Design of synthetic zeolites as catalysts in organic reactions

Applied Catalysis, 49 (1989) 109-123 Elsevier Science Publishers B.V., Amsterdam -

109 Printed in The Netherlands

Design of Synthetic Zeolites as Catalysts in Organic Reactions Acylation of Anisole by Acyl Chlorides or Carboxylic Acids Over Acid Zeolites AVELINO CORMA* Znstituto de Cat&Isis y Petroleoquimica, CSZC, Serrano 119,28006 Madrid (Spain) and MARIA JOSE CLIMENT, HERMENEGILDO

GARCIA and JAIMO PRIM0

Departamento de Quimica, ETSZZ, Universidad Politdcnica, Apartado 22012,46071 Valencia (Spain) (Received 10 August 1988, revised manuscript received 28 November 1988)

ABSTRACT Acylation of anisole with phenylacetyl and phenylpropanoyl chlorides and phenylacetic and 3phenylpropionic acids was carried out in a batch reactor at 50-l 15’ C over a series of zeolites Y at 21,50 and 100% Na+ exchange, with a framework Si-to-Al ratio from 9 to 24 and on HZSM-5 and H-P zeolites. The products observed were the corresponding acids and the para-substituted aryl ketones. Only trace amounts of the ortho isomers could be detected, and phenol and cresols were absent from the reaction mixtures. When phenylpropanoyl chloride and phenylpropionic acid were used as the acylating agent, I-indanone was also formed. From the relative adsorption constants of phenylacetyl chloride and anisole on HY-100 zeolite, and the absence of any anisole demethylation and rearrangement products, it was concluded that the interaction of the acid sites with the anisole oxygen must not be very strong. On the other hand, the influence of the level of Nat exchange on the initial rate of formation of l- (4-methoxyphenyl)-2-phenyl-phenylethanone shows that the activity of the catalyst is independent of its acid strength distribution. This was confirmed by studying the influence of the framework Si-to-Al ratio on the turnover numbers (TON), The relatively small TON differences could be related to the hydrophobicity of the samples. By using zeolites with different pore size and configuration it was observed that while Y and p zeolites show similar behaviour, the medium-pore ZSM-5 fails to catalyse the acylation of pcresol with phenylacetic acid for steric reasons. Finally, it was observed that zeolites give a much higher ratio of inter- to intramolecular acylation than a typical Lewis acid such as AlCl,.

INTRODUCTION

Two general problems are associated with the classical del-Crafts acylations of arenes: the requirement for more of an acid which will be completely destroyed during the and the low selectivity when the formation of para/ortho 0166-9834/89/$03.50

0 1989 Elsevier Science Publishers B.V.

catalysis of the Friethan molar amounts separation work-up, isomers is possible.

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In principle, the above limitations could be easily overcome by means of nonconventional heterogeneous catalysis with solid acids that do not form stable complexes with the resulting aryl ketones. Synthetic zeolites provide the opportunity of being tailored to suit the desired reaction and therefore are among the most promising heterogeneous catalysts [l-4]. Although in their acid form they have been widely used as catalysts for aromatic hydrocarbon alkylations [5,6] and isomerization of alkylarenes [ 71, much less attention has been given to the possibility of employing these materials as suitable catalysts for the analogous aromatic acylations [4,8,9]. One of the main advantages of zeolites over other types of superacids is the possibility of preparing them in a variety of samples with different acid strength distributions, framework Si-to-Al ratios and crystalline structures. These factors would be especially valuable when the acylation competes with other side-reactions, as with alkoxybenzenes, which may undergo dealkylations and rearrangements with the usual Friedel-Crafts catalysts [ 10,111. In this paper we report the results obtained in the acylation of anisole (1) with phenylacetyl chloride (2a) in the presence of a series of acid zeolites at different levels of sodium exchange (HY-21, HY-50 and HY-loo), framework Si-to-Al ratios (HYD-1 and HYD-2) and structures (HZSM-5 and H-j?), and the reaction of anisole and p-cresol with phenylacetic (3a) and 3-phenylpropanoic (3b) acids and 3-phenylpropanoyl chloride (2b) in carbon tetrachloride or toluene. EXPERIMENTAL

