Zeolite-catalysed acetylation of alkenes with acetic anhydride

99

Zeolite-Catalysed Acetylation of Alkenes with Acetic Anhydride Keith Smith, *a Zhao Zhenhua,^ Lionel Delaude,^ and PhiHp K. G. Hodgson^ ^Department of Chemistry, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK ^BP Chemicals Ltd, Sunbury Research Laboratory, Chertsey Road, Sunbury-onThames, TW16 7LN, UK

Abstract Among various microporous adsorbents such as alumina, silica, clays, molecular sieves, etc., the HY zeolite was found to be best at promoting the acylation of 2,3-dimethyl-2-butene with acetic anhydride. The influence of numerous experimental p a r a m e t e r s on the course of the reaction was investigated. Variations in the silica/alumina ratio of the zeolite, or in the relative proportions of reagents and catalyst, markedly affected the yield of 3,3,4trimethyl-4-penten-2-one, whereas the reaction time and temperature were less influential. The procedure was extended to various other alkenes and it was possible to regenerate and to reuse the solid catalyst without significant loss of activity. 1.

INTRODUCTION

Friedel-Crafts acylation is one of the most important methods for the synthesis of ketones [1]. To achieve satisfactory reaction rates, "catalysts" such as aluminium chloride are usually needed in more than stoichiometric amounts because of complexation to starting materials and/or products. Work-up often involves hydrolysis, which leads to loss of the catalyst and causes problems with corrosion and disposal of potentially toxic wastes. Also, reactions are not always clean and may lead to mixtures of products. Recourse to recoverable and regenerable solid catalysts can overcome many problems of these types [2]. Therefore, the development of new heterogeneous catalytic procedures for the acylation of organic compounds has become a priority for the chemical industry. Significant advances resulting from the use of aluminosilicate solids were made during the last few years [3-6] and the first industrial application of zeolites in large scale Friedel-Crafts acylations was reported very recently [7]. However, most of the efforts devoted so far focused on the acylation of aromatic compounds. To the best of our knowledge, recourse to heterogeneous aluminosilicate catalysts for the acylation of alkenes has not yet been reported. Conventional methods for alkene acylation [8] involve the use of Br0nsted or Lewis acids such as sulfuric acid [9], boron trifluoride [10], zinc chloride [11], or

100

tin(IV) chloride [12]. In this pubhcation, we present the results obtained in the acetylation of various alkenes with acetic anhydride in the presence of zeolites. 2. EXPERIMENTAL 2.1 Materials Commercially available alkenes (Aldrich) were used as supplied. Acetic anhydride (Aldrich, 99%) was refluxed overnight over P2O5 and distilled under dry N2. Unless otherwise specified, zeolite HY refers to a sample supplied by Merck Ltd UK (product code B-157, Si02/Al203 = 29, specific surface area 700 m^/g). Hp, H-mordenite, H-ZSM5, and HY zeolites with other Si/Al ratios were gifts fi:om PQ Zeolites. All solid catalysts were calcined in air at 400 or 550°C for 2-5 h prior to use and cooled to room temperature in a desiccator over silica gel. 2.2 Acylation procedure Afi:*eshlycalcined zeolite catalyst was added to a mixture of 2,3-dimethyl-2butene (1), acetic anhydride, and chlorobenzene (internal standard). The suspension was stirred at room temperature or heated for a few hours (see Tables for details). The solid was filtered off with suction and rinsed with acetone. The filtrate was analysed by GC on a Pye Unicam Series 104 chromatographic system using a glass column packed with SE-30 stationary phase. A sample of pure 3,3,4-trimethyl-4-penten-2-one (2) was prepared and characterised according to literature indications [13] and used for calibration. Yields were determined using the internal standard method. 3. RESULTS AND DISCUSSION To start our investigations, we examined the conversion of 2,3-dimethyl-2butene (1) into 3,3,4-trimethyl-4-penten-2-one (2) as a model reaction (eq. 1). The choice of acetic anhydride as the acetylating agent was made in the light of related studies on the acylation of aryl ethers. Our work in this field had shown that acetic anhydride was the most efiective reagent for the Friedel-Crafts acylation of anisole in the presence of Hp zeolite. A lower degree of conversion was achieved with acetyl chloride, while hardly any reaction occurred with ethyl acetate or acetic acid [6].

