Highly efficient and simple catalytic system for the N-arylation of some hindered aza-heterocycles in water

Article abstract of DOI:10.1080/00397911.2011.555219

A highly efficient and simple protocol for the N-arylation of some hindered aza-heterocycles in water has been developed with readily available basic copper carbonate as the catalyst using bis(3,5-dimethyl-1H-pyrazol-1-yl)methane as ligand. This mild catalytic system possesses excellent tolerance for a large variety of functional groups. A total of 11 previously unknown coupling products have been synthesized by this methodology.

Full text of DOI:10.1080/00397911.2011.555219

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Highly Efficient and Simple Catalytic
System for the N-Arylation of Some
Hindered Aza-Heterocycles in Water
Chhanda Mukhopadhyay a & Pradip Kumar Tapaswi a
a Department of Chemistry , University of Calcutta , Kolkata , India
Accepted author version posted online: 19 Dec 2011.Published
online: 09 Apr 2012.
To cite this article: Chhanda Mukhopadhyay & Pradip Kumar Tapaswi (2012) Highly Efficient and
Simple Catalytic System for the N-Arylation of Some Hindered Aza-Heterocycles in Water, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
42:15, 2217-2228, DOI: 10.1080/00397911.2011.555219
To link to this article: http://dx.doi.org/10.1080/00397911.2011.555219
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Synthetic Communications1, 42: 2217–2228, 2012
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2011.555219
HIGHLY EFFICIENT AND SIMPLE CATALYTIC SYSTEM
FOR THE N-ARYLATION OF SOME HINDERED
AZA-HETEROCYCLES IN WATER
Chhanda Mukhopadhyay and Pradip Kumar Tapaswi
Department of Chemistry, University of Calcutta, Kolkata, India
GRAPHICAL ABSTRACT
Abstract A highly efficient and simple protocol for the N-arylation of some hindered
aza-heterocycles in water has been developed with readily available basic copper carbonate
as the catalyst using bis(3,5-dimethyl-1H-pyrazol-1-yl)methane as ligand. This mild
catalytic system possesses excellent tolerance for a large variety of functional groups. A
total of 11 previously unknown coupling products have been synthesized by this methodology.
Keywords Aqueous medium; basic copper carbonate; bis(3,5-dimethyl-1H-pyrazol-1-
yl)methane; hindered aza-heterocycles; N-arylation
INTRODUCTION
N-arylazoles are important compounds that have extensive applications in
biochemical, pharmaceutical, and material fields.[1] Traditionally, these moieties were
prepared by nucleophilic aromatic substitution of azoles with activated aryl halides or
the classical Ullmann-type coupling reactions.[2] For both these cases, the reactions
suffered from several shortcomings such as high reaction temperature, use of stoichio-
metric copper reagents, moderate yields, and poor substrate generality, thereby limit-
ing their applications.[3] The development of a mild, ecofriendly, and highly efficient
protocol for the synthesis of N-arylazoles over the classical Ullmann-type or nucleo-
philic aromatic substitution reactions has recently gained considerable interest both
in academia and industry.[4] Because of the economic attractiveness of copper[5]
and by using N,N- and N,O-bidentate compounds as ligands, many copper-catalyzed
C–N,[6] C–O,[7] C–S,[8] and C–C[9] bond-formation reactions have received growing
Received October 8, 2010.
Address correspondence to Chhanda Mukhopadhyay, Department of Chemistry, University of
Calcutta, 92 APC Road, Kolkata 700 009, India. E-mail: cmukhop@yahoo.co.in
2217

