Aromatics mercuration

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. 

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. 

Aromatic mercuration (Scheme 1), the oldest SEAr metalation reaction known, has been the subject of several reviews and monographs [7-11]. 

Numerous studies have revealed an electrophilic mechanism of aromatic mercuration. For example, the reactivity of a mercurating agent HgX2 strongly... 

Similarly, unactivated arenes readily react with thallium(III) trifluoroacetate in TFA to give the corresponding arylthallium trifluoroacetates, ArTl(02CCF3)2, which are stable and do not readily decompose to aryl trifluoroacetates and T1(I) 282-286 The rate of aromatic mercuration is increased by a factor of 7 X 10s in TFA relative to acetic acid as solvent.292... 

The carboxylates of indium and thallium are obtained by dissolving the oxides in acid. Acetate and trifluoroacetate salts are used extensively as reagents in organic synthesis. Certain other thallium compounds have been used also. The trifluoroacetate, T1(02CCF3)3, will directly thallate aromatic compounds to give arylthal-lium species, for example, C6H5T1(02CCF3)2 (cf. aromatic mercuration, Section 15-15) and oxidize arenes to biaryls. 

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

The kinetics and mechanisms of these aromatic mercurations are known, e.g., for Hg(OCOCF3)2 ° and Hg(OAc)2 . The most straightforward for kinetic purposes is the Hg(0C0CF3)2-CF3C02H system. Clean second-order reactions (first order in Hg salt and in aromatic) are found at all concentrations. For the Hg(02CCHj)2-CH3C02H system (with or without HCIO4.), there is departure from second-order kinetics at other than low concentrations. Another problem with reaction in AcOH is solvent mercura-tion. 

Perchloric acid strongly accelerates aromatic mercuration in acetic acid solution it converts mercuric acetate into a positively charged electrophile.24b,c,d... 

In reactions like aromatic mercuration, palladium(II) compounds will metaUate aromatic rings via an electrophilic substitution, hence electron-rich systems are the most reactive. ortfto-Palladation assisted by electron-releasing chelating groups has been used frequently."°... 

Preparation of aromatic mercuric halides from aromatic amines via diazonium salts ... 

Thallium carboxylates, particularly the acetate and trifluoroacetate, which can be obtained by dissolution of the oxide in the acid, are extensively used in organic chemistry.14 Both Tl metal and Tl1 salts such as the acetylaceton-ate also have specific uses. One example is the use of thallium(m) acetate in controlled bromination of organic substances such as anisole. The trifluoroacetate will directly thallate (cf. aromatic mercuration, Section 18-9) aromatic compounds to give aryl thallium ditrifluoroacetates, e.g., C6H5Tl(OOCCF3)2. It also acts as an oxidant, inter alia converting para-substituted phenols into p-quinones. 

This reaction has been modified for preparing aromatic mercuric halide. In addition, a similar procedure for preparing aryl halides from dry arenediazonium o-benzenedisulfonimides and quaternary ammonium halides in the presence of copper can be considered a further modification of the Gattermann reaction. ... 

This st example of C-H activation with a metal compound is the electrophilic aromatic mercuration of benzene using Hg(OAc)2 which was reported by Dimroth in 1898. This reaction is the analog of Friedel-Crafts reactions that use organic... 

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