Aromatic compounds acetoxylation

Three oxidative reactions of benzene with Pd(OAc)2 via reactive rr-aryl-Pd complexes are known. The insertion of alkenes and elimination afford arylalk-enes. The oxidative functionalization of alkenes with aromatics is treated in Section 2.8. Two other reactions, oxidative homocoupling[324,325] and the acetoxylation[326], are treated in this section. The palladation of aromatic compounds is possible only with Pd(OAc)2. No reaction takes place with PdCl2. 

Aromatic compounds can be hydroxylated, benzoxylated and acetoxylated and the isomer distributions and substituent effects indicate that the reaction is an electrophilic substitution132. Very little kinetic work has been carried out so that the nature of the electrophile is in some doubt. 

The anode acetoxylation of aromatic compounds in solutions of acetic acid carrying alkali or tetraalkylammonium acetates takes the same route. As shown (Eberson 1967, Eberson and Jonsson 1981), the process starts with one-electron oxidation at the anode and then passes through the same stages as in oxidation with cobalt trifluoroacetate. The reaction takes place at potentials sufficient to oxidize the substrate but not sufficient to convert acetate ion into acetoxy radical. 

Side-chain acetoxylations of alkyl aromatic compounds can be performed selectively by use of internally electrogenerated cobalt(ni) acetate (Eq. (17))... 

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... 

A typical example is the nuclear acetoxylation of an aromatic compound [23] ... 

Fuchigami, Marken and coworkers also reported self-supported anodic methoxy-lation and acetoxylation of several aromatic compounds using a simple thin-layer flow cell reactor (interelectrode gap of 80 pm) (Scheme 4.41) [56]. The current efficiency (CE) of this process was 10% at best because of oxidation of methanol at flow rates lower than 0.03 ml/min. Even though CE increased at a faster flow rate (0.5 ml/min), the yield decreased sharply. The importance of selecting an appropriate choice of electrode material also was noted. 

Investigation of the use of acetyl hypofluorite in acetic acid for the regiocontrolled monofluorination of aromatic compounds starting from the corresponding mercurated derivatives has been carried out by Visser and coworkers29 (equation 19). On the basis of the observed fluorinated (7), acetoxylated (8) and methylated (9) products, a one-electron-transfer mechanism leading to an intermediate radical cation was proposed which might... 

S.C.E.) and, because diphenylacetylene is preferentially oxidized, it is likely that the aromatic compound is discharged, as for cyanation, at ca. 2 0 V. From the figures given in Table 3 it seems unlikely that localized oxidation of the triple bond is taking place even though it is the triple bond which is acetoxylated. The probable mechanism of formation of the products is indicated in Scheme 14. 

Yields of acetoxylation products vary greatly [4,9-12,114,121], depending on the oxidation potential of the product(s) formed. As a general rule, they tend to be low in nuclear acetoxylation of aromatic compounds, and fair to good in acetoxylations at activated C atoms. 

A characteristic feature of the reaction is that nuclear acetoxylation of aromatic compounds requires the presence of AcO [122], whereas acetoxylation of the other types of substrates takes place with either AcO or other anions present [40,70,123], Both nuclear and side-chain acetoxylation may be rationalized according to the general mechanism given in Eq. (42). 

In the context of the direct mechanism, several perplexing features concerning nuclear and side-chain substitution of aromatic compounds have been highlighted [124-126], and in the search for explanations attempts have been made to estimate, by thermo-chemically based calculations, free energies of activation for reactions between radical cations and nucleophiles. Long-standing puzzles in this area include the dependence of the ratio of side chain/nuclear substituted products on the nucleophile for alkylbenzenes, side-chain acetoxylation predominates in acetic acid (AcOH) unless AcO is present, when nuclear substitution becomes important. 

Isomer distribution for cyanation of aromatic compounds has been determined in a number of cases [217-220]. The selectivity of the cyanation reaction is higher than that of the anodic acetoxylation process. This high regioselectivity has been put to good use for the preparation of 2-cyanopyrroles and 2-cyanoindoles [226] ... 

A. Olefinic compounds Acetylenic compounds Aromatic compounds Carbonyl compounds F/c-Oxygen compounds Nitrogen compounds Sulfur compounds Halogen compounds Other heteroatom compounds Organometallic compounds Stereoselective and Stereospecific Electrooxidation A. Carboxylic acids Acetoxylation Methoxylation Acetamidation... 

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. 

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