Materials The partially sodium-exchanged HY zeolites (HY-21, HY-50 and HY-100, where the number indicates the percentage of sodium exchange) were prepared starting from a commercial NaY sample (SK-40, Union Carbide) in the following way. HY-21 was obtained treating NaY with a 0.25 M solution of ammonium acetate, using a solid-liquid weight ratio of 1:4 at 25’ C for 30 min, followed by drying at 110°C for 6 h and ulterior calcination at 550” C for 3 h. HY-50 was obtained from HY-21 by further exchange of Na+ ions by NH4+ with 0.4 M ammonium acetate solution in the above weight ratio and subsequent drying and calcination. HY-100 zeolite was obtained from HY-50 following the same procedure, but using 0.60 M ammonium acetate solution and a final deep-bed calcination at 550°C for 3 h. The dealuminated zeolites (HYDl - 2) were prepared by treating the commercial NaY samples with SiCl, at 400 at 450’ C for 3 h. Their unit cell sizes were determined following a procedure described previously [ 121. After wash-

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ing thoroughly with water, sodium ion exchange was carried as for the HY-100 sample. The TEA-P zeolite was obtained following a published procedure [ 131. Tetraethylammonium was eliminated by calcination in air at 55O”C, then the solid was exchanged twice with ammonium acetate and calcined at 550’ C. The initial TEA-P sample had an Si-to-Al ratio of 10. After the calcinations the zeolite was partially dealuminated and the resulting framework Si-to-Al ratio was 14, as measured by magic angle spinning (MAS ) spectroscopy. The sample showed regular crystal sizes of 0.30 pm, determined by scanning electron microscopy. HZSM-5 was synthesized as reported [ 141. After calcination to remove the organic materials, it was exchanged twice with ammonium acetate, followed by calcination at 550’ C. The average crystal size was 2 pm. The acid strength distribution was established by the amount of the pyridine retained by the samples after evacuation at increasing temperatures [ 151, and measured from the intensity of the IR bands on a Perkin-Elmer 580 B spectrophotometer equipped with a data station. Gaschromatographic (GC) analyses were performed on a Hewlett-Packard 5890 apparatus with a 25-m capillary column of cross-linked 5% phenyl methyl silicone. A Hewlett-Packard 5965 A FT-IR detector was coupled to the GC system; absorption in wavenumbers (cm-’ ) are given only for the carbonyl bands. ‘H NMR spectra were recorded in CDC& solutions with a Varian 360 EM 60-MHz spectrometer using tetramethylsilane as internal standard; chemical shifts are reported as ppm downfield (S) . Mass spectra were measured on a Hewlett-Packard 5988 A mass spectrometer; m/z values and relative abundances (in parentheses) are given only for the significant peaks. Chemicals

The solvents employed were carbon tetrachloride and toluene (both of analytical-reagent grade) with specified water contents of 0.02 and 0.05%, respectively and, except where indicated, were used without further purification. Carbon tetrachloride was dried by means of molecular sieves. Phenylacetyl chloride (2a) was prepared treating phenylacetic (1.4 g, 10 mmol) with thionyl chloride (1.43 g, 12 mmol) in Ccl, (50 ml) at reflux temperature. The progress of the reaction was followed periodically by ‘H NMR spectroscopy, comparing the intensity of the singlets corresponding to the methylenes of the phenylacetyl chloride (6~4.41 ppm) and the starting carboxylic acid (6=3.58 ppm). The reaction was allowed to continue until the yield of phenylacetyl chloride was more than 90%. The mixture was then concentrated in vacua and, in order to eliminate the excess of thionyl chloride, a new aliquot of carbon tetrachloride (100 ml) was added and again eliminated.

