Phenyl acetate phenol acylation

An additional example is the observed moderate acceleration in the cleavage of particular phenyl esters in the presence of a cyclodextrin. In such cases, the bound ester is attacked by an hydroxyl group on the cyclodextrin to yield a new ester. There was found to be a significant enhancement of phenol release from meta-substituted phenyl acetate on interaction with cyclodextrin (relative to other esters which do not fit the cavity so well) (Van Etten, Clowes, Sebastian Bender, 1967). During the reaction, the acyl moiety transfers to an hydroxyl group on the... 

Fries rearrangement—that is, the transformation of phenolic esters to isomeric hydroxyphenyl ketones—is related to Friedel-Crafts acylations.392,393 Olah et al.394 have found a convenient way to perform the Fries rearrangement of a variety of substituted phenolic esters in the presence of Nafion-H in nitrobenzene as solvent [Eq. (5.153)]. A catalytic amount of Nafion-H is satisfactory, and the catalyst can be recycled. In contrast, Nafion-silica nanocomposites, in general, exhibit low activities in the Fries rearrangement of phenyl acetate to yield isomeric hydroxyacetophe-nones.239,395 In a recent study, BF3-H20 was found to be highly efficient under mild conditions (80°C, 1 h) to transform phenolic esters of aliphatic and aromatic carboxylic acids to ketones (71-99% yields).396 In most cases the para-hydroxyphenyl isomers are formed with high (up to 94%) selectivity. 

Figure 3.6 Yields, X (%) of phenyl acetate, PA ( ), o-hydroxyacetophenone, o-HAP (o) and p-hydroxyacetophenone, p-HAP ( ) as a function of the conversion of phenol, Xp (%), in an equimolar mixture with acetic acid over HMFI at 553 K. Reprinted from Journal of Molecular Catalysis, Vol. 93, Neves et al., Acylation of phenol with acetic acid over a HZSM-5 zeolite, reaction scheme, pp. 169-179, Copyright (1994), with permission from Elsevier...

The exact mechanism has still not been completely worked out. " Opinions have been expressed that it is completely intermolecular, ° completely intramolecular, and partially inter- and intramolecular. One way to decide between inter- and intramolecular processes is to run the reaction of the phenolic ester in the presence of another aromatic compound, say, toluene. If some of the toluene is acylated, the reaction must be, at least in part, intermolecular. If the toluene is not acylated, the presumption is that the reaction is intramolecular, though this is not certain, for it may be that the toluene is not attacked because it is less active than the other. A number of such experiments (called crossover experiments) have been carried out sometimes crossover products have been found and sometimes not. As in 11-17, an initial complex (68) is formed between the substrate and the catalyst, so that a catalyst/substrate molar ratio of at least 1 1 is required. In the presence of aluminum chloride, the Fries rearrangement can be induced with micro-wave irradiationSimply heating phenyl acetate with microwave irradiation gives the Fries rearrangement. " The Fries rearrangement has been carried out in ionic melts. 

The ratio of (II) to (I) was 3.36 at 204° at 93°, (II) was the only product. In the reaction of phenyl acetate over Aids (140), the para isomer is favored at lower temperatures (25°), and the ortho isomer, at higher ones (165°). Catalyst aging, largely from degradation of acyl residues to coke and water, was rapid. CaX was a less active catalyst for this reaction. The major product in all runs was phenol, which presumably arose from hydrolysis of the starting acetate. 

A kinetic study of the acylation of phenol with phenyl acetate was carried out in liquid phase at 160°C over HBEA zeolite samples, sulfolane or dodecane being used as solvents. The initial rates of hydroxyacetophenone (HAP) production were similar in both solvents. However the catalyst deactivation was faster in dodecane, most likely because of the faster formation of heavy reaction products such as bisphenol A derivatives. Moreover, sulfolane had a very positive effect on p-HAP formation and a negative one on o-HAP formation. To explain these observations as well as the influence of phenol and phenyl acetate concentrations on the rates of 0- and p-HAP formation it is proposed that sulfolane plays two independent roles in phenol acylation solvation of acylium ions intermediates and competition with phenyl acetate and phenol for adsorption on the acid sites. Donor substituents of phenyl acetate have a positive effect on the rate of anisole acylation, provided however there are no diffusion limitations in the zeolite pores. 

Under mild conditions (liquid phase, 160°C) HBEA zeolites can catalyse the acylation of phenol with phenyl acetate. High selectivity to p-hydroxyacetophenone is obtained by using sulfolane as a solvent, which can be explained by a better dissociation of phenyl acetate into acylium ions due to a solvation effect. However a competition between sulfolane and phenyl acetate for adsorption on the active acid sites is also demonstrated. A preliminary investigation of the effect of the acylating agent shows that generally, donor groups in aromatic acetates have a positive effect on the rate of acylation provided they do not block the access of the acetate to the acid sites of the zeolite pores. 

Most studies in which acid solids were used concern the synthesis of hydroxyacetophenones either by Fries rearrangement of phenyl acetate [3,5-15] or by acylation of phenol with acetic acid or acetic anhydride [11,16-21]. These reactions were conducted in the gas or liquid phase, zeolites being generally chosen as catalysts (Section 5.3.1). These shape-selective catalysts can also be used... 