2218
C. MUKHOPADHYAY AND P. K. TAPASWI
Figure 1. Proposed ligands for the N-arylation of azoles in water.
interest in carbon–heteroatom coupling reactions, and their applications seem to
possess more importance[10,11] in the present scenario.
Recently, N-arylation has been carried out with various N-containing hetero-
cycles using diverse organic ligands such as diamine derivatives,[12] amino acids,[13]
N-hydroxyimides,[14] phosphine oxides,[15] imidazole derivatives,[16] diketones or
diphenols,[17] fluoraphite,[18] per-6-amino-b-cyclodextrin,[19] and even pyridine
N-oxides.[20(a)] On the other hand, several N-heterocycles such as bis(3,5-dimethyl-1-
H-pyrazol-1-yl)methane,[21] bis(1,2,4-triazol-1H-yl)methane,[22] and 2-(2-pyridyl)
benzimidazole[23] long have been recognized as ligands in coordination chemistry
(Fig. 1), but applications of these N-heterocyclic ligands for copper-catalyzed
N-arylation of aza-heterocycles have not yet been reported.
Despite significant progress in the copper-catalyzed N=O-arylation with aryl
halides, very few reports are available describing N-arylation of azoles in aqueous
medium.[20] Therefore, enough room for exploration of an efficient ligand-assisted
catalytic system in the aqueous medium still remains open.
In general, almost all the organic-solvent-mediated syntheses of organic
compounds have the inherent problem of pollution. In recent years, water-mediated
organic synthesis without using organic solvents has become one of the most
important developments in present-day organic chemistry to meet the environmental
demands.[24] Carrying out organic synthesis in water is extremely challenging both
from synthetic and environmental viewpoints. Additional problems of N-arylation
of azoles in aqueous medium are the water tolerance for the catalyst and solubility
of the substrates and ligands in the aqueous phase. In this regard, the development
Scheme 1. N-arylation of aza-heterocycles in water.

N-ARYLATION OF AZA-HETEROCYCLES IN WATER
2219
Scheme 2. Optimization of N-arylation of imidazole with iodobenzene in water.
of less expensive and more sustainable catalysts in water remains an attractive goal in
modern synthetic organic chemistry.
RESULTS AND DISCUSSION
In continuation of our endeavors toward the synthesis and functionalization of
biologically important N-containing heterocycles,[25] we herein describe N-arylation
of azoles with aryl halides catalyzed by readily available Cu(OH)2 ꢀ CuCO3 with
N-heterocyclic ligands in water (Scheme 1).
Imidazole and iodobenzene were chosen as model substrates (Scheme 2) to
optimize the reaction conditions such as amount of ligands, base, copper source,
and reaction temperature. Imidazole (1 mmol), iodobenzene (1.2 mmol), base
(2 mmol), Cu source (15 mol%), and ligand (20 mol%) in water were taken for initial
optimization. After thorough screening several combinations, the best result was
obtained after 12 h of reflux using bis(3,5-dimethyl-1H-pyrazol-1-yl)methane (L1)
as ligand and Cu(OH)2 ꢀ CuCO3 as copper source in aqueous medium. It should be
mentioned here that when Cu(OH)2 ꢀ CuCO3 (1.2 equivalents) was used both as
Table 1. Optimization of reaction condition for the N-arylation of imidazole in water
Copper source
(15 mol%)
Ligand
(20 mol%)
Base
(2 eqv.)
Time
(h)
Yield (%)
(isolated)
Entry
1
2
—
—
—
—
L1
L2
L3
L1
L1
L1
L1
L1
L1
L1
L1
K2CO3
—
24
24
24
12
12
12
12
12
12
12
12
12
12
12
Nil
Nil
55
92
82
46
88
70
50
75
65
60
50
55
CuCO3 ꢀ Cu(OH)2
CuCO3 ꢀ Cu(OH)2
CuCO3 ꢀ Cu(OH)2
CuCO3 ꢀ Cu(OH)2
CuCO3 ꢀ Cu(OH)2
CuCO3 ꢀ Cu(OH)2
CuCO3 ꢀ Cu(OH)2
CuCO3 ꢀ Cu(OH)2
Cu(OH)2
3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
K3PO4
DABCO
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
4
5
6
7
8
9
10
11
12
13
14
CuSO4
CuI
Cu(OAc)2
CuCl2

2220
C. MUKHOPADHYAY AND P. K. TAPASWI
Table 2. N-Arylation of aza-heterocycles in water
Aryl
halides
Time Yields (%)
(h) (isolated) References
Entry
1
Product no.
Products
3a
12
92
19
19
2
3
3a
3b
14
14
88
91
26
—
4
5
3c
3d
15
13
85
92
—
27
6
7
3e
3f
13
15
92
82
27
8
9
3g
3h
13
14
86
82
—
—
10
3i
16
82
—
(Continued )

