Acetoxylation of aromatics

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. 

N03)j, a newcomer to the arena of oxidants, is useful for the acetoxylation of aromatic side chains in benzylic positions [415, 416] and for the oxidation of methylene or methyl groups that are adjacent to aromatic rings to carbonyl groups [238, 415, 417]. The reagent also oxidizes alcohols to aldehydes [418, 419, 420, 421] and phenols to quinones [422, 423], cleaves vicinal diols to ketones and a-hydroxy ketones to acids [424, 425], and converts diaryl sulfides into sulfoxides [426]. A specialty of ammonium cerium nitrate is the oxidative recovery of carbonyl compounds from their oximes and semicarbazones [422, 427] and of carboxylic acids from their hydrazides [428] under mild conditions. 

Eberson has compared the isomer ratios for the acetoxylation of aromatic substrates at the anode with those obtained from different oxidants (Table 1) [26]. The anodic reaction proceeds by an initial electron transfer, thus forming a radical cation, which is then attacked by acetate ion. The homogeneous reactions, which come closest to the results of the anodic oxidation, are the Co(III)Wi2O40 and the Ag(II) oxidation. These and also the Ce(IV) oxidant, which still exhibits some similarities in the isomer distribution, have been characterized as electron transfer oxidants, whereas the Pd(II)-catalyzed acetoxylation has been shown to proceed via organopalladium species [27]. The isomer distributions in the latter, where predominantly meta orientation is being observed, are drastically different from those of ET-mediated reactions. 

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). 

Acetoxylation of aromatics (eq. (25)) was first carried out by Davidson and Triggs [92] to produce phenyl acetate, which could give rise to the development of a new phenol synthesis. The formation of phenyl acetate is accompanied by the formation of biphenyl (see Section 3.3.14.7.2). In the presence of oxidants such as Pb(OAc)4, NaN02, NaNOs, KMn04, K2CrOv [93] and P-Mo-V heteropolyacids [94], phenyl acetate is the main product. The favored ring acetoxylation with high-oxidation-state Pd catalyst over dimerization with low-oxidation-state catalyst has recently been confirmed [95]. With toluene, probably in an allylic-like oxidation, benzyl acetate is obtained [92]. 

Biphenyls are also by-products of acetoxylation of aromatics [92]. Their formation is favored with a palladium metal catalyst in the absence of oxidants [93-95]. Vinyl acetate undergoes oxidative coupling under similar conditions to form 1,4-diacetoxy-1,3-butadiene [99], and aromatics and heterocycles can substitute an olefinic H-atom [100] according to eq. (28) (with X = H, CN, AcO, EtO) [100-102]. 

Similar to mercuration, Pd(OAc)2 undergoes facile palladation of aromatic com-povmds. The palladation product 162 is an unstable intermediate. It can be isolated only when stabilized by chelation. The palladation products of aromatics as reactive intermediates undergo three reactions. The reaction with alkenes to afford styrene derivatives 164 is the first one. Pd(II)-promoted alkenylation of aromatic compovmds, discovered by Fujiwara, is a stoichiometric Heck reaction. The second one is homocoupling to form biaryls. The acetoxylation of aromatic rings is the third reaction. These latter two reactions are treated in Chapter 2.7. 

FT mechanisms coupled with addition of nucleophiles operate, for example, in acetoxylation of aromatic compounds with the well-known FT agent, K S Og, in acetic acid [36] or aerobic Cu-catalyzed orf/io-acetoxylation of aryl C—H bonds [39]. 

The addition of sulphuric acid increased the rate of nitration of benzene, and under the influence of this additive the rate became proportional to the first powers of the concentrations of aromatic, acetyl nitrate and sulphuric acid. Sulphuric acid markedly catalysed the zeroth-order nitration and acetoxylation of o-xylene without affecting the kinetic form of the reaction. ... 

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. 

In the oxidation of aromatic substances at the anode, radical cations or dications are formed as intermediates and subsequently react with the solvent or with anions of the base electrolyte. For example, depending on the conditions, 1,4-dimethoxybenzene is cyanized after the substitution of one methoxy group, methoxylated after addition of two methoxy groups or acetoxylated after substitution of one hydrogen on the aromatic ring, as shown in Fig. 5.55, where the solvent is indicated over the arrow and the base electrolyte and electrode under the arrow for each reaction HAc denotes acetic acid. 

BASF has developed a direct electrochemical process based on anodic acetoxylation for the production of aromatic aldehydes on industrial scale [40,146,147]. The reaction passes smoothly through the benzyl acetate stage. 

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] ... 

Acetoxylation of Alkenes, Dienes and Aromatic Hydrocarbons 61.3.4.3.1 Vinyl acetate from ethytene... 

Nuclear597 or side-chain588,598 acetoxylation of arenes can be performed with good yields by persulfate and copper(II) salts in acetic acid (equations 268 and 269). As previously shown for cyclohexene (equation 263), persulfate oxidizes the aromatic ring to a radical cation which loses a proton to give a carbon radical, which is further oxidized by copper(II) acetate to the final acetoxylated product. 

The effects of added species. The rate of nitration of benzene, according to a rate law kinetically of the first order in the concentration of aromatic, was reduced by sodium nitrate, a concentration of io 3 mol l-1 of the latter retarding nitration by a factor of about 4.llc>28 Lithium nitrate anticatalysed the nitration and acetoxylation of o-xylene in solutions of acetyl nitrate in acetic anhydride. The presence of 6 x io-4 mol 1 1 of nitrate reduced the rate by a factor of 4, and modified the kinetic form of the nitration from a zeroth-order dependence on the concentration of aromatic towards a first-order dependence. However, the ratio of acetoxylation to nitration remained constant.146 Small concentrations of sodium nitrate similarly depressed the rate of nitration of anisole and again modified the reaction away from zeroth to first-order dependence on the concentration of the aromatic.116... 

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