Acylation anisole

Friedel-Crafts acylation is widely used for the production of aromatic ketones applied as intermediates in both fine chemicals and pharmaceutical industries. The reaction is carried out by using conventional homogenous catalysts, which represents significant technical and environmental problems. The present work reports the results obtained in the Friedel-Crafts acylation of aromatic substrates (anisole and 2-methoxynaphthalene) catalyzed by Beta zeolite obtained by crystallization of silanized seeds. This material exhibits hierarchical porosity and enhanced textural properties. For the anisole acylation, the catalytic activity over the conventional Beta zeolite is slightly higher than with the modified Beta material, probably due to the relatively small size of this substrate and the weaker acidity of the last sample. However, the opposite occurred in the acylation of a bulky substrate (2-methoxynaphthalene), with the modified Beta showing a higher conversion. This result is interpreted due to the presence of a hierarchical porosity in this material, which favors the accessibility to the active sites. 

Both materials were tested as catalysts in the anisole acylation (Scheme 1). The conventional Beta sample showed a slightly higher activity than the Beta (PHAPTMS). At 3 hours, the conversions were 26.8 and 22.8 % for the conventional and seed silanized catalysts, respectively. This behavior is explained as a consequence of the relatively small size of the anisole molecule, which allows this compound to diffuse without significant hindrances through the zeolitic micropores, and of the slightly weaker acidity of the Beta (PHAPTMS) sample. In both cases, p-methoxyacetophenone (p-MAP) was the main reaction product, being obtained with a high selectivity (> 97%). 

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. 

The rate of anisole acylation depended on the acetate (Table 2). Initially it was about 1.5 times greater with p-tolyl acetate and with 2-methoxyphenyl acetate than with phenyl acetate, slightly lower with 2-methoxyhydroquinone diacetate, 2.5 times lower with the hydroquinone diacetate and very low with 2,4,6-trimethylphenyl acetate. The low reactivity of this latter acetate can be related to limitations in the rate of diffusion of this bulky compound in the BEA zeolite pores. Furthermore, a greater reactivity of this acetate was found with HFAU zeolites whose pore size is greater. Curiously, with hydroquinone diacetate (but not with the 2-methoxyhydroquinone acetate), there was a quasi immediate deactivation. We are carrying out additional experiments so as to understand how the reactivity of aromatic acetates changes with their nature and the zeolite acidity and porosity. 

Table 4.12 Anisole acylation with different carboxylic acids catalyzed by zeolite ZSM-5...

Table 4.19 Anisole acylation with anhydrides in the presence of SZ...

Polymer-immobilized aluminium or copper tungstophosphates and tungstosilicates as new catalysts in anisole acylation... 

Acylation is the most important method for the synthesis of aryl ketones that, in turn, are used for the preparation of many fine chemicals. In order to test the performance of the prepared catalysts, previous to their use in 2-methoxynaphtalene acylation, anisole acylation with acetic anhydride in a batch reactor is carried out, using the immobilized aluminium or copper salts of tungstophosphoric and tungstosilicic acids as catalysts. 

The catalytic activity of AITPApva-peg, CuTPApva-peg, AITSApva-peg, and CuTSApvA-PEG catalysts in anisole acylation is displayed in Table 1. Although o-methoxyacetophenone was detected, the main product of the reaction was p-methoxyacetophenone (/7-MAP), with selectivity higher than 90%. Only monoacylated products were detected. 

The acylation of anisole with HZSM-5 zeolite (Si/Al = 30) as a catalyst proceeds differently.With C2 - C3 acids, at 120°C, PhOMe/acid = 4 and 20% HZSM-5, the phenyl esters are the main products no methyl esters have been found. At 150"C and otherwise the same conditions, a 2 1 - 5 1 mixture of acylated anisole and phenyl ester forms at an 87 - 100% acid conversion. The conversion drops sharply for the acids higher than C3, down to 0.6% for Cl2, probably because of restricted access into zeolite pores. Thus Cs2 5H0 5[PWi204o] is a more active as well as more selective catalyst than HZSM-5 for the anisole acylation. 

Strong inhibition of HPA-catalysed process with reaction products takes place both in homogeneous and heterogeneous systems like in anisole acylation. Addition of more HPA catalyst allows reaching a higher PhOAc conversion. Some irreversible catalyst deactivation is also observed. ... 

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