N-ARYLATION OF AZA-HETEROCYCLES IN WATER
2221
Table 2. Continued
Aryl
halides
Time Yields (%)
(h) (isolated) References
Entry
11
Product no.
Products
3j
14
15
92
88
—
—
12
13
3k
3l
12
14
92
85
25 (d)
25 (d)
14
15
3m
3n
12
16
90
86
25 (d)
16
17
3o
—
3p
16
84
—
18
3q
12
90
—
(Continued )

2222
C. MUKHOPADHYAY AND P. K. TAPASWI
Table 2. Continued
Aryl
halides
Time Yields (%)
(h) (isolated) References
Entry
Product no.
Products
19
3r
13
88
—
20
3s
12
90
12(a)
source of metal and as base, almost complete conversion of imidazole to 1-phenylimi-
dazole occurred after 16 h of reflux in aqueous conditions. However, the isolated yield
was poor (50%), which may be due to the partial conversion of imidazole to corre-
sponding Cu(ii) complex of imidazole. Then we decided to use Cu(OH)2 ꢀ CuCO3 as
the metal source only and not as a base. Under this condition, the product yield
was better than in the previous case. The arylation reaction is highly sensitive to
reaction temperature and time. With lower temperature (below 90 ꢁC) and shorter
reaction time (less than 12 h), rate of conversion of imidazole to 1-arylimidazole
decreases sharply. Therefore, the optimized conditions for the N-arylation of imidaz-
ole in water are the combination of Cu(OH)2 ꢀ CuCO3 (15 mol%), L1 (20 mol%), and
K2CO3(2 mmol) at 100 ꢁC for 12 h in air. The results are listed in Table 1.
After optimizing the reaction condition, it was tested with different aza-
heterocycles. The scope of this coupling reaction was further expanded to diverse aryl
bromides, and the overall yields were little bit lower with aryl bromides than with the
aryl iodides. The results indicate clearly that this methodology is quite general and
applicable for the reaction of a wide variety of imidazoles, benzimidazoles, triazoles,
benzotriazoles, indoles, pyrroles, and pyrazoles. Substrates possessing electron-rich
groups such as 4-bromotoluene, 3,4-dimethyliodobenze, and 2,5-dichloroiodoben-
zene afford coupling products in good yields but with larger reaction time than
3-nitroiodobenzene. The reaction is also less sensitive to steric factors and gives good
yields, as is evident in the case of the reaction of 2,5-dichloroiodobenzene with
2-methylimidazole. This methodology also provides good results in the case of the
reaction of sterically hindered 5,6-dimethylbenzimidazole and benzotriazoles. The
complete results are shown in Table 2.
To our delight, most of the imidazole and triazole derivatives afforded the cor-
responding arylated products in good yields. Notably, 2-methylimidazole (Table 2,
entries 3 and 4), 2-methylbenzimidazole (Table 2, entries 6 and 8), and even more
bulkier 2-dichloromethylbenzimidazole (Table 2, entries 10 and 11) gave satisfactory

N-ARYLATION OF AZA-HETEROCYCLES IN WATER
2223
Scheme 3. Probable mechanism for N-arylation of nitrogen-containing heterocycles (imidazole) with basic
copper carbonate.
yields under the present reaction conditions. All the coupling products were well
1
characterized by H NMR, 13C NMR, and infrared (IR) spectroscopic data.
A probable mechanism for the basic copper carbonate–catalyzed N-arylation
of nitrogen-containing heterocycles has been proposed in Scheme 3, in which the first
step is the rapid coordination of copper hydroxide and the ligand (L1) with the
N-containing heterocycles such as imidazole to form a four-coordinated copper(ii)
complex A. The second step involves the oxidative addition of the aryl halide to form
the five-coordinated complex (intermediate B). Complex B subsequently undergoes
reductive elimination to form the N-aryl products.
CONCLUSION
In summary, we have utilized cheap and commercially available Cu(OH)2 ꢀ
CuCO3 as an efficient catalyst for the N-arylation of aza-heterocycles with wide a
range of aryl iodides and aryl bromides in aqueous medium using bis(3,5-dimethyl-1-
H-pyrazol-1-yl)methane (L1) as the ligand. This methodology avoids the use of
hazardous, toxic organic solvents and an inert atmosphere, which are common
requirements in previous methods. Therefore, this methodology is very simple for
N-arylation of aza-heterocycles.
EXPERIMENTAL
All solvents were distilled before use. All the chemicals were purchased from
Aldrich Chemical Company and Spectrochem, Pvt. Ltd. (Mumbai, India). Silica
gel G with binder from Spectrochem, Pvt. Ltd., Mumbai, India, was used for thin-
layer chromatography (TLC). 1H and 13C NMR spectra were obtained on a Bruker

