Palladium complex containing two sterically hindered ligands as highly efficient catalyst for Suzuki-Miyaura reaction

Article abstract of DOI:10.1002/aoc.3376

A new palladium(II) complex containing two sterically hindered ligands, a P,P-bonded diphosphine and N,N-bonded Schiff base, within the same coordination sphere has been synthesized and investigated as a catalyst for the Suzuki-Miyaura cross-coupling reactions of aryl halides with arylboronic acids. The reaction was highly efficient with aryl bromides in water at room temperature and aryl chlorides in dimethylformamide under relatively mild conditions. Excellent yields of coupling products were obtained for a wide range of aryl halides including heteroaryl halides with a low loading of catalyst.

Full text of DOI:10.1002/aoc.3376

Full paper
Received: 8 May 2015
Revised: 1 August 2015
Accepted: 9 August 2015
Published online in Wiley Online Library: 22 September 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3376
Palladium complex containing two sterically
hindered ligands as highly efficient catalyst for
Suzuki–Miyaura reaction
Nasifa Shahnaz and Pankaj Das*
A new palladium(II) complex containing two sterically hindered ligands, a P,P-bonded diphosphine and N,N-bonded Schiff base,
within the same coordination sphere has been synthesized and investigated as a catalyst for the Suzuki–Miyaura cross-
coupling reactions of aryl halides with arylboronic acids. The reaction was highly efficient with aryl bromides in water at room
temperature and aryl chlorides in dimethylformamide under relatively mild conditions. Excellent yields of coupling products were
obtained for a wide range of aryl halides including heteroaryl halides with a low loading of catalyst. Copyright © 2015 John Wiley
&
Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web site.
Keywords: palladium complex; DPEphos; Schiff base; Suzuki–Miyaura reaction; water; aryl chloride
explored for the Suzuki–Miyaura reactions of aryl bromides in water
Introduction
at room temperature and aryl chlorides in dimethylformamide
(DMF) under slightly harsh conditions. It is pertinent to mention
that there are a few reports of the employment of Schiff base
Palladium-catalysed Suzuki–Miyaura cross-coupling reactions of
aryl halides with boronic acids have become one of the most essen-
tial tools in modern chemistry for the construction of unsymmetric
biaryls that are used as building blocks in the pharmaceutical, agro-
[
13,14]
ligands in Suzuki–Miyaura reactions;
however, to the best
of our knowledge, this is the first example of a Pd complex con-
taining both bulky phosphine and bidentate Schiff base ligands
within the same coordination sphere. It is expected that the intro-
duction of two bulky ligands within the same coordination sphere
would increase the steric congestion around the Pd centre which
would facilitate both the oxidative addition and reductive elimina-
tion steps in the cross-coupling mechanism. It is worth mention-
[
1]
chemical and material industries. In recent years, significant
efforts have been made towards the synthesis of new and highly
efficient palladium catalysts that can promote these reactions in a
[2]
sustainable way. Like many other transition metal-catalysed
reactions, in the Suzuki–Miyaura reaction also the nature of ligands
coordinated to the palladium centre plays a significant role in the
overall performance of a catalyst.