H (1)

COCH3

ACoO

•

V

C

+

AcOH

(1)

(2)

The ability of numerous microporous adsorbents to catalyse the acylation of (1) was scrutinised. Amorphous materials such as alumina, silica-alumina, or zinc oxide afforded only traces of the product (2) or were totally devoid of catalytic activity. KIO montmorillonite clay and two types of aluminophosphate or silicoaluminophosphate molecular sieves were equally inefficient in promoting acylation, whereas acidic forms of zeolites were much better catalysts. As can be seenfi:-omTable 1, proton-exchanged aluminosilicates with the ZSM-5, p, or

101

faujasite Y structures led to significant amounts of the desired ketone within 2 h at 60°C. Only HX zeolite, which lacks strongly acidic sites, and H-mordenite, which has monodimensional pores, gave very low yields of (2). Table 1 Comparison of activity between various proton-exchanged zeolite catalysts Catalyst

Si02/Al203

GCYieldof2(%)

2.4 HX zeolite H-mordenite 35 H-ZSM5 80 Hp zeolite 25 HY zeolite 12 HY zeolite 40 All reactions were carried out using a l/Ac20/zeolite ratio mmol/0.05 g at 60°C for 2 h.

0.1 0.9 9 22 41 49 of 1 mmol/1.2

Since the most encouraging results were obtained with catalysts possessing the faujasite Y structure, various other ion-exchanged forms of this molecular sieve were prepared and their catalytic activity assessed. Replacement of the proton counter-ions with either sodium, magnesium, aluminium, iron(III), copper(II), lanthanum(III), or mixed rare earths reduced the yields of (2) to trace amounts. Conversely, impregnation of HY zeolite with ZnCl2 or FeCls led to highly active catalysts. Preliminary experiments with these composite materials revealed, however, t h a t they did not withstand h e a t t r e a t m e n t , t h u s compromising their chances of recycling and reuse. Therefore, research in this direction was abandoned and unmodified HY zeolite was used as a catalyst for all our subsequent studies. To complement the data in Table 1, we investigated further the influence of the Si02/Al203 ratio of zeolite HY on the outcome of the reaction (Fig. 1). A sharp increase in the yield of (2) was first observed when the Si02/Al203 ratio increased from 5.4 to 10.5. This threshold effect probably indicates that a specific high acidic strength must be reached in order for the catalyst to play its role. An optimum efficiency was attained with the sample having a Si02/Al203 ratio of 29, then the conversion rate slowly decreased as the number of acidic sites per unit cell of the crystalline aluminosilicate dropped. In order to investigate the influence of the amount of catalyst on the acylation rate, the proportion of zeolite HY (Si02/Al203 = 29) was varied between 0.02 and 0.4 g per mmol of (1) and the model reaction, carried out at 25°C, was monitored by GC. The results after 2 h are plotted in Fig. 2. The yield of (2) steadily increased with the proportion of catalyst. In addition, analyses of the reaction mixtures at various time intervals indicated that the acylation was almost complete within one hour or less. Extending the reaction time to 24 h did not result in any significant improvement. The 5delds increased by only a few per-cent or remained unchanged after 2 h at room temperature. As an alternative to altering the amount of catalyst, we examined the influence of the alkene/acetic anhydride molar ratio on the course of the reaction

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g 60CM

•D 0

> 0 0

40-

20-

/•

0 - flC—

100 Si02/Al203 Figure 1. Effect of the Si02/Al203 ratio of zeolite HY on the 5deld of 2 (all reactions were carried out using a I/AC2O/HY ratio of 1 mmol/ 10 mmol/O.l g at 25°C for 2 h).

0

0.1

1 0.2

, 0.3

, 0.4

0.5

Amount of HY (g/mmol of 1) Figure 2. Effect of the amount of zeolite HY on the 5nLeld of 2 (all reactions were carried out using a I/AC2O molar ratio of 1/15 at 25°C for 2 h).

(Table 2). In a first set of experiments, a fixed amount of alkene (1) was acetylated using a 5-, 10-, 15-, or 21-fold excess of acetic anhydride. Taking into accoimt the experimental errors, identical 5delds of ketone (2) were obtained in the three latter cases. Thus, an optimum was reached for an AC2O/I molar ratio close to 10/1. Exceeding this limit only added to the cost of the process without any practical advantage. In a second set of reactions, the amount of anhydride was kept constant and the amount of alkene was progressively increased. This modification of the proportions of reagents also led to increased formation of the acylated product. The yield of (2) was maximum for a I/AC2O molar ratio of 8/1 and slightly decreased when larger excesses were used. The use of a ten-fold excess of alkene led to a higher 5deld than when acetic anhydride was used in the same excess under otherwise identical conditions. Excess amounts of the low boiling point alkene could also be separated from the reaction mixture by distillation and recycled more easily than acetic anhydride (b. p. 73°C and 138-140°C respectively). Therefore, recourse to an excess of alkene would be more efficient and more economic than the use of excess acetic anhydride in an industrial process. To continue our systematic study of the acylation of 2,3-dimethyl-2-butene, we examined the influence of the temperature on the reaction course (Table 3). Using various amounts of the HY catalyst, the acylation of (1) was carried out at 25 or 65°C. Surprisingly, increasing the temperature had only a minor effect on the yield of (2), which increased by just a few per-cent. This confirmed previous indications that the acylation proceeds very quickly and that the time allowed for the reaction (2 h) is sufficient to afford completion at room temperature. To confirm this h5T3othesis, we followed the time course of the model reaction at

