Comparison of the catalytic properties of protonic zeolites and exchanged clays for Diels-Alder...

Applied Catalysis A: General, 101 (1993) 253-267

Elsevier Science Publishers B.V., Amsterdam

253

APCAT A2525

Comparison of the catalytic properties of proton and exchanged clays for Diels-Alder synthesis

.ic zeo ites

C. Cativiela

Depto de Quimica Organica, Znstituto de Ciencia de Materiales de Aragon, Universidad de

Zaragoza, C.S.I.C., 50009 Zaragoza (Spain)

F. Figueras

Laboratoire de Chimie Organique Physique et Cindtique Chimique Appliqtkes (URA 418

CNRS), E.N.S.C.M., 8 .rue Ecole Normale, 34053 Montpellier Cedex 1 (France)

J.M. Fraile, J.I. Garcia and J.A. Mayoral

Depto de Quimica Organica, Instituto de Ciencia de Materiales de Aragon, Uniuersidad de


L.C. de MQnorval

Laboratoire de Chimie Organique Physique et Cindtique Chimique AppliquCes (URA 418

CNRS), E.N.S.C.M., 8 rue Ecole Normale, 34053 Montpellier Cedex I(France)

and

E. Pires a

Depto de Quimica Organica, Instituto de Ciencia de Materiales de Aragon, Universidad de


Received 4 December 19’32, revised manuscript received 18 March 1993

Abstract

Zn*+- and Fe3+-exchanged KlO montmorillonites have been compared to HY and HBEA zeolites for Diels-Alder condensation of methyl and (-)-menthyl acrylates with cyclopentadiene. In the case of clays, the endo/exo and diastereofacial selectivities of the reaction increase with the temperature of dehydration of the solid. The catalysts dried at 393 K show lower selectivities and some deactivation as a function of time. The initial selectivity of the zeolitic catalysts is good but decreases rapidly with time. This decrease is attributed to the formation of polymers of cyclopentadiene, which deactivate the catalyst. FT.IR studies of pyridine adsorption on the catalysts showed that this polymerization reaction increases when the proportion of Bransted acid sites increases.

Key words: catalytic properties; Diels-Alder synthesis; exchanged clays; protonic zeolites

Correspondence to: Dr. F. Figueras, Laboratoire de Chimie Organique Physique et Cinktique Chi-

mique Appliqukes, URA 418 CNRS, E.N.S.C.M., 8 rue Ecole Normale, 34053 Montpellier Cedex

1, France. Fax: (+33)67144353.

0926-860X/93/$06.00 0 1993 El sevier Science Publishers B.V. All rights reserved.

254 C. Catioiela et al./Appl. Catal. A 101 (1993) 253-267

INTRODUCTION

The Diels-Alder cycloaddition of dienes and dienophiles is both a model reaction of organic chemistry [ 11, and a powerful tool for the total synthesis of a variety of natural products [ 2-61.

The non-catalysed reaction is well known to be very sensitive to solvent effects: fast rates and better selectivities can be obtained using water as solvent [7,8], but the use of water restricts the temperature range for the reaction. This reaction has long been known to be catalysed by Lewis acids [ 9,101, which activate the dienophile and increase both regio and endo/exo selectivities. Fur- thermore, the control of the conformation of the enoate moiety of the dienophile by complexation with the Lewis acid permits excellent diastereofacial selectivities to be attained in asymmetric Diels-Alder reactions with chiral dienophiles [ 11-131.

This methodology of homogeneous catalysis shows, however, many drawbacks such as the need for large quantities of non-recycled and corrosive Lewis acids. The use of solid acid catalysts introduces then the classical advantages of heterogeneous catalysis: easy separation of the products, reduction of cor- rosion and environmental problems, and recycling and regeneration of the catalyst. Activated clays, such as KlO montmorillonite [ 14-201 and zeolites [ 21,221 have been reported as good catalysts for Diels-Alder synthesis.

In the clay catalysed Diels-Alder reactions it is also observed that the solvent plays a major role, toluene and dichloromethane being suitable solvents [ 23 1. Indeed, side reactions of the diene can occur, and the solvent must be chosen to minimize them, in function of the catalyst: anisole is the best solvent for %n2+ -doped KlO, but is not as good when the exchanged cation is Fe3+. In this case, the interaction of anisole with Fe3+ cations forms radical cations, and the radical-promoted Friedel-Crafts reaction of cyclopentadiene with anisole and oligomerisation of cyclopentadiene take place, decreasing the yield

[241. When a correct choice of catalyst and solvent is made, the asymmetric re-

action of cyclopentadiene with (-) -menthyl acrylate can be realized with high endo/exo selectivitiy (a ratio of endo/exo of 24 with a 88% yield in cycloadduct) and a reasonable diastereoisomeric excess of 52% [25]. Calcination of the catalyst above 5OO”C, which induces some dehydroxylation of the solid, with conversion of Brijnsted to Lewis acidity, increases catalytic activity, endo/ exo and diastereofacial selectivities [ 261.