[
13a,14c]
ing that, barring a few examples,
the majority of the
Bulky phosphines are known to be some of the most influential
ligands for this reaction. Among various bulky phosphines, biphe-
reported Pd–Schiff base catalysts were not effective in water as
a reaction medium. In fact, from environmental and economic
perspectives, water is the most desirable solvent for organic syn-
theses, and hence many research groups including those of
[3]
[4]
[5]
nyl phosphines, phosphapalladacycles, phosphabarrelenes,
[
6]
[7]
hemilabile phosphines,
indolyl phosphine,
ferrocenyl
[8]
[9]
[15a,b]
[15c]
[15d]
[15e]
[15f]
phosphines, resorcinarenyl phosphines, etc., have proven to
be of great value in activating difficult substrates including less
reactive aryl chlorides. For many years, the ligand bis
Leadbeater,
SanMartin and Domínguez,
Eppinger,
Zhang,
Luo,
Vanelle,
Plenio
Bora,
and Shao
[
15g,h]
[15i]
[15j]
[15k]
have developed some excellent catalytic systems for Suzuki reac-
tions in water. However, the majority of those protocols required
[2-(diphenylphosphino)phenyl]ether (DPEphos) has been used as
[
15a]
a sterically hindered wide-bite-angle diphosphine that has
attracted tremendous attention in catalysis because of its commer-
either severe reaction conditions
and/or addition of phase
[
15a,b,d,e]
[15f]
[15f]
transfer catalyst
/additive
or high catalyst loading,
[
10]
cial availability and interesting chemistry. In fact, this ligand has
been extensively studied for various transition metal-catalysed re-
actions including palladium-catalysed cross-coupling reactions like
which limits the applications of these catalytic systems on an in-
dustrial scale. Thus, developments of new catalytic systems that
can effectively perform the cross-coupling reactions in aqueous
media, especially under mild conditions without using any addi-
tive, are of great significance.
[
11]
Negishi, Kumada–Corriu, etc. Unfortunately, the potentiality of
this ligand in the Suzuki–Miyaura cross-coupling reaction is virtually
unexplored, although other related bulky diphosphines have
[
4c,9a,12]
shown promising cross-coupling activity.
[
13]
Herein, as a part of our continuing efforts to develop efficient
Pd catalysts for Suzuki–Miyaura reactions, we report the synthesis
of a new Pd complex containing a sterically hindered diphosphine
*
Correspondence to: Pankaj Das, Department of Chemistry, Dibrugarh University,
Dibrugarh 786004, Assam, India. E-mail: pankajd29@yahoo.com
(DPEphos) and a bidentate Schiff base ligand in the same coordina-
tion sphere. The efficacy of the complex as a catalyst has been
Department of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India
Appl. Organometal. Chem. 2015, 29, 829–833
Copyright © 2015 John Wiley & Sons, Ltd.

N. Shahnaz and P. Das
Experimental
Chemicals, instrumentation and analysis
PdCl and DPEphos were purchased from Acros Organics. Silver
2
perchlorate was purchased from Sigma-Aldrich. The substrates
and other chemicals used for the experiments were procured from
Sigma-Aldrich, Merck, Spectrochem and Rankem. The solvents
were purchased from various Indian firms and distilled prior to
use. Fourier transform infrared (FT-IR) spectra were recorded with
1
Scheme 1. Synthetic pathway for the preparation of complex 2.
a Shimadzu IR Prestige-21 FTIR spectrophotometer. The H NMR
3
1
and P NMR spectra of the complex were recorded with a Bruker
1
3
Avance II 400 and C NMR spectra with a JEOL ECS-400. The mass
spectrum of the complex was obtained with a Waters ZQ-4000
1
13
1
31
1
analyses, ESI-MS and FT-IR, H NMR, C{ H} NMR and P{ H} NMR
1
spectroscopies. In the FT-IR spectrum of complex 2, the C¼N
ESI/MS. The H NMR spectra of the products were recorded with
ꢀ
1
stretching band appears at 1589 cm which is significantly lower
an AV400 and a Bruker Avance III 500. Gas chromatographic studies
of the products were carried out using an Agilent 7820A GC system.
ꢀ
1
than the C¼N stretching band of complex 1 (1609 cm ). The imine
1
signal in the H NMR spectrum of complex 2 appears at 8.05 ppm,
which, compared to the parent complex, is an upfield shift of 0.78
Preparation of palladium catalyst
13
ppm. In the C NMR spectrum, a downfield shift of 5.04 ppm in
31
A solution of ligand DPEphos (0.175 g, 0.325 mmol) in 15 ml of di-
chloromethane was added dropwise to a solution of complex 1
the imine-carbon signal is observed. The P NMR spectrum of the
complex shows a singlet at 2.05 ppm.