103 Table 2 Influence of the alkene/anhydride ratio on the yield of ketone I/AC2O (mol/mol)

GCYieldof2(%)a

Conditions

A 38 1/5 A 45b 1/10 46 A 1/15 1/21 A 45 B 18 1/1 B 27 2/1 B 34 5/1 B 44 8/1 B 42 10/1 15/1 B 37 Conditions A: reactions were carried out using a 1/HY zeolite ratio of 1 mmol/0.14 g at 22°C for 4 h. Conditions B: reactions were carried out using a AC2O/HY zeolite ratio of 1 mmol/0.1 g at 25°C for 2 h. ^Yield based on the reagent in deficiency. ''The yield was 32% using conditions B. 23°C (Fig. 3). The results clearly demonstrate that conversion occured rapidly and that almost no more reaction took place after 1 h. Table 3 Influence of the temperature on the yield of ketone Amount of HY Zeohte (g/mmol of 1)

GC Yield of 2 (%) Reaction at 25°C

0.2 0.3 0.4 All reactions were carried out using a

Reaction at 65°C

55 56 59 62 64 61 I/AC2O molar ratio of 1/10 for 2 h.

Next, we tried to introduce a solvent in our system, instead of using only neat liquid reagents. Three experiments were carried out in ethyl acetate, dichloromethane, and chloroform respectively (Table 4). Dilution of the reaction mixtures in these polar organic solvents did not have any beneficial influence on the acylation, as the yields of (2) were consistently lower than that obtained in the absence of any solvent. Ethyl acetate had the most negative effect, probably because this Lewis base competes with acetic anhydride for coordination to the acidic sites of the zeolite catalyst. A series of reactions was also performed using different grades of acetic anhydride, viz., (i) not purified before use, (ii) refluxed over P2O5 and distilled

104

Table 4 Influence of solvents on the yield of ketone Solvent

GCYieldof2(%)

AcOEt 28 CH2CI2 41 CHCI3 50 none 58 All reactions were carried out using 1 (1 mmol), AC2O (15 mmol), HY zeolite (0.2 g) in 1.718 g of solvent at 25°C for 5 h.

0

2 4 6 Reaction Time (h)

24

Figure 3. Time course of the acetylation of 1 (the reaction was carried out using a I/AC2O/ HY ratio of 1 mmol/15 mmol/0.138 g at 23°C).

under N2 prior to use, (iii) contaminated with small known amounts of acetic acid (5 or 10 molar %). The results were unambiguous: while satisfactory yields are obtained if acetic anhydride is purified before use, only traces of the ketone (2) were obtained in the presence of even relatively small amounts of acetic acid. This impurity is inevitably foimd in acetic anhydride left in contact with humidity. Therefore, it is essential to remove the acid accompan5dng the anhydride as completely as possible before starting the acylation of (1) in the presence of zeolite HY as the catalyst. The generation of acetic acid during the reaction also explains why the catalyst is deactivated before conversion is complete. As an alternative to prior removal of acetic acid, we performed this operation in situ by adding phosphorus pentoxide to our reaction mixtures. The results showed that the association of P2O5 and unpurified acetic anhydride led to inferior 5delds compared against purified anhydride alone. Yet, adding the drying agent to already purified anhydride boosted the yield of ketone (2), but made the work-up more cimabersome. Having established the influence of the various experimental parameters on the acylation of 2,3-dimethyl-2-butene, we extended the procedure to a few other alkenes (Table 5). Unsubstituted cyclohexene gave a mixture of 1-acetyl-lcyclohexene and 3-acetyl-l-cyclohexene in almost equimolar amounts. The best overall yield was obtained by reacting a five-fold excess of acetic anhydride and 0.1 g of zeolite HY per mmol of alkene for 1 h at 25°C. Increasing the reaction time or the proportions of acylating agent and catalyst had a detrimental effect on the yields of the a,p- and p,Y-unsaturated ketones. Acid-catalysed side reactions and degradations probably account for these observations. The acylation of the trisubstituted double bond of 1-methylcyclohexene was easier to carry out and afforded a 60% yield of 6-acetyl-1-methylcyclohexene within 3 h. Similarly, ethylidenecyclohexane led to a satisfactory 66% 5deld of 3-(lcyclohexenyl)-2-butanone after 1 h. In the case of 2,4,4-trimethyl-1-pentene, three isomeric ketones were obtained, viz., 4-(2,2-dimethylpropyl)-4-penten-2-one and (E)- or (Z)-4,6,6-trimethyl-4-hepten-2-one in an overall 72% yield. The first isomer, with the terminal double bond, was the major product of the reaction, but

