A Polymeric Heterogeneous Catalyst Based on ...

Iranian Polymer Journal

Available online at: http://journal.ippi.ac.ir

15 (4), 2006, 331-339

A Polymeric Heterogeneous Catalyst Based on Polyacrylamide for Knoevenagel Reaction in Solvent Free and Aqueous Media Bahman Tamami* and Abdulhamid Fadavi Chemistry Department, Shiraz University, Shiraz-71454, Iran Received 21 December 2005; accepted 4 February 2006

ABSTRACT odified polyacrylamide containing amino functionality was synthesized by transamidation of polyacrylamide with 1,6-diamino hexane. The catalyst was easily characterized by routine analytical methods. It proved to be an efficient and chemoselcetive heterogeneous catalyst for Knoevenagel condensation of aromatic and aliphatic aldehydes with ethyl cyanoacetate, malononitrile and cyanoacetamide in water and solvent-free system to afford the desired alkenes in good purity and yields. Ketones were almost unreactive. The reactivity of the catalyst increased considerably in protic polar solvents such as water and ethanol. The order of reactivity of methylene compounds was malononitrile> cyano ethylacetate> cyanoacetamide. The results showed that the polymeric catalyst could be regenerated and used many times. The green and mild reaction condition, medium to short reaction time, high to excellent yields, low cost, readily preparation, and easy work up of the reactions are the main advantages of this catalyst.

M

Key Words: amino modiefied; polyacrylamide; Knoevenagel condensation; solvent free.

INTRODUCTION

(*) To whom correspondence shxould be addressed. E-mail: [email protected]

Nowadays, chemical processes employ large amounts of hazardous and toxic solvents. One of the challenges for the chemists is to come up with new approaches that are less hazardous to human and environment. The choice of pursuing a low

waste route and reusable reaction media, and minimize the economic cost and environmental impact of a chemical process, are becoming ever more urgent for the future. One of the most promising approaches uses water as reaction medium. Advan-

Tamami B. et al.

A Polymeric Heterogeneous Catalyst Based on...

tages of catalytic reactions in green aqueous or solvent free systems include low cost, inflammability, no toxicity, availability, and safety. The use of aqueous medium in organic synthesis is not long-standing and, because of its advantages, a rapid development is expected in the future. The Knoevenagel condensation of aldehydes with active methylene compounds is an important and widely employed method for carbon-carbon bond formation in organic synthesis [1-3], with numerous applications in the synthesis of fine chemicals [4], hetero DielsAlder reactions [5,6], and in synthesis of carboxylic as well as heterocyclic [7] compounds of biological significance. These reactions are usually catalysed by bases [8,9] such as amines, ammonia or sodium ethoxide in organic solvents. Heterogeneous catalysts have also been used for the Knoevenagel reaction thus, the reaction has been catalyzed by heterogeneous catalysts based on alumina [10-12], silica [13], zinc and magnesium oxides [14], resins [15], and other catalysts with more or less success [16-28]. The use of environmentally benign solvents like water [28] and solvent-free reactions represent very powerful green chemical technology procedures from both the economical and synthetic point of view. They not only reduce the burden of organic solvent disposal, but also may enhance the rate of many organic reactions. Efforts have been made to perform the Knoevenagel condensation in aqueous media as well as in the absence of solvents [24,31,32] which are usually catalysed by Lewis acids [31] and require drastic conditions [24,32]. Some of these reactions are performed on solid supports, promoted by infrared [33], ultrasound or microwave [34] heating. Recently we have focused our attention on the use of modified polyacrylamides as catalysts in organic synthesis [35-42]. Very recently we reported in a short communication [42] the Knoevenagel condensation of aromatic aldehydes with ethyl cyanoacetate in water and solvent free systems. Here, we report fully in details very simple and highly efficient methods for the condensation of various aromatic and aliphatic aldehydes with a variety of active methylene compounds such as malononitrile, cyanoacetamide and ethyl cyanoacetate under aqueous and solvent free conditions in the presence of catalytic amount of a modified polyacrylamide.

