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Modified clay catalysts for acylation of crown compounds
Heterogeneous Catalysis and Fine Chemicals IV H.U. Blaser, A. Baiker and R. Prins (editors) © 1997 Elsevier Science B.V. All rights reserved.
83
Modified clay catalysts for acylation of crown compounds S. Bekassy^ K. Bir6^ T. Cseri''^'^ B. Agai' and F. Figueras^ ^Department of Organic Chemical Technology, Technical University of Budapest, H-1521 Budapest, Hungary ^Institut de Recherches sur la Catalyse du CNRS, 2 Avenue Albert Einstein, F-69626 Villeurbanne, France
1. SUMMARY Heterogeneous catalytic acetylation of benzo-15-crovm-5 was investigated using different ion exchanged KIO catalysts. The nature of the transition metal ions introduced in the mesoporous clay plays an important role in the activity of the catalysts. Fe^^-KlO is particularly advantageous for practical preparative purposes. 4'-Acetyl-benzo-15-crown-5 was isolated with 55% yield under optimal conditions.
2. INTRODUCTION Crown ethers are well known for their ability to form strong complexes with alkali metal and organic cations. Introduction of lipophilic substituents into crown ethers has a great importance e. g. for their application as electroactive substances in ion-selective electrodes [1] and as complexing agents for recovering precious or radioactive metal cations [2]. These substituents can be introduced into benzo annelated crown ethers by S^ reactions (alkylation, acylation, nitration, halogenation). The introduced substituents modify the complexing properties of the crown ether macroring through the aromatic ring. We have taken an advantage of this phenomenon at the planning of the K-selective ionophore BME-44 (2) and at its synthesis starting from benzo-15-crown-5 (B15C5) (1). In order to study how the NH group of the molecules of type 2 influences the complexation it was necessary to synthesize the analogue of type 4 [3]. To clear up the role of the NO2 group we have synthesized the derivatives of type 5 [4,5] having the similar electron-withdrawing ester group which is also sufficiently lipophilic in the case of an appropriate R substituent. Both these types as target molecules have the same 3 acetyl derivative (4'Ac-B15C5) as key intermediate. The obvious way to prepare the latter compound is the acetylation of _!, therefore it was extremely important to develop a well controllable procedure for the preparation of 3. Acylation of B15C5 in a Friedel-Crafts type reaction with the classical Lewis acid AICI3 can be performed only with poor yields (about 30%) in the presence of a large excess of ' present address: Institiit FrauQais du Petiole, F-92506 Rueil-Mabnaison, France •This work was supported by the VARGA JOZSEF Foundation of the Technical University of Budapest and by the Hungarian Science Foundation OTKA (grant No. T-015673 and T-015677).
84 catalyst (catalyst/substrate ratio is about 5.5) [6] because of the formation of a stable AlCl,crown ether complex [7]. In reality the acetylation can be carried out with P2O5 or in polyphosphoric acid [8,9]. Both homogeneous catalytic reactions give rise to serious corrosion and environmental problems and their practical application is therefore very limited. Substitution of mineral acids by solid acids as catalysts can reduce these difficulties. Moreover solid catalysts offer by their nature other advantages such as an easier separation and work-up because of the heterogeneous system.
C"" N^V^"^ """^W
"^
<^o
^
^^^^"
H N ^ o
BME.44 FLUKA product No. 60397
.y...
o - c^ -?0
85 Mesoporous clays, such as the commercial catalyst KIO, ion exchanged with transition metal cations (Me"^-K10) were active for benzylation of aromatic compounds [10]. The reaction rate appeared to be related not to the acidity of the solid, but to its redox properties. In other words, the formation of the carbocation involved a redox step of the type: O-CH2CI + Fe^^ -^ Fe"^ + O-CHjCl"^"
a)-CH2Ci'^" -^ o-CH2'^ + c r Cr + F e ' ^ - > F e ' V c r Such a mechanism for C-Cl activation should be valid for acyl chlorides and we reported recently a heterogeneous catalytic acetylation of B15C5 with an acceptable 32% yield of isolated product, using Cu^^-KlO [11].
