A highly efficient Schiff-base derived palladium catalyst for the Suzuki-Miyaura reactions of aryl chlorides

Article abstract of DOI:10.1016/j.tetlet.2013.03.115

A new palladium complex derived from a bidentate Schiff-base ligand showed excellent activity as catalyst for the Suzuki-Miyaura cross-coupling reactions of less reactive aryl chlorides with arylboronic acids. Under an optimized condition, moderate-to-excellent yields of biaryls were obtained with a wide range of substrates at a relatively low loading of catalyst (0.2 mol %).

Full text of DOI:10.1016/j.tetlet.2013.03.115

Tetrahedron Letters 54 (2013) 2886–2889
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Tetrahedron Letters
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A highly efficient Schiff-base derived palladium catalyst for the Suzuki–Miyaura
reactions of aryl chlorides
Nasifa Shahnaz a, Biplab Banik a,b, Pankaj Das a,c,
⇑
a Department of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India
b Department of Chemistry, Tinsukia College, Tinsukia 786125, Assam, India
c Department of Chemistry, Rajiv Gandhi University, Doimukh, Rono Hills, Itanagar 791112, Arunachal Pradesh, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new palladium complex derived from a bidentate Schiff-base ligand showed excellent activity as cat-
alyst for the Suzuki–Miyaura cross-coupling reactions of less reactive aryl chlorides with arylboronic
acids. Under an optimized condition, moderate-to-excellent yields of biaryls were obtained with a wide
range of substrates at a relatively low loading of catalyst (0.2 mol %).
Received 11 December 2012
Revised 24 March 2013
Accepted 26 March 2013
Available online 6 April 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Suzuki–Miyaura reaction
Homogeneous catalysis
Palladium complex
Schiff-base ligand
Aryl chloride
Over the past few years, there has been an exponential rise
in the number of publications in palladium-catalyzed Suzuki–
Miyaura cross-coupling reactions of aryl halides with arylboronic
acids as this method is one of the most powerful and simplest
methods for carbon–carbon bond formation in organic synthesis.1
Today, literally thousands of palladium catalysts are known that
effectively promote this reaction under a variety of conditions.
Despite considerable advancements, a major challenge which still
remains in the Suzuki reaction is to develop catalysts that can acti-
vate aryl chlorides as substrates as these chlorides are highly
accessible and less expensive compared to corresponding bro-
mides or iodides. Although a sizable number of catalysts are known
that can activate aryl chlorides in Suzuki reaction, in most of the
cases phosphines such as PtBu3,2 PCy3,3 biphenyl-based phos-
phines,4 phosphapalladacycles,5 hemilabile phosphines,6 indole-
based phosphines,7 and ferrocenyl phosphines8 are employed as
ligands. Despite considerable success, the major drawbacks associ-
ated with such phosphines are their handling problems, synthetic
difficulties, high costs, inherent toxicity, air and moisture sensitiv-
ity, etc. In this respect, nitrogen-based ligands are generally
advantageous as they are easy-to-handle, less expensive than their
phosphine counterparts, usually stable to air/moisture, etc. As a
result, a plethora of nitrogen-based ligands such as amines,9
N-heterocyclic carbenes (NHC),10 oximes,11 hydrazones,12 and
others13 have been tested as possible alternatives to phosphines
in Suzuki reactions. However, barring a few NHC10a–d,f or oxime-
based palladacycles,11a,b a majority of such protocols are incompat-
ible with aryl chlorides. It is worth to note that irrespective of the
ligand being either phosphine-based or phosphine-free, aryl chlo-
ride activation usually required a high catalyst loading. For in-
stance, recently Sarkar and co-workers reported a highly efficient
phosphine-based catalytic system that required 4 mol % catalyst
loading.7a Similarly, a few years back, Mino’s group had developed
a phosphine-free catalytic system that required 10 mol% catalyst
loading to activate a simple substrate like 4-chloroacetophenone.12
However, from the economic point of view, such high loading of
catalyst is unacceptable, as the advantage associated with the
use of less expensive aryl chlorides is often negated by the high
cost of palladium. Thus, efforts are being continued to design alter-
native catalytic systems that can activate aryl chlorides smoothly
with low catalyst loading.
For decades, Schiff-bases have played a key role as chelating
multidentate ligands, used in coordination chemistry and catalysis,
because of their easy synthesis and high stability.14 In fact, there
have been a number of reports about employment of Schiff-base li-
gands in palladium-catalyzed Suzuki reactions,15 but in a majority
of the cases good results were usually obtained with aryl bromides
or iodides as substrates. Nevertheless, there exist a very few excep-
tions where Schiff-base derived catalysts showed good results with
aryl chlorides also. For example, a few years back, Cui et al.15e
developed an amino-salicylaldimine based Pd catalyst that could
⇑
Corresponding author. Tel.: +91 9435394373; fax: +91 3732370323.
