-dibromocyclohexene by sodium/mercury in the presence of a nickel-bromide complex afforded the Ni-alkyne complex 66 as a thermally stable and isolable compound (Scheme 22).36 Complex 66 smoothly reacted with C02 under atmospheric pressure to give nickelacycle 67 in good yield. Dimethyl acetylenedicarboxylate was inserted into the vinyl-nickel bond in 67 to give the seven-membered oxanickelacycle 68. 

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... 

Thiols react directly with non-activated alkynes [15] and with 1-alkynyl thioethers [16] to yield alkenyl thioethers in good yield (>76%), whereas thiocyanate anions only add to non-activated alkynes under acidic phase-transfer catalytic conditions on the addition of mercury(II) thiocyanate. Terminal alkynes are converted into vinyl thiocyanates, but disubstituted alkynes also form vinyl isothiocyanates [17]. Major by-products are the ketones formed by solvolysis of the alkynes. 

Radical intermediates are also trapped by intramolecular reaction with an alkene or alkyne bond. At a mercury cathode this process competes with formation of the dialkylmercury [51], At a reticulated vitreous carbon cathode, this intramolecular cyclization of radicals generated by reduction of iodo compounds is an important process. Reduction of l-iododec-5-yne 5 at vitreous carbon gives the cyclopentane... 

The rhodium-entrapped cage compound which is formed using a stoichiometric amount of [RhCl(CO)2]2 is a notable paradigm of the rhodium-catalyzed [2-I-2-I-1] al-kyne-alkyne and CO coupling [35]. Heating 57 in acetone at 50 °C for 8 h or irradiation by a tungsten or mercury lamp provided the cage compound in 50% yield based on NMR spectroscopy. However, due to mechanical losses it was isolated in only 16% yield from the reaction mixture, by crystallization as the hexafluorophosphate salt 58 (Eq. 13). 

In Ref 169, some peculiarities associated with adsorption of alkyne peroxides from DM F-water solutions onto the mercury electrode in the presence of tetraethylammonium cations have been described. Polarography and electrocapillary measurements were employed as the experimental techniques. It has been shown that interfacial activity of these peroxides was determined by the species generated as a result of associative interactions between peroxides and DMF and tetraethylammonium cations. 

Hydration and Hydroalkoxylation of Alkynes Gold compounds were first applied to catalyze these types of reactions by Utimoto et al. in 1991, when they studied the use of Au(III) catalysts for the effective activation of alkynes. Previously, these reactions were only catalyzed by palladium or platinum(II) salts or mercury(II) salts under strongly acidic conditions. Utimoto et al. reported the use of Na[AuCI41 in aqueous methanol for the hydration of alkynes to ketones [13]. 

The generally accepted pathway for the hydration of alkynes are the generation and subsequent tautomerization of an intermediate enol. The use of fairly concentrated acids, usually H2S04, is necessary to achieve suitable reaction rates. Addition of catalytic amounts of metal salts, however, greatly accelerates product formation. In most cases mercury(II) salts are used. Mercury-impregnated Nafion-H [with 25% of the protons exchanged for Hg(II)] is a very convenient reagent for hydration 35... 

Enol ethers and acetals are formed when alcohols add to alkynes. The reaction of alcohols may be catalyzed by mercury(II) salts 43,44 Hg(OAc)2 with or without TosOH allows the synthesis of enol ethers, acetals, or ketones under appropriate reaction conditions.44 Au3+ was found to be an effective catalyst in the synthesis of acetals 37... 

Mercury(II) salts317 and Tl(OAc)3318 may be used as catalyst to add aliphatic and aromatic amines to alkynes to yield imines or enamines. Selective addition to the carbon-carbon triple bond in conjugated enynes was achieved by this reaction 319... 

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