Open-air N-arylation of N–H heterocycles with arylboronic acidscatalyzed by copper(II) Schiff base complexes

S. M. Islam • Ram Chandra Dey • Anupam Singha Roy •

Sumantra Paul • Sanchita Mondal

Received: 5 June 2014 / Accepted: 5 September 2014 / Published online: 13 September 2014

� Springer International Publishing Switzerland 2014

Abstract Two copper Schiff base complexes, in both

homogeneous and heterogeneous forms, were prepared and

characterized by using elemental analysis, FTIR, UV–Vis

spectroscopy and scanning electron microscopy. The cat-

alytic performances of these complexes were studied in the

N-arylation of N–H heterocycles with arylboronic acids in

methanol without any added base at 40 �C under open air.

The effects of various parameters such as solvent and

temperature on the reaction system were studied. The

reaction is applicable to a wide variety of N–H heterocy-

cles and arylboronic acids. The heterogeneous catalyst was

recovered by simple filtration, and reusability experiments

showed that this catalyst can be used five times without

much loss in the catalytic activity.

Introduction

Copper-mediated N-arylation reactions play an important

role in organic synthesis since the products, arylamines and

N-arylheterocyclic compounds are ubiquitous in pharma-

ceuticals, crop protection chemicals and materials science

[1–4]. In 1997, a copper-mediated heteroatom arylation

reaction using arylboronic acids as aryl donors was dis-

covered independently by Chan, Evans and Lam [5, 6].

Ullmann and Goldberg arylation of amines, amides and

anilines with aryl halides using copper catalysts is a pop-

ular methodology for the synthesis of N-arylated products

[7]. These reactions have been carried out at high

temperatures, and many functional groups are not tolerated.

In addition, they often require the use of stoichiometric

amounts of copper reagents, which, on scale-up, leads to

problems of waste disposal. To overcome these drawbacks,

several Pd- and Ni-catalyzed protocols have been devel-

oped by Hartwig and Buchwald [8, 9]. Also, Chan et al. [5],

Evans et al. [6], Lam et al. [10] and others [11] have

reported the preparation of arylamines and N-arylhetero-

cycles using boronic acids and cupric acetate under milder

conditions than conventional methods. There are reports

describing arylsiloxanes [12], diaryliodonium salts [13] or

aryllead triacetates [14] as aryl donors using cupric acetate

and a base. Some of these processes include use of stoi-

chiometric quantities of Cu(OAc)2, excess arylboronic acid

or base and extended reaction times. Recently, N-arylation

reactions have been reported using copper salts in the

absence of base in protic solvents [15, 16]. However,

copper-mediated couplings are still the reaction of choice

for large- and industrial-scale formation of C–N bonds.

Chan and Lam established an efficient approach to

N-arylimidazoles via Cu(OAc)2-mediated coupling of

imidazoles with readily available arylboronic acids [10].

Xie et al. [15] have demonstrated the simple copper-cata-

lyzed coupling of imidazoles with arylboronic acids in

protic solvent without added base. Recently, Buchwald

used a diimine as ligand for effective copper-mediated

cross-coupling reactions under mild conditions [17]. Very

recently, Buchwald and others have reported a copper-

based protocol for the formation of N-aryl bonds [18–20].

There are, however, very few reports of N-arylation reac-

tions under open-air conditions [21]. Despite the significant

progress made in the development of copper-catalyzed

coupling reactions of this type, there still exists a need for

new methods that involve cheap and environmentally

benign catalysts.