(0.15 g, 0.325 mmol) in 15 ml of dichloromethane. To this solution,
AgClO (0.135 g, 0.65 mmol) was added in small portions and then
4
Suzuki–Miyaura cross-coupling reactions
stirred at room temperature for 4 h. A curdy white precipitate of
AgCl deposited at the bottom of the solution which was filtered
off to afford a yellow solution. After evaporation of the solvent un-
der reduced pressure and washing with hexane, complex 2 was
isolated as a bright yellow solid which was recrystallized from
dichloromethane.
In order to investigate the efficacy of our complex as a catalyst, we
performed the cross-coupling reaction using 4-bromoanisole and
phenylboronic acid as model substrates. The reaction was per-
2 3
formed in water at room temperature using K CO as base with
0.2 mol% of complex 2. We were delighted to find that the reaction
proceeds smoothly in water and almost quantitative formation of
cross-coupling product is achieved after 8 h of reaction time (Fig. 1).
In order to investigate the efficacy of our catalytic system, we per-
formed the model reaction using complex 2, parent Schiff base
Yield 90%. Anal. Calcd for C56
.93; N, 2.48. Found (%): C, 59.76; H, 3.94; N, 2.49. ESI-MS (CH
H
44
N
2
P
2
Cl
2
O
9
Pd (%): C, 59.62; H,
Cl ):
3
2
2
ꢀ1
2
+
2+
m/z 1164 [M + 2NH
4
] , 1185 [M + NH
4
+ K] . FT-IR (KBr, cm ):
, δ, ppm): 8.05 (2 H, s,
CH¼N), 6.59–7.74 (42 H, br m, Ph). C{ H} NMR (400 MHz, CDCl
δ, ppm): 159.17 (CH¼N), 134.27, 134.10, 132.33, 131.56, 130.88,
1
1
589 (ν
C
¼
N 3
: imine). H NMR (400 MHz, CDCl
13
1
2
complex 1 and the phosphine complex Pd(DPEphos)Cl (3) as cata-
3
,
lysts using various inorganic bases (Fig. 1). Interestingly, under the
same set of experimental conditions, complex 2 shows higher per-
formance than complexes 1 and 3 which may be attributed to the
presence of both Schiff base and DPEphos ligands in the same
31
1
1
30.23, 129.66, 129.28, 129.11, 128.94, 126.37, 126.03 (Ph). P{ H}
NMR (161.97 MHz, CDCl , δ, ppm): 2.08 (2P, s).
3
complex. With complex 2 as catalyst, K
efficient base (Fig. 1). Good yields are also obtained with NaHCO3
and Na CO . On optimizing the catalyst quantity we find that 0.1
2 3
CO is found to be the most
General procedure for Suzuki–Miyaura reaction
A 50 ml round-bottomed flask was charged with a mixture of aryl
2
3
halide (0.5 mmol), arylboronic acid (0.75 mmol), K
CO
2 3
(1.5 mmol)
mol% of complex 2 is sufficient to achieve reaction completion
and Pd catalyst (0.1 mol% for aryl bromide or 0.5 mol% for aryl chlo-
ride). The mixture was stirred for required times at room tempera-
ture in water (4 ml) for aryl bromides or at 80°C in DMF (4 ml) for
aryl chlorides. The progress of the reaction was monitored using
TLC with aluminium-coated TLC plates (Merck) under UV light or
using GC. After completion, the reaction mixture was diluted with
water (20 ml) and extracted with ether (3 × 20 ml). The combined
extract was washed with brine (3 × 20 ml) and dried over Na SO .
(Table 1, entry 2).
2
4
After evaporation of the solvent under reduced pressure, the resi-
due was chromatographed (silica gel, ethyl acetate–hexane 1:9) to
obtain the desired product. Finally the separated products were
1
characterized using H NMR and mass spectral analyses.
Results and discussion
Synthesis and characterization of catalyst
Figure 1. Effect of base on the catalytic activities of complexes 1, 2 and 3
The new Pd complex 2 was prepared via halide abstraction reaction
(
reaction conditions: 4-bromoanisole (0.5 mmol), phenylboronic acid (0.75
[
13b]
with a previously reported complex 1
according to Scheme 1.
mmol), base (1.5 mmol), catalyst (0.2 mol%),
temperature, 8 h).
