Aromatic acyloxylation

Aromatic acyloxylation can be achieved via the decomposition of diacyl peroxides in the presence of aromatic substrates. By careful choice of reaction conditions, either acyloxylation or trifluoromethylation of the ring system can be achieved, using trifluoroacetyl peroxide generated in situ from trifluoroacetic anhydride and sodium percarbonate (Figure 3.117).492... 

Free-radical acyloxylation of aromatic substrates has been accomplished with a number of reagents including copper(II) acetate,benzoyl peroxide-iodine, silver(II) complexes, and cobalt(III) trifluoroacetate. ... 

Depending on the oxidation conditions, benzene and its substituted derivatives, and polycyclic aromatic hydrocarbons may be converted to phenols and quinones. Alkoxylation and acyloxylation are also possible. Addition reactions may afford dihydrodiols, epoxides, and endoperoxides. 

Ring Acyloxylation. Soluble and supported palladium catalysts are suitable reagents used to achieve ring acyloxylation of aromatic compounds.499 503 693 780 Reaction conditions, however, are critical since coupling and side chain substitution also occur. 

Highly selective formation of phenyl acetate was observed in the oxidation of benzene with palladium promoted by heteropoly acids.694 Lead tatraacetate, in contrast, usually produces acetoxylated aromatics in low yields due to side reac-tions. Electrochemical acetoxylation of benzene and its derivatives and alkoxylation of polycyclic aromatics789 790 are also possible. Thermal or photochemical decomposition of diacyl peroxides, when carried out in the presence of polycyclic aromatic compounds, results in ring acyloxylation.688 The less reactive... 

Several studies have been reported about Pd(II)-cataIyzed acetoxylation. Pd(OAc)2, which catalyzes aromatic coupling and nuclear acyloxylation as well (see Section 9.4.1), is an efficient catalyst of benzylic acetoxylation in the presence of alkali acetates. Stoichiometric oxidation in the absence of oxygen857,858 and a catalytic method858 are known. A twofold rate increase was observed when a mixture of Pd(OAc)2, Sn(OAc)2, and charcoal was employed in acetoxylation.859 Xylenes are converted to the a,a -diacetyloxy compounds in high yields.860... 

Another reaction of industrial interest is the nuclear acyloxylation of aromatics, opening up a new synthetic route to phenols. The reaction was first used by Mobil146) for the synthesis of naphthyl acetate. The principal problem in this reaction was the... 

Anodic acetoxylation has also been used for the synthesis of diphenyl esters 150-151) and acyloxyalkoxyaromatics 152 1S5). Aromatics with electron acceptors can also be acyloxylated in the presence of trifluoroaeetic acid (Hooker 156-158>) ... 

In the case of alkyl-substituted aromatics, the predominant reaction path very often is side-chain acyloxylation. By using Pd-coated cathodes in an undivided cell, it is possible to avoid the formation of the side chain-substituted products, because under these conditions the benzyl ester undergoes cathodic cleavage into the starting compounds 161) ... 

Nuclear and side chain substitution in aromatics or substitution of a -hydrogen in alkylamines is — in most cases — best rationalized by postulating radical cations as intermediates. For anodic nuclear substitution of aromatics, especially for acyloxylation, cyanation or bromination a ECnECb3 -mechanism is assumed 37,4 9,50,226,227). jc-oxidation of the aromatic to the radical cation 28, which reacts with a nucleophile Nu, e.g., acetate, cyanide, alkoxide, followed by a second electron transfer and deprotonation (Eq. (98) ) ... 

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