The catalytic properties of zeolitic catalysts for Diels-Alder additions have been reported for both gas phase [27] and liquid phase reactions [ 21,221. It was therefore interesting to compare both types of catalysts in order to inves- tigate the possible influence of the pore structure: KlO montmorillonite under- went treatment in air at a high temperature as well as acid leaching and showed a low microporosity, whereas zeolites were characterized by a well defined in-

C. Catiuiela et al./Appl. Catal. A 101 (1993) 253-267 255

tracrystalline porosity. Since the reaction yields rather bulky bicyclic com- pounds, only large pore zeolites HY and HBEA were considered.

EXPERIMENTAL

Preparation and churacterization of the catalysts

Zeolite j? (BEA ) was synthesized using tetraethylammonium hydroxide (TEAOH) as an organic template, following the procedure described by Ni- colle [ 281. The crystals appeared as spheroids with an average size of 0.6 pm. These samples were calcined at 773 K for 8 h under dry air (120 ml min-’ ), then converted to the ammonium form by ion exchange in a 1 M NH,NO, solution at 373 K.

The Y zeolite was Linde LZ-Y62 (Si/Al= 2.5 ), at that time a NH, form. The sample (1.25 g) was calcined in air for 5 h at 773 or 823 K to decompose the ammonium form into the protonic form.

Cation exchange of montmorillonite was effected by gradually adding the original KlO (10 g) supplied by Siid Chemie to stirred ferric or zinc chloride solutions (125 ml, 1 M) at room temperature, following the procedure reported by Laszlo and Moison [ 181. The slurries were stirred for 24 h, filtered and washed chloride free with deionised water. The resulting solids were dried overnight in a thin bed at 393 K in an oven, then ground in a mortar. The final colour of the samples was pale grey for the Zn2+ clay and yellowish brown for the Fe3+ sample. The catalysts were equilibrated over salt solutions in order to give reproducibite water contents, which were measured by thermogravime- try in a Perkin-Ehmer TGS-2 apparatus equipped with a System 4 micropro- cessor controller. ‘These samples were eventually calcined in dry air (25-30 ml/min) for 10 h at 823 K. Chemical analyses (Tables 1 and 2) were obtained by plasma emissio,n spectroscopy in a Perkin-Elmer plasma 40 emission spec- trometer. The lower Al content and higher Fe content of Zn-KlO reflects the fact that the acid s’olution obtained by dissolution of ZnCl, leaches some amor- phous Al phase out of the solid during the process of ion exchange.

The micropore .volume and BET surface area were deduced from the isotherms of adsorption of nitrogen at 77 K. X-ray diffraction patterns were re-

TABLE 1

Physico-chemical characteristics of the zeolites used in this work

Sample Si/Al Mesoporous vol.

(ml g-l)

Micropore vol.

(ml g-‘)

Acidity

(meq g-‘1

HY 2.5 0.04 0.36 1.5 P 15 0.04 0.27 0.85

256 C. Cativiela et al./Appl. Catal. A 101 (1993) 253-267

TABLE 2

Physico-chemical characteristics of the clays used in this work

Sample

KlO Zn-K10 Fe-K.10

Al (%) Fe (%) Zn (W) Water (%) BET surface Micropore vol.

(m* 9-l) (ml g-‘)

6.89 2.12 4.69 240 0.1 6.43 2.32 1.77 3.85 226 0.1 6.72 3.95 4.69 244 0.1

corded on a Philips computer-driven X-ray diffractometer using Cu Karl ra- diation. The two zeolites exhibited high crystallinity. In the case of clays, the X-ray diffraction spectrum is much less informative. Judged from the changes in the XRD pattern or surface area, the cation-exchanged clay samples re- tained the structure of the initial KlO solid.

The acid properties were determined by adsorption of pyridine as probe mol- ecule on self-supported wafers obtained by compressing the powders under 5 tona cm-‘. A determination of the Lewis/Brnnsted ratio was carried out by integrating the surface area corresponding to the infrared lines at 1450 and 1545 cm-l, normalized to the bands at 1920-1827 cm-’ relative to montmorillonite.