S. M. Islam (&) � R. C. Dey � A. S. Roy � S. Paul � S. Mondal

Department of Chemistry, University of Kalyani,

Kalyani, Nadia 741235, WB, India

e-mail: manir65@rediffmail.com

123

Transition Met Chem (2014) 39:961–969

DOI 10.1007/s11243-014-9881-2

Immobilization of soluble catalysts onto an insoluble

matrix can allow for easy separation and recyclability of

the catalyst. In this direction, we have already reported

reusable palladium catalysts for C–C cross-coupling reac-

tions [22, 23]. Many of the disadvantages of homogeneous

catalysts can be overcome by anchoring onto a polymer

[24]. There are many examples of heterogeneous catalysts

for C–N coupling reactions, and they can be prepared by

different approaches such as encapsulation or immobili-

zation of a catalytically active metal complex on a solid

support [25–28].

In this work, we found that N-arylation can be effi-

ciently catalyzed by copper(II) Schiff base complexes

under open-air conditions. Furthermore, a variety of imi-

dazoles, benzimidazoles, amides, imides and sulfonamides

can be reacted with arylboronic acids using this catalytic

system to give the corresponding N-arylation products in

moderate to excellent yields.

Results and discussion

The synthesis of the copper complexes is outlined in

Scheme 1. These copper complexes were synthesized and

characterized according to our earlier study [29]. The

reaction of 2-aminophenol and salicylaldehyde in methanol

leads to the formation of a Schiff base. Reaction of the

Schiff base with copper acetate in methanol gave a Cu(II)

Schiff base complex which we designate as [Cu(am-

p)(OAc)]. For the heterogeneous catalyst, first, aminopo-

lystyrene was stirred with copper acetate, and then, the

Schiff base ligand was reacted with this metal-loaded

polymer to produce PS–Cu–amp–OAc.

Elemental analysis of the soluble copper complex was

quite comparable to the calculated values. The copper

content of the polymer-supported catalyst was estimated by

atomic absorption spectroscopy as 1.52 %.

The formation of a Schiff base was indicated by an

absorption band at 1,639 cm-1 assigned to the C=N

stretching vibration in the spectrum of Schiff base. In the

spectrum of copper complex, this band is shifted to lower

frequency at 1,606 cm-1, indicating coordination of

the azomethine nitrogen to the metal [30]. The phenolic

(C–O) stretching frequency was observed in the region

of 1,270 cm-1 (free Schiff base), compared with

1,294–1,315 cm-1 for the complex, indicating coordina-

tion through phenolic oxygen [31, 32]. In addition, the

spectrum of the copper complex includes a new feature

around 520–580 cm-1, which is assigned to the Cu–N

bond [33]. The complexation of copper(II) also resulted in

bands around 630–688 cm-1 assigned to the Cu–O bond

[33]. A weak band in the 3,435 cm-1 region indicates the

presence of –OH in copper complex. In spectrum of the

[Cu(amp)(OAc)] catalyst, a medium intensity band at

1,318 cm-1 suggests monodentate coordination of the

acetate groups. The vC=O stretching vibration of the ace-

tate groups is observed at 1,720 cm-1 in copper complex.

The FTIR spectra of poly(4-aminostyrene) and the poly-

mer-supported immobilized complex were also studied.

The intensity of the spectrum of the polymer-supported

metal complex is weak, due to its low concentration in the

polymer matrix. However, it appears to be quite similar to

the neat metal complex. The band at 1,619 cm-1 for the

primary amine in poly(4-aminostyrene) is shifted to

1,590–1,600 cm-1 in the supported complex. The presence

of the Schiff base moiety in the immobilized complex is

indicated by a band at 1,641 cm-1 assigned to the v(C=N)

stretch.

The electronic spectra of the free Schiff base and its

copper complex were measured in acetonitrile (Fig. 1a). In

the UV spectrum of the free Schiff base, absorption bands

at 265 and 348 nm are assigned to the n?p* and p?p*

transition, respectively. The electronic spectrum of the

copper complex exhibits a shoulder around at 430 nm that

is assigned to d$d transitions [34]. A band at 275 nm is

presumably caused by charge transfer. The thermal sta-

bility of heterogeneous copper complex was investigated

ANH2

OH

OHC

HO

MeOHReflux

N

OH

OH

Cu(OAc)2MeOH

KOH

N

O

OH

CuOAc

B

NO

OH

Cu

OAcNH2P

Scheme 1 Synthesis of the homogeneous (a) and polymer-anchored

(b) copper(II) Schiff base catalysts

962 Transition Met Chem (2014) 39:961–969

123

by TGA (heating rate, 10 �C/min in air from 30 to 500 �C,

Fig. 1b). The polymer-supported Cu(II) Schiff base com-

plex decomposes at 230 �C.