C "
AcCl, 83°C (ill 1,2-dichloroethane)
»
o
CH,
Cu^^-K 10 catalyst
This work presents an optimization of the reaction and an extension to other catalysts in order to clarify the role of acidity and achieve a yield high enough for production purposes. 3. RESULTS AND DISCUSSION Previous work was performed with a fairly diluted reaction mixture, and it was first attempted to increase the total concentration in order to reduce the volume/product ratio. Under unchanged component ratios two steps of increase could be realized (Table 1) with the limiting amount of the solid material (about 17 g catalyst for 100 cm^ solvent) provided that sufficient stirring was used. The gained 9% B15C5 concentration in the reaction mixture is already a useful level for fine chemical synthesis. Table 1 Development in reaction conditions for Cu^^-KlO catalyst B15C5 concentration g/lOOcm^
Component ratio mol/mol CU^VB15C5
ACC1/B15C5
4'Ac-B15C5 % By-products (time to reach the final level) %
0.18
0.2
20
42
(3h)
traces
3.6
0.2
20
66
(Ih)
11
oa
5
60
(2h)
10
0.1
5
64
(1.5h)
liiH^^^^
8
After the first concentration jump it was established that both the catalyst/B15C5 ratio and the AcCl excess could be reduced, at practically unchanged yield (determined by HPLC) and by-product level. The amount of the by-products could not be reduced by decreasing the reaction temperature to 60°C. We have also examined how catalysts exchanged with other transition metal cations can be used for this acetylation reaction. ZnClj is good catalyst of special Friedel-Crafts reactions (activated aromatic compounds, e.g. phenols) therefore Zn -exchanged KIO was one of the selection. The results of two hours reaction time are presented in Table 2. The reaction conditions are the same as found best for Cu'^^-KlO (3.6 g B15C5/100cm', 0.1 mol Me"Vmol B15C5, 5 mol AcCl/mol B15C5). The optimum conditions for Fe^^-KlO were identical to those determined with Cu^^-KlO. Table 2 Comparison of catalysts containing different exchanged metals 4'Ac-B15C5 By-products
Acidity
rel. int.
%
%
Bronsted
Cu^'^-KlO
60
10
0.13
0.68
Zn^'^-KlO
55
10
0.16
0.73
Fe^'^-KlO
85
14
0.35
0.34
KIO(H^)
45
8
0.33
0.28
Lewis
Cu^^-KlO and Zn^^-KlO catalysts result in practically the same reaction rate and product yield. On the other hand Fe ^-KIO leads the reaction substantially faster (Fig. 1) and till a practically complete conversion, although the amount of the by-products is also higher. With a double quantity of Fe -KIO catalyst the reaction is completed in 30 minutes. For comparison the performance of the original KIO (without ion exchange) is also presented in Table 2. The differences are always significant in favour of the ion exchanged catalysts, so the introduced metal ions play an important role in the activity of the catalyst. These metal ions increase the Lewis acidity of the catalysts [10] but the efficiency does not depend directly on this acidity: Fe -KIO is particularly active although it has a lower Lewis acidity than the two other ion-exchanged catalysts. At the same time the Lewis acidity of the salts in homogeneous phase diminishes in the sequence Fe^^> Zn^^> Cu^^; CuCl2 itself is not used in this type of organic reactions. Nevertheless, the Lewis acidity could play a role because the heat treatment of the catalysts at 250°C is advantageous: in the case ofFe^'^-KlO the reached product yield increases from 55% (with only dried catalyst) to 85%. The question arises why Fe -KIO catalyst has a special reactivity in this reaction. The activity pattern observed on the same series of catalyst samples for the alkylation of aromatics by benzyl chloride i.e. Fe^^-KlO > Zn^'^-KlO > Cu^'^-KlO » KIO is compatible with the activities observed here, and suggests that the mechanism of formation of the carbocation could be similar, therefore occiu-s by oxidation.
87 4'Ac-B15C5 (%) UU -| ^,
80 60-
•—•
_.
^Zi
__ ^ — 5
4020 0 i (.
