Polyaniline supported CuI: An efficient catalyst for C–N bond formation by N-arylation of N(H)-heterocycles and benzyl amines with aryl halides and arylboronic acids, and aza-Michael reactions of amines with activated alkenes

Catalysis Communications 9 (2008) 2226–2230

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Polyaniline supported CuI: An efficient catalyst for C–N bond formation by N-arylation of N(H)-heterocycles and benzyl amines with aryl halides and arylboronic acids, and aza-Michael reactions of amines with activated alkenes M. Lakshmi Kantam a,*, Moumita Roy a, Sarabindu Roy a, Bojja Sreedhar a, Rajib Lal De b a b

Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Uppal Road, Hyderabad 500007, India Department of Chemistry, Jadavpur University, Kolkata 700032, India

a r t i c l e

i n f o

Article history: Received 23 November 2007 Received in revised form 5 April 2008 Accepted 29 April 2008 Available online 13 May 2008

a b s t r a c t Polyaniline supported CuI catalyst was prepared, characterized and effectively used in the N-arylation of N(H)-heterocycles and benzylamines with aryl halides and arylboronic acids and in aza-Michael reaction of amines with activated alkenes. The catalyst was recovered by simple filtration and reused for several cycles with consistent activity. Ó 2008 Elsevier B.V. All rights reserved.

Keywords: Polyaniline Copper N-arylation Aza-Michael reaction Reusable catalyst

1. Introduction Carbon–nitrogen bond forming reactions are one of the most widely used methodologies for the synthesis of biologically active molecules. N-arylheterocycles and benzyl amines are common motifs in pharmaceutical research [1]. Usually these compounds were synthesized via SNAr substitution with aryl halides bearing electron-withdrawing substituent or via the Ullmann type coupling at high temperatures [2]. After the initial reports of Chan and Lam, the Cu-catalyzed cross coupling between N-heterocycles and arylboronic acids has become an important synthetic methodology in modern organic synthesis [3,4]. Later the discovery and development of the catalytic path for N-arylation of heterocycles by Buchwald with bromo- and iodoarenes using copper in the presence of basic ligands [5–8] generated greater interest in industry. Afterwards Taillefer and coworkers reported oxime type as well as Schiff base ligands [9–10] and Ma and coworkers a- and b-amino acids as ligands for effective N-arylation of N-heterocycles with aryl halides [11]. Despite the synthetic elegance these coupling protocols suffer from serious limitations of (1) non-reusability of the catalysts, (2) possible contamination of the product with metal, and (3) using toxic and/or expensive ancillary ligands. One of the most promising solutions to this problem seems to be the * Corresponding author. Tel./fax: +91 40 2716 0921. E-mail address: [email protected] (M.L. Kantam). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.04.023

immobilization of the soluble catalysts onto an insoluble matrix using a simplified protocol, which will allow easy separation and recyclability of the catalyst with minimal amount of product contamination with metal. In this direction, we have already reported reusable copper catalysts such as Cu-exchanged fluoroapatite [12], Cu-exchanged Y-zeolite [13], cellulose supported Cu(0) [14], modified silica supported Cu(OAc)2 [15] and Son et al. [16] reported nano Cu2O coated Cu particles for N-arylation of N(H)-heterocycles with aryl halides and arylboronic acids. Recently Punniyamurthy and coworkers reported ligand free reusable nano CuO [17] and Huang et al. reported commercial Cu2O catalyzed N-arylation of amines [18]. Aza-Michael, (i.e., conjugated addition of nitrogen nucleophiles to a,b-unsaturated compounds) another important C–N bond forming reaction, is used for the synthesis of b-amino carbonyl functionality which serves as an essential intermediate in the synthesis of b-amino ketones, b-amino acids and b-lactam antibiotics [19–20]. Aza-Michael reaction usually requires basic [21–22] or acidic catalysis [23]. But most of the methods reported so far are homogeneous and only handful heterogeneous catalytic systems are reported [24–27]. Polyaniline (PANI), one of the most widely studied conducting polymers for electronic and optical applications [28], is also getting considerable attention in modern organic synthesis as a support and also as a promoter for metal catalyzed organic transformations for its easy preparative protocol from inexpensive starting