E-mail address: pankajd29@yahoo.com (P. Das).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.03.115

N. Shahnaz et al. / Tetrahedron Letters 54 (2013) 2886–2889
2887
activate aryl chlorides elegantly at
a
low catalyst loading
(0.5 mol %). Excellent conversions were seen with substrates
comprising electron-withdrawing groups (viz. aldehyde, acetyl,
and nitro), but electron-rich substrates such as 4-chlorotoluene
and 4-chlorophenol gave only low yields. Recently, Hanhan
et al.13b designed a ferrocene-based palladium Schiff-base catalyst
that showed remarkable activities with aryl chlorides bearing both
electron-rich and electron-poor substituents. The higher activities
of this catalyst were attributed to the sterically demanding nature
and electron-rich property of the ligand caused by the presence of
two ferrocene moieties. Very recently, we have reported a binu-
clear palladium complex (Fig. 1) with a N4-type Schiff-base ligand
that showed excellent activity as catalyst for the Suzuki reactions
of aryl bromides and iodides in water at room temperature at a
low loading of catalyst (0.2 mol %).16 However, despite fivefold
increasing the catalyst loading, changing the solvent systems,
and raising the temperature to 80 °C, we were able to get only
moderate yields with aryl chlorides. In continuation with our ef-
forts, herein we have developed a new palladium Schiff-base cata-
lyst that shows excellent activity for the Suzuki reactions of aryl
chlorides with a low catalyst loading (0.2 mol %).The Schiff-base li-
gand, N,N0-bis(benzylidine)-o-phenylenediamine (L) was previ-
ously reported,17 however its coordination chemistry with
palladium is unknown. In this work, we have prepared a new pal-
ladium complex (2) by reacting PdCl2 with the ligand L in acetoni-
trile (Scheme 1). The complex 2 was isolated as brown solid and its
identity was confirmed by elemental analyses, ESI-mass, FTIR, 1H
and 13C NMR spectroscopy.18 In the FTIR spectra of the complex
N
N
N
N
PdCl2
Pd
Cl
Cl
Acetonitrile
70°C, 3 h
Ligand L
Complex 2
Scheme 1. Synthesis of complex 2.
Table 1
Effects of temperatures in the Suzuki–Miyaura reactionsa of 4-chloronitrobenzene
with phenylboronic acid using complex 2 as catalyst
2
Complex (1 mol%)
K2CO3, DMF, Temp.
+
Cl
NO2
B(OH)2
NO2
Entry
Temperature (°C)
Time (h)
Yieldb,c (%)
1
2
3
4
5
26 (rt)
60
80
100
120
24
5
3
1
1
30
70
85
98
98
a
Reaction conditions: 0.5 mmol 4-chloronitrobenzene, 0.75 mmol phenylboronic
acid, 1.5 mmol K2CO3 DMF (6 mL).
b
c
Isolated yield.
Yields are of average of two runs.
Table 2
Effects of solvents in the Suzuki–Miyaura reactionsa of 4-chloronitrobenzene with
2, the
m
C@N band appears at 1609 cmꢀ1 which is marginally higher
C@N stretching of the free ligand (1604 cmꢀ1). The imine
phenylboronic acid using complex 2 as catalyst
than the
m
signal in the 1H NMR spectra appears at d 8.83 ppm and compared
to free ligand a downfield shift of 0.21 ppm was observed. A similar
downfield shift was also observed for the corresponding carbons of
the 13C NMR spectra of the complex. However, no prominent shifts
were observed in the other aromatic protons or carbon resonances.
To evaluate the efficiency of our complex as catalyst for the Su-
zuki–Miyaura reaction,19 we have chosen 4-chloronitrobenzene
and phenylboronic acid as model substrates. Initial reaction was
conducted at room temperature using DMF as solvent and K2CO3
as base with 1.0 mol % of the catalyst. We are quite satisfied to
see that our reaction proceeded at room temperature, although
only 30% product was isolated after a reaction time of 24 h (Table
1, entry 1). However, the product yield could be improved signifi-
cantly by increasing the reaction temperature. The best result was
obtained at 100 °C as quantitative product formation was achieved
within 1 h of reaction time (Table 1, entry 4). It is well established
that in the Suzuki reaction the choice of solvent often plays a cru-
cial role in the overall performance of a catalyst, and to investigate
this effect we have screened different solvents with our model
reaction (Table 2). Our study reveals DMF is the best solvent for
our catalyst (Table 2, entry 2). The other solvents including water
(entry 4) and mixed-aqueous solvents (entries 7–9) gave only poor
yields. We have also screened the effects of different bases (Table
3) on our model reaction and noticed that the reaction can tolerate
2
Complex (1 mol%)
+
Cl
NO2
B(OH)2
NO2
K2CO3, 100 0C, Solvent
Entry
Solvent
Time (h)
Yieldb,c (%)
1
2
3
4
5
6
7
8
9
iPrOH
DMF
Toluene
H2O
Glycerol
THF
5
1
4
6
4
4
5
6
5
50
98
16
12
26
12
23
10
34
iPrOH:H2O
Toluene:H2O
DMF:H2O
a
Reaction conditions: 0.5 mmol 4-chloronitrobenzene, 0.75 mmol phenylboronic
acid, 1.5 mmol K2CO3 6 mL solvent.
b
c
Isolated yield.