Field emission SEM for poly(4-aminostyrene) and the

polymer-supported copper complex was recorded to

examine the morphological changes that occur in the

polystyrene beads during various stages of synthesis.

The SEM images of poly(4-aminostyrene) (A) and the

immobilized copper complex (B) are shown in Fig. 2. The

pure poly(4-aminostyrene) bead has a smooth surface.

After metal loading on the polymer, a change in mor-

phology of the polymer surface is observed. The presence

of copper metal along with oxygen and chlorine on

polymer-anchored complex was verified by EDX

(Fig. 2c).

Catalytic activity

Initially, we chose imidazole and phenylboronic acid as

model substrates to optimize the catalytic conditions for the

cross-coupling reaction (Scheme 2). Several simple copper

salts were tested as copper sources to promote the coupling

reaction in methanol as solvent. As shown in Table 1, most

of the copper salts that were used gave the desired products

in low to moderate yields (Table 1, entries 2–4). Copper

chloride and copper iodide (entries 2 and 3) were less

reactive than copper(II) acetate and gave a result among

our best attempts with 79 % yield (entry 4). Between the

homogeneous- and polymer-anchored Cu(II) Schiff base

catalysts, the polymer-anchored catalyst obtained from

copper(II) acetate was found to be most effective (entry 7).

Reaction temperature plays a crucial role in such cross-

coupling reactions [35]. It is reported that reaction times

may sometimes be dramatically affected by changing the

reaction temperature. We found that increasing the tem-

perature remarkably accelerated the reaction, and further-

more, yields were also increased (Table 2, entries 1–5). A

high yield was obtained when the reaction was carried out

in methanol at 40 �C within 6 h (entry 3). A still higher

temperature of 60 �C did not much improve the results and

showed a large decrease in reactivity (entry 5). Further-

more, higher temperature was unfavorable as more of the

biphenyl byproduct was obtained.

The effect of solvent was also examined (Table 3,

entries 1–8). We first selected toluene and acetonitrile as

reaction solvents (entries 1 and 2). However, low to

moderate yields were obtained with these solvents. Mod-

erate yield of the product was obtained when water was

used as solvent (entry 3). However, when protic solvents

such as methanol and ethanol were employed (entries 4 and

5), the desired products were obtained in high yields.

Reaction in DMF and DMSO gives trace amounts of the

coupled product (entries 6 and 7). Here, methanol is found

to be the best solvent for the N-arylation.

Under the optimized reaction conditions, a wide range

of structurally diverse arylboronic acids were coupled with

imidazole using both the homogeneous and heterogeneous

copper(II) Schiff base catalysts in methanol at 40 �C to

produce the corresponding substituted N-aryl imidazoles in

good to excellent yields. In an endeavor to expand the

scope of this methodology, the catalytic system was

applied to imidazole, benzimidazole, imides, amides and

sulfonamides. N-arylation of imidazole with phenylboronic

acid using the copper(II) catalysts was carried out in the

absence of added base. Our method was successfully

amenable to a wide range of arylboronic acids, allowing

preparation of N-arylimidazoles and N-arylbenzimidazoles

in high yields, as shown in Table 4. Arylboronic acids with

electron-donating groups afforded better yields with

Fig. 1 Electronic spectra of Schiff base, copper(II) Schiff base

complex (a) and thermogravimetric weight loss plot for polymer-

supported Cu(II) complex (b)