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material, high environmental stability, easy acid-base doping–dedoping and redox properties [29–30]. Recently, we have reported polyaniline supported Os, Sc, In, Pd catalyzed organic transformations [31]. In continuation of our work on polyaniline supported catalysts, herein we wish to report the preparation, characterization and catalytic properties of polyaniline supported cuprous iodide in the N-arylation of N(H)-heterocycles and benzyl amines with aryl halides and arylboronic acids and aza-Michael reaction of amines with electron-deficient alkenes.

δ+ HN

N

PANI-Cu was prepared by stirring CuI and polyaniline in acetonitrile for 24 h followed by filtration. Thus prepared PANI-Cu was fully characterized by using FTIR, XPS, ICP-AES, SEM, and EDAX. The most important bands in the FTIR spectrum of PANI are located at 1584, 1494, 1376, 1308, 1163 and 830 cm 1 (Fig. 1). They are due to the stretching vibrations of quinoid (mC@N + mC@C), benzenoid (mC@C) units of the polymer, deformations of the C–N bond, stretching vibrations of the C–N bond, in plane deformations of CH bonds presents in the aromatic rings of the undoped polymer and the out of plane deformations of CH bonds in 1,4-substituted aromatic ring, respectively [32]. Upon incorporation of CuI into PANI no appreciable shifts in the quinoid or benzenoid ring bands positions have been observed. XPS analysis of PANI-Cu showed Cu 2p3/2 line at 932.4 eV and Cu 2p1/2 line at 952 eV, which confirmed the oxidation state of copper in PANI-Cu to be +I [33]. We analyzed the PANI by XPS for the N 1 s and it showed three types of N namely -N (398 eV, share 45%), -NH- (399.4 eV, share 45%), and -N+ (402 eV, share 10%). Similarly, PANI-Cu showed the presence of -N (397.7 eV, share 13%), -NH- (400 eV, share 65%), and -N+ (401.9 eV, share 22%). The decrease in share of imine nitrogen indicates that most of the copper was bound with the PANI via imine nitrogen than the amine nitrogen and the possible structure of PANI-Cu is shown in Scheme 1. The SEM images of PANI and PANI-Cu were almost similar indicating that after doping with CuI, morphology did not change appreciably. The EDAX analysis of PANI-Cu indicates the presence of copper and iodine in 1:1 ratio. The loading of copper in PANI-Cu was measured by ICP-AES and found to be 0.25 mmol g 1.

NH

N H

CuI

HN

N

N

NH

δ+

N H

Scheme 1. Possible structure of PANI-Cu.

2. Results and discussion 2.1. Characterization of the polyaniline supported cuprous iodide (PANI-Cu)

N

2.2. Catalytic activity of PANI-Cu in N-arylation reaction using aryl halides To optimize the conditions for the N-arylation reaction, we have chosen the reaction between iodobenzene and imidazole as a model reaction (Scheme 2) and various solvents and bases were screened (Table 1). From Table 1, it can be seen that the best yield was obtained by using Cs2CO3 (3 equiv.) in acetonitrile solvent in presence of PANI-Cu (2.5 mol% with respect to Cu) (Table 1, entry 1). It is noteworthy that when homogeneous CuI (2.5 mol%) was used instead of PANI-Cu, very low amount of product formation was observed (Table 1, entry 10). Here most probably polyaniline is playing the dual role of support as well as a macro ligand which facilitates the reaction as reported by using other nitrogen containing ligands [5–11]. By lowering the amount of catalyst to 1 mol%, we obtained 47% of the product in 12 h (Table 1, entry 11). To explore the scope and limitations of the current catalytic protocol, several haloarenes and nitrogen containing heterocycles were allowed to react under the optimized conditions and the results are summarized in Table 2. It was observed that iodoarenes with electron-withdrawing group (Table 2, entry 4 and 5) reacted at a faster rate than iodoarenes with electron donating group (Table 2, entry 2 and 3). Sterically hindered 2-iodotoluene took longer duration to afford a good yield (Table 2, entry 6). Benzyl amine with electron donating moieties found to be more active than simple benzyl amine (Table 2, entry 11 and 12). When diben-

I +

N

PANI-Cu (2.5 mol%)

N H

solvent, base, 80 °C, 12h, N2

N

N

Scheme 2. PANI-Cu catalyzed N-arylation of imidazole with iodobenzene.

