Mercuration, of aromatic

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 ... 

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. 

Perchloric acid increased the electrophilicity of Pd(II). A strong parallel between palladation and mercuration of aromatic hydrocarbons was drawn.568 (The effect of strong acids in increasing the electrophilicity of metal acetates has been discussed earlier.) Arylmercury complexes are relatively stable and do not afford biaryl readily. Analogous arylthallium(III) complexes only afford biaryls on photolysis in the presence of arenes via the following sequence ... 

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. 

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... 

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. 

Recently an excellent study has appeared on the mechanism of the mercuration of aromatic hydrocarbons (132a). These processes are strongly catalyzed by the addition of inert salts such as NaC104. Correlations of observed rate with physical parameters of the solution such as Hammett acidity functions fail, but the rate is inversely proportional to the activity of water in the environment, suggesting that it is the function of the perchlorate anion to aid in removal of solvating water from mercuric ion, increasing its activity in attack on the aromatic nucleus. 

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... 

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. 

---