y
, 0.5
,
,
Fe3+-K10 -Cu2+-K10 - - 3 K - Zn2+-K10 "ik' •KIO -
->Zi--
,
1.5 Reaction time (h)
Figure 1. Activity of the investigated catalysts. In order to evaluate the reactivity of Fe^^-KlO it is worth mentioning that KIO itself contains Fe in the octahedral layer (1.97%, Table 3), which has become at least partly accessible upon steam treatments or acid leaching during the manufacturing process. This Fe could be one reason of its activity. The exchanged Fe ^ represents a smaller amount (0.95%) but it is deposited at the surface, certainly more accessible then far more effective since the final conversion rises from 45 to 85%. The usual work-up procedure of the reaction mixture made with Fe^^-KlO under optimum conditions (Table 2) resulted in 4'Ac-B15C5 with 55% yield. This result gives a practical preparative method which is really competitive with the 65-70% yield of the P2O5 or polyphosphoric acid acylation methods and eliminates their drawbacks: difficult handling and mixing because of high viscosity, costly neutralization. 4. EXPERIMENTAL KIO clay is manufactured by high temperature acidic treatment from bavarian montmorillonite and was purchased from Siid Chemie as original sample. Cation exchange was performed by gradually adding 10 g KIO clay to 125 cm^, 1 mol/1 stirred solution of CuClj, FeClg or ZnCl2 at room temperature and stirring the suspension for 24 hours. After exchange, the suspensions were filtered and washed with deionised water. The resulting solids were dried on a thin bed at 100°C and ground. Specific surface areas were calculated fi'om BET nitrogen isotherms determined at -196°C on samples degassed at 250°C for 12h before the experiment. Chemical analyses were obtained by plasma emission spectroscopy. Acidity of the catalysts was measured by intensity of the IR bands of pyridine coordinated to Bronsted and Lewis sites respectively. For a quantitative characterization the area of the absorption bands was related to the area of a structural band of the clay in the same spectral region (values indicated as rel. int.) [10].
88
Table 3 Some characteristic data of the catalysts Catalyst
Specific surface Surface area of Fe content Metal retained Charge exchanged area micropores m^/g m^/g wt % wt% meq/g cat
KIO
229
2.5
1.97
Cu^'^-KlO 236
-
Cu''^=1.24
0.39
Zn^'^-KlO 213
-
Zn^'^=1.19
0.36
Fe^'^-KlO 239
10
Fe^"^ = 0.95
0.54
2.92
General reaction conditions [11]: Acetylation was made by acetyl chloride in 15 cm^ 1,2dichloroethane at 83°C (boiling point) using a batch reactor. The catalysts were normally heat treated at 250°C. After filtration the solvent was evaporated in vacuo, from the residue 3x15 cm^ dichloroethane was distilled to eliminate the traces of acetyl chloride. The dark, thick residual oil was repeatedly extracted with boiling n-heptane. After cooling the crystals were filtered. Yield: 55% isolated product (in the case ofFe' -KIO catalyst). The reactions were monitored by HPLC (CI8 reversed phase column, eluent methanolwater 50:50 v/v, UV-detection at 254 nm). 5. CONCLUSIONS Benzo-15-crown-5 has been acetylated efficiently using KIO clays ion-exchanged by Cu ^, Fe^^ or Zn^^. The best results were obtained with Fe^^-KlO. The catalytic properties are not related to the acidity of the solid and it can therefore be admitted that the C-Cl bond is activated by a redox mechanism, as proposed earlier for the alkylation of aromatic compounds by benzyl chloride. The results give a practical preparative method, competitive with the P2O5 or polyphosphoric acid acylation processes. The method can be generalized and was successfully applied in acetylation of other benzo-crown compounds.
REFERENCES 1. E. Lindner, K. Toth, J. Jeney, M. Horvath, E. Pungor, I. Bitter, B. Agai and L. Toke, Microchim. Acta I., (1990) 157. 2. J. Beger and M. Meerbote, J. prakt. Chem., 327 (1985) 2. 3. L. Toke, I. Bitter, B. Agai, E. Csongor, K. Toth, E. Lindner, M. Horvath, S. Harfouch and E. Pungor, .Justus Liebigs Annalen der Chemie, (1988) 349. 4. L. Toke, L Bitter, B. Agai, Z. Hell, E. Lindner, K. Toth, M. Horvath, S. Harfouch and E. Pungor, Justus Liebigs Annalen der Chemie, (1988) 549.
89 5. K. Toth, E. Lindner, M. Horvath, J. Jeney, I. Bitter, B. Agai, T. Meisel and L. Toke, Anal. Lett., 22 (1989) 1185. 6. K. Szabo, Diploma Thesis, Technical University of Budapest, 1990. 7. F. Wada and T. Matsuda, Bull. Chem. Soc. Jpn., 53 (1980) 421. 8. W.W. Paris, P.E. Stott, C.W. McCausland and J.S. Bradshaw, J. Org. Chem., 43 (1978) 4577. 9. S. Kano, T. Yokomatsu, H. Nemoto and S. Shibuya, Tetrahedron Lett., 26 (1985) 1531. lO.T. Cseri, S. Bekassy, F. Figueras and S. Rizner, J. Mol. Catal. A: Chemical, 98 (1995) 101. 1 l.T. Cseri, S. Bekassy, Z. Bodas, B. Agai and F. Figueras, Tetrahedr. Lett., 37 (1996) 1473.