Table 1 Optimization of reaction conditions for the N-arylation of imidazole with iodobenzenea

Fig. 1. FTIR spectra of (a) PANI and (b) PANI-Cu.

Entry

Solvent

Base

Isolated yield (%)

1 2 3 4 5 6 7 8 9 10b 11c

CH3CN Dioxane MeOH Dimethoxyethane EtOH CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN

Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 Cs2CO3 K2CO3 NaOH NaOAc K3PO4 Cs2CO3 Cs2CO3

92 25 10 10 10 78 60 32 88 5 47

a Reaction conditions: iodobenzene (1 mmol), imidazole (1.2 mmol), base (3 mmol), solvent (3 mL), PANI-Cu (100 mg, 2.5 mol%), 80 °C, 12 h, N2 atmosphere. b CuI (2.5 mol%) used as catalyst. c PANI-Cu (1 mol%) used as catalyst.

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Table 2 N-arylation of N(H)-heterocycles and benzyl amines with aryl halides using PANI-Cua Entry Aryl halide

N(H)-heterocycle/Amine

Time (h)

Yield (%)

1

C6H5-I

Imidazole

12

2 3 4 5 6 7 8 9 10 11 12 13 14c 15c 16c 17c 18c

4-Me-C6H4-I 4-MeO-C6H4-I 4-MeCO-C6H4-I 4-NO2-C6H4-I 2-Me-C6H4-I C6H5-I C6H5-I C6H5-I C6H5-I C6H5-I C6H5-I C6H5-I C6H5-Br 4-MeCO-C6H4-Br 4-NO2-C6H4-Br 4-Me-C6H4-Br 4-MeO-C6H4-Br

Imidazole Imidazole Imidazole Imidazole Imidazole Benzimidazole Pyrazole Pyrrole Benzyl amine 4-Methyl benzyl amine 4-Methoxy benzyl amine Dibenzyl amine Imidazole Imidazole Imidazole Imidazole Imidazole

16 18 8 6 24 16 14 12 15 12 10 24 15 10 8 18 30

92, (90, 89, 90, 87)b 85 82 98 95 65 84 92 88 90 92 90 N.R. 90 (80, 65)b 95 90 90 88

a Reaction conditions: aryl halide (1 mmol), amine (1.2 mmol), PANI-Cu (2.5 mol%), Cs2CO3 (3 mmol), CH3CN (2 mL), 80 °C, N2 atmosphere, isolated yield. b Yields after consecutive cycles. c DMF used as solvent and reaction temperature 100 °C.

Table 3 Comparison of activity of different heterogeneous copper catalysts in the N-arylation reaction of iodobenzene and imidazole Entry

Catalyst

Reaction conditions

Yield (%)

Reference

1

Cellulose-Cu(0)

95

14

2

Cu-FAP

92

12

3

89

15

4

SiO2-PyCu(OAc)2 Cu-Y zeolite

Cu: 0.92 mol%, 130 °C, DMSO, 12 h Cu: 12.5 mol%, 110 °C, DMSO, 6h Cu: 5 mol%, 100 °C, Toluene, 8 h

99

13

5 6

Cu2O Nano-CuO

90 91

18 17

7

PANI-Cu

Cu: 10.8 mol%, 120 °C, DMF, 24 h Cu: 10 mol%, 110 °C, DMSO, 24 h Cu: 2.5 mol%, 110 °C, DMSO, 24 h Cu: 2.5 mol%, 80 °C, CH3CN, 12 h