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EXPERIMENTAL Materials Solvents, reagents and chemicals were obtained from Merck (Germany) and Fluka (Switzerland) Chemical Companies. Acrylamide was recrystallized from chloroform. Divinylbenzene (55%) was washed with sodium hydroxide solution (1%, 2 × 10 mL) and water (3 × 20 mL) to remove the inhibitor. Benzoyl peroxide was purified. Gas chromatography analysis was recorded on a Shimadzu GC 14-A. IR Spectra were run on a FTIR-8000 Shimadzu Spectrometer. 1H NMR Spectra were recorded on a Bruker Avance DPX instrument (250 MHz). All products characterized by comparison of their IR and NMR spectra and physical data with those reported in the literature. All yields refer to the isolated products. Preparation of Cross-linked Polyacrylamide In a round bottom flask was placed divinylbenzene (3.9 g, 0.03 mol) and acrylamide (60.4 g, 0.85 mol) in ethanol (250 mL). Benzoyl peroxide (350 mg, 1.4 mmol) was added and the mixture while stirring, was heated to 70-75oC for 5 h. The resulting white fine powder polymer was collected by filtration, washed several times with water, ethanol, benzene, and tetrahydrofuran, and finally dried at 60oC under reduced pressure. The IR spectrum of the polymer showed the characteristic absorption of amide (N-H) at 3200 and 3300 cm-1. amide I and amide II bands at 1662 cm-1 and 1556 cm-1 can also be seen [36]. Preparation of Poly [N- (6-aminohexyl)-acrylamide] Polyacrylamide (10.0 g) was immerged in dioxan (50 mL) for 12 h. 1,6-Diaminohexane (16.0 g) was added to the suspension and the mixture kept at 100oC for 2 days. The reaction mixture was cooled, filtered and washed with hot water followed by cold water to remove the unreacted 1,6-diaminohexane. The modified polymer was finally washed with chloroform, methanol and acetone and finally dried at 50oC in vacuum to constant weight. The IR spectrum of the amino functionalized polyacrylamide showed the characteristic absorption of (N-H) amino group at 3400 cm-1 and amide I and amide II bands at 1660 cm-1 and 1550 cm-1, respectively (Figure 1). The amino group content was determined by back titration and gravimetric methods and was

Iranian Polymer Journal / Volume 15 Number 4 (2006)

Tamami B. et al.


3.2 mmol amino group per gram of polymer. Typical Procedure for the Solvent-free Knoevenagel Reaction

Figure 1. IR Spectrum of 2% cross-linked poly[N-(6-aminohexyl)-acrylamide].

found to be equal to 3.2 mmol/g. Poly [N-(2-aminoethyl)-acrylamide] (APA) was also prepared and characterized according to the same procedure using ethylenediamine. Its amino content was 3.7 mmol/g. Typical Procedure for The Determination of Capacity of Poly[N-(6-aminohexyl)-acrylamide]: Gravimetric Method:

The reaction procedure was repeated using 2% crosslinked polyacrylamide (1.00 g). The product was weighed carefully and was found to be 1.55 g. The calculated amine value was 3.2 mmol of NH2 per gram of polymer. Reverse Titration Method:

HCl (10 mL, 0.5M) (Merck) was added to 2% crosslinked poly[N-(6-aminohexyl)-acrylamide] (0.10 g) and the mixture stirred at room temperature for 12 h. Then the mixture was filtered and washed with water (3 × 10 mL). The filtrate was titrated for the remaining HCl against freshly prepared solution of NaOH (0.165M, standardized against 0.50M HCl, Merck). The average amount of NaOH used was 29.25 mL, and the calculated average amine value of the resin was

In a test tube was placeed a mixture of an aromatic aldehyde (5 mmol), cyanoacetamide (5 mmol) and poly [N-(6-aminohexyl)-acrylamide] (0.31 g, 1.0 mmol). The mixture was heated at 60oC for the lengths of time shown in Table 3. Completion of the reaction was determined by TLC, using n-hexane/THF (5:1) as eluent and/or by GC. The mixture was poured in the ethanol and stirred for 3 min. Workup accomplished by filtration and evaporation. The product was identified with 1H NMR and IR techniques. Typical Procedure for the Knoevenagel Reaction in Aqueous Media

In a round bottom flask was placed a mixture of an aromatic aldehyde (1mmol), malononitrile (1 mmol) in H2O (10 mL) and poly [N- (6-aminohexyl) acrylamide. (0.06 g, 0.2 mmol). The suspension was stirred at 80oC for the lengths of time shown in Table 3. Progress and completion of the reaction was monitored by TLC, using n-hexane/THF (5:1) as eluent and/or by GC. The mixture was cooled to 10oC, for solidification of the product, and water was removed by Buchner filtration and the residual solids were heated in ethanol for 5 min. The catalyst was removed by filtration and washed with ethanol. Solvent was evaporated, and the pure product was obtained. When the reaction was incomplete the product was isolated by coloumn chromatography using n-hexane/THF (5:1) as eluent. The product was identified with 1H NMR and IR spectroscopy techniques.

RESULTS AND DISCUSSION Cross-linked polyacrylamide was prepared by free radical solution polymerization of the monomer mixture in ethanol using benzoylperoxide and divinylbenzene as

Scheme I.