2224
C. MUKHOPADHYAY AND P. K. TAPASWI
300-MHz instrument at 300 and 75 MHz, respectively. CDCl3 was purchased from
Aldrich Chemical Company. Melting points were determined on an electrical
melting-point apparatus with an open capillary and are uncorrected. IR spectra were
recorded on a Perkin-Elmer spectrophotometer RX=FT-IR system. The elemental-
analyses were carried out on a 2400 series II CHNS analyzer, Perkin-Elmer, USA.
General Experimental Procedure
Azole (1 mmol), aryl halide (1.2 mmol), Cu(OH)2 ꢀ CuCO3 (15 mol%), ligand
(L1) (20 mol%), K2CO3 (2 mmol), and water (2 mL) were mixed in an Erlenmeyer flask
(5 mL) fitted with a reflux condenser and refluxed on an oil bath for the stipulated
period of time. After the completion of the reaction (monitored by TLC), 20 mL of
EtOAc was added to the reaction mixture after transfer to a beaker (50 mL) and
stirred for several minutes. The mixture was then filtered through a bed of celite,
and the organic layer was separated by the separating funnel, which was washed suc-
cessively with water (2 ꢂ 10 mL) and brine (2 ꢂ 10 mL). The organic layer was dried
with anhydrous Na2SO4 and concentrated by reduced pressure in a rotary evaporator.
Finally, the products were purified by column chromatography using neutral alumina
as the column material and EtOAc and petroleum ether (60–80 ꢁC) as eluant. The
IR, 1H NMR, 13C NMR, and analytical data of all previously unknown compounds
are given.
1-(2,5-Dichlorophenyl)-2-methyl-1H-imidazole (3c, Table 2, Entry 4)
White crystalline solid, mp 60–62 ꢁC (EtOAc); IR (KBr): 3052, 2980, 2926,
1
2213, 1609, 1517, 1448, 1392, 1279 and 744 cmꢃ1; H NMR (300 MHz, CDCl3) d:
7.29 (dd, J ¼ 9.0 and 3.6 Hz, 1H), 7.22–7.15 (m, 2H), 6.87 (s, 1H), 6.70 (s, 1H),
2.06 (s, 3H); 13C NMR (75 MHz, CDCl3) d: 145.4, 136.1, 133.4, 131.4, 130.7,
130.5, 129.1, 127.3, 120.5, 12,9. Anal. calcd. for C10H8N2Cl2: C, 52.89: H, 3.55: N,
12.34%. Found: C, 52.76: H, 3.65: N, 12.37%.
1-(3,4-Dimethylphenyl)-1H-benzimidazole (3d, Table 2, Entry 5)
Pale yellow low-melting solid; IR (KBr): 3042, 2923, 1605, 1504, 1452, 1293,
1
1210 and 736 cmꢃ1; H NMR (300 MHz, CDCl3) d: 8.02 (s, 1H), 7.88–7.82 (m,
1H), 7.48–7.43 (m, 1H), 7.29–7.23 (m, 2H), 7.20 (d, J ¼ 7.7 Hz, 2H), 7.14 (d,
J ¼ 7.7 Hz, 1H), 2.24 (s, 6H); 13C NMR (75 MHz, CDCl3) d: 143.6, 141.9, 138.1,
136.2, 133.5, 133.4, 130.5, 124.6, 123.0, 122.1, 120.8, 120.0, 110.1, 19.4, 19.0. Anal.
calcd. for C15H14N2: C, 81.05: H, 6.35: N, 12.60%. Found: C, 80.92: H, 6.44: N,
12.64%.
1-(3,4-Dimethylphenyl)-2-methyl-1H-benzimidazole (3g, Table 2,
Entry 8)
Pale yellow solid; mp 84–86 ꢁC (EtOAc); IR (KBr): 3029, 2934, 1598, 1466,
1
1289, 1225 and 744 cmꢃ1; H NMR (300 MHz, CDCl3) d: 7.68 (d, J ¼ 7.8 Hz, 1H),
7.25 (d, J ¼ 7.8 Hz, 1H), 7.19 (t, J ¼ 7.8 Hz, 1H), 7.12 (t, J ¼ 7.8 Hz, 1H), 7.08–6.98