105

the GC conditions adopted for analysis did not allow us to fully separate the different compounds and to obtain quantitative determinations. Table 5 Acetylation of various alkenes Substrate

o o O-' XA

I/AC2O/HY (mmol/mmol/g) 1/5/0.1

Time GC Yield (h) (%) 1

23

Product(s) (See Text for Isomer Distributions) {

^—COCH3 /

1/10/0.2

3

V-COCH3

60 COCH3

1/10/0.25

1

66

0^

1/10/0.25

2

72

>O^C0CH3

^

^

COCH3 .COCH3

COCH3 All reactions were carried out at 25°C. To conclude this study, we examined the possibility of recycling and reusing the HY catalyst. Initially, the spent solid was simply recovered by filtration, washed with acetone, dried at 110°C, and reused. Under these conditions, the recycled aluminosilicate exhibited only poor catalytic activity, and the conversion of the alkene to ketone was limited to a few per-cent. When an additional calcination step was performed before reuse, on the other hand, the recycled material was almost as active as a fresh sample of zeolite HY. For instance, the yield of (2) dropped only firom 45 to 39% when catalyst regenerated by calcination in air at 400°C was used instead of the original fresh molecular sieve. Furthermore, it was possible to reemploy the same catalyst in a third, and even a fourth run, without any further decrease in yield, provided t h a t the solid was regenerated by calcination between each successive reaction.

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4.

CONCLUSION

The above results clearly demonstrate that proton-exchanged Y zeolite is an efficient heterogeneous catalyst for the acylation of alkenes with acetic anhydride. The ease of separation and of regeneration of the spent solid is particularly attractive in view of possible industrial applications and contributes to the environmental friendliness of the process, together with the absence of any solvent. Nevertheless, a careful optimisation of the experimental parameters is required in order to achieve high 5delds of ketones. In the case of 2,3-dimethyl-2butene, we were able to obtain a 79% yield of 3,3,4-trimethyl-4-penten-2-one (based on acetic anhydride) by adopting the following conditions: 10/1 alkene/anhydride molar ratio, 0.2 g of HY zeolite per mmol Ac20, reaction temperature 25°C, reaction time 4 h. No alkene oligomerisation was observed. ACKNOWLEDGEMENT We wish to thank the British Government for an ORS Award to Z. Z. and PQ Zeolites for gifts of zeolite samples. Financial support from BP Chemicals and from the European Union within the Human Capital and Mobility Programme (Contract CHRX CT 940564) is gratefully acknowledged, as is the use of the EPSRC's Chemical Database Service at Daresbury [14] and the EPSRC Mass Spectrometry Service in Swansea. REFERENCES 1. G. A. Olah, Friedel-Crafbs Chemistry, Wiley-Interscience, New York, 1973. 2. K. Smith (ed). Solid Supports and Catalysts in Organic Synthesis, Ellis Horwood, Chichester, 1992. 3. H. van Bekkum, A. J. Hoefhagel, M. A. van Koten, E. A. Gunnewegh, A. H. G. Vogt and H. W. Kouwenhoven, Stud. Surf Sci. Catal., 83 (1994) 379 and references cited therein. 4. Q. L. Wang, Y. Ma, X. Ji, H. Yan and Q. Qiu, J. Chem. Soc, Chem. Commun., (1995) 2307. 5. F. Jayat, M. J. Sabater Picot and M. Guisnet, submitted. 6. K. Smith, Z. Zhenhua and P. K. G. Hodgson, manuscript in preparation. 7. M. Spagnol, L. Gilbert, R. Jacquot, H. Guillot, P. J. Tirol and A.-M. Le Govic, preprints 4th Int. Symp. on Heterogeneous Catalysis and Fine Chemicals, Basel, 1996, p. 92. 8. For reviews see C. D. Nenitzecsu and A. T. Balaban, in Friedel-Crafts and Related Reactions, Vol. Ill, G. A. Olah (ed), Wiley, New York, 1964, pp 10331052; J. K. Groves, Chem. Soc. Rev., 1 (1972) 73. 9. A. C. Byrns and T. F. Doumani, Ind. Eng. Chem., 35 (1943) 349. 10. K. Hideo, N. Yoshinori and H. Yasno, Jpn Pat. 05 163 189; Chem. Abstr. 119 (1993) 249581s. 11. P. Beak and K. R. Berger, J. Am. Chem. Soc, 102 (1980) 3848. 12. J. E. Dubois, I. Saumtally and C. Lion, Bull. Soc. Chim. Fr., II (1984) 133. 13. E. F. Kiefer and D. A. Carlson, Tetrahedron Lett., (1967) 1617. 14. D. A. Fletcher, R. F. McMeeking and D. Parkin, J. Chem. Inf. Comput. Sci., 36(1996)746.