Transition Met Chem (2014) 39:961–969 963

123

imidazole than arylboronic acids with electron-withdraw-

ing groups (entries 2–9). The coupling reactions of

2-methylphenylboronic acid, 2-methoxyphenylboronic

acid and 3,4-dimethoxyphenylboronic acid afforded lower

yields, presumably due to steric effects (entries 4–6). The

coupling of 3-nitrophenylboronic acid which is a difficult

substrate also proceeded smoothly under the present con-

ditions (entry 8). Similar observations were made when

benzimidazole was used in place of imidazole to obtain the

corresponding N-arylbenzimidazoles (entries 10–12). A

series of substituted arylboronic acids were coupled with

phthalimide (entries 13–16) and succinimide (entries 17

and 18) under the optimized reaction conditions to afford

the corresponding N-aryl imides in good to excellent

yields. The reaction of amides and sulfonamides with

phenylboronic acid afforded the corresponding products in

higher and lower yields, respectively, (entries 19 and 20).

Fig. 2 FE SEM images of poly(4-aminostyrene) (a), polymer-supported Cu(II) Schiff base complex (b) and EDX plot of polymer-supported

Cu(II) Schiff base complex (c)

N

NH

methanol 40 0C

N

N

B(OH)2

, ,6 h

Copper (II) Schiffbase complex

Scheme 2 Homogeneous copper(II) Schiff base catalyzed N-aryla-

tion of imidazole with phenylboronic acid

Table 1 Effect of copper source on the coupling of imidazole with

phenylboronic acida

Entry Copper source Yield (%)b

1 None No reaction

2 CuCl2 48

3 CuI 44

4 Cu(OAc)2 79

5 Homogeneous copper(II)c 92

6 Homogeneous copper(II)d 72

7 Heterogeneous copper(II)c 96

8 Heterogeneous copper(II)d 74

a Reaction conditions: phenylboronic acid (0.0182 g, 1.5 mmol),

imidazole (0.082 g, 1.2 mmol), catalyst (1.5 9 10-5 mol), methanol

(10 ml), 40 �C, 6 h, open airb Yield refers to GC and GC–MS analysisc Catalyst prepared from Cu(OAc)2

d Catalyst prepared from CuCl2

964 Transition Met Chem (2014) 39:961–969

123

A possible reaction mechanism can be outlined for the

copper-catalyzed N-arylation of N–H heterocycles with

arylboronic acid, based on the previously reported mech-

anism [1, 36, 37]. The first step is transmetalation of

arylboronic acid with the copper catalyst. Then, in the

presence of base, coordination of N–H heterocycles to CuII

species causes a decrease in the reduction potential of the

CuIII/CuII couple. Air or oxygen oxidizes CuII–CuIII spe-

cies. Finally, in a reductive elimination pathway, the

product is eliminated, and the CuI species is ready to

continue the catalytic cycle.

Heterogeneity and recycling tests

To determine whether the catalyst was actually functioning

in a heterogeneous manner, a hot-filtration test was per-

formed in the N-arylation of imidazole with phenyl boronic

acid. The solid catalyst was filtered out after the reaction

had proceeded for 2 h, and the yield determined by GC

analysis was 46 %. The liquid phase of the reaction mix-

ture was collected at the reaction temperature. Atomic

absorption spectrometric analysis of this liquid phase

confirmed that Cu was absent from the reaction mixture.

The filtrate was then stirred under the reaction conditions.

After 6 h, the yield was determined to be still 46 %. This

result indicated that the reaction was caused by the solid

catalyst and suggested that the Cu was not being leached

out from the catalyst during the N-arylation reactions.

A key advantage of heterogeneous catalysis is the pos-

sibility of recovering and reusing the catalyst. The capa-

bility of recycling of the catalyst was confirmed after five

consecutive N-arylation reactions of imidazole with phen-

ylboronic acid in MeOH medium. After the first run, the

catalyst was separated by filtration, washed, dried under

vacuum and then subjected to further runs under the opti-

mized reaction conditions. The results summarized in

Fig. 3 demonstrate that there was almost no change in

catalytic activity even after the fifth recycle. The metal

content of the recycled catalyst remained unaltered, indi-

cating no leaching of the metal from the polymer support.