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The residue was submitted to distillation under reduced pressure, which yielded the expected phenylacetyl chloride (2a) (90 mg, 58% ). 3-Phenylpropanoyl chloride (2b) was obtained following the same procedure for phenylacetyl chloride. The yield was 72%. Reaction procedures Zeolite-catalysed acylations Activation of the catalysts was performed just before their use by heating the solid (1 g) at 150°C under vacuum of 1 Torr for 3 h. The system was cooled to room temperature and (i) a previously heated solution of nitrobenzene ( 120 mg) as internal standard, (ii) the acyl chloride or the carboxylic acid in the appropriate solvent (50 ml) were poured onto the activated catalyst. The resulting suspension was stirred magnetically at the required temperature. The course of the reaction was followed by GC at several times ranging from 0.1 to 17 h. At the end of the reaction the catalyst was filtered and washed with dichloromethane (50 ml) and the organic solutions were concentrated under vacuum and analyzed by GC-MS and lH NMR. The inorganic solid was submitted to continuous Soxhlet extraction with dichloromethane until the total material recovered was more than 90% of the theorical weight. After evaporation of the solvent, the residue was analysed by GC-MS and ‘H NMR. Although the relative amount of each compound extracted from the zeolite compared with the material dissolved in the liquid phase changes in each run, the following average values were found: anisole 0.5, aryl ketone 0.8 and carboxylic acid 7.8. Conversions measured in the liquid phase were corrected taking into account the percentage of each compound adsorbed on the zeolite. AK&-catalysed acylation The alternative reaction of 3-phenylpropanoyl chloride (170 mg, 1 mmol) in toluene (50 ml) with aluminium chloride (160 mg, 12 mmol) was accomplished stirring the suspension at 50’ C for 1 h. The organic layer was washed with concentrated hydrochloric acid, dried over anhydrous sodium sulphate and the solvent removed in vacua. ‘H NMR and GC-MS analysis of the reaction mixture showed the formation of l- (2-methylphenyl) -3-phenyl-l-propanone (3%), 1-(4-methylphenyl)-3-phenyl-1-propanone (5b) (8%) and l-indanone (6) (78%). Spectral data of the products 1-(4Methoxyphenyl)-2-phenylethanone (4a.JIR (cm -’ ): 1693. ‘H NMR (6 ppm): 7.95 (d, 2H, J=9 Hz, H at C-2 and C-6), 7.30 (s, 5H, Ph-H), 6.93 (d, 2H, H at C-3 and C-5), 4.15 (s, 2H, CH2), 3.80 (s, 3H, CH3. MS (m/z): 226 (3), 135 (loo), 91 (20).

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1-(4-Methylphenyl)-2-phenylethunone(5a) IR (cm-‘): 1695. ‘HNMR (6ppm): 7.93 (d, 2H, J=9 Hz, H at C-2 and C-6), 7.29 (s, 5H, Ph-H), 7.17 (d, ZH, H at C-3 and C-5), 4.13 (s, 2H, CH2), 2.36 (s, 3H, CH,). MS (m/z): 210 (lo), 119 (loo), 91 (50). 1-(4-Methoxyphenyl)-3-phenyl-1 -propanone (4b) IR (cm -’ ): 1693. ‘H NMR (6 ppm): 7.92 (d, 2H, J=9 Hz, H at C-2 and C-6), 7.28 (s, 5H, Ph-H), 6.87 (d, 2H, J=9Hz, H at C-3 and C-5), 3.78 (s, 3H, CH,) 3.32-2.98 (m, 4H, CH2CH*). MS (m/z): 240 (20), 135 (loo), 91 (40). l-(4-Methylphenyl,J-3-phenyl-1 -propanone (5b) IR (cm -l): 1696 ‘H NMR (6 ppm): 7.89 (d, 2H, J=9 Hz, H at C-2d and C-6), 7.46-7.09 (m, 7H, Ph-H+H at C-3 and C-5), 3.34-2.96 (m, 4H, CH,--CH,), 2.40 (s, 3H, CH3). MS (m/z): 224 (30), 119 (loo), 91 (40). RESULTS AND DISCUSSION

The main characteristics of the zeolites used are summarized in Table 1. Products observed in all the reactions were phenylacetic acid (3a) and l- (4methoxyphenyl) -2-phenylethanone (4a). The total yields of these products after 17 h of reaction time under the different conditions studied are given in Table 2. No appreciable amounts of unidentified materials were present in the reaction mixtures, and only traces of the corresponding ortho acylation isomer could be detected. Neither phenol nor cresols could be found under the experimental conditions used. Althoughpara substitution generally predominates in classical Friedel-Crafts acylations [ 161, the exclusive formation of the para isomer is rare and can be explained by a shape selectivity of zeolites operating in the product formation. The ortho compound would require a greater volume than the space available within the channels or cavities of the catalyst. Similar para selectivity in a TABLE I Main structural characteristics of the zeolites used Zeolite

U.C.b CA,

Si/AI

Al/U.C.

