Arenes with aryllead tricarboxylate

The reaction of arenes with aryllead tricarboxylates performed in trifluoroacetic acid, affording biarenes, takes place via a cationic 7t-complex.162 Since azulenes form 7r-donor/acceptor complexes with various 7r-acids, the arylation of 4,6,8-trimethylazulene 108 was attempted with />-methoxyphenyllead triacetate 1 (Equation (135)).163 Only one isomer of 1-arylazulene 109 was formed although in a modest 27% isolated yield. Based on recovered unreacted azulene, the effective yield was 43%. A dimer 110, 3,3 -dianisyl-l,l -biazulene (4% yield), was suggested to result from the one-electron oxidation of the intermediate 4-methoxyphenyl cation in the 7t-complex. 

Lead tetraacetate reacts with arenes to lead either to aromatic nucleus substitution or to substitution on the benzylic position of the side chain. Substitution on the nucleus involves electrophilic attack of (AcO)3Pb " to give aryllead tricarboxylates. Subsequently, these aryl species react with acid to afford eventually the corresponding aryl esters. ... 

Aryllead(IV) tricarboxylates, ArPb(OOCR)3 [64], undergo the arylation of electron-rich arenes under very mild reaction conditions, usually at room temperature within a few hours, to produce the respective biaryls in moderate to high yields. The reaction involves the electrophilic aromatic substitution, and with mono-substituted benzenes, all three isomeric biaryls are obtained, with ortho-isomer as the major product [65-69], Scheme 30. 

The aryllead(IV) tricarboxylates can be prepared, among several possible routes, the most conveniently by the reaction of aryltri-n-butylstannanes [66], or arylboronic acids [67] with lead(IV) acetate in the presence of mercury(II) trifluoroacetate as the catalyst, or, in the case of electron-rich arenes such as polyalkoxybenzenes, by direct plumbylation with Pb(OAc)4 or its derivatives. Scheme 32. 

Beside haloacetic acids, this reaction can be strongly accelerated by aluminum chloride, trifluoroacetate, Al(OCOCF2CF3)3, or Al(OCOCF2CF2CF3)3 (8 eq. to ArPb(OAc)3). These fluorinated carboxylic salts were readily prepared by careful addition of the given acid to resublimed aluminum chloride [65]. These catalysts allow to perform the arylation with less reactive arenes such as toluene and benzene. The main side-reaction which occurs during the aryllead(IV) tricarboxylates involving reactions in trifluoroacetic acid is the protodeplumbylation. This reaction produces the parent arene by protonation of aryllead(IV) compound, and may become the major process [65,66]. 

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