Phenol acylation with acetic anhydride over MFI catalysts is also very o-HAP selective, although with this acylating agent o-HAP would result from direct C-acylation of phenol rather than secondary transformation of phenyl acetate [21]. 

Chemical Properties of Phenols.—Phenols exhibit the behavior of hydroxyl compounds when treated with the substances which react with the hydroxyl group. They react with acyl chlorides and acid anhydrides. Phenol and acetyl chloride, for example, react to form phenyl acetate and hydrogen chloride ... 

Sample Solution (a) The oxygen that has a single bond to the carbonyl carbon is the alcohol oxygen, and the carbonyl carbon is part of the acyl chloride or anhydride. The compound in part (a) is phenyl acetate, and it can be prepared from phenol and acetyl chloride, or acetic anhydride. 

The photodissociative pathway was confirmed by Meyer and Hammond, who foimd that essentially no o- or p-hydroxyacetophenone was formed in the photolysis of phenyl acetate in the gas phase. Instead, all products could be rationalized by recombination of phenoxy and methyl radicals (formed from decarbonylation of acyl radicals). Similarly, a photodissociative mechanism for the photo-Claisen reaction was supported by observation of products expected from the recombination of radicals produced by photodissociation of 3-methyl-l-phenoxybut-2-ene (113, Table 12.6). In addition to phenol, products of the reaction are the rearranged ether 114, the two y,y-dimethylallyl phenols 115 and 116, and the two rearranged allyl phenols... 

It must also be observed that the mode of p-HAP formation is probably different form that of o-HAP the ortho isomer is a primary product, while the para-one seems to be a secondaiy product. Of course, other ways for the formation of o-HAP can result from the acylation of phenol with phenyl acetate, which is a better acylating agent than acetic acid. 

Solid acid catalysts such as clays and zeolites are also utilized for phenol acylation however, these processes suffer from catalyst deactivation problems and lack C-selectivity. In the acylation of phenol with acetic anhydride, HZSM-5 zeolite shows a very high ort/io-selectivity (48% o-HAP yield, <1% p-HAP yield), although phenyl acetate is isolated in only approximately 20% yield [115]. The SAR value has a remarkable influence on the selectivity of the process when the reaction is carried out in the presence of HZSM-5(30), HZSM-5(150), and HZSM-5(280) zeolites, the o-HAP yields are 42,40, and 15%, respectively, whereas the O-acylation is noticeably increased. These results mean that C-acylation requires higher Brpnsted acidity and that lower acidity leads to phenyl acetate formation. It must be noted that the reaction performed with an amorphous aluminosilicate acid catalyst gives mostly phenyl acetate without isomer selectivity. These results suggest that the C-acylation of phenol occurs in the channels of zeolites and not on the external surface. 

The HBEA(20) deactivation in the reaction between phenol and phenyl acetate has also been studied [118]. In this case, the organic material trapped in the zeolite can be recovered following two methods (i) Soxhlet extraction of the zeolite [Ext] and (ii) extraction of coke by dissolution of the zeolite itself in a 40% solution of hydrofluoric acid [Coke]. The acylation reactions are carried out in two classical solvents, dodecane and sulfolane, and in both cases, a significant lowering of the rate of HAPs formation with time is observed. This deactivation is faster in dodecane ( 1 h) than in sulfolane ( 2h).Whatever the solvent, the two reactants are the main components of the material retained in the catalyst nevertheless, in the case of sulfolane, their contents in Ext and Coke are similar to that of the reaction mixture, whereas when the less polar solvent dodecane is employed, their contents in Ext and Coke are greater than that in the reaction mixture. In addition. 

The archetypal PFR is represented by phenyl acetate. The reaction has been extended to a wide variety of aromatic esters, where a number of structural modifications have been introduced in both the phenolic and acyl moieties. 

The rearrangement is found to be of intramolecular nature. The radical pair remains in the solvent cage and their recombination in the cage affords the acyl migration products, while hydrogen abstraction by the phenoxy radical from the solvent leads to the formation of phenol as by-product. When the reaction of phenyl acetate was carried out with deuterated phenol in methanol, the major products were ortho- and para-hydroxy acetophenones and phenol. Only trace amounts of crossover products were obtained [42]. The presence of methoxy substiment at meta- and para-positions increases the yield of ortho-Vries product [43]. For example, 39 gives 40. 

The acylation of anisole with C2 - C12 acids was carried out under the same conditions as that of toluene, except a shorter reaction time (5 h). The acylated anisole formed as the major product para/ortho = 59 1 - 96 1 and no meta isomers) together with esterification products - methyl esters of carboxylic acids and phenol. No phenyl esters formed. The selectivity to esters increases from acetic to dodecanoic acid, reaching 40% for the latter. The acylation of anisole, in contrast to that of toluene, is most efficient with C2 - C6 acids, giving a 62 - 65% yield of acylated products and only 2 - 6% of methyl esters. 

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