HY-100 HYD-1 HYD-2 H-P HZSM-5

24.40 24.31 24.30

9.2 21 24 14” 40

20.0 8.7 7.7

“Measured by MAS NMR spectroscopy. ‘Unit cell size.

114 TABLE 2 Results obtained in the reaction of anisole ( 1) and phenylacetyl chloride (2a)in Ccl, at 78” C in the presence of acid zeolites Yields are given after reaction for 17 h. Substrates (mmol)

1

2a

1

1 1 1 2 4 1 1 1 1

2 4 1 1 1 1 1 1

Catalyst

HY-100 HY-100 HY-100 HY-100 HY-100 HYD-1 HYD-2 HZSM-5 H-P

Product (% ) 3a”

4a

r,of 4a

TON”

66 51 27 62 59 61 47 57 67

25 42 71 61’ 66” 29 46 16 25

6.2 10.2 22.7 9.5 14.8 3.2 3.1 0.9 3.5

62

73 90 37 53

“Based on 2a. “Calculated as ro/ [Al/ (Al + Si) ] ‘Based on 1.

large-pore zeolite has been reported previously in the acylation of toluene over CeY [8,9]. It is also worth mentioning that anisole does not rearrange to cresols or undergo demethylation to give phenol in the presence of any of the zeolites studied here. This behaviour contrasts with the results reported with NafionH superacid [17], where electrophilic acylations of anisole are accompanied by demethylations and methyl rearrangements [ 181. In fact, Nafion-H is capable of catalysing efficiently these reactions involving the oxygen site [ 191. Therefore, the absence of phenol or cresols can be considered as indicative that the interaction of the zeolites with anisole in the liquid phase at 78°C must not be very strong, thus allowing the substitution reaction on the aromatic ring. Considering the nature of the product observed, it was of interest to establish if phenylacetic acid (3a) is also capable of acylating anisole under the experimental conditions used, especially taking into account that in reported Friedel-Crafts reactions promoted by zeolites the acylating agents were aliphatic carboxylic acids. Fig. 1 shows the yields of the products 3a and 4a vs. reaction time when an equimolar mixture of anisole and phenylacetyl chloride are heated in carbon tetrachloride solution at 78°C in the presence of HY-100. It can be seen that whereas the percentage of the aryl ketone 4a increases and then levels off, the yield of 3a shows a maximum, and consequently it is a primary unstable product. This indicates that anisole is acylated not only by the acyl chloride 2a, but also by the carboxylic acid 3a (Scheme 1). This possibility was confirmed by carrying out an alternative acylation using

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8

8

HV-21

H V-l 00

HV-50

(%)

N&I* exchanged

Fig. 2. Influence af the percentage of Nat exchange on the initial rate of formation of l- (4-methoxyphenyl)-Z-phenylethanone (4a) ( ro). iOH PhCH*-C-Cl

K-1

ii

+ H’-Zeol-

7

PhCH 2 -!I!-C,

7 OCti3

K-2 OCH3

+ H+-Zeal-

d

(I)

Zeal-

+

Zeal

-

(III

+OlPhCH2-C-CL

II

k3 Zeal-

+

OCH3

_

0CH3

e

OCH3

Several

*

OCH1

Zeal-

+ HCI

+ H*-Zeal-

(III)

(IV)

steps

Scheme 2. Mechanism of the acylation of anisole over zeolites.