Conclusions

In summary, we have successfully prepared homogeneous

and heterogeneous copper(II) Schiff base acetate com-

plexes. These compounds show high catalytic activity in

the N-arylation of various nitrogen-containing compounds,

in methanol under open-air conditions. The catalytic sys-

tems are base-free, economical, easy to handle and do not

need addition of oxygen or nitrogen. The N-arylation

products were generally obtained in moderate to excellent

yields.

Experimental

Materials

Analytical grade reagents and freshly distilled solvents

were used throughout. All reagents and substrates were

purchased from Merck. Liquid substrates were distilled and

dried with the appropriate molecular sieve. Solid reagents

were recrystallized before use. Copper salts and other

organic reagents were purchased from Merck and used

without further purification.

Physical measurements

A Perkin-Elmer 2400 C elemental analyzer was used to

collect microanalytical data (C, H and N). The transition

metal contents of the samples were measured with a

Table 3 Effect of solvent on the coupling of imidazole with phen-

ylboronic acida

Entry Solvent Temperature �C Time h Yield (%)b

Cu–amp–OAc/

PS–Cu–amp–OAc

1 PhCH3 120 16 51/53

2 CH3CN 50 12 60/64

3 H2O 50 12 75/78

4 EtOH 40 6 91/94

5 MeOH 40 6 92/96

6 DMF 140 24 45/46

7 DMSO 140 24 41/45

a Reaction conditions: copper(II) catalyst (1.5 9 10-5 mol), phen-

ylboronic acid (0.0182 g, 1.5 mmol), imidazole (0.082 g, 1.2 mmol),

solvent (10 ml), open airb Yield refers to GC and GC–MS analysis

Table 2 Effect of temperature on the coupling of imidazole with

phenylboronic acida

Entry Temperature

(�C)

Time

(h)

Yield (%)b

Cu–amp–OAc/

PS–Cu–amp–OAc

1 20 48 83/87

3 30 24 88/90

5 40 �C 6 92/96

6 50 �C 10 92/97

7 60 �C 24 74/78

a Reaction conditions: copper(II) catalyst (1.5 9 10-5 mol), phen-

ylboronic acid (0.0182 g, 1.5 mmol), imidazole (0.082 g, 1.2 mmol),

methanol (10 ml), open airb Yield refers to GC and GC–MS analysis

Transition Met Chem (2014) 39:961–969 965

123

Table 4 N-arylation of N–H

heterocycles using

homogeneous copper(II)

catalyst with various

arylboronic acidsa

Entry N-H

heterocycles

Arylboronic

Acids

Products Isolated Yield (%)b

Cu-amp-OAc/PS-

Cu-amp-OAc

1 Imidazole B(OH)2

H

NN

92/96

2 Imidazole B(OH)2

Me

Me

NN

87/90

3 Imidazole B(OH)2

OMe

OMe

NN

89/91

4 Imidazole B(OH)2Me

NN

Me 80/83

5 Imidazole B(OH)2OMe

NN

OMe 82/87

6 Imidazole B(OH)2

OMeOMe

OMe

OMe

NN 77/78

7 Imidazole B(OH)2

Cl

Cl

NN

88/90

8 Imidazole B(OH)2

NO2

NN NO2

82/86

9 Imidazole B(OH)2

COMe

COMe

NN

80/84

10 Benzimidazole B(OH)2

H

NN 93/95

11 Benzimidazole B(OH)2

Me

NN

Me

88/93

966 Transition Met Chem (2014) 39:961–969

123

Varian AA240 atomic absorption spectrophotometer

(AAS). FTIR spectra were recorded on a Perkin-Elmer

FTIR 783 spectrophotometer using KBr pellets. Electronic

spectra were recorded on a Shimadzu UV/3101 PC

spectrophotometer. NMR spectra were recorded at

400 MHz for 1H NMR and 100 MHz for 13C NMR,

Table 4 continued

a Reaction conditions:

