Pd(l -proline)2 complex: An efficient catalyst for Suzuki-Miyaura coupling reaction in neat water

Article abstract of DOI:10.1002/aoc.3129

An efficient catalytic system for Suzuki-Miyaura coupling reactions in neat water has been developed by using a water-soluble Pd(l-proline)₂ catalyst. Under the optimized conditions, various biaryl compounds were obtained in good to excellent yields and a wide range of functional groups on the tested substrates were well tolerated. The catalytic system could be reused at least six times with no significant loss in its activity.

Full text of DOI:10.1002/aoc.3129

Full Paper
Received: 27 November 2013
Revised: 18 January 2014
Accepted: 29 January 2014
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3129
Pd(L-proline)2 complex: an efficient catalyst
for Suzuki–Miyaura coupling reaction in
neat water
Guofu Zhang, Yuxin Luan, Xingwang Han, Yong Wang, Xin Wen
and Chengrong Ding*
An efficient catalytic system for Suzuki–Miyaura coupling reactions in neat water has been developed by using a water-soluble
Pd(L-proline)2 catalyst. Under the optimized conditions, various biaryl compounds were obtained in good to excellent yields
and a wide range of functional groups on the tested substrates were well tolerated. The catalytic system could be reused at
least six times with no significant loss in its activity. Copyright © 2014 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; N,O-ligand; Suzuki–Miyaura reaction; biaryls; water
hydrophilicity, we were curious as to whether L-proline could be
Introduction
used as an effective N,O-ligand for palladium-catalyzed Suzuki–
Miyaura coupling reactions in water.[12] In this context, we wish
During the past decades, there has been an increasing interest in
the development of green and efficient reaction procedures in
both academic and industrial research.[1] Among various systems,
those organic transformations that have been conducted in
water have attracted much attention,[2] because water is nontoxic,
nonflammable and readily available. Meanwhile, the utilization of
water as a green solvent offers such benefits as simplification of
workup procedures and acceleration of reaction rates.[3]
to develop a novel aqueous Suzuki–Miyaura coupling system
catalyzed by an L-proline-coordinated palladium complex.
Results and Discussion
The palladium complex[13] was simply prepared as shown in
Scheme 1. Et3N was added to a methanol solution of L-proline
and mixed for 1 h. Palladium acetate, which had been dissolved
in methanol, was then introduced. The mixture was stirred
vigorously at room temperature under N2 until the coordinated
complex precipitated, after which the desired complex was
obtained by filtration. In addition, the complex showed good
water solubility, as expected.
The Suzuki–Miyaura coupling reaction[4] has been widely
applied in the construction of biaryl compounds, which are ubiq-
uitous structural motifs presented in numerous biologically active
molecules, functional materials and natural products.[5] However,
most of the reaction media were traditionally confined to organic
solvents such as toluene and DMF, which may cause environ-
mental problems.[6] With respect to these issues, continuous
efforts have recently been made to achieve aqueous Suzuki–
Miyaura reactions and various protocols were developed.[7] Among
them, the application of water-soluble ligand/palladium complexes
as catalysts turned out to be one of the most effective methods.[8]
Generally, hydrophilic ligands were nitrogen/phosphine-containing
molecules which were coupled to polyethylene glycol, sugar,
quaternary ammonium salts or sulfonates.[9] However, multistep
synthetic procedures were always needed for the preparation of
the above catalysts, which hampered their extensive application
to some degree. Hence, from an environmental and economic
perspective, the exploration of effective hydrophilic palladium
catalysts employing commercially available hydrophilic molecules
would be an ideal choice to make a step forward in the field of
aqueous Suzuki–Miyaura coupling systems.
To determine the coordination state of the palladium complex,
1H NMR analysis was first adopted. Gratifyingly, all the proton
signals shifted in a certain scale compared with pure L-proline,
indicating that L-proline was coordinated with palladium ion.
Additionally, the spectrum showed no proton signals of acetate,
suggesting that both acetates of Pd(OAc)2 were substituted.
Moreover, the FT-IR spectrum of Pd(L-proline)2 exhibited two
strong peaks at 1622 and 1352 cmÀ1, which may arise from νas
(COO) and νs(COO) of the carboxyl group, respectively. The
separation between the two peaks (Δ = 1622 À 1352 = 270 cmÀ1
)
was larger than 200 cmÀ1, demonstrating the monodentate
coordination of the carboxyl group to the palladium ion.[14]
A
*
Correspondence to: Chengrong Ding, College of Chemical Engineering and
Materials Science, Zhejiang University of Technology, Hangzhou 310014,
China. E-mail: dingcr@zjut.edu.cn
L-Proline, a common α-amino acid, has been used as an
effective organocatalyst for organic transformations such as
Aldol, Mannich and Michael addition reactions.[10] Notably, it
has also been regarded as an excellent N,O-ligand for copper-
catalyzed carbon–nitrogen bond formation.[11] Considering its
College of Chemical Engineering and Materials Science, Zhejiang University of
Technology, Hangzhou, 310014, China
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.