zeolites have a wide range of acid strength distribution, this linearity indicates that all the acid sites, i.e., sites of weak, medium and strong acidity, are active in catalysing the reaction considered here. Relative adsorption constants of the reactants on the zeolite surface In the more general mechanism of the Friedel-Crafts acylation, the acid catalyst reacts with the acylating agent to give an acylium ion or an acylium-

118

YIELD

(“h]

100

80

80

40

20

+

(“1

Fig. 3. Yields of ( n ) phenylacetic acid (3a) and ( l ) l- (bmethoxyphenyl)-2-phenylethanone (4a) versus reaction time for the reaction of anisole (1) (110 mg, 1 mmol) and phenylacetyl chloride (2a) (150 mg, 1 mmol) in Ccl, solution (50 ml) over HYD-2d zeolite (1 g).

like complex. Whatever the electrophilic species generated through this interaction, the second step involves the attack of this electron-deficient intermediate on the aromatic ring (Scheme 2 ). In the above discussions we have noted that in general anisole acylation must be promoted by adsorption of phenylacetyl chloride on the zeolite surface, but is complicated by side-reactions arising from the adsorption of the anisole. In order to advance our understanding of the mechanism of the reaction over zeolites, it was of interest to determine the value of the relative adsorption constants of the reagent on the solid surface. For this purpose we measured the initial rates of formation of 4a at different initial concentrations of 1 and 2a. We assume that rapid protonation occurs, i.e., that the protonated species are in equilibrium with the reagents at the surface of the solid. Accepting that the reaction will take place by a Eley-Rideal-type process, i.e., a molecule of the adsorbed acyl chloride 2a reacts with anisole in the liquid phase, as has been represented in Scheme 2, then the following rate equation, based on a Hougen-Watson-Langmuir-Hinselwood formalism, can be written: r=

k3 [ H+-zeol- ]K,’ [PhCH,COCl] [anisole] 1 +Ki’ [PhCH,COCl] +K,’ [anisole]

(1)

If the initial rates are measured at different initial concentrations of anisole while the concentration of phenylacetyl chloride (2a) is kept constant

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(P’hCH&OCllo), and vice

versa, then eqn. 1 can be simplified and converted

into

$a [ anisole ] b+& [ anisole]

r=

(2)

and k,c [ PhCH,COCl] ‘= d+K,’

[PhCH,COCl]

where a= [H+-Zeal-]K,’ [PhCH&OCllO, b=l+K1’ c= [H+-Zeal-l]K,’ [ anisole], and d= l+&’ [anisole],. By linearizing eqns. 2 and 3 we obtain M 1 -= +n rO [ anisole] 1 r,-

[PhCH$OCl],,,

(4)

P [ PhCH,COCl]

(3)

i-s

(5)

with s K,’ ---. n-K,’

[PhCH2COCl10 [ anisole ]

where m = b/k3a, nK,’ /k3a,p = d/kg and s = K,’ /kg, and the adsorption constant ratio ( K1’ /K2’ ) can then be calculated. By least-squares fitting of the results given in Table 2, a K,’ /K2’ ratio of 2.3 was found. This value indicates that the extent of the adsorption of the acyl chloride 2a on the zeolite sites is more than double that of the oxygenated aromatic compound 1. Although it is obvious that adsorption per se does not guarantee the occurrence of any reaction (as must be the case with the adsorbed anisole), this step is a prerequisite in the catalysis and therefore the relative adsorption constant found is favourable to the acylation reaction, as in most of the occupied sites it could be possible to generate the electrophilic acylating species. Influence of the framework Si-to-Al ratio of the zeolites As can be seen in Scheme 1, although phenylacetic acid can also acylate anisole (process iii), its formation (process ii) is competitive with the direct acylation by the acyl chloride (process i) and, owing to the differences in reactivity on an acyl halide and carboxylic acid (compare also the respective yields of 4a in Tables 2 and 3)) the hydrolysis of phenylacetyl chloride must be considered to be an undesirable side-reaction. In an attempt to gain more control over the competitive processes taking