Reaction procedures

Methyl acrylate and acryloyl chloride were purchased from Merck and used without further purification. (-)-Menthol was purchased from Aldrich. Or- ganic solvents were dried and purified according to standard procedures.

The reactions were carried out in Schlenk flasks, at 293 K. Overall yields and endo/exo ratios were determined from the solution obtained by filtration and washing of the catalyst with the reaction solvent.

Reaction between methylacrylate and cyclopentadiene

A preweighed (1.25 g) amount of catalyst was dried at 393 K overnight or calcined. The flask was charged with the catalyst and methylene chloride (15 ml) under argon atmosphere at 293 K. Methyl acrylate (0.645 g, 7.5 mmol) and freshly distilled cyclopentadiene (1.5 g, 22.5 mmol) were added via a syringe. Under these conditions a complete dissolution of the reagents was achieved. The reaction flask was shaken for 24 h and the reaction monitored by gas chromatography (cross-linked methyl silicone column 25 mx 0.2 mm x 0.33 mm, helium as carrier gas 16 psi, injector temperature 503 K, detector temperature 523 K, oven temperature programme: 323 K (3 min), 25 K/ min up to 373 K (9 min). In these conditions the retention times are: 2.7 min

C. Cativiela et al./Appl. Catal. A 101 (1993) 253-267 251

for methyl acrylate, 12.7 for the exo adduct and 12.9 for the endo adduct. The flame ionisation detector was calibrated by repetitive injections of synthetic mixtures of the cycloadducts.

Reaction between (-)-menthyl acrylate and cyclopentadiene

(-)-Menthyl acrylate was prepared according to a procedure described in the literature [ 291. A preweighted amount of Zn2+-clay (1.5 g) was dried at 393 K overnight or calcined. The flask was charged with the catalyst and methylene chloride (15 ml) under argon atmosphere at the working temperature.

(-) -Menthyl acrylate (0.63 g, 3 mmol) and freshly distilled cyclopentadiene were added via a syringe. The reaction was monitored by gas chromatography (cross-linked methyl silicone column 25 m x 0.2 mm x 0.33 mm, helium as carrier gas 19 psi, injector temperature 503 K, detector temperature 523 K, oven temperature programme: 463 K (1 min), 2’ /min, 453 K (0 min) , lo /min, 443 K (5 min). The retention times are: (-)-menthyl acrylate: 3.9 min, 6a+ 6b: 18.9 min, 5a: 19.6 min, 5b: 20.0 min. Absolute configurations were assigned by comparison of the gas chromatograms obtained in clay catalysed reactions, with those in Lewis acid catalysed reactions described previously by Oppolzer et al. [30].

RESULTS AND DISCXJSSION

Characterisation of the catalysts

From their N, adsorption isotherms the two series of catalysts can be divided into mesoporous clays on one side (peak of the distribution of pore sizes at 1.5-2 nm) and microporous zeolites.

From X-ray diffraction spectra, the original KlO contains mainly silica impurities in the form of quartz and cristobalite which are commonly associated with clays. These minerals give sharp diffraction lines, and are then in the form of big crystals of low-surface area ( < l-2 m”/g) which simply dilute the active phase since the surface area of these clays determined by nitrogen adsorption was determined to be 240, 226 and 244 m2/g respectively for KlO and the Zn and Fe samples. From the comparison of the intensities of the lines obtained on KlO and a pure quartz sample the amount of these impurities has been estimated to lo-15%.

The original clay and the samples exchanged by either zinc or iron show the same X-ray diffraction spectrum, and the same crystallinity. This is not really surprising because of the bidimensional character and disordered structure of clays which restrict the amount of information which can be extracted from the spectrum.

The environment of Al and Si atoms in the material was investigated on KlO


and Zn-KlO, calcined or not, using high-resolution solid state NMR. Neither the introduction of Zn ions, nor calcination changes the spectra.

The 27A1 NMR spectrum contains two resonance lines at 4 and 67 ppm to- gether with additional side spinning bands (Fig. 1). The line at 4 ppm is usu- ally) attributed to Al ions in octahedral symmetry located either in the octahedral sheet of the clay or in alumina impurities. The line at 67 ppm is attributed to Al ions in tetrahedral symmetry [ 311. The Al spectrum of montmorillonites before and after heating at 500” C has been reported by Tennakoon et al. [ 321. Al is found to be in octahedral symmetry, with a resonance line at 2 ppm in both samples. By contrast, Lussier [ 331 recently reported the reincorporation of Al in tetrahedral coordination for kaolins calcined at high temperature (1173 K) then treated by HCl. The presence of tetrahedral Al in KlO is then probably due to this activation treatment.