Yields are of average of two runs.
common inorganic bases. K2CO3 and Cs2CO3 are found to be the
two most effective bases that can complete the reactions in quick
time (Table 3, entries 2 and 5). Other bases usually require ex-
tended reaction time to produce good results. Among the bases
screened, MgCO3 was found to be the least effective and gave only
52% yield in 6 h (Table 3, entry 4). Studies on optimization of cat-
alyst quantities (Table 4) reveal that 0.2 mol % palladium loading is
essential for completion of the reaction (entry 3). On decreasing
the catalyst loading to 0.1 mol %, 56% biphenyl formation was
achieved after 4 h (entry 4). A further decrease in catalyst loading
resulted in poor conversion even after increasing the reaction time
up to 24 h.
Under the optimized condition (DMF, K2CO3, 100 °C, 0.2 mol %
complex 2), we have examined the cross-coupling reactions of a
wide range of electronically and sterically diverse aryl chlorides
with arylboronic acids. As illustrated in Table 5, the results showed
that Suzuki-coupling of aryl chlorides with arylboronic acids took
place smoothly to get moderate-to-excellent yields of coupling
products. However, the steric and electronic natures of substitu-
N
N
N
N
Pd
Pd
AcO
OAc
AcO
OAc
Complex 1
Figure 1. Schematic representation of the previously reported complex.16
ents have
a significant influence on the overall catalytic

2888
N. Shahnaz et al. / Tetrahedron Letters 54 (2013) 2886–2889
Table 3
Table 5
Effects of bases in the Suzuki–Miyaura reactionsa of 4-chloronitrobenzene with
Suzuki–Miyaura Reactionsa of various aryl chlorides with arylboronic acids with
phenylboronic acid using complex 2 as catalyst
complex 2 as catalyst
Complex (1 mol%)
Complex 2 (0.2 mol%), 100 0
K2CO3, DMF
C
2
+
Cl
NO2
B(OH)2
NO2
Base, DMF, 100 0
C
Cl
B(OH)2
+
R
R'
R'
R
Entry
Base
Time (h)
Yieldb,c (%)
Entry
R
R0
Time (h)
Yieldb,c (%)
1
2
3
4
5
6
7
NaOH
K2CO3
KF
MgCO3
Cs2CO3
LiOH
5
1
5
6
1
6
6
90
98
85
52
96
87
84
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
4-NO2
4-CHO
4-COCH3
H
4-OCH3
4-CH3
4-CH2OH
3-CH2OH
2-OCH3
2-NO2
3-NO2
2-CH3
4-NO2
4-NO2
4-NO2
4-CH3
H
H
H
H
H
H
H
H
H
H
H
H
4-Cl
3-NO2
4-CH3
4-CH3
1
2
2
4
4
4
4
6
6
6
6
6
6
6
6
6
98 (63)d
88
92
82 (42)d
86
82 (34)d
NaHCO3
83
66
57
a
Reaction conditions: 0.5 mmol 4-chloronitrobenzene, 0.75 mmol phenylboronic
acid, 1.5 mmol base, DMF (6 mL).
b
c
65 (Trace)d
Isolated yield.
Yields are of average of two runs.
78
52
91
44
82
86
Table 4
Effects of catalyst quantity in the Suzuki–Miyaura reactionsa of 4-chloronitrobenzene
with phenylboronic acid using complex 2 as catalyst
a
Reaction conditions: 0.5 mmol aryl chloride, 0.75 mmol arylboronic acid,
1.5 mmol K2CO3 DMF (6 mL).
2
Complex (0.05-1 mol%)
+
Cl
NO2
B(OH)2
NO2
K2CO3, DMF, 100 0
C
b
c
Isolated yield.
Yields are of average of two runs.
Entry
Catalyst (mol %)
Time (h)
Yieldb,c (%)
d
The values in the parenthesis indicate yields using complex 1 as catalyst under
similar experimental conditions.