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place, and as water is required or formed in the reactions involving phenylacetic acid (processes ii and iii), the acylation was studied over zeolite Y with different framework $%-to-Al ratios. It is well established that highly siliceous materials show a marked hydrophobicity on their surfaces [ 221, and therefore these catalysts may have a pronounced effect when water is involved in the reaction. Table 2 gives the yields of 4a after reaction for 17 h when an equimolar mixture of 1 and 2a is heated at 78°C in the presence of HYD-1 and HYD-2. In order to establish a valid comparison among the different catalysts, and as the total number of acid sites per unit of solid weight changes with the aluminium content, the results are also expressed as a value proportional to the turnover numbers (TON ) , which are calculated by dividing the initial rates of formation of 4a (~~‘0)by the Al-to- (Si + Al) ratio, which is related to the number of aluminium atoms per unit cell. Table 2 indicates that there is a small increase in the TON when the framework Si-to-Al ratio changes from 9 to 24, those zeolites with a high silicon content being slightly more active. This result can be explained by considering that all the acid sites are active, and therefore a linear correlation between activity and Sanderson’s global electronegativity [23] exists, this conclusion being in agreement with the results obtained with the NaHY samples. The small variations found in the TON values could be rationalized by taking into account the differences in the hydrophobicity of these highly siliceous materials. In fact, hydrolysis and acylation are competitive processes, both being catalysed by zeolites. As in heterogeneous catalysis the real reaction medium that must be considered is the solid surface, and the concentration of water is lower in the more dealuminated samples, acylation becomes comparatively favoured over hydrolysis when more hydrophobic catalysts are used. Moreover, the equilibrium of the secondary reaction of anisole with phenylacetic (Scheme 1)) which also involves the formation of a molecule of water, must also be shifted in the direction of water surface evacuation, i.e., towards the formation of the phenone 4a, by the same hydrophobic catalysts. In fact, a comparison of Figs. 1 and 3, together with the final yields of phenylacetic acid (3a) and the aryl ketone 4a in the presence of HY-100 and HYD-2 (the ratio of the percentage of 4a to that of 3a increases 2.6-fold over the dealuminated catalyst) strongly support the occurrence of these changes in the equilibrium constants. Influence of the crystalline structure of the zeolites Friedel-Crafts acylations were also carried out on a three-dimensional 12membered ring zeolite (H-P), with large cavities at the intersection of the channels [ 241, and with a medium-pore two-dimensional 10 membered ring

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zeolite (HZSM-5). In Table 2 the TON calculated for H-P with a framework Si-to-Al ratio of 14 is slightly lower than that for the Y zeolites. This lower value may be due to spatial constraints on acylation in H-/I and/or to the facts that a decrease in crystallinity is always observed after calcination. We have shown previously [ 251 that H-P shows more limitations than Y zeolite for the formation of multi-branched products during cracking of alkanes indicating that the size of the internal cavities at the intersection of the channels is smaller than the a cages in HY zeolite. A higher constrain index has also been observed for H-j? than for HY zeolite. In any case, if such constraint exists for acylation reactions, it is small. With HZSM-5 zeolite the TON is much smaller than those for HY or H-P zeolites. It is known that the intrinsic Bronsted acidity of an HZSM-5 zeolite is higher than that of HY or H-P. We can see no chemical reason that could explain those results. There is, however, another explanation for the observed TON, namely that HZSM-5 encloses channels and cavities of appreciably lower dimensions than those in HY or H-j?. Therefore, only the most external ones will be acessible to the reactants, and consequently only part of the framework aluminium atoms will produce accessible protons. Influence of the structure of the organic compounds In order to obtain a more complete picture of the Friedel-Crafts reactions catalysed by zeolites, we carried out a series of acylations using 3-phenylpropanoyl chloride (2b) and phenylacetyl (3a) and 3-phenylpropionic acids (3b) using carbon tetrachloride at 78°C or toluene at 115°C as solvent over HY100, HYD-2 and HZ in the presence and absence of anisole or p-cresol. Table 3 indicates the products obtained and the yields measured after reaction for 17 h under the conditions described in the general procedure under Experimental. From Table 3 it can be concluded that HY zeolites are general catalysts for the Friedel-Crafts acylation of aromatic compounds using either acyl chlorides or carboxylic acids, yielding only the para isomer. With HZSM-5 zeolite more information about the presumably steric limitations that occur in the acylation catalysis can be easily obtained by carrying out the reaction using a more substituted aromatic compound. In order to establish a valid comparison, effects associated with the substitution pattern other than the size of the molecule must be avoided. For this purpose, we chose p-cresol as an alternative substrate, as the main difference from anisole is the presence on the aromatic ring of a weakly activating methyl group, and the Cacylation ofp-cresol by phenylacetic acid in the presence of HYD-2 has already been described [ 261. In addition, the nuclear acylation ofp-xylene, which must have a molecular size comparable to that ofp-cresol, in the presence of a largepore zeolite has been also reported [8,9]. As can be seen in Table 3, the attempted reaction of p-cresol with phenyl-