This structural modification of the clay by the combination of heat and acid treatment is also evidenced by the 2gSi MAS NMR spectrum, showing the presence of three lines at -91, - 100 and - 110 ppm (Fig. 2). In their study of aluminosilicates, Maggi et al. [ 341, assigned the signal at - 110 ppm to sili- cious impurities associated with the clay, and a peak at - 93 ppm (close to our peak at -91 ppm) to Si atoms in the tetrahedral layer of the clay, linked to

1”“““‘I““““‘I““““‘I”“““‘I”“““‘I””””’ 100 50 Pi4 -50 -100

Fig. 1. 27 Al MAS-NMR spectrum of K10 montmorillonite. (*) Spinning side bands.


r---e I 1 I . ’ I ’ . -60 -60 -100 -120

PPM

Fig. 2. ” Si MAS-NMR spectrum of KlO montmorillonite.

three other Si atoms and either to an Al or Mg atom in the tetrahedral layer. The peak at - 100 ppm has been observed on 2/l clays calcined at high temperatures and is characteristic of dehydroxylated phases [ 311. This peak may be explained by the presence of Q” (1Al) structural units in three-dimen- sionally connected. aluminosilicate-like structures formed by SiOAl tet linkages between the silicate sheet and the gibbsite sheet during dehydroxylation. This interpretation, is supported by the observation of the 27A1 NMR signal at about 65 ppm which is typical of tetrahedral Al in aluminosilicates. In the case of kaolin calcined at approximately 1200 K, then treated by hydrochloric acid, Lussier [33] observed only a broad band of Si at - 108ppm, reflecting the destruction of the structure of the clay. Rhodes and Brown [35] recently described a montmorillonite which had suffered long acid treatments (20 h). 2gSi MAS-NMR and elemental analysis suggested in this case, that there is little residual clay structure left. The structure of the final clay is then largely de- pendent on the intensity of this acid treatment.

This NMR study of the original KlO, and of the clay exchanged by zinc, therefore shows that after the activating treatment of calcination and acid leaching, montmorillonite is converted to a solid which retains still in part the structure of the original clay, but possesses some tetrahedral aluminium atoms

260 C. Cativiela et al.fAppl. Catal. A 101 (1993) 253-267

which are a source of acidity. The protons associated to these Al atoms can be substituted by Zn or Fe cations. Adams [36] reported previously that clays exchanged by metallic ions can be strong acids.

Zn-KlO and Fe-KlO, simply dried at oven temperature contain both Lewis and Bronsted acidity, as illustrated in the IR study of adsorbed pyridine reported in Fig. 3-A. The important difference between these two clays is a different Lewis/Bronsted ratio: 20 for Zn-KlO and 3 for Fe-KlO. Calcination of

.i I I

, v '50.0 1627. e 112s.o 1412.2 17%x. 0 1227 .s 162a.o 1412.e

W.r.numtx~r (cm-l) wav~numow- km-i)

Fig. 3. FT-IR spectra of pyridine adsorbed. 3-A: on Zn-KlO (a) and Fe-K10 (b) samples initially

evacuatedat 413 K. 3-B: on Zn-KlO (a) andFe-KlO (b) samples calcinedat 823 K, then evacuated

at 773 K. Pyridine is adsorbed at 413 K and the sample is then evacuated at the same temperature.

3-C: on Fe-K10 treated at 413 (a) or 673 K (b) in vacuum after adsorption of pyridine at 393 K

then desorption at the same temperature. 3D: on Zn-K10 treated at 413 K (a) or 673 K (b) in

vacuum after adsorption of pyridine at 393 K then desorption at the same temperature.

C. Cotiviela et al./Appr!. Catal. A 101 (1993) 253-267 261

these solids at 823 K induces the preferential disappearence of Bronsted acidity, and yields pure Lewis acids, but the two samples show, however, a different behaviour when treated in vacuum around 773 K (Fig. 3-B).

Dehydration of Fe-K10 (Fig. 3C ) induces an increase of the intensity of the Lewis band at 1450 cm-’ (integrated intensity of 1.25 for the sample dried at 393 K, and 2.43 for the same sample evacuated at 673 K) with a decrease of the intensity of the band at 1545 cm-’ of Bronsted acidity (intensity decreasing from 0.54 to 0.05 ).

