Mercuration of aromatic compounds

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. 

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... 

Another electrophilic substitution reaction which has been examined for both a free ligand and its metal complexes is mercuration. The rate of mercuration of aromatic compounds can generally be given by a second order expression of the type ... 

The mercuration of aromatic compounds is of sufficiently general applicability to be classed with such processes as bromination, nitration, and sulfonation, and is particularly easily carried out in the case of phenols and amines. 

There are two main processes for direct preparation of arylmercuric salts, namely, mercuration of aromatic compounds and the so-called Nesmeyanov reaction. 

Arylmercuric salts can be obtained with extraordinary ease by mercuration of aromatic compounds with mercury salts, the reaction involving direct replacement of hydrogen by mercury ... 

Mercury(II) trifluoroacetate [13257-51-7] M 426.6, m 171-173°. It is prepared from HgO and trifluoroacetic acid and is recrystallised from trifluoroacetic anhydride/trifluoroacetic acid. It is a useful reagent for mercuration of aromatic compounds [Lau Kochi J Am Chem Soc 108 6720 1986]. Store it in a tightly stoppered bottle away from light, [cf Aylett in Comprehensive Inorganic Chemistry (Bailer et al. eds) Vol 3 pl87 1973, Beilstein 2 IV 458.] It is very TOXIC and hygroscopic. 

Mercuration of aromatic compounds comparison with thallation... 

Kinetic studies of the mercuration of aromatic compounds show that the reaction is first order both in aromatic compound and in mercuric BCCtStC I... 

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-... 

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]. 

Essentially similar methods can be employed to prepare non-homoleptic organometallic compounds of the Main Group elements, EXmR etc. Some procedures unique to particular classes of compound are important, for example the mercuration of aromatic hydrocarbons by mercuric acetate ... 

Discovered over a century ago, electrophilic mercuration is probably the oldest known C-H bond-activation reaction with a metal compound. The earliest examples of aromatic mercuration were reported by Volhard (mercuration of thiophene) [1], Pesci (mercuration of aromatic amines) [2], and Dimroth [3], who was the first to mercurate benzene and toluene, generalize the reaction, and assign the correct structures to the products originally observed by Pesci. Since the work of Dimroth electrophilic aromatic metalation reactions with compounds of other metals, for example Tl(III), Pb(IV), Sn(IV), Pt(IV), Au(III), Rh(III), and Pd(II), have been discovered [4], In this chapter, we will focus on intermolecular SEAr reactions involving main-group metal electrophiles and resulting in the formation of isolable metal aryls which find numerous important applications in synthesis [5], Well-known electrophilic cyclometalation reactions, for example cyclopalla-dation can be found in other chapters of this book and will not be reviewed here. 

Hundreds of reports have been published on electrophilic mercuration of a variety of aromatic compounds and uses of the resulting organomercurials in synthesis [5, 7-11]. Although in this chapter, we will not discuss synthetic applica-... 

Investigation of the use of acetyl hypofluorite in acetic acid for the regiocontrolled monofluorination of aromatic compounds starting from the corresponding mercurated derivatives has been carried out by Visser and coworkers29 (equation 19). On the basis of the observed fluorinated (7), acetoxylated (8) and methylated (9) products, a one-electron-transfer mechanism leading to an intermediate radical cation was proposed which might... 

The simplest reaction is the mercuration reaction of aromatic compounds, which is commonly achieved by the action of mercuric acetate in methanol, e.g.,... 

Electrophilic mercuration and thallation of aromatic compounds, alkenes and alkynes are considered in Chapter 3. 

The formation of organomercury compoimds by the use of halides and sodium amalgam, diazo compounds, diaizonium salts, decarboxylation reactions, mercuration of aromatic compoimds, and reactions between mercury halides (or acetates) and olefins or acetylenes has been discussed in Chapter 2 and is not further elaborated here. 

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]. 

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