2
H O (4 ml), room
The identity of the complex was confirmed using elemental
wileyonlinelibrary.com/journal/aoc
Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2015, 29, 829–833

New and efficient palladium catalyst for Suzuki–Miyaura reaction
Table 1. Effects of catalyst quantity in Suzuki–Miyaura reactions of 4-
Table 2. Suzuki–Miyaura reactions of various aryl and heteroaryl bro-
mides with arylboronic acids using complex 2 as catalyst
a
a
bromoanisole with phenylboronic acid using complex 2 as catalyst
b
Entry
Catalyst (mol%)
Time (h)
Yield (%)
Entry
R–Br
R′
Time Yield
b
(
h)
(%)
1
2
3
4
5
0.2
8
11
15
24
48
100
100
89
0.1
1
2
3
4
5
6
7
8
9
4-CHOC
4-COCH
4-OCH
4-NO
3-OCH
2-CHOC
4-CH
Br
4-COPhC
2,5-OCH
6
H
4
Br
C
C
C
C
C
C
C
C
C
C
C
C
6
6
6
6
6
6
6
6
6
6
6
6
H
H
H
H
H
H
H
H
H
H
H
H
5
5
5
5
5
5
5
5
5
5
5
5
11
8
95
94
0.075
0.05
0.025
3 6
C
H
4
Br
Br
Br
Br
Br
Br
74
3
6
C H
4
8
97
c
61
2
C
6
H
4
8
68/96
87
3
6
C H
4
14
19
12
12
20
20
a
Reaction conditions: 4-bromoanisole (0.5 mmol), phenylboronic acid
CO (1.5 mmol), H O (4 ml).
H
6 4
86
(
0.75 mmol), K
2
3
2
b
3
C
6
H
4
94
GC yields.
C H
6 5
91
6
H
4
Br
Br
93
10
11
12
13
14
15
3
C
6
H
3
81
c
c
5-Bromopyrimidine
20 72/96
20 74/92
In order to widen the scope of our catalyst, various aryl bromides
including heteroaryl bromides were coupled with arylboronic acids,
using 0.1 mol% of complex 2 in aqueous media. The results are
summarized in Table 2. Usually, para-substituted aryl bromides with
electron-withdrawing groups (–CHO, –COCH , –COPh) as well as
electron-donating groups (–CH , –OCH ) give excellent yields
2-Bromo-3-methylthiophene
4-NO
4-NO
2
C
C
6
H
H
4
Br
Br
4-ClC
6
H
4
20
20
20
91
98
90
58
2
6
4
4-CH
3 6 4
C H
6 5
C H Br
3-NO
2 6 4
C H
3
16
C H Br
2-Thenylboronic 20
acid
6
5
3
3
(
Table 2, entries 1–3, 7, 9). Surprisingly, the coupling reaction
a
Reaction conditions: aryl bromide (0.5 mmol), arylboronic acid (0.75 mmol),
CO (1.5 mmol), H O (4 ml).
Isolated yields (average of two runs).
between para-nitrobromobenzene and phenylboronic acid gives
a relatively poor yield in water. However, the yield can be substan-
tially increased by using PrOH as co-solvent with water (entry 4).
K
b
2
3
2
i
c
2
iPrOH–H O (1:1) used as solvent.
Interestingly, our catalyst can also tolerate sterically demanding
substrates like 3-bromoanisole, 2-bromobenzaldehyde and 1-
bromo-2,5-dimethoxybenzene and gives the desired products in
good yields (entries 5, 6 and 10, respectively). It may be noted that
heteroaryl halides are usually difficult to activate in Suzuki–Miyaura
reactions and often produce poor yields. However, in our case,
the heteroaryl bromides 5-bromopyrimidine and 2-bromo-3-
methylthiophene give reasonably good yields in neat water which
[
16]
i
can be further enhanced by using PrOH as co-solvent with water
(
entries 11 and 12). Also, in addition to phenylboronic acid, our
catalyst can also tolerate other representative boronic acids like
-chlorophenylboronic acid (entry 13) and 4-tolylboronic acid
entry 14). The coupling reaction with heteroarylboronic acid, how-
4
(
ever, results in moderate yield (entry 16).