On Zn-KlO (Fig. 3D), both bands decrease, with a larger diminution (0.18 to 0.03) of the pyridinium band compared to that of pyridine coordinated at Lewis sites (3.51 to 1.28). This might be the result of the reaction of some Zn’~+ ions with the clay forming an aluminate. The fewer remaining Lewis sites retain, however, pyridine after evacuation at 673 K, then appear as strongly acid sites.

It is well known that protonic Y zeolites possess strong Bronsted acidity, which can be converted to Lewis type by dehydroxylation at high temperatures [ 37 11. It is also well known that BEA zeolite contains after activation an ap- preciable amount Iof extra framework Al, and Lewis acid centres in addition to normal Bronsted centres associated with framework Al [ 38,391. The FT-IR spectra of pyridine adsorbed on a sample calcined at 773 K then dehydrated at the same temperature, show the retention of a significant amount of protonic acidity while exchanged clays behave as pure Lewis acids (Fig. 4 ) .

The number of acid centres of the two zeolites was also determined by step-

Fig. 4. FT-IR spectra of pyridine on NH4Y (a) and H-BEA (b) zeolites calcined at 823 K, then

evacuated at 773 K. Pyridine is adsorbed at 413 K then the samples are evacuated at the same

temperature.

262 C. Catiuiela et al./Appl. Catal. A 101 (1993) 253-267

wise thermal desorption of ammonia. The catalyst was first activated in situ, in flowing nitrogen, saturated with ammonia at 373 K, then swept by a flow of dry nitrogen while the temperature was raised by steps of 50 K. The amount of ammonia evolved by the solid was monitored by conductometry. The num- bers of relatively strong sites, retaining ammonia above 523 K are reported in Tables 1 and 2 for the two zeolitic catalysts. With the exchanged clays, the amount of ammonia desorbed by the solids was too small for an accurate measurement.

Catalytic properties:

Table 3 reports the results obtained in the reactions of methyl acrylate ( 1) and cyclopentadiene (Scheme 1). In spite of a small number of acid sites, as evidenced from the respective intensities of the absorption bands in the infrared spectrum (Figs. 3 and 4), the exchanged clays show equal or better activities than zeolites. Indeed the activities of the two zeolites are roughly pro- portional to the number of acid sites as determined by desorption of ammonia above 523 K, and this proportionality suggests that diffusional limitations do not control the kinetics of Diels-Alder cycloaddition.

In the case of clay catalysts, dehydroxylation leads to an improvement of the reaction rates and selectivities. The main difference between clays and zeolites is the decrease in selectivity as a function of conversion. It can be proposed that this fact is related to the protonic acidity of zeolites, which can promote cationic oligomerisation of cyclopentadiene. Indeed, a polymer frac- tion was detected by precipitation with methanol from the reaction medium. At the low temperature used for the reaction these oligomers inhibit the access of the dienophile to the surface, and the reaction is then realised in the liquid phase, at a slow rate and lower selectivity. Furthermore, an extensive polymerisation consumes the diene and the Diels-Alder reaction then stops, as happened in the reaction catalysed by H-BEA. In this case the reaction stops after 7 h and the final yield is lower than that obtained in the non-catalysed reaction.

It can be pointed out that BEA shows much smaller particles than Y (0.6 vs. 2 pm), and a higher external surface area. If catalysis is restricted to the external surface area, the catalytic activity of BEA calculated per proton is higher, and a slower deactivation is expected. However, this is not the case: H-BEA,

Scheme 1

C. Cativiela et al./Appl. Catal. A 101 (1993) 253-267

TABLE 3

263

Results obtained from the reaction of methyl acrylate ( 1) on cyclopentadiene”

Catalyst Activation

temperature

Time

(h)

Yield

(%)

endojexo

(2:3)*

Zn’+-KlO 393 0.5 35 9.4

2 63 9.4

24 82 9.1

393 0.5 26 9.7

2 44 10.0

24 51 9.8

Fe3+ -KlO

0.5 60 14.7

2 94 14.6

24 100 14.7

Zn’+-KlO 823

0.5 47 15.3

2 75 15.0

24 97 14.8

Fe3+-KlO 823

2 65 13.7

24 81 11.7 HY 823

HBE:A 823 0.5 11 10.3

2 22 6.7

7 26 6.2

24 26 6.2

None 0.5 2 3.7

24 54 3.7

n In CH&12 at 293 K using a 3:l diene dienophile molar relationship and 0.17 g of catalyst per

mmol of dienophile.