1
2
3
4
5
1.0
0.5
0.2
0.1
0.05
1
1
1
4
24
98
98
98
56
22
aryl chlorides, including sterically demanding ortho-substituted
chlorides, could conveniently be coupled with arylboronic acids.
a
Reaction conditions: 0.5 mmol 4-chloronitrobenzene, 0.75 mmol phenylboronic
acid, 1.5 mmol K2CO3 DMF (6 mL).
Isolated yield.
Acknowledgement
b
c
Yields are of average of two runs.
We gratefully acknowledge the University Grants Commission
(UGC), New Delhi for a research Grant (No: 36-73/2008). The Tez-
pur University and the SAIF, NEHU, Shillong are gratefully
acknowledged for various analytical services.
performance. For instance, para-substituted aryl chlorides bearing
electron withdrawing groups such as nitro (entry 1), aldehyde (en-
try 2), and acetyl (entry 3) showed excellent yields within 2 h of
reaction time, while aryl chlorides bearing electron donating
groups such as methoxy (entry 5) or methyl (entry 6) gave the de-
sired biphenyls in very good yields within a reaction time of 4 h. It
may be noted that under slightly extended reaction time, our cat-
alyst can also tolerate sterically bulky ortho-substituted chlorides
such as 2-chloroanisole (entry 9), 2-chloronitrobenzene (entry
10), or 2-chlorotoluene (entry 12) that gave the desired product
in reasonably good yields. In comparison to ortho-substituted chlo-
rides the meta-substituted chlorides gave slightly better yields (en-
tries 8 and 11). No major differences in yields were observed for
the coupling reaction of 4-chloronitrobenzene, when phenylbo-
ronic acid was replaced by 4-chloroboronic acid (entry 13) or 4-tol-
ylboronic acid (entry 15). However, the coupling reaction between
4-chloronitrobenzene and 3-nitrophenylboronic acid resulted only
in moderate yield (entry 14). To compare the catalytic activity of
complex 2 with complex 1, we performed cross-coupling reactions
of some selected aryl chlorides with complex 1 using the same set
of experimental conditions (Table 5, entries 1, 4, 6, and 10). Inter-
estingly, a marked difference in catalytic activity has been noticed.
For instance, as compared to 98% nitrobiphenyl formation by com-
plex 2, the coupling reaction with complex 1 gave only 63% yield
(entry 1). Similarly, substantially lesser yields were also obtained
with chlorobenzene and 4-chlorotoluene (entries 4 and 6). These
results demonstrate that complex 2 is a better catalyst than com-
plex 1.
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N. Shahnaz et al. / Tetrahedron Letters 54 (2013) 2886–2889
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18. Synthesis of the complex 2: A solution of the ligand L (0.14 g, 0.5 mmol) in 20 mL
acetonitrile was added drop wise to a solution of the PdCl2 (0.89 g, 0.5 mmol)
in 20 mL acetonitrile. The reaction mixture was heated at 70 °C for 3 h and
during that time the colour of the solution changed from yellow to yellow–
brown. The solvent was then evaporated under reduced pressure to get a solid
mass which was then washed with hexane. Finally the complex was obtained
as brown solid which was recrystallized from dichloromethane. Yield: 88%, mp
188 °C; Anal. Calcd for C20H16N2Cl2Pd: C, 52.06; H, 3.47; N:,6.07. Found: C,
51.89; H, 3.44; N, 6.09. MS-ESI (CH2Cl2): m/z: 463 [M+2H]+; 386 [M-Ph+2H]+;
305 [M-2Ph-2CH+Na+H]+ (100%). FTIR (cmꢀ1, KBr): 1609 (
NMR (400 MHz, CDCl3) d/ppm: 8.83 (s, 2H, CH@N), 7.88 (d, J = 7.6 Hz, 2H, Ar),
7.69 (d, J = 7.2 Hz, 4H, Ar), 7.22–7.48 (m, 6H, Ar), 7.10 (d, J = 6.8 Hz, 2H, Ar). 13
m
C@N: imine); 1H
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C
{1H} NMR (100.62 MHz, CDCl3) d/ppm: imine carbons, 154.13; aromatic
carbons, 142.9, 136.32, 129.97, 127.78, 125.94, 123.09, 119.91, 110.54.
19. General procedure for the Suzuki–Miyaura reaction: A 50 mL round bottom flask
was charged with a mixture of aryl chloride (0.5 mmol), arylboronic acid
(0.75 mmol), K2CO3 (1.5 mmol), Pd catalyst (0.2 mol %), solvent (6 mL) and the
mixture was stirred for required times at appropriate temperature. After
completion, the reaction mixture was diluted with water (20 mL) and
extracted with ether (20 mL ꢁ3). Combined extract was washed with brine
(20 mL ꢁ3) and dried over Na2SO4. After evaporation of the solvent under
reduced pressure, the residue was chromatographed (silica gel, ethyl acetate/
hexane 1:9) to obtain the desired product. The products were confirmed by
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