92

This study

2.3. Catalytic activity of PANI-Cu in N-arylation using arylboronic acids N-arylation of N(H)-heterocycles with arylboronic acids is complementary to the N-arylation with aryl halides as it requires very mild conditions and less time but uses expensive arylboronic acids. To check the generality and scope of the PANI-Cu, we tried the N-arylation of N(H)-heterocycles with arylboronic acids in methanol using 2.5 mol% catalyst (Scheme 3) and the results are summarized in Table 4. It took almost two days to complete the reaction of imidazole and phenylboronic acid at room temperature (Table 4, entry 1). By raising the temperature to 80 °C, the reaction was complete within 1.5 h affording 94% yield. Several boronic acids were reacted with imidazole using PANI-Cu and the results are summarized in Table 4. It was observed that boronic acids with electron-withdrawing group required longer time compared to boronic acids with electro-donating group. Here also the catalyst was used for five cycles with consistent activity (Table 4, entry 2). 2.4. Catalytic activity of PANI-Cu in aza-Michael reaction of amines with a, b-unsaturated compounds

zyl amine was employed, no product formation was observed (Table 2, entry 13). This is may be due to the steric hindrance of the benzyl amine. Bromoarenes were found to be unreactive but an increase in temperature from 80 °C to 100 °C and a change in solvent from acetonitrile to DMF afforded excellent yields (Table 2, entry 16–22). For any supported catalyst, it is important to know its ease of separation and possible reuse. PANI-Cu can be easily separated by filtration. The recovered catalyst after washing with acetone followed by drying at 80 °C was used in the next run. Almost consistent activity was observed over five cycles in the reaction of iodobenzene with imidazole (Table 2, entry 1). The difference between the copper content of the fresh catalyst and the used catalyst (5th cycle) is only 2.4%. But in case of bromobenzene, the yield dropped considerably after the second cycle (Table 2, entry 14). This is due to the partial solubility of the catalyst in DMF sol-

NH-heterocycle + Arylboronic acid

vent. Almost 13% of the charged copper leached into the solution. To check the activity of the leached copper, we terminated the reaction between bromobenzene and imidazole after 2 h at 38% conversion and filtered off the catalyst. Then we continued the reaction with the filtrate for an additional 10 h. The conversion increased to 50%. In the presence of the fresh catalyst, the conversion reached almost 98%. These studies showed that the leached copper is catalytically active but less effective than the PANI-Cu. More importantly polyaniline is also soluble to some extent in hot DMF, so the leached copper is not purely CuI, but it is a combination of polyaniline and CuI. We have performed the same reaction in presence of CuI (2.5 mol%) in DMF under the identical conditions and found only 20% product formation after 15 h. Further we compared the activity of the PANI-Cu in the N-arylation reaction with the other reported heterogeneous catalysts (Table 3). From Table 3, it can be seen that the activity of PANICu is more or less similar to the other reported systems and in the present system the reaction is conducted at lower temperature (80 °C).

Recently Verma et al. reported [34] the Cu-nanoparticles as a chemoselective catalyst for the aza-Michael reactions of N-alkyl and N-arylpiperazines with acrylonitrile and simple copper salts as catalysts for aza-Michael reactions in water by Xu et al. [35]. These results prompted us to investigate the catalytic activity of PANI-Cu in aza-Michael reaction of amines with a, b-unsaturated compounds (Scheme 4). The reaction between imidazole and acrylonitrile was chosen as model reaction to optimize the reaction conditions and the best results were obtained by reacting imidazole and acrylonitrile in methanol solvent at 60 °C for 8 h (Table 5, entry 1). In absence of PANI-Cu, only 10% product formation was observed (Table 5, entry 1). To widen the scope of PANI-Cu catalyzed aza-Michael reaction, various other activated alkenes such as acrylonitrile, methyl acrylate, and methyl vinyl ketone were reacted with several other amines. Aliphatic amines were found to be fairly reactive at room temperature affording the products in excel-

PANI-Cu (2.5 mol%)

Aryl-N-heterocycle

MeOH, 80 °C

Scheme 3. N-Arylation of N(H)-heterocycles with arylboronic acids by using PANI-Cu.

