Aromatic substitution mercuration

Aromatic compounds react with mercuric salts to give arylmercury compounds. The reaction shows substituent effects that are characteristic of electrophilic aromatic substitution." Mercuration is one of the few electrophilic aromatic... 

Nitration in sulphuric acid is a reaction for which the nature and concentrations of the electrophile, the nitronium ion, are well established. In these solutions compounds reacting one or two orders of magnitude faster than benzene do so at the rate of encounter of the aromatic molecules and the nitronium ion ( 2.5). If there were a connection between selectivity and reactivity in electrophilic aromatic substitutions, then electrophiles such as those operating in mercuration and Friedel-Crafts alkylation should be subject to control by encounter at a lower threshold of substrate reactivity than in nitration this does not appear to occur. 

Whereas most reactions in the isoxazole series are undoubtedly those of electrophilic substitution, mercuration of isoxazoles, as well as some cases of mercuration of aromatic compounds, could be considered as homolytic reactions. However, the ready mercuration of... 

Mercuration of aromatic compounds can be accomplished with mercuric salts, most often Hg(OAc)2 ° to give ArHgOAc. This is ordinary electrophilic aromatic substitution and takes place by the arenium ion mechanism (p. 675). ° Aromatic compounds can also be converted to arylthallium bis(trifluoroacetates), ArTl(OOCCF3)2, by treatment with thallium(III) trifluoroacetate in trifluoroace-tic acid. ° These arylthallium compounds can be converted to phenols, aryl iodides or fluorides (12-28), aryl cyanides (12-31), aryl nitro compounds, or aryl esters (12-30). The mechanism of thallation appears to be complex, with electrophilic and electron-transfer mechanisms both taking place. 

Aromatic compounds react with mercuric salts to give arylmercury compounds.69 Mercuric acetate or mercuric trifluoroacetate are the usual reagents.70 The reaction shows substituent effects that are characteristic of electrophilic aromatic substitution.71 Mercuration is one of the few electrophilic aromatic substitutions in which proton loss from the a complex is rate determining. Mercuration of benzene shows an isotope effect kB/kD = 6,72 which indicates that the [Pg.1026]

A specific case of the carbonium ion mechanism [Eq. (5)] with reasonable plausibility is decarboxylation of metal arenoates by classic electrophilic aromatic substitution [Eq. (12)]. This mechanism would be favored by electron-donating substituents and has been invoked to explain the relative ease of decarboxylation of p-methoxybenzoic acid in molten mercuric trifluoroacetate (77) as well as the very facile decarboxylation on reaction of polymethoxybenzoic acids with mercuric acetate (18) (see below). 

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. 

Successful decarboxylations where the substituents favor classic electrophilic aromatic substitution are known. Reaction of 2-hydroxy-l-naphthalenecarboxylic acid with mercuric acetate in cold acetic acid has been reported to give 2-hydroxynaphthalen-l-ylmercuric acetate [Eq. (71)] (84). Although the result probably requires reinvestigation,... 

The selective electrophilic aromatic substitution carried out by displacement of a metallic substituent (Hg, Sn) ( F-fluorodemetallation) using [ F]p2 or [ F]AcOF remains a method of choice to introduce a fluorine atom on a specific position. In the early preparations of [6- F]fluoro-L-DOPA, the reaction of a 6-substituted mercuric derivative with [ F]acetyl hypofluorite yielded the expected compound in 11 % yield [73,74]. Reaction of a mercuric precursor, free or on a modified polystyrene support P-CH2-COOHg(DOPA precursor) allows the preparation of [ F]fluoro-L-DOPA in an overall yield up to 23 %. The polymer supports are easily prepared, require no special treatment for storage and are convenient to use in automated production [75]. 

The partial rate factors af and /3f for the a- and /3-positions of thiophene have been calculated for a wide range of electrophilic reactions these have been tabulated (71 AHC(13)235, 72IJS(C)(7)6l). Some side-chain reactions in which resonance-stabilized car-benium ions are formed in the transition states have also been included in this study. A correspondence between solvolytic reactivity and reactivity in electrophilic aromatic substitution is expected because of the similar electron-deficiency developed in the aromatic system in the two types of reactions. The plot of log a or log /3f against the p-values of the respective reaction determined for benzene derivatives, under the same reaction conditions, has shown a linear relationship. Only two major deviations are observed mercuration and protodemercuration. This is understandable since the mechanism of these two reactions might differ in the thiophene series from the benzene case. 

Aromatic ketones arylations, 10, 140 asymmetric hydrogenation, 10, 50 G—H bond alkylation, 10, 214 dialkylzinc additions, 9, 114-115 Aromatic ligands mercuration, 2, 430 in mercury 7t-complexes, 2, 449 /13-77-Aromatic nitriles, preparation, 6, 265 Aromatic nucleophilic substitution reactions, arene chromium tricarbonyls, 5, 234... 

The linear free enogy relationship obsoved for arene donors relates the activation barrier AG for aromatic substitution directly to the CT transiticxi oiergy Aver of the EDA complex. Since Aver pertains to the energetics of the photoionizadons in equations (27) and (28), the correlation suggests that these arene contact ion pairs ate reasonable approximatims to the transition states for both mercuration and thallation, e.g. Scheme 6. 

R. Taylor Kinetics of electrophilic aromatic substitution. Sections 7 (Mercuration) and 9 (Protodemetallation) are noteworthy for their comprehensive coverage. 

The selective mono-mercuration of aromatic rings with mercury acetate followed by Hg NMR spectroscopic analysis has been used to elucidate the aromatic substitution patterns in lignins [8]. Direct and indirect (HMQC 2D Hg- H spectra) Hg NMR spectra of the formed derivatives were recorded 7( Hg, H) coupling constants provided information about the position of the mercury... 

On irradiation at the C-T band, (15) was transformed to CgMegCH2Hg02CCp3. The activation of ArH in mercurations proceeds vU Tr-complexes quantitative spectrophotonietric analysis of the formation constants of the ir-complexes and the 2nd order rate constants establish that Hg(02CCF3)2 is the reactive electrophilic species, both in 7r-complexation formation and in the aromatic substitution. ... 

Complex 13 undergoes electrophilic substitution with aromatic substrates. Thus, treatment with benzene in dichloromethane at ambient temperature results in the formation of the diphenyl complex 15 (Scheme V. Reaction of 13 with pyridine (5-6 equivs) in dichloromethane affords a new complex that is the result of pyridine a-CH activation. The NMR data clearly show two chemically equivalent coordinated pyridines and pyridine that has lost one of the a-hydrogens. Structure 16 is proposed from the preliminary data. The formation of 15 and 16 was quantitative by NMR monitoring, but these compounds are reactive and have not been isolated as pure solids. While main group Lewis acids are well known to undergo aromatic substitutions (e.g., mercurations, thallations, etc.) (33), relatively little is known about the ability of transition metal complexes to undergo electrophilic aromatic substitution (34). 

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