Highly selective Friedel–Crafts acylation of 2-methoxynaphthlene catalyzed by H-BEA zeolite

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Abstract

The Friedel–Crafts acylation of 2-methoxynaphthalene with acetic and propionic anhydrides is actively catalyzed by H-BEA zeolite in batch conditions. Zeolite pre-treatment at increasing temperature leads to increasing selectivity of the less bulky 6-acyl-2-methoxy isomer. The selectivity toward the 6-isomer also increases if the catalyst/substrate ratio is increased. Catalyst activity and selectivity are correlated with the extra framework aluminum formation during thermal pretreatments of the zeolite.

Introduction

The Friedel–Crafts acylation is a key step in the production of aromatic ketones largely used as intermediates in the fine chemicals and pharmaceutical industry [1]. The conventional synthetic procedure is the Friedel–Crafts acylation in homogeneous phase with carboxylic acid derivatives using as catalyst Lewis acid anhydrous metal halides like AlCl3. The catalyst is used in more than stoichiometric amount and must be hydrolized after the reaction producing large amounts of waste products that cause serious technological and environmental problems. The use of recoverable and regenerable solid catalysts such as zeolites can overcome many of these problems [2]. Ring acylation of aromatic ethers has been successfully carried out on large pore HY, H-BEA, ZSM-12 zeolites [3], [4], [5], [6], [7], [8], [9], [10], [11] and MCM-41 type molecular sieves [12]. The heterogeneously catalyzed acylation of 2-methoxynapthalene has been the object of several studies because the product of the acylation in 6-position is of particular interest for the production of the anti-inflammatory drug Naproxen. The reaction is actively catalyzed by zeolites [4], [6] or MCM-41 molecular sieves [12], nevertheless in all cases the major product was the kinetically favored 1-isomer. Recently some patents were filed on this subject [13], [14] but no claims have been made on particular selectivities of the process.

In this paper we report the highly selective acylation of 2-methoxynaphtalene with acetic and propionic anhydride catalyzed by H-BEA zeolite.

Shortly after this manuscript was sent to the Editor a paper was published on the same subject [15] and it will be considered in the discussion.

Section snippets

Materials

Propionic anhydride, 2-methoxynaphtalene, 1,2-dichlorobenzene and nitrobenzene were supplied by Aldrich Chimica. Acetic anhydride was obtained by Carlo Erba and used after purification by standard methods.

β-Zeolite in ammonium form (NH4-BEA) was the CP814E zeolite (SiO2/Al2O3=25), surface area (SA)=680 m2/g and was kindly provided by Zeolyst International.

Preparation of catalysts

The conversion of NH4-BEA in H-BEA was carried out by calcination in air at various temperatures: 673 K (H-BEA673), 773 K (H-BEA773), 923 K

Results

In the following section we present the results of the acylation of 2-methoxynaphtalene with acetic anhydride catalyzed by H-BEA thermally treated at 773 K.

Discussion

Zeolite β was synthesized in 1967 [19] but, because its structural disorder, its framework structure was determined later by a combination of techniques [20], [21], [22] and described as a three-dimensional intersecting channel system. Two mutually perpendicular straight channels, each with a cross-section of 0.76 nm×0.64 nm, run in the a- and b-directions. A sinusoidal channel of 0.55 nm×0.55 nm runs parallel to the c-direction. Crystallografic faulting are frequently observed in this zeolite. The

Conclusions

Thermally activated H-BEA zeolite was found to actively catalyze the 2-methoxynaphtalene acylation with both acetic and propionic anhydride, giving a mixture of 1- and 6-isomers. The reaction proceeded with good conversion and selectivity toward ketones. The 6/1 isomer ratio ranged between 2.5 and 9, depending from the pretreatment temperature and from the catalyst/substrate ratio. It was in fact observed that the selectivity toward the 6-isomer increases with the increasing pretreatment

Acknowledgements

Financial support of the MURST is gratefully acknowledged. The authors thank A. Talon for the analytical determinations and Dr. Oreste Piccolo for helpful discussion and useful suggestions.

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