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M.L. Kantam et al. / Catalysis Communications 9 (2008) 2226–2230 Table 4 N-arylation of N(H)-heterocycles with arylboronic acids using PANI-Cua Entry 1 2 3 4 5 6 7 8 9 10

N(H)-heterocycle Imidazole Imidazole Imidazole Imidazole Imidazole Imidazole Imidazole Benzimidazole Benzimidazole Pyrrazole

Arylboronic acid C6H5-B(OH)2 C6H5-B(OH)2 4-Me-C6H4-B(OH)2 2-Me-C6H4-B(OH)2 4-F-C6H4-B(OH)2 3-NO2-C6H4-B(OH)2 4-MeO-C6H4-B(OH)2 C6H5-B(OH)2 4-Me-C6H4-B(OH)2 C6H5-B(OH)2

Time (h)

Table 5 Aza-Michael addition of various amines with activated alkenesa Yield (%) b

48 1.5 1 2 3 5 1 2 2 3

95 94, 89c 93 88 90 80 88 91 94 90

a Reaction conditions: N(H)-heterocycle (1 mmol), arylboronic acid (1.2 mmol), PANI-Cu (2.5 mol%), MeOH (2 mL), 80 °C, air, isolated yield. b Room temperature. c Yields after 5th cycle.

Entry

Amine

Alkene

1

3. Experimental

Isolated yield (%)

8

95, 10b, 91c

N

NH

2

N

NH

CO2Me

8

90

3

N

NH

CO2Bun

8

92

4

N

NH N

COMe

6

90

COMe

10

88

0.3

95

0.5

92

0.5

95

CO2Me

0.3

90

CO2Me

0.3

92

0.3

95

5

N H

6d

lent yields within very short duration (Table 5, entry 6–11). The catalyst was recovered by simple filtration and reused with for several cycles with consistent activity (Table 5, entry 1).

CN

Time (h)

d,e 7

8d

NH

9d

NH

CH2NH2

CN

CH2NH2

CO2Me CN

3.1. Materials Aniline and other solvents were distilled before use. All other chemicals were procured from commercial sources and used as such without further purification. Iodoarenes, bromoarenes and arylboronic acids were purchased from Aldrich. All other chemicals were procured from commercial sources and used as such without further purification. Polyaniline base (PANI) was synthesized according to literature procedure [31]. 3.2. Preparation of polyaniline supported CuI (PANI-Cu) PANI (1000 mg) was charged into a round-bottom (RB) flask containing an acetonitrile solution (25 mL) of cuprous iodide (190 mg, 1.0 mmol) and stirred under nitrogen atmosphere for 48 h. The resultant catalyst was filtered off and washed with acetonitrile followed by acetone. The residue was dried in air for 24 h to afford the black catalyst.

10d

O

11d

n-Bu NH n-Bu

NH

CO2Me

a Reaction conditions: amine (1 mmol), alkene (1.1 mmol), PANI-Cu (2.5 mol%), MeOH (2 mL), 60 °C. b Without catalyst. c Yield after 5th cycle. d Reaction conducted at room temperature. e 5% bis adduct formed.

3.4. N-arylation of amines with arylboronic acids 3.3. N-arylation of amines with aryl halides In an oven dried 10 mL RB flask, PANI-Cu (2.5 mol%, 100 mg), aryl halide (1 mmol), amine (1.2 mmol), Cs2CO3 (3 mmol), and acetonitrile/DMF (for bromoarenes) (3 mL) were stirred under nitrogen atmosphere at 80 °C/100 °C (for bromoarenes) and the reaction was monitored by using TLC. After the completion of the reaction, the catalyst was filtered off and washed with water followed by acetone and dried at 80 °C in oven. The filtrate was diluted with ethyl acetate and washed with saturated NaCl solution. The organic layer was dried over anhydrous Na2SO4 and concentrated to get the crude product. The crude product was column chromatographed using mixture of hexane and ethyl acetate as eluent. All the products were characterized by using NMR and mass spectroscopy and compared with the literature data [5–10].

