Alkynes mercury salts

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... 

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. 

Mercury salts, such as mercury(II) acetate,521-525 mercury(II) oxide,524,526-528 metcury(II) trifluoroace-tate,529,530 mercury(II) sulfate524,531 and mercury(II) phosphate531 catalyze the addition of carboxylic acids to alkynes. Acetic anhydride in the presence of boron trifluoride etherate can also be effectively used in this reaction (equation 292).521,522 Alkynoic acids undergo mercury-catalyzed cyclization to lactones (equation 293).523,532,533... 

The direct hydration of a terminal alkyne, with dilute sulphuric acid in the presence of a mercury salt, yields initially an enol which rearranges to the more stable ketone. The regioselectivity of the reaction is consistent with that predicted on the basis of mechanistic theory. 

The addition of water to alkynes is also aided by the presence of mercury (II) salts. The reaction is usually conducted in water, with the presence of a strong acid, such as sulfuric acid, and a mercury salt, such as HgS04 oi HgO. In this case the mercury is spontaneously replaced by hydrogen under the reaction conditions, so a second step is not necessary. The addition occurs with a Markovnikov orientation stereochemistry is not an issue. 

Dimethyl acetals. The addition of methanol to alkynes requires 1 mol% of (Ph3P)AuMe and MsOH or HBF. The catalyst system is far superior to that comprising stoichiometric mercury salts which are toxic. 

However, hydration of alkynes by acid alone is less important than addition of water under the influence of mercury salts. For this purpose solutions of mercury(n) sulfate in sulfuric acid or of mercury(n) acetate in acetic acid are generally used. When the products are water-soluble, the hydration can be effected with aqueous catalyst solutions for water-insoluble acetylenes it is advisable to add solvents such as 70% methanol, 70% acetone, or acetic acid.83... 

Mercury salts can react directly with hydrocarbons exchanging hydrogen for mercury. This reaction is an electrophilic substitution (equation 5) and hence can take place with arenes, cyclopentadienyls, terminal alkynes, and also with aliphatic hydrocarbons that contain activated carbon-hydrogen bonds (e.g. carbonyl or nitrile compounds). When the hydrocarbon contains several equivalent hydrogen atoms, polymercuration is often observed. 

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). 

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). 

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... 

Except for very reactive alkynes, acid-catalyzed hydrations are usually sluggish. This slow hydration can be overcome by the addition of catalytic amounts of mercury(II) salts. Such hydrations are generally mild and will tolerate the presence of other functional groups. Specific examples of mercury-catalyzed hydrations are discussed in the next section. 

The addition of alcohols to alkynes is catalyzed by bases,411,412 or salts of palladium,413 silver414 or mercury413 (equations 254-256). The mercury-catalyzed processes are summarized in the following section. 

The acyloxymercuration of alkynes has been reported to produce a wide variety of products. Terminal alkynes afford either dialkynylmercurials516,517 or polymercurated products whose structures have not been well established (equation 290). Internal alkynes usually afford vinylic mercurials in which the mercury(II) salt has added in an anti fashion (equation 291 ).518-520 Only sodium borohydride has been used to demercurate a few of these mercurials.520... 

Several cyclofunctionalization reactions of alkynic alcohols are synthetically useful. Metal ion-promoted cyclofunctionalization of ris-2-propargylcyclopentanol systems proceeds by the 5-exo mode (equation 77 and Table 23).197 Protiodemetallation or reductive demetallation provides the cyclic enol ether in high yields. This method has been used by Noyori in the synthesis of prostacyclin (PGh).197b,197c Reactions with catalytic amounts of mercury(II) or palladium(II) salts gave the endocyclic enol ether as the major product.197 -198 A related cyclization with Ag2C03 has been reported by Chuche.191 Schwartz... 

The electrochemical generation of hydrogen in aqueous acid or alkaline solutions reduces unactivated alkynes 46 a b). This process is similar to catalytic hydrogenation, however, and does not involve electron transfer to the substrate. The electrochemical generation of solvated electrons in amine solvents or HMPA has also been effective in reducing these compounds 29). The focus of this section, however, is the electrolysis of these difficult to reduce alkynes and alkenes at mercury cathodes with tetraalkyl-ammonium salts as electrolytes. Specific attention is also given to competitive reductions of benzenoid aromatics and alkynes or alkenes. 

Indeed only alkynes could be reduced in (CH3)4N+ solutions while alkenes were inactive. Reduction of 1-hexyne, propargyl alcohol and 1,4-butyne diol were performed 13 at a mercury cathode with (CH3)4NC1 as the electrolyte. The corresponding olefins were formed and the respective yields were 45 %, 62 % and 82 %. The diacetate of 15 behaved similarly. However only the trans isomer 16 was formed from 15 while a mixture of trans and cis (6 4) isomers resulted from the reduction of the diacetate. Polarography of several alkynes in methanol with (C4H9)4N+ electrolytes showed, 3) that they react close to background decomposition. It was therefore proposed 14) that (CH3)4N+-mercury may be involved in the cathodic reduction of alkynes when (CH3)4N+ salts serve as the electrolytes. 

Mercury(II) salts also catalyze the hydration of alkynes through similar mercurinium intermediates, with the overall process being... 

Gold(l) complexes of alkenes and alkynes appear to play an important role as intermediates of the gold(l)-catalyzed addition of water, alcohols, carboxylic acids, or amines to these substrates. The elfect of this 7r-complexation (22) is superior to the performance of mercury(ll) in this type of reactions. " In a similar way, gold(III) salts have found applications in homogeneous catalysis. ... 

Direct mercuration of alkynes has also been achieved. " The interest in mercury alkynyls stems from their documented applications in the rapid separation and detection of toxic organomercurials (particularly, methylmercury derivatives). Upon reaction with alkynes, samples containing methylmercury salts yield Hg(C=CR)2 or Hg(Me)(C=CR) species that can then be analyzed using chromatographic techniques. 

Mercury(II) salts react with alkynes in two different ways. As discnssed in Section 2.2, terminal alkynes react with mercnry nnder basic conditions to yield species with general formula Hg(C=CR)2 or Hg(X)(C=CR). Alternatively, under neutral or acidic conditions, alkynes undergo solvomercuration reactions to yield anti addition products (for example, see equation 17). ... 

Another synthetic route to monoorganothallium compounds is the reaction of aryl or vinyl derivatives of mercury(II) or tin(IV) with TIX3 (X = halide, carboxy-late). Monoalkylthallium derivatives are intermediates m the oxidation of alkenes and alkynes by thallium(III) salts (oxythallation) (see Section 7). 

Vinyltin derivatives react with lead tetraacetate to yield usually the alkynes, and vinyl-mercurials react with lead tetraacetate to yield the corresponding enol acetates. However, addition of mercury(II) salts to the vinyltin reactions draw the reaction towards formation of the enol acetate. The involvement of an alkylidenecarbene intermediate as an alternative decomposition pathway has been excluded. ... 

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