N-ARYLATION OF AZA-HETEROCYCLES IN WATER
2225
(m, 3H), 2.39 (s, 3H), 2.30 (s, 3H), 2.28 (s, 3H); 13C NMR (75 MHz, CDCl3) d: 151.5,
141.4, 138.5, 137.7, 136.3, 133.2, 130.9, 127.8, 124.2, 122.7, 122.5, 118.5, 110.1, 19.8,
19.5, 14.1. Anal. calcd. for C16H16N2: C, 81.32: H, 6.82: N, 11.85%. Found: C, 81.19:
H, 6.93: N, 11.87%.
2-Dichloromethyl-1-phenyl-1H-benzimidazole (3h, Table 2, Entry 9)
Oil; IR (neat): 2935, 1577, 1499, 1381, 1278 and 1088 cmꢃ1 1H NMR
;
(300 MHz, CDCl3) d: 8.12 (s, 1H), 7.90–7.87 (m, 1H), 7.59–7.42 (m, 6H),
7.36–7..25 (m, 2H); 13C NMR (75 MHz, CDCl3) d: 142.2, 136.3, 130.0, 128.0,
124.0, 123.6, 122.7, 120.5, 118.1, 110.4. Anal. calcd. for C14H10N2Cl2: C, 60.67: H,
3.64: N, 10.11%. Found: C, 60.58: H, 3.70: N, 10.14%.
2-Dichloromethyl-1-(4-methylphenyl)-1H-benzimidazole (3i, Table 2,
Entry 10)
1
Oil; IR (neat): 3033, 2928, 1601, 1467, 1288, 1221 and 754 cmꢃ1; H NMR
(300 MHz, CDCl3) d: 8.01 (s, 1H), 7.80–7.77 (m, 1H), 7.42–7.39 (m, 1H),
7.29–7.16 (m, 6H), 2.35 (s, 3H); 13C NMR (75 MHz, CDCl3) d: 143.5, 142.1,
138.0, 133.7, 133.5, 130.4, 123.8, 123.5, 122.6, 120.2, 110.4, 20.9. Anal. calcd. for
C15H12N2Cl2: C, 61.87: H, 4.15: N, 9.62%. Found: C, 61.79: H, 4.22: N, 9.63%.
1-(3,4-Dimethylphenyl)-1H-[1,2,4]triazole (3j, Table 2, Entry 11)
Pale white solid; mp 52–54 ꢁC (MeOH); IR (KBr): cmꢃ1; 1H NMR (300 MHz,
CDCl3) d: 8.45 (s, 1H), 8.01 (s, 1H), 7.38 (s, 1H), 7.31–7.25 (m, 1H), 7.15 (d,
J ¼ 8.1 Hz, 1H), 2.25 (s, 3H), 2.22 (s, 1H); 13C NMR (75 MHz, CDCl3) d: 152.1,
140.6, 138.2, 136.7, 134.7, 130.4, 121.1, 117.2, 19.7, 19.2. Anal. calcd. for
C10H11N3: C, 69.34: H, 6.40: N, 24.26%. Found: C, 69.25: H, 6.47: N, 24.28%.
1-(2,5-Dichlorophenyl)-1H-[1,2,4]triazole (3k, Table 2, Entry 12)
White crystalline solid, mp 102–104 ꢁC (EtOAc); IR (KBr): 3099, 2927, 2374,
1
1581, 1513, 1477, 1141, 1091, 1029 and 820 cmꢃ1; H NMR (300 MHz, CDCl3) d:
8.66 (s, 1H), 8.16 (s, 1H), 7.65 (d, J ¼ 2.1 Hz, 1H), 7.50 (d, J ¼ 8.7 Hz, 1H), 7.39
(dd, J ¼ 8.7 Hz and 2.3 Hz, 1H); 13C NMR (75 MHz, CDCl3) d: 152.4, 144.4,
135.5, 133.7, 131.6, 130.1, 127.4, 126.3. Anal. calcd. for C8H5N3Cl2: C, 44.89: H,
2.35: N, 19.63%. Found: C, 44.77: H, 2.44: N, 19.66%.
5,6-Dimethyl-1-phenyl-1H-benzotriazole (3o, Table 2, Entry 16)
Pale yellow crystalline solid; mp 88–90 ꢁC (EtOAc): IR (KBr): 3046, 2965,
1
2373, 1590, 1498, 1455, 1063 and 760 cmꢃ1; H NMR (300 MHz, CDCl3) d: 7.82
(s, 1H), 7.74 (d, J ¼ 8.1 Hz, 2H), 7.56 (t, J ¼ 8.7 Hz, 2H), 7.46–7..41 (m, 2H), 2.39
(s, 6H); 13C NMR (75 MHz, CDCl3) d: 145.6, 138.5, 137.2, 134.0, 131.1, 129.6,
128.7, 122.5, 119.1, 109.7, 20.8, 20.2. Anal. calcd. for C14H13N3: C, 75.31: H, 5.87:
N, 18.82%. Found: C, 75.22: H, 5.94: N, 18.84%.