copper(II) catalyst (0.005 g,

1.5 9 10-5 mol), 1.5 mmol of

arylboronic acids, 1.2 mmol of

N–H heterocycles, MeOH

(10 ml), 40 �C, 6 h, open airb Isolated yield after column

chromatography. All products

were characterized by NMR

12 Benzimidazole B(OH)2

OMe

NN

OMe

87/90

13 Phthalimide B(OH)2

H

N

O

O

90/93

14 Phthalimide B(OH)2

Me

N

O

O

Me

89/92

15 Phthalimide B(OH)2

Cl

N

O

O

Cl

88/90

16 Phthalimide B(OH)2

COMe

N

O

O

COMe

86/89

17 Succinamide B(OH)2

H

N

O

O

90/92

18 Succinamide B(OH)2

Me

N

O

O

Me

87/89

19 Benzamide B(OH)2

H

CONH

(S)

93/95

20 Sulfonamide B(OH)2

H

SO2NH

(T)

71/76

Entry N-H

heterocycles

Arylboronic

Acids

Products Isolated Yield (%)b

Cu-amp-OAc/PS-

Cu-amp-OAc

Transition Met Chem (2014) 39:961–969 967

123

respectively. Characterization of the products was carried

out by 1H NMR spectroscopy using Bruker DPX-400 in

CDCl3 with TMS as internal standard. Chemical shifts are

reported as d values with reference to tetramethylsilane

(TMS) as internal standard. The reaction products were

quantified (GC data) with a Varian 3,400 gas chromato-

graph equipped with a 30 m CP-SIL8CB capillary column

and a flame ionization detector and identified by a Trace

DSQ II GC–MS equipped with a 60 m TR-50MS capil-

lary column.

Synthesis of the Schiff base and complex

N-(hydroxyphenyl) salicyldimine was prepared by mixing

2-aminophenol and salicylaldehyde in (1:1) molar ratio

[38]. The copper complex was prepared as follows. To a

solution of the Schiff base (0.01 mol) in methanol (20 ml)-

containing KOH (0.01 mol) was added copper acetate

(0.01 mol). The resultant mixture was stirred for 1 h at

room temperature under air, whereupon a solid precipitated

out. This was filtered off and washed thoroughly with water

and ether and then dried over fused CaCl2

Cu ampð Þ OAcð Þ½ � ¼ C% 53:9 53:80ð Þ; H% 4:0 3:9ð Þ;½N% 4:2 4:2ð Þ�:

Synthesis of the immobilized catalyst

First, poly(4-aminostyrene) was prepared according to the

literature [39]. The poly(4-aminostyrene) was stirred with a

solution of excess copper acetate in methanol for 12 h and

then washed thoroughly with water. The metal-loaded

poly(4-aminostyrene) samples were then reacted with

solutions of Schiff base ligand in methanol to get the

immobilized metal complex (PS–Cu–amp–OAc).

General procedure for N-arylation reaction

In a 100-ml RB flask, copper catalyst (1.5 9 10-5 mol),

arylboronic acids (1.5 mmol), N–H heterocycles

(1.2 mmol) and 10 ml methanol were stirred under open

air, at 40 �C for 6 h. The reaction mixtures were collected

at different time interval and identified by GC–MS and

quantified by GC analysis. After the completion of the

reaction, the catalyst was filtered off and washed with

water followed by acetone and dried in oven. The filtrate

was extracted with ethyl acetate (3 9 20 ml), and the

combined organic layers were dried with anhydrous Na2

SO4 by vacuum. The filtrate was concentrated by vacuum,

and the resulting residue was purified by column chroma-

tography on silica gel to provide the desired product.