83
Modified clay catalysts for acylation of crown compounds S. Bekassy^ K. Bir6^ T. Cseri''^'^ B. Agai' and F. Figueras^ ^Department of Organic Chemical Technology, Technical University of Budapest, H-1521 Budapest, Hungary ^Institut de Recherches sur la Catalyse du CNRS, 2 Avenue Albert Einstein, F-69626 Villeurbanne, France
1. SUMMARY Heterogeneous catalytic acetylation of benzo-15-crovm-5 was investigated using different ion exchanged KIO catalysts. The nature of the transition metal ions introduced in the mesoporous clay plays an important role in the activity of the catalysts. Fe^^-KlO is particularly advantageous for practical preparative purposes. 4'-Acetyl-benzo-15-crown-5 was isolated with 55% yield under optimal conditions.
2. INTRODUCTION Crown ethers are well known for their ability to form strong complexes with alkali metal and organic cations. Introduction of lipophilic substituents into crown ethers has a great importance e. g. for their application as electroactive substances in ion-selective electrodes [1] and as complexing agents for recovering precious or radioactive metal cations [2]. These substituents can be introduced into benzo annelated crown ethers by S^ reactions (alkylation, acylation, nitration, halogenation). The introduced substituents modify the complexing properties of the crown ether macroring through the aromatic ring. We have taken an advantage of this phenomenon at the planning of the K-selective ionophore BME-44 (2) and at its synthesis starting from benzo-15-crown-5 (B15C5) (1). In order to study how the NH group of the molecules of type 2 influences the complexation it was necessary to synthesize the analogue of type 4 [3]. To clear up the role of the NO2 group we have synthesized the derivatives of type 5 [4,5] having the similar electron-withdrawing ester group which is also sufficiently lipophilic in the case of an appropriate R substituent. Both these types as target molecules have the same 3 acetyl derivative (4'Ac-B15C5) as key intermediate. The obvious way to prepare the latter compound is the acetylation of _!, therefore it was extremely important to develop a well controllable procedure for the preparation of 3. Acylation of B15C5 in a Friedel-Crafts type reaction with the classical Lewis acid AICI3 can be performed only with poor yields (about 30%) in the presence of a large excess of ' present address: Institiit FrauQais du Petiole, F-92506 Rueil-Mabnaison, France •This work was supported by the VARGA JOZSEF Foundation of the Technical University of Budapest and by the Hungarian Science Foundation OTKA (grant No. T-015673 and T-015677).
84 catalyst (catalyst/substrate ratio is about 5.5) [6] because of the formation of a stable AlCl,crown ether complex [7]. In reality the acetylation can be carried out with P2O5 or in polyphosphoric acid [8,9]. Both homogeneous catalytic reactions give rise to serious corrosion and environmental problems and their practical application is therefore very limited. Substitution of mineral acids by solid acids as catalysts can reduce these difficulties. Moreover solid catalysts offer by their nature other advantages such as an easier separation and work-up because of the heterogeneous system.
C"" N^V^"^ """^W
"^
<^o
^
^^^^"
H N ^ o
BME.44 FLUKA product No. 60397
.y...
o - c^ -?0
85 Mesoporous clays, such as the commercial catalyst KIO, ion exchanged with transition metal cations (Me"^-K10) were active for benzylation of aromatic compounds [10]. The reaction rate appeared to be related not to the acidity of the solid, but to its redox properties. In other words, the formation of the carbocation involved a redox step of the type: O-CH2CI + Fe^^ -^ Fe"^ + O-CHjCl"^"
a)-CH2Ci'^" -^ o-CH2'^ + c r Cr + F e ' ^ - > F e ' V c r Such a mechanism for C-Cl activation should be valid for acyl chlorides and we reported recently a heterogeneous catalytic acetylation of B15C5 with an acceptable 32% yield of isolated product, using Cu^^-KlO [11].