R1 R2

NH +

EWG

PANI-Cu (2.5 mol%) MeOH, 60 °C /RT

R1 R2

EWG N

Scheme 4. PANI-Cu catalyzed aza-Michael reaction.

In an oven dried 10 mL RB flask, amine (1 mmol), arylboronic acid (1.2 mmol), PANI-Cu (2.5 mol%, 100 mg) and methanol (2 mL) were taken and stirred at 80 °C under air. After the completion of the reaction as monitored by TLC, the catalyst was filtered off. The filtrate was diluted with ethyl acetate and washed with 10% NaOH solution followed by saturated aqueous NaCl solution. The organic layer was dried over anhydrous Na2SO4 and concentrated to get the crude product. The crude product was column chromatographed using mixture of hexane and ethyl acetate as eluent. 3.5. General procedure for aza-Michael reaction of amines with electron-deficient alkenes In an oven dried 10 mL RB flask, amine (1 mmol), alkene (1.1 mmol), PANI-Cu (2.5 mol%) and MeOH (2 mL) were taken and stirred at 60 °C or room temperature. After the completion of the reaction as monitored by TLC, the catalyst was filtered off. The filtrate was diluted with ethyl acetate and washed with saturated aqueous NaCl solution. The organic layer was dried over anhydrous Na2SO4 and concentrated to get the crude product. The crude product was column chromatographed using mixture

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of hexane and ethyl acetate as eluent. Spectroscopic data of some of the representative products:  1-Phenyl-1 H-imidazole (Table 2, entry 1) 1H NMR (300 MHz, CDCl3): d 7.83 (s, 1 H), 7.50–7.30 (m, 5H), 7.25 (bs, 1H), 7.18 (bs, 1H); EI-MS: 144 (M+), 142, 92, 57.  1-o-Tolyl-1H-imidazole (Table 2, entry 6) 1H NMR (200 MHz, CDCl3): d 7.67 (br s, 1H), 7.41–7.15 (m, 5H), 7.04 (br s, 1H), 2.21 (s, 3H); EI-MS: 158 (M+).  N-Phenylbenzylamine (Table 2, entry 10) 1H NMR (300 MHz, CDCl3): d 7.38–7.29 (m, 3H), 7.28–7.18 (m, 2H), 7.15–7.06 (m, 2H), 6.65 (t, 1H, J = 7.55 Hz), 6.57 (d, 2H, J = 8.03 Hz), 4.30 (s, 2H), 3.93 (br s, 1H); EI-MS: 183 (M+), 155, 141, 91, 77, 55, 43.  3-Imidazol-1-yl-propionic acid methyl ester (Table 5, entry 2) 1 H NMR (300 MHz, CDCl3): d 7.42 (br s, 1H), 6.97 (br s, 1H), 6.85 (br s, 1H), 4.23 (t, 2H, J = 7.03 Hz), 3.66 (s, 3H), 2.72 (t, 2H, J = 7.03 Hz); EI-MS: 154 (M+), 122, 95, 81, 68, 54.  4-Benzoimidazol-1-yl-butan-2-one (Table 5, entry 5) 1H NMR (300 MHz, CDCl3): d 7.90 (br s, 1H), 7.75 (br s, 1H), 7.15–7.38 (m, 3H), 4.42 (t, 2H, J = 6.8 Hz), 2.94 (t, 2H, J = 6.8 Hz), 2.09 (s, 3H); EI-MS: 188 (M+), 131, 118, 77, 43. 4. Conclusion In conclusion, we have prepared polyaniline supported copper catalyst for effective N-arylation of N(H)-heterocycles and benzyl amines with aryl halides as well as arylboronic acids and aza-Michael reaction of amines with a, b-unsaturated compounds. Moreover the catalyst can easily be separated by simple filtration and reused for several cycles with consistent activity. Acknowledgements M.R. thanks UGC and S.R. thanks CSIR for providing fellowship. References [1] P. Cozzi, G. Carganico, D. Fusar, M. Grossoni, M. Menichincheri, V. Pinciroli, R. Tonani, F. Vaghi, P. Salvati, J. Med. Chem. 36 (1993) 2964.

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