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Table 2. The effect of molar ratio of the polymeric catalyst on the Knoevenagel condensation in aqueous mediaa,b. Time

Yield

(h:m)

(%)

No catalyst

5:00

----

0.05

5:00

60

0.10

5:00

75

0.20

2:00

95

Cat/Substrate

Scheme II.

cross-linking agent (Scheme I) [30]. The IR spectrum of the polymer showed peaks at 3500 cm-1 (NH amide) and 1660 cm-1 (CO amide). Amino modified polyacrylamide was obtained by transamidation reaction of cross-linked polyacrylamide with excess amount of 1,6-diaminohexane (Scheme I). The amino resin was characterized by IR spectroscopy (Figure 1) and semiquantitative ninhydrin reaction. The characteristic peaks of amides I and II and amino group were observed at 1550, 1660 and 3400 cm-1, respectively. The resin gave a deep blue color on heating with ninhydrin reagent. The content of amino functionality of the resin was determined by back titration as 3.2 mmol/g. To develop the synthetic utility of this modified polyacrylamide as a heterogeneous catalyst for the Knoevenagel reaction, several aromatic and aliphatic aldehydes were reacted with ethyl cyanoacetate, cyanoacetamide, and malononotrile Scheme II. The effects of solvent and molar ratio of the polymeric catalyst on a typical knoevenagel reaction were investigated (Tables 1 and 2). The effect of different solvents was investigated by carrying out the reactions in tetrahydrofuran, toluene, dichloromethane, cyclohexane, dioxane, Table 1. The effect of solvent on the Knoevenagel condensationa with the polymeric catalyst (APA)b. Time

Yield

(h:m)

(%)

-----

0:15

100

H 2O

2:30

100

EtOH

2:00

100

CH3CN

5:00

89

CH2Cl2

12:00

5

CHCl3

12:00

5

THF

12:00

10

Cyclohexane

12:00

5

Solvent

(a) for reaction of benzaldehyde and cyanoacetamide; (b) Poly [N-(6-amino-

hexyl)-acrylamide] as catalyst.

334

(a) for reaction of benzaldehyde and cyanoacetamide in water; (b) Poly [N-(6-

aminohexyl)-acrylamide] as catalyst.

ethanol ,and water. The two last solvent proved to be the best. Surprisingly it was observed that although Knoevenagel condensation is a net dehydration reaction, our reaction easily occurred in water, the fact that has been also reported previously by other authors with different catalyst systems [43-45]. In addition, unexpected results were obtained when we examined the same reactions under solvent-free condition. The reaction occurred much faster with higher yields and easier workup compared to aqueous system. It was found that the optimum molar ratio of the polymeric catalyst to aldehyde for both conditions is 0.2:1. The desired knoevenagel products were obtained in excellent yields in the presence of the polymeric catalyst for various aromatic aldehydes containing electron donating and withdrawing groups in aqueous and solvent-free systems using three active methylene compounds (Table 3). However the reaction was slower and low yielding for aliphatic aldehydes compared to aromatic aldehydes in the case of cyanoacetamide and ethyl cyanoacetate in aqueous medium. This effect was not observed for malononitrile under both conditions. Therefore, it was possible to do chemoselective Knoevenagel condensation of aliphatic and aromatic aldehydes in the case of cyanoacetamide and ethyl cyanoacetate in aqueous system only, examples are shown by Schemes III and IV. The Knoevenagel condensation failed to proceed with ketones for the case of cyanoacetamide and ethyl cyanoacetate under both conditions but not for malononitrile (Table 3) probably due to weaker electron withdrawing property of amido and ester groups compared to the cyano group. It is notable that we did not observe any side reactions such as Michael addition and hydrolysis of amide and cyano moieties. It is obvi-


Tamami B. et al.


Table 3. Knoevenagel condensation of different carbonyl compound with malononitrile, cyanoacetamide and cyano ethylacetate in the presences of the polymeric catalysta,b,c. Solvent-free

Water

Time

Yield

(h:m)

(%)

Carbonyl compound

b

Time

Yieldb

h:m)

(%)