C "
AcCl, 83°C (ill 1,2-dichloroethane)
»
o
CH,
Cu^^-K 10 catalyst
This work presents an optimization of the reaction and an extension to other catalysts in order to clarify the role of acidity and achieve a yield high enough for production purposes. 3. RESULTS AND DISCUSSION Previous work was performed with a fairly diluted reaction mixture, and it was first attempted to increase the total concentration in order to reduce the volume/product ratio. Under unchanged component ratios two steps of increase could be realized (Table 1) with the limiting amount of the solid material (about 17 g catalyst for 100 cm^ solvent) provided that sufficient stirring was used. The gained 9% B15C5 concentration in the reaction mixture is already a useful level for fine chemical synthesis. Table 1 Development in reaction conditions for Cu^^-KlO catalyst B15C5 concentration g/lOOcm^
Component ratio mol/mol CU^VB15C5
ACC1/B15C5
4'Ac-B15C5 % By-products (time to reach the final level) %
0.18
0.2
20
42
(3h)
traces
3.6
0.2
20
66
(Ih)
11
oa
5
60
(2h)
10
0.1
5
64
(1.5h)
liiH^^^^
8
After the first concentration jump it was established that both the catalyst/B15C5 ratio and the AcCl excess could be reduced, at practically unchanged yield (determined by HPLC) and by-product level. The amount of the by-products could not be reduced by decreasing the reaction temperature to 60°C. We have also examined how catalysts exchanged with other transition metal cations can be used for this acetylation reaction. ZnClj is good catalyst of special Friedel-Crafts reactions (activated aromatic compounds, e.g. phenols) therefore Zn -exchanged KIO was one of the selection. The results of two hours reaction time are presented in Table 2. The reaction conditions are the same as found best for Cu'^^-KlO (3.6 g B15C5/100cm', 0.1 mol Me"Vmol B15C5, 5 mol AcCl/mol B15C5). The optimum conditions for Fe^^-KlO were identical to those determined with Cu^^-KlO. Table 2 Comparison of catalysts containing different exchanged metals 4'Ac-B15C5 By-products
Acidity
rel. int.
%
%
Bronsted
Cu^'^-KlO
60
10
0.13
0.68
Zn^'^-KlO
55
10
0.16
0.73
Fe^'^-KlO
85
14
0.35
0.34
KIO(H^)
45
8
0.33
0.28
Lewis
Cu^^-KlO and Zn^^-KlO catalysts result in practically the same reaction rate and product yield. On the other hand Fe ^-KIO leads the reaction substantially faster (Fig. 1) and till a practically complete conversion, although the amount of the by-products is also higher. With a double quantity of Fe -KIO catalyst the reaction is completed in 30 minutes. For comparison the performance of the original KIO (without ion exchange) is also presented in Table 2. The differences are always significant in favour of the ion exchanged catalysts, so the introduced metal ions play an important role in the activity of the catalyst. These metal ions increase the Lewis acidity of the catalysts [10] but the efficiency does not depend directly on this acidity: Fe -KIO is particularly active although it has a lower Lewis acidity than the two other ion-exchanged catalysts. At the same time the Lewis acidity of the salts in homogeneous phase diminishes in the sequence Fe^^> Zn^^> Cu^^; CuCl2 itself is not used in this type of organic reactions. Nevertheless, the Lewis acidity could play a role because the heat treatment of the catalysts at 250°C is advantageous: in the case ofFe^'^-KlO the reached product yield increases from 55% (with only dried catalyst) to 85%. The question arises why Fe -KIO catalyst has a special reactivity in this reaction. The activity pattern observed on the same series of catalyst samples for the alkylation of aromatics by benzyl chloride i.e. Fe^^-KlO > Zn^'^-KlO > Cu^'^-KlO » KIO is compatible with the activities observed here, and suggests that the mechanism of formation of the carbocation could be similar, therefore occiu-s by oxidation.
87 4'Ac-B15C5 (%) UU -| ^,
80 60-
•—•
_.
^Zi
__ ^ — 5
4020 0 i (.