2226
C. MUKHOPADHYAY AND P. K. TAPASWI
5,6-Dimethyl-1-(4-methylphenyl)-1H-benzotriazole (3p, Table 2,
Entry 17)
Pale yellow crystalline solid; mp 132–134 ꢁC (EtOAc); IR (KBr): 2950, 2372,
1
1512, 1457, 1058, 996 and 817 cmꢃ1; H NMR (300 MHz, CDCl3) d: 7.71 (s, 1H),
7.50 (d, J ¼ 7.8 Hz, 2H), 7.33 (s, 1H), 7.25 (d, J ¼ 7.8 Hz, 2H), 2.32 (s, 3H), 2.28
(s, 6H); 13C NMR (75 MHz, CDCl3) d: 145.4, 138.4, 138.3, 134.6, 134.0, 131.3,
130.1, 122.4, 119.0, 109.7, 21.0, 20.8, 20.2. Anal. calcd. for C15H15N3: C, 75.92: H,
6.37: N, 17.71%. Found: C, 75.80: H, 6.47: N, 17.73%.
3,4-Dimethylphenyl-1H-indole (3q, Table 2, Entry 18)
1
Oil; IR (neat): 2954, 2934, 1601, 1487, 1243, 1190 and 845 cmꢃ1; H NMR
(300 MHz, CDCl3) d: 7.91 (d, J ¼ 7.8 Hz, 1H), 7.77 (d, J ¼ 7.8 Hz, 1H), 7.48–7.35
(m, 6H), 6.86 (br. d, J ¼ 3.0 Hz, 1H), 2.49 (s, 6H). Anal. calcd. for C16H15N: C,
86.84: H, 6.83: N, 6.33%. Found: C, 86.72: H, 6.92: N, 6.36%.
1-(3,4-Dimethylphenyl)-1H-pyrrole-2-carbaldehyde (3r, Table 2,
Entry 19)
Dark brown semisolid; IR (KBr): 2923, 2853, 2376, 1657, 1383, 1040 and
1
750 cmꢃ1; H NMR (300 MHz, CDCl3) d: 9.54 (s, 1H), 7.21 (d, J ¼ 7.9 Hz, 1H),
7.16–7.10 (m, 2H), 7.07 (d, J ¼ 7.9 Hz, 1H), 7.03 (s, 1H), 6.37 (t, J ¼ 3.0 Hz, 1H),
2.30 (s, 6H); 13C NMR (75 MHz, DMSO-d6) d: 178.7, 137.2, 136.2, 136.1, 132.0,
131.6, 129.8, 126.7, 123.1, 121.5, 110.6, 19.2, 18.9. Anal. calcd. for C13H13NO: C,
78.36: H, 6.58: N, 7.03%. Found: C, 78.24: H, 6.67: N, 7.06%.
ACKNOWLEDGMENTS
One of the authors (P. K. T.) thanks the University Grants Commission (UCG),
New Delhi, for a fellowship. We thank the CAS Instrumentation Facility, Depart-
ment of Chemistry, University of Calcutta, for spectral data. We also acknowledge
a grant received from UGC for Major Project F No. 37-398=2009 (S.R.).
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1. Schlummer, B.; Scholz, U. Palladium-catalyzed C—N and C—O coupling—A practical
guide from an industrial vantage point. Adv. Synth. Catal. 2004, 346, 1599–1626.
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