A

B

C

A

B

C

A

B

C

A

B

C

Ph-CHO

0:05

0:15

0:10

100

100

100

0:30

2:30

2:0

100

100

98

p-Cl-C6H4-CHO

0:05

0:15

0:08

100

100

100

0:30

2:30

1:35

100

100

99

o-Cl- C6H4-CHO

0:07

0:20

0:12

100

98

95

0:40

3:0

2:30

100

95

90

p -NO2 C6H4-CHO

0:05

0:15

0:08

100

100

100

0:30

2:30

1:35

100

100

94

m-NO2 C6H4-CHO

0:05

0:15

0:10

100

100

99

0:30

2:30

1:40

100

100

93

o-NO2- C6H4-CHO

0:07

0:20

0:14

100

100

95

0:40

3:0

1:55

100

100

86

p-OMe- C6H4-CHO

0:10

0:25

0:16

95

92

99

0:50

3:0

2:20

96

93

97

p-Me- C6H4-CHO

0:10

0:25

0:12

97

94

99

0:50

3:0

2:30

99

98

98

PhCH=CHCHO

0:15

0:25

0:18

98

97

99

1:0

3:30

3:20

98

96

94

p-NMe2- C6H4-CHO

0:15

0:25

0:18

98

96

96

1:0

3:30

3:50

97

95

90

p-OH- C6H4-CHO

0:15

0:25

0:20

97

96

97

1:0

3:30

4:20

95

91

90

n-Pr-CHO

0:10

0:30

0:25

95

93

92

0:30 12:0

10:0

96

15

27

i-Pr-CHO

0:10

0:30

0:30

94

92

89

0:30 12:0

12:0

96

15

23

Furfural

0:05

0:15

0:10

100

100

100

2:0

100

100

93

PhCOCH3

0:30

6:0

6:0

85

0

0

2:0

12:0

6:0

80

0

0

Cyclohexanone

0:20

6:0

6+:0

91

0

0

1:0

12:0

6:0

85

0

0

A= CH2(CN)2

B= CH2(CONH2)CN

0:30

2:30

C=CH2(CO2Et)CN

(a) molar ratio of cat/subst is 0.2; (b) Yields refer to pure isolated products. The products were identified by comparision of other IR and 1H NMR

spectra and physical data with those of the authentic samples; (c) Poly [N-(6-aminohexyl)-acrylamide] as catalyst.

ous that chemoselective reactions can also be done between a ketone and aldehyde, in the case of cyanoacetamide and ethyl cyanoacetate. Effect of the length of side-chain spacers was also

studied. There was a moderate increase in knoevenagel reaction rate when poly[N-(6-aminohexyl)-acrylamide] was used compared to poly [N-(2-aminoethyl)-acrylamide]. This may be related to more freedom of the amino

Scheme III. Chemoselective Knoevenagel condensation of cyano acetamide in water by APA catalyst.


335

Tamami B. et al.


Scheme IV. Chemoselective Knoevenagel condensation of ethyl cyanoacetate in water by APA catalyst.

The activity of our basic polymeric catalyst under solvent-free and aqueous condition is compared with several other reported catalysts in the literature for the knoevenagel condensation (Table 4). As seen our catalyst is advantageous with respect to time and yield of reac-

groups in the former case. According to the literature [46] and our previous studies [47] the proposed mechanism for this type of amine catalyzed knoevenagel reaction via imine intermediate is given in Scheme V.

Table 4. Comparison of the polymeric catalyst (APA) under solvent-free condition with several heterogeneous catalysts in Knoevenagel condensation of Benzaldehyde with malononitrile (A), cyanoacetamide (B) and ethyl cyanoacetate (C). A Solid catalyst

B

C

Condition Time (min) Yield (%) Time (min) Yield (%) Time (min) Yield (%)

(APA) solvent less (APA) water

p

N

NH [14]

CHCl3/MW

p

N

NH [16]

EtOH

5

100

15

100

10

100

30

100

150

100

120

100

10

78

____

____

10

78

120

87

120

79

120

96

7

94

____

____

15

94

60

100

____

____

120

50

FAP/NaNO3 [20]

solvent less

Mn(III) salen [17]

PhCH3

Al2O3-K90 [8]

solvent less

3

96

10

85

____

____

MgO [12]

solvent less

5

94

200

60

30

94

NaF [18]

solvent less/MW

1

96

ZnCl2 [15]

solvent less

10

88

____

____

90

85

Aminosilica [21]

PhCH3

15

99

____

____

15

100

AlPO4Al2O3 [10]

solventless

15

80

____

____

60

74

ZeoliteHZSM-5 [23]

CH2Cl2

300

80

____

____

360

75

Ionic Liquid [25]

120

93

____

____

60

95

Weak acidic resin [13]

300

92

____

____

300

87

336


1.5

92

1.5

94

Tamami B. et al.


Scheme V. The proposed mechanism for the knoevenagel condensation by the polymeric catalyst.

tions and especially the fact that the reaction medium is water.

ACKNOWLEDGEMENT The authars are thankful to Shiraz University Research Council for partial support of this work.

CONCLUSION Amino functionalized polyacrylamide is presented as an effective and chemoselective heterogeneous catalyst for Knoevenagel condensation of aromatic and aliphatic aldehydes with active methylene compounds in solventfree and aqueous media. The green and mild reaction conditions, medium to short reaction times, high to excellent yields, low cost and easy preparation and handling of the polymeric catalyst (as a bench top catalyst) are the obvious advantages of the present catalyst. Also the work-up is reduced to a mere filtration and evaporation of the solvent. Finally, this polymeric catalyst can be recovered by washing with aqueous solution and uses again at least three times with negligible loss in its activity.

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