Tricarboxylates, aryllead

Aromatic compounds can also be arylated by aryllead tricarboxylates. Best yields ( 70-85%) are obtained when the substrate contains alkyl groups an electrophilic mechanism is likely. Phenols are phenylated ortho to the OH group (and enols are a phenylated) by triphenylbismuth dichloride or by certain other Bi(V) reagents. O-Phenylation is a possible side reaction. As with the aryllead tricarboxylate reactions, a free-radical mechanism is unlikely. ... 

Aromatic compounds can also be arylated by aryllead tricarboxylates.33 Best yields (—70 to 85%) are obtained when the substrate contains alkyl groups an electrophilic mechanism... 

C-Arylation. These aryllead tricarboxylates are useful for arylation of ji-illkrloncs and /l-keto esters. Yields are high with substrates containing only one acidic hydrogen. 

In recent years, there has been much interest in the use of aryllead compounds in organic synthesis, because of the ability of compounds of the type ArPb(02CR)3 to smoothly arylate a variety of nucleophiles [34—37]. Aryllead tricarboxylates can be prepared by direct plumbylation of aromatic compounds (Scheme 5), a reaction that has been known for a long time [38],... 

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. 

T -Keto esters and aryllead tricarboxylates react in chloroform in the presence of pyridine to give a-arylated products (Scheme 13.3) [32]. A variety of substrates can be used in this arylation reaction an exception is keto esters with two a-hydrogen atoms. 

Aryllead tricarboxylates have been shown to react in trifluoroacetic acid with a range of aromatic compounds to give biaryls [31c]. With aromatic compounds that are more electron-rich than toluene the yields are generally good and the method is a useful synthesis of unsymmetrical biaryls (Scheme 13.18). It has been reported that a cationic intermediate generated from aryllead tricarboxylates was probably involved. 

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. ... 

Direct electrophilic plumbation of aromatic compounds can be used to prepare a small range of aryllead tricarboxylates.limited to substrates which are more electron rich than halobenzenes. Plumbation of halobenzenes can be conveniently performed by treatment with lead tetraacetate in the presence of trichloroacetic or trifluoroacetic acid. For the plumbation of compounds of intermediate reactivity between the halobenzenes and 1,3-dimethoxybenzene, the use of haloacetic acid (mono-, di- or trichloroacetic acid) is required to optimize the yield of aryllead triacetates. For 1,3-dimethoxybenzene and the more electron-rich aromatics, the reaction can be suitably performed in acetic acid. 

In a series of outstanding papers, Pinhey et al have shown that aryllead tricarboxylates react with soft nucleophiles to afford C-arylation products. These aryllead derivatives behave as aryl cation equivalents in reactions which involve a ligand coupling mechanism (see section 7.5).9 2 ju most cases, the reactions proceed in chloroform at 40-60 C in the presence of pyridine as a base with a ratio of substrate to organolead derivative to pyridine of 1 1 3. The substrates which easily undergo C-arylation include phenols, p-dicarbonyl compounds and their vinylogues, a-cyanoesters, a-hetero-substituted ketones, enamines and nitroalkanes. A very limited number of non-carbon nucleophiles has also been reported to react. 

In parallel to their work on the arylation reactions with aryllead tricarboxylates, Pinhey et al have examined, although in much less detail, the reactivity of vinyllead tricarboxylates as vinylation reagents for soft nucleophiles. 9,11... 

Pol merization reactions in [MeSi(OEt)3] have been studied using Si NMR spectroscopy. y-Glycidoxypropyltrimethoxysilane in aqueous solutions has been characterized using Si NMR spectroscopy. " Pb NMR chemical shifts of aryllead tricarboxylates have been found to depend on both electronic and steric... 

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