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acetic acid over HZSM-5 was unsuccessful, thus providing additional evidence that the pore size and channel dimensions of HZSM-5 are in the range of the substituted benzene ring size, and consequently may create steric constraints that limit the course of the reaction. Finally, it is worth mentioning the surprising product shape selectivity observed for the reactions involving the 3-phenylpropanoyl moiety. This moiety can acylate its own phenyl group, giving rise to the intramolecular cyclization product, or react with the aromatic ring of another molecule, yielding intermolecular products. The intramolecular acylations to form 5- or g-membered rings are so efficiently promoted by the usual Lewis acids that they are of substantial synthetic utility, and can be carried out even using aromatic compounds such as benzene, toluene or anisole as solvents [ 271. In principle, it might have been expected that acylations catalysed by zeolites would strongly favour the intra- versus the intermolecular products, as the reaction is taking place inside a cage, and therefore a similar overwhelming predominance of lindanone should be achieved with both types of catalysts. For the sake of clarity the AlCl,-catalysed reaction of 3-phenylpropanoyl chloride in toluene at 50’ C was also performed and the results are included in Table 3. A considerable difference in the product distribution between AlC&and HY-100-catalysed reactions is observed. Thus, whereas the yield of indanone (6) (intramolecular acylation) with AlC& is seven times greater than the total yield of intermolecular acylation products (o&o plus para isomers), in the zeolite-catalysed reaction this ratio is close to unity. Although possible differences in the reactivities of toluene and anisole towards electrophilic aromatic substitutions must be considered when we compare the results obtained in acylations over AlCl, and acidic zeolites in Table 3, as far as we are aware this relative efficiency of the zeolites to catalyse, inside a cage, bimolecular versus unimolecular processes can be understood by assuming a concentration effect inside the pores. Indeed, owing to the high adsorption capacity of large-pore zeolites, the bimolecular reaction is enhanced with respect to the monomolecular process. However, other possibilities, such as substantial modifications in the entropy of the transition states of the competing reactions, or the existence of a product shape selectivity, cannot be ruled out. Further studies are in progress in order to establish the scope, mechanism and possible synthetic applications of this limitation to intramolecular cyclizations inside the zeolite work. In conclusion, anisole can be acylated in the liquid phase and at moderate temperatures in the presence of acid zeolites by acyl halides or carboxylic acids without undergoing demethylation or rearrangement, giving rise only to the para isomer. From the point of view of the design of a zeolite tailored as an appropriate catalyst for the acylation of anisole by acyl halides, a suitable material does not need to have very strong acid sites, as we have shown that there is no influence of the acid strength distribution on the initial rates of acylation.

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However, it is better to use zeolites with a high framework Si-to-Al ratio in order to achieve an appreciable hydrophobicity. Large-pore zeolites are more generally applicable than the medium-pore zeolites, which are just in the range of the benzenoid ring size and, depending on the organic substrate, may fail to catalyse the acylation for steric reasons. Finally, an effect favouring the intermolecular para acylation compound 4b or 5b versus the intramolecular cyclization product 6 has been described with large-pore zeolites. ACKNOWLEDGEMENT

Financial support by the Comision Asesora de Investigation Cientifica y Tecnica of Spain (Project PA 85 -0284 and MAT 88-0147) is gratefully acknowledged. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

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