y
, 0.5
,
,
Fe3+-K10 -Cu2+-K10 - - 3 K - Zn2+-K10 "ik' •KIO -
->Zi--
,
1.5 Reaction time (h)
Figure 1. Activity of the investigated catalysts. In order to evaluate the reactivity of Fe^^-KlO it is worth mentioning that KIO itself contains Fe in the octahedral layer (1.97%, Table 3), which has become at least partly accessible upon steam treatments or acid leaching during the manufacturing process. This Fe could be one reason of its activity. The exchanged Fe ^ represents a smaller amount (0.95%) but it is deposited at the surface, certainly more accessible then far more effective since the final conversion rises from 45 to 85%. The usual work-up procedure of the reaction mixture made with Fe^^-KlO under optimum conditions (Table 2) resulted in 4'Ac-B15C5 with 55% yield. This result gives a practical preparative method which is really competitive with the 65-70% yield of the P2O5 or polyphosphoric acid acylation methods and eliminates their drawbacks: difficult handling and mixing because of high viscosity, costly neutralization. 4. EXPERIMENTAL KIO clay is manufactured by high temperature acidic treatment from bavarian montmorillonite and was purchased from Siid Chemie as original sample. Cation exchange was performed by gradually adding 10 g KIO clay to 125 cm^, 1 mol/1 stirred solution of CuClj, FeClg or ZnCl2 at room temperature and stirring the suspension for 24 hours. After exchange, the suspensions were filtered and washed with deionised water. The resulting solids were dried on a thin bed at 100°C and ground. Specific surface areas were calculated fi'om BET nitrogen isotherms determined at -196°C on samples degassed at 250°C for 12h before the experiment. Chemical analyses were obtained by plasma emission spectroscopy. Acidity of the catalysts was measured by intensity of the IR bands of pyridine coordinated to Bronsted and Lewis sites respectively. For a quantitative characterization the area of the absorption bands was related to the area of a structural band of the clay in the same spectral region (values indicated as rel. int.) [10].
88
Table 3 Some characteristic data of the catalysts Catalyst
Specific surface Surface area of Fe content Metal retained Charge exchanged area micropores m^/g m^/g wt % wt% meq/g cat
KIO
229
2.5
1.97
Cu^'^-KlO 236
-
Cu''^=1.24
0.39
Zn^'^-KlO 213
-
Zn^'^=1.19
0.36
Fe^'^-KlO 239
10
Fe^"^ = 0.95
0.54
2.92
General reaction conditions [11]: Acetylation was made by acetyl chloride in 15 cm^ 1,2dichloroethane at 83°C (boiling point) using a batch reactor. The catalysts were normally heat treated at 250°C. After filtration the solvent was evaporated in vacuo, from the residue 3x15 cm^ dichloroethane was distilled to eliminate the traces of acetyl chloride. The dark, thick residual oil was repeatedly extracted with boiling n-heptane. After cooling the crystals were filtered. Yield: 55% isolated product (in the case ofFe' -KIO catalyst). The reactions were monitored by HPLC (CI8 reversed phase column, eluent methanolwater 50:50 v/v, UV-detection at 254 nm). 5. CONCLUSIONS Benzo-15-crown-5 has been acetylated efficiently using KIO clays ion-exchanged by Cu ^, Fe^^ or Zn^^. The best results were obtained with Fe^^-KlO. The catalytic properties are not related to the acidity of the solid and it can therefore be admitted that the C-Cl bond is activated by a redox mechanism, as proposed earlier for the alkylation of aromatic compounds by benzyl chloride. The results give a practical preparative method, competitive with the P2O5 or polyphosphoric acid acylation processes. The method can be generalized and was successfully applied in acetylation of other benzo-crown compounds.
REFERENCES 1. E. Lindner, K. Toth, J. Jeney, M. Horvath, E. Pungor, I. Bitter, B. Agai and L. Toke, Microchim. Acta I., (1990) 157. 2. J. Beger and M. Meerbote, J. prakt. Chem., 327 (1985) 2. 3. L. Toke, I. Bitter, B. Agai, E. Csongor, K. Toth, E. Lindner, M. Horvath, S. Harfouch and E. Pungor, .Justus Liebigs Annalen der Chemie, (1988) 349. 4. L. Toke, L Bitter, B. Agai, Z. Hell, E. Lindner, K. Toth, M. Horvath, S. Harfouch and E. Pungor, Justus Liebigs Annalen der Chemie, (1988) 549.
89 5. K. Toth, E. Lindner, M. Horvath, J. Jeney, I. Bitter, B. Agai, T. Meisel and L. Toke, Anal. Lett., 22 (1989) 1185. 6. K. Szabo, Diploma Thesis, Technical University of Budapest, 1990. 7. F. Wada and T. Matsuda, Bull. Chem. Soc. Jpn., 53 (1980) 421. 8. W.W. Paris, P.E. Stott, C.W. McCausland and J.S. Bradshaw, J. Org. Chem., 43 (1978) 4577. 9. S. Kano, T. Yokomatsu, H. Nemoto and S. Shibuya, Tetrahedron Lett., 26 (1985) 1531. lO.T. Cseri, S. Bekassy, F. Figueras and S. Rizner, J. Mol. Catal. A: Chemical, 98 (1995) 101. 1 l.T. Cseri, S. Bekassy, Z. Bodas, B. Agai and F. Figueras, Tetrahedr. Lett., 37 (1996) 1473.