Encouraged by the results of coupling reactions of aryl bromides,
we extended the scope of our catalyst for the cross-coupling reac-
tions of aryl chlorides. To evaluate the efficiency of our complex
as a catalyst for the Suzuki–Miyaura reaction of aryl chlorides, we
chose 4-chloronitrobenzene and phenylboronic acid as model sub-
strates. Studies of the effect of various inorganic bases and solvents
Figure 2. Effect of base and solvent on the conversion of 4-
chloronitrobenzene (reaction conditions: 4-chloronitrobenzene (0.5 mmol),
phenylboronic acid (0.75 mmol), base (1.5 mmol), complex 2 (1 mol%),
solvent (4 ml), 80°C, 5 h).
2 3
on the reaction reveal that K CO is the most effective base and
DMF is the most effective solvent. Using this combination, quantita-
tive biaryl formation is achieved with 4-chloronitrobenzene (Fig. 2)
with 1 mol% catalyst loading at 80°C. Unfortunately, unlike aryl bro-
mides, in this case the coupling reaction does not seem to proceed
very well in neat water and poor yields are observed. Optimization
of temperature and catalyst quantity was also done and the results
are summarized in Table 3. Our results indicate that 0.5 mol% cata-
lyst loading is enough to achieve quantitative biaryl formation at
To study the scope and limitations of our present catalytic sys-
tem, a series of Suzuki coupling reactions of a wide range of
electronically and sterically diverse aryl chlorides with arylboronic
acids were performed under the optimized conditions (DMF,
K CO , 80°C, 0.5 mol% complex 2). As evident from Table 4, both
2
3
8
0°C (entry 4). Further reduction of catalyst quantity and tempera-
electron-rich and electron-poor aryl chlorides are successfully
coupled with boronic acids resulting in the desirable products
in good to excellent yields. Usually, aryl chlorides bearing
electron-withdrawing para substituent groups like –CHO and
–NO2 give excellent yields (94 and 98%, respectively) in 6 h
ture causes the product yields to decrease considerably. It is worth
mentioning that activation of aryl chloride as substrate is quite
challenging due to the higher Ar–Cl bond strength and generally
harsh reaction conditions are required.
Appl. Organometal. Chem. 2015, 29, 829–833
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

N. Shahnaz and P. Das
Table 3. Optimization of temperature and catalyst quantity for Suzuki–
Conclusions
Miyaura cross-coupling reaction of 4-chloronitrobenzene with
a
phenylboronic acid with complex 2 as catalyst
In summary, we have developed a highly stable Pd complex
composed of phosphine and Schiff base ligands, and successfully
performed the cross-coupling reactions of various aryl bromides
in water and aryl chlorides in DMF with a low catalyst loading
b
Entry
Catalyst (mol%)
Temp. (°C)
Time (h)
Yield (%)
(<0.5 mol %). The catalytic system could also effectively activate
sterically demanding aryl halides.
1
2
3
4
5
6
1
26
60
80
80
80
80
24
12
6
34
76
1
1
100
100
57
Acknowledgments
0.5
0.25
0.1
6
12
12
The Council of Scientific and Industrial Research (CSIR), New Delhi,
is gratefully acknowledged for financial support (grant no.
02/0081/12). N.S. thanks the University Grants Commission (UGC),
New Delhi, for a BSR-RFSMS fellowship.
32
a
Reaction conditions: 4-chloronitrobenzene (0.5 mmol), phenylboronic
acid (0.75 mmol), K
2
CO
3
(1.5 mmol), DMF (4 ml).
b
GC yields.
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3
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Appl. Organometal. Chem. 2015, 29, 829–833

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