Mercuration, aromatic

Mercuration, aromatic, 2 71 Mercurials containing Hg—NlCFjlj bond, 16 2-7... 

The same eom >ound is also formed when mercuric benzoate is heateortance in ttse preparation of mercurated aromatic acids. Mercury salieylatf" yicdds an t- rirano-niercuri imhyddde, wlien heated for a pro-... 

Iv) Aromatic Mercuration . Aromatic compounds are metallated by electrophilic Hg salts ... 

Primary aromatic amides are crystaUine sohds with definite melting points. Upon boiling with 10-20 per cent, sodium or potassium hydroxide solution, they are hydrolysed with the evolution of ammonia (vapour turns red htmus paper blue and mercurous nitrate paper black) and the formation of the alkah metal salt of the acid ... 

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. 

Similarly to mercuration reactions, Pd(OAc)2 undergoes facile palladation of aromatic compounds. On the other hand, no reaction of aromatic compounds takes place with PdClj. PdCl2 reacts only in the presence of bases. The aro-... 

The transmetallation of various organometallic compounds (Hg, Tl, Sn, B, Si, etc.) with Pd(II) generates the reactive cr-aryl, alkenyl, and alkyl Pd compounds. These carbopalladation products can be used without isolation for further reactions. Pd(II) and Hg(II) salts have similar reactivity toward alkenes and aromatic compounds, but Hg(II) salts form stable mercuration products with alkenes and aromatic rings. The mercuration products are isolated and handled easily. On the other hand, the corresponding palladation products are too reactive to be isolated. The stable mercuration products can be used for various reactions based on facile transmetallation with Pd(II) salts to generate the very reactive palladation products 399 and 400 in rim[364,365]. 

Thallation of aromatic compounds with thallium tris(trifluoroacetate) proceeds more easily than mercuration. Transmetallation of organothallium compounds with Pd(II) is used for synthetic purposes. The reaction of alkenes with arylthallium compounds in the presence of Pd(Il) salt gives styrene derivatives (433). The reaction can be made catalytic by use of CuCl7[393,394], The aryla-tion of methyl vinyl ketone was carried out with the arylthallium compound 434[395]. The /9-alkoxythallium compound 435, obtained by oxythallation of styrene, is converted into acetophenone by the treatment with PdCh[396]. 

The aHphatic iodine derivatives are usually prepared by reaction of an alcohol with hydroiodic acid or phosphoms trHodide by reaction of iodine, an alcohol, and red phosphoms addition of iodine monochloride, monobromide, or iodine to an olefin replacement reaction by heating the chlorine or bromine compound with an alkaH iodide ia a suitable solvent and the reaction of triphenyl phosphite with methyl iodide and an alcohol. The aromatic iodine derivatives are prepared by reacting iodine and the aromatic system with oxidising agents such as nitric acid, filming sulfuric acid, or mercuric oxide. 

The exchange of aromatic protons can be effected in the absence of any -OH or —NH2 activating group during the course of a Clemmensen reduction in deuteriochloric and deuterioacetic acid mixture (see section Ill-D). This reaction has been carried out with various tricyclic diterpenes and is best illustrated by the conversion of dehydroabietic acid into its 12,14-d2-labeled analog (40 -+ 41).Amalgamated zinc is reportedly necessary for the exchange reaction since the results are less satisfactory when a zinc chloride-mercuric chloride mixture is used. 

Acetyl hypofluorite is very effective m the fluorination of the aryl-metal (Hg, Ge, or Si) bond, but yields are frequently low. With aryl silicon compounds some competition exists for replacement of an aromatic hydrogen [5i, 52, 55, 54] (equations 25-27). Fluoroxytrifluoromethane fluorinates p-methoxypheny 1 mercuric acetate to givep-fluoroanisole in 86% yield [52]... 

The previous sections have dealt with stable C=N-I- functionality in aromatic rings as simple salts. Another class of iminium salt reactions can be found where the iminium salt is only an intermediate. The purpose of this section is to point out these reactions even though they do not show any striking differences in their reactivity from stable iminium salts. Such intermediates arise from a-chloroamines (133-135), isomerization of oxazolidines (136), reduction of a-aminoketones by the Clemmensen method (137-139), reductive alkylation by the Leuckart-Wallach (140-141) or Clarke-Eschweiler reaction (142), mercuric acetate oxidation of amines (46,93), and in reactions such as ketene with enamines (143). 

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... 

Mercuric Nitrate (Mercury Nitrate, Mercury Pemitrate). Hg(NO3)2 0, mw 342.61, OB to HgO and N2 +26.4% wh deliq powd or colorl crysts, mp 79°, bp dec, d 4.39g/cc. Sol in w and nitric acid, insol in ale. Prepd by action of hot nitric acid on ale. Highly toxic and a dangerous fire risk in contact with organic materials, It has been used for the nitration of aromatic organic compds and in the prepn of MF... 

The acceleration by anions under both conditions was attributed to displacement of one of the water molecules presumed to be tetrahedrally coordinated with the mercuric ion, the subsequent reaction being then envisaged as displacement of the anion or water molecule by the aromatic the anions which cause reaction to take place more slowly were presumed to be more tightly bound to the mercuric ion than water. It has, however, been pointed out that less tightly bound anions would be unlikely to displace the more tightly bound water molecules in the first place438. 

The rates of mercuration of a number of aromatic hydrocarbons have been determined using mercuric acetate in acetic acid, in some cases using perchloric acid as catalyst. Rate coefficients for benzene and alkylbenzene have been determined under both conditions at a range of temperatures and the data are gathered in Tables 117 and 118441 -4 8 in the absence of perchloric acid, mercuric acetate reacted with the solvent at a rate that was not insignificant compared with the rate of aromatic mercuration, and a correction was made for this side reaction. 

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. 

Iodination of moderately reactive aromatics can be effected by mixtures of iodine and silver or mercuric salts.31 Hypoiodites are presumably the active iodinating species. Bis-(pyridi nc)iodonium salts can iodinate benzene and activated derivatives in the presence of strong acids such as HBF4 or CF3S03H.32... 

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,... 

Organomercuric compounds require nucleophilic assistance by soft iodide anions (cf. Section 9.6.3.2.2) for participation in cross-coupling reactions.158 The reaction is useful for aryl-aryl cross-coupling in those cases in which the organomercury species are obtained by direct mercuration of aromatic compounds (38).159... 

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