* Determined by gas chromatography.

Scheme 2.


suffers a very fast deactivation, and H-Y is moderately poisoned, which also suggests that intra-crystalline catalysis did occur.

Further evidence of the importance of the nature of zeolitic acidity can be found in the results obtained in only slightly different experimental conditions by Narayana Murthy and Pillai [ 221, using Ce-Y and Zn-Y zeolites which are known to show Lewis acidity [ 371. With these catalysts the selectivity is no- ticeably increased at high conversion.

The importance of the polymerisation side reaction is even clearer with the less reactive (-) -menthyl acrylate (4 ) (Scheme 2 ). The results obtained (Ta- ble 4) show that a greater amount of catalyst is needed in order to avoid its

TABLE 4

Results obtained from the reaction of (- ) -menthyl acrylate (4 ) and cyclopentadiene”

Catalyst Activation

temperature

Zn’+-K10 393

Zn”+-KlO 393

Fe3+-KlO 393

Zn’+-KlO 823

Fe3+-KlO 823

HY 823

None

Catalyst:

&lmmol)

0.17

0.5

0.5

0.5

0.5

0.5

Time Yield endo/exo d.eh,’

(h) (So) (5:6) (“0)

0.5 8 8.8 33

2 24 8.1 32

24 55 7.3 26

48 66 6.4 23

0.5 16 9.9 30

2 34 9.7 30 24 63 9.6 32

0.5 7 7.9 39

3d 17 7.0 39

24 49 6.0 35

0.5 35 11.0 38 2 54 10.5 37

17 90 10.0 36

0.5 22 8.2 40 2 36 8.0 39

24 76 6.4 33

0.5 19 9.6 41

2 40 8.3 38

24 64 5.8 27

51 60 3.8 6.0

a In CH,Cl, at 293 K using a 6:l diene dienophile molar relationship.

b Determined by gas chromatography.

’ Diastereoisomeric excess in endo cycloadduct, 5a is the major product.

d After this time 6 additionnal mmol of diene are added.

C. Cativiela et al.fAppl Catal. A 101 (1993) 253-267 265

deactivation and the subsequent reduction of endo/exo selectivity and diastereofacial excess caused by the non-catalysed reaction. In spite of the higher catalyst/dienophile ratio, the solids with a higher number of Bronsted acid sites, namely non-calcined Fe-K10 and Y zeolite, show the drawbacks related to this kind of acidity. Therefore, with the former catalysts, the diene is elim- inated from the reaction medium and an additional amount must be intro- duced. Although both selectivities decrease when the conversion increases, the decrease is more pronounced in the case of Y zeolite. The different porosity of both solids, that makes the deactivation of the Y zeolite easier, can account for this difference.

In the reaction catalysed by calcined Fe-K10 a small decrease in selectivities is also observed. G.iven that this clay does not possess Brernsted acid sites, this side reaction can be attributed to the radical-promoted polymerisation of cyclopentadiene previously described [ 241.

In heterogeous catalysis, as in homogeneous catalysis, the Diels-Alder reaction of carbony’l-containing dienophiles is rather specific of Lewis acidity. Both mesoporous clays and microporous zeolites give good results when converted to Lewis forms. The Diels-Alder reaction is very sensitive to electronic factors but it has also been shown that the endo transition state is more com- pact [ 401, hence some shape selectivity is expected with zeolites. However, this is not the case and selectivity is here probably controlled by the interaction between the dienophile and the solid, which is related to the strength of the Lewis acid. This also agrees with the lack of influence of solvophobicity on the endo/exo and diastereofacial selectivities recently reported for the reaction of (-) -menthyl acrylate and cyclopentadiene [ 411.

In conclusion, from the comparison of protonic zeolites with clays or zeolites exchanged with transition cations, it appears that good selectivities for Diels- Alder condensation of methyl acrylate with cyclopentadiene can be obtained in the presence of solid Lewis acids. The presence of protons on the solid induces side reactions of cyclopentadiene oligomerisation, and lower selectivities and deactivation of the catalyst.

ACKNOWLEDGEMENTS

The financial support of the Comision Interministerial de Ciencia y Tec- nologia (Project MATgO-0778) of the Subdireccion General de Cooperation International in Spain, and of CNRS in France (Action Integrada Hispano- Francesa‘243B) is gratefully ackownledged. J.M. Fraile thanks the Minister of Education and Science for a grant.


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Comparison of the catalytic properties of protonic zeolites and exchanged clays for Diels-Alder...

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