Mercury substituent

Oxymercaration (2, 265-267 3. 194). Brown and Geoghegan have investigated the relative reactivities of a number of olefins in the oxymercuration reaction in a 20 80 (v/v) mixture of water and THF. The following reactivity is observed terminal disubstituted > terminal monosubstituted > internal disubstituted > internal trisub-stituted > internal leirasubstitu ted. Thus steric factors play a major role in the reactivity of olefins. Increased substitution on the double bond and increased steric hindrance at the site of hydroxyl or mercury substituent attachment decrease the rate of reaction. In the case of olefins of the type RCH CHR, cts-oleiins are more reactive than the corre.sponding rra/i.s-olefins. Inclusion of the double bond in ring systems causes a moderate rate increase which varies somewhat with structure cyclohcxenc > cyclo-pentenc cyclooctene norbornene bicyclo[2.2.2]-octene-2. 

A wide variety of electrophiles can effect aromatic substitution. Usually, it is a substitution of some other group for hydrogen that is of interest, but this is not always the case. Eor example, both silicon and mercury substituents can be replaced by electrophiles. Scheme 9.1 lists some of the specific electrophiles that are capable of carrying out substitution of hydrogen. Some indication of the relative reactivity of the electrophiles is given as well. Many of these electrophiles are not treated in detail until Part B. Nevertheless, it is important to recognize the very broad scope of electrophilic aromatic substitution. 

The occurrence of a hydrogen isotope effect in an electrophilic substitution will certainly render nugatory any attempt to relate the reactivity of the electrophile with the effects of substituents. Such a situation occurs in mercuration in which the large isotope effect = 6) has been attributed to the weakness of the carbon-mercury bond relative to the carbon-hydrogen bond. The following scheme has been formulated for the reaction, and the occurrence of the isotope effect indicates that the magnitudes of A j and are comparable ... 

Anthraquinone can be sulfonated, nitrated, or halogenated. Sulfonation is of the greatest technical importance because the sulfonic acid group can be readily replaced by an amino or chloro group. Sulfonation with 20—25% oleum at a temperature of 130—135°C produces predominandy anthraquinone-2-sulfonic acid [84-48-0]. By the use of a stronger oleum, disulfonic acids are produced. The second sulfonic acid substituent never enters the same ring a mixture of 2,6- and 2,7-disulfonic acids is formed (Wayne-Armstrong rule). In order to sulfonate in the 1-, 1,5-, or 1,8-positions, mercury or one of its salts must be used as a catalyst. 

Mercury(II) acetate tends to mercurate all the free nuclear positions in pyrrole, furan and thiophene to give derivatives of type (74). The acetoxymercuration of thiophene has been estimated to proceed ca. 10 times faster than that of benzene. Mercuration of rings with deactivating substituents such as ethoxycarbonyl and nitro is still possible with this reagent, as shown by the formation of compounds (75) and (76). Mercury(II) chloride is a milder mercurating agent, as illustrated by the chloromercuration of thiophene to give either the 2- or 2,5-disubstituted product (Scheme 25). 

Remarkable cis diastereoselectivity occurs in the addition of l-alkoxy-l-(trialkylsilyloxy)-ethenes to y-silyloxy-substituted cyclo-2-alkenones using mercury(II) iodide as a catalyst265,266. C-C Bond formation syn to the electron-withdrawing silyloxy substituent has been attributed to the stabilization of the emerging cr -orbital at the -carbon by interaction with the ct(CH) bond at the y-carbon atom267. 

The effects of substituents have been determined in the cleavage of diaryl-mercury compounds by hydrogen chloride in dimethyl sulphoxide-dioxan at 32 °C. Rates were measured within the temperature range 12.8-75.0 °C, though over a range of not more than 18 °C for each compound611 (Table 184). The... 

Usually, C-mercury substituted phosphorus ylides are monomers and in order to stabilize these complexes the presence of a second substituent on the carbon is necessary to balance the electron-donating effect of the metal. However a dimeric complex 85 has been obtained by the reaction of mercuric halides HgX2... 

Several studies confirm an ortho effect leading to a dominating aryl sulphur bond cleavage. For example, the introduction of a bulky ortho substituent will provide the formation of the aliphatic sulphinic acid. A series of cyclic sulphones was studied at the mercury cathode and the results (see Table 2) appear to be fully in agreement with those expected when considering the preliminary works presented above. 

Most of the synthetic applications of organomercury compounds are in transition metal-catalyzed processes in which the organic substituent is transferred from mercury to the transition metal in the course of the reaction. Examples of this type of reaction... 

Nitration of the surface of polypyrrole and the subsequent reduction of the nitrate groups has been reported [244] and Bidan et al. [306, 307] have investigated the electrochemistry of a number of polymers based on pyrroles with /V-substituents which are themselves electrochemically active. Polypyrrole has also been successfully deposited onto polymeric films of ruthenium complexes [387], and has been used as an electrode for the deposition and stripping of mercury [388], As with most conducting polymers, several papers have also appeared on the use of polypyrrole in battery systems (e.g. [327, 389] and Ref. therein). 

Successful thermal decarboxylation of metal arenoates other than poly-halogenoarenoates are restricted to mercury compounds and fall into three categories, namely (i) those where electron-withdrawing substituents other than halogens are present in the organic groups, (ii) those where substituents and/or conditions are used which favor a different mechanism, e.g., classic electrophilic aromatic substitution, or (iii) those where the conditions are sufficiently forcing for both mercuration and decarboxylation to occur. 

Syntheses are limited to mercuric salts of weak acids (2,110). Generally, increasing the length of the straight alkyl chain decreases the extent of decarboxylation (e.g., Ref. 133). Electron-withdrawing substituents suppress decarboxylation. For example, mercurials are not formed with Me02C, Cl, and Me(CH2)nO substituents on the a carbon (137,148,149), but some decarboxylation occurs with these on the j8 carbon (135-137). Chain decarboxylation predominated in reactions in benzene, butyric acid [R = Me(CH2)2] (150), or acetic acid (R = Me) (124). The chain reaction was also observed for R = Me(CH2)2 in the absence of solvent and in ethylacetate or heptane solution, but in these media the radical displacement reaction was dominant (2,150). When benzene was used as solvent... 

Application of catalysts allows sometimes executing this addition/elimination process even with alkenes without any electron-deficient substituent attached. Such case is illustrated by an example in Scheme 15. In the presence of mercury-(n) acetate and trifluoroacetic acid, 1,2,3-triazoles 146 react with vinyl acetate at 70 °G to give vinyl derivatives 148 in good yields (70-88%) <2002RJ01056>. Adducts 147 are presumed to be intermediates in this process. 

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