Spectroscopic data for selected products

1-Phenyl-1H-imidazole (Table 4, entry 1)

1H NMR (400 MHz, CDCl3) d: 7.83 (s, 1H), 7.52–7.44 (m,

2H), 7.41–7.34 (m, 3H), 7.28 (bs, 1H), 7.21 (bs, 1H); 13C

NMR (100 MHz, CDCl3) d: 136.0, 134.3, 129.7, 129.5,

127.4, 121.3, 118.0.

1-(4-Methylphenyl)-1H-imidazole (Table 4, entry 2)

1H NMR (400 MHz, CDCl3) d: 7.77 (s, 1H), 7.24 (m, 4H),

7.20 (bs, 1H), 7.13 (bs, 1H), 2.40 (s, 3H); 13C NMR

(100 MHz, CDCl3) d: 137.4, 134.9, 134.8, 130.5, 130.0,

121.1, 118.3, 20.9.

1-(4-(1H-imidazol-1-yl)phenyl)ethanone (Table 4,

entry 9)

1H NMR (400 MHz, CDCl3) d: 8.10 (d, 2H), 7.97 (bs, 1H),

7.49 (d, 2H), 7.34 (bs, 1H), 7.24 (bs, 1H), 2.63 (s, 3H); 13C

NMR (100 MHz, CDCl3) d: 196.4, 141.1, 135.8, 135.4,

131.2, 130.3, 120.7, 117.7, 26.6.

1-Phenyl-1H-benzimidazole (Table 4, entry 10)

1H NMR (400 MHz, CDCl3) d: 8.07 (s, 1H), 7.81–7.86 (m,

1H), 7.44–7.62 (m, 6H), 7.5–7.3 (m, 2H); 13C NMR

(100 MHz, CDCl3) d: 144.0, 142.6, 136.7, 133.6, 130.2,

128.0, 126.0, 124.0, 123.0 (2C), 120.3, 110.1 (2C).

1-(4-Methylphenyl)-1H-benzimidazole (Table 4, entry

11)

1H NMR (400 MHz, CDCl3) d: 8.02 (br s, 1H), 7.77–7.86

(m, 1H), 7.40–7.47 (m, 1H), 7.20–7.33 (m, 6H), 2.41 (s,

3H); 13C NMR (100 MHz, CDCl3) d: 145.0, 142.0, 138.0,

135.0 (2C), 130.6, 124.0, 123.9, 123.0 (2C), 120.3, 110.0

(2C), 21.0.

Fig. 3 Recycling efficiency for the N-arylation of imidazole with

phenylboronic acid by polymer-supported Cu(II) Schiff base complex

968 Transition Met Chem (2014) 39:961–969

123

2-Phenylisoindoline-1,3-dione (Table 4, entry 13)

1H NMR (400 MHz, CDCl3) d: 7.41–7.47 (m, 3H),

7.49–7.56 (m, 2H), 7.79–7.83 (m, 2H), 7.98 (m, 2H).

N-Phenylbenzamide (Table 4, entry 19)

1H NMR (400 MHz, CDCl3) d: 7.13–7.16 (m, 1H),

7.36–7.41 (m, 2H), 7.49–7.54 (m, 2H), 7.55–7.57 (m, 1H),

7.64–7.65 (m, 2H), 7.83 (s, 1H), 7.86–7.88 (m, H).

N-Phenylsulfonamide (Table 4, entry 20)

1H NMR (400 MHz, CDCl3) d: 7.73–7.76 (m, 2H), 7.52

(t, 1H), 7.40 (t, 2H), 7.20 (t, 2H), 7.09–7.06 (m, 4H).

Acknowledgments We thank the Indian Association for the Culti-

vation of Science, Kolkata for providing the instrumental support. MI

acknowledges Department of Science and Technology (DST),

Council of Scientific and Industrial Research (CSIR) and University

Grant Commission (UGC), New Delhi, India for funding.

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