Lead tetraacetate aromatic compounds

The procedure described here serves to illustrate a new, general method for effecting the < -arylation of g-dicarbonyl compounds by means of an aryllead triacetate under very mild conditions. Although the first synthesis of an aryllead triacetate was reported relatively recently, a wide range of these compounds can now be readily prepared. The most direct route to these compounds is plumbation of an aromatic compound with lead tetraacetate, and in the procedure reported here p-methoxyphenyllead triacetate has been prepared in this way. It may also be obtained by reaction of the diarylmercury with lead tetraacetate, a longer, but more general method of synthesis of aryllead triacetates. 

This reaction is most often carried out with R = aryl, so the net result is the same as in 14-17, though the reagent is different. It is used less often than 14-17, but the scope is similar. When R = alkyl, the scope is more limited. Only certain aromatic compounds, particularly benzene rings with two or more nitro groups, and fused ring systems, can be alkylated by this procedure. 1,4-Quinones can be alkylated with diacyl peroxides or with lead tetraacetate (methylation occurs with this reagent). 

In a related reaction, primary aromatic amines have been oxidized to azo compounds by a variety of oxidizing agents, among them Mn02, lead tetraacetate, O2 and a base, barium permanganate, and sodium perborate in acetic acid, tert Butyl hydroperoxide has been used to oxidize certain primary amines to azoxy compounds. 

The oxidation of A-acyl-A-arylhydroxylamines with lead tetraacetate is very rapid even at very low temperatures. The product obtained is the corresponding aromatic nitroso compound. The most favorable reaction conditions involve propionic acid or ethanol-acetic acid as a solvent and reaction times of less than 10 sec at temperatures of —20°C or lower [87]. The use of ethanol-acetic acid is particularly recommended for several reasons. First, since the product is best isolated by steam distillation, the solvent assists in steamdistilling rapidly. The ethanol in the distillate helps minimize clogging of the condenser and also solubilizes small quantities of impurities that may be entrained. 

On the pages which follow, general methods are illustrated for the synthesis of a wide variety of classes of organic compounds including acyl isocyanates (from amides and oxalyl chloride p. 16), epoxides (from reductive coupling of aromatic aldehydes by hexamethylphosphorous triamide p. 31), a-fluoro acids (from 1-alkenes p. 37), 0-lactams (from olefins and chlorosulfonyl isocyanate p. 51), 1 y3,5-triketones (from dianions of 1,3-diketones and esters p. 57), sulfinate esters (from disulfides, alcohols, and lead tetraacetate p. 62), carboxylic acids (from carbonylation of alcohols or olefins via carbonium-ion intermediates p. 72), sulfoxides (from sulfides and sodium periodate p. 78), carbazoles... 

Nitro groups also enable free radical reactions to occur with aromatic rings. As Fieser s investigations [71a] have shown, aromatic nitro compounds can be methylated when reacted with lead tetraacetate ... 

Diaryl tellurium compounds are cleanly oxidized at 20° to diaryl tellurium diacetates by lead tetraacetate in benzene or toluene, The isolation of the diaryl tellurium diacetates, which are soluble in aromatic hydrocarbons, is facilitated by the precipitation of lead diacetate8. 

Methods of oxidation of hydrazo to azo compounds and hydroxylamino to nitroso compounds have been reviewed. Reagents which oxidize aromatic primary amines to azo compounds are also suitable for the oxidation of aromatic hydrazo compounds, since the hydrazo compounds are intermediates in the oxidation of the amines. Thus, manganese dioxide, mercury(II) oxide and lead tetraacetate are all suitable oxidants. Silver carbonate on Celite rapidly oxidizes both diarylhydrazines and acylhydrazines to the corresponding azo compounds in good yield. Another supported oxidant which can convert hy-drazobenzene into azobenzene in high yield is sodium periodate on silica gel. ... 

Several other examples of the oxidation of hydroxyamino and diamino aromatic compounds with silver oxide [368, lead dioxide [368], lead tetraacetate [441], and sodium hypochlorite [694] are documented in equations 534-536. 

Although the reaction is of no preparative value, it is of interest that aromatic nitro compounds can be op-methylated by radicals generated by the decomposition of lead tetraacetate. ... 

Thallinm(ni) °, particnlarly as its triflnoroacetate salt , has been successfully used for the synthesis of phenols. This method can be carried out in a single step and is subject to isomer orientation control" . The aromatic compound to be hydroxylated is first thallated with thallium trifluoroacetate (TTFA)" and, by treatment with lead tetraacetate followed by triphenylphosphine and then dilute NaOH, it is converted to the corresponding phenol (equation 57). Table 1 shows some examples of these transformations . ... 

Aromatic compounds with a C-H group at the benzylic position are readily oxidized by lead tetraacetate to the corresponding benzyl acetates. Benzylic acetoxyla-tion is preferably performed in acetic acid (Scheme 13.35) [58]. 

Aromatic primary amines react with lead tetraacetate to give symmetrical azo compounds in varying yields, via hydrazo intermediates." " However, in the case of 2,4,6-tri-tert-butylaniline, reaction with lead tetraacetate in benzene at 5°C led to a mixture of three products, the formation of which can be explained by a ligand coupling process. (Scheme 7.2)... 

The reaction of aromatic compounds with lead tetraacetate can be catalysed by boron trifluoride to give mixtures of dimerisation and acetoxylation products with the dimeric products being usually predominant. 

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. 

Azoo and Grimshaw2 prepared a finely divided form of lead by reaction of lead tetraacetate with aqueous sodium borohydride. This form converted benzyl chloride in benzene (reflux) into dibenzyl in 70% yield. However, a commercial lead powder ( 100 mesh to dust ) was equally effective. Both forms of lead were also effective for reduction of aromatic nitro compounds to azoxy compounds yields are generally in the range 40-70%. Only the trims isomer is formed. 

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