G. Zhang et al.
coupling performance decreased obviously when the reactions
were performed with 0.01 mol% and 0.001 mol% palladium
catalysts (entries 6 and 7). In view of these results, 0.02 mol%
catalyst was used for the further optimization of other parame-
ters. Considering the pivotal role of base in promoting transme-
tallation and/or formation of active palladium intermediates
during the catalytic cycle,[15] different bases including sodium/
potassium inorganic salts and organic base were then screened.
It was found that base had a significant influence on the coupling
efficiency (entries 8–15). When the reactions were carried out in
the presence of Na2HPO4 or K2CO3, quantitative conversions of
starting materials were observed (entries 8 and 11), while the
use of NaOAc and K3PO4 resulted in lower yields of 85% and
80%, respectively (entries 9 and 13). Strong bases like NaOH
and KOH significantly affected the coupling reactions (entries
10 and 12). Notably, NEt3 was also a competent organic base,
facilitating the formation of the desired product with a high yield
(96%, entry 14). A control experiment in the absence of base
provided trace products, which confirmed the essentiality of base
for the smooth coupling reaction in this system (entry 15).
Further investigations indicated that K2CO3 was superior to
Na2CO3 and Na2HPO4, giving an excellent yield after only 3 h
(entries 5, 8, 11). Finally, 0.02 mol% Pd(OAc)2 was also tested as
the catalyst for comparison, and a lower conversion of 65% was
obtained with palladium black (entry 16). Moreover, after the
product was extracted from the system, the aqueous phase could
not be reused, providing a significantly decreased yield of 27% of
the desired product (entry 17). Thus we gained the optimal condi-
tions: aryl halide (1.0 mmol), arylboronic acid (1.5 mmol), catalyst
(0.02 mol%), K2CO3 (1.0 mmol) and water (3.0ml) under reflux.
Having obtained the optimized conditions (Table 1, entry 11),
we next surveyed the substrate scope for aryl halides of the
catalytic system. Representative results are shown in Table 2.
Generally, a wide range of coupling products were obtained with
excellent isolated yields (90–98%, entries 1–18). The reactions of
aryl halides with substituents such as formyl, acetyl or carboxyl
Scheme 1. Preparation of the Pd(L-proline)2 complex.
high-resolution mass spectrum was also obtained to confirm the
structure and accurate molecular mass of the desired complex,
with 335.0223 assigned to [M + H]+. The characterization date
can be found in supporting information.
With the palladium complex in hand, we embarked on
evaluating its catalytic activity in aqueous Suzuki–Miyaura
reaction and the results are summarized in Table 1. Initially,
3-nitrobromobenzene and phenylboronic acid were selected
as the model substrates to optimize the reaction conditions.
We were pleased to obtain a 98% yield of 3-nitrobiphenyl when
the reaction was carried out in refluxed water with 0.5 mol%
catalyst and 1.0 equivalent base for 5 h (entry 1). Interestingly,
when the amount of the catalyst was reduced, high yields of
3-nitrobiphenyl were still observed (entries 2–5). However, the
Table 1. Optimization of reaction conditionsa
Entry
n
Base
Yieldb (%)
proceeded smoothly, even with
a lower catalyst loading
1
0.5
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2HPO4
NaOAc
NaOH
K2CO3
KOH
98
98
(0.01 mol%). Interestingly, electron-rich substitutes including
hydroxyl and hydroxymethyl proceeded readily with phenyl-
boronic acid as well, delivering the corresponding products
4-phenylphenol and 4-phenylbenzyl alcohol with high yields
(entries 2 and 9). The coupling reactions were also successful with
aryl halides bearing a hydrophobic group such as 4-iodoanisole
or 4-bromotoluene, though with a somewhat longer reaction
time and/or higher catalyst loading (entries 3 and 10). It is worth
mentioning that 4-bromoaniline, which could lead to catalyst
inactivation in some cases,[16] afforded a good yield of the
desired product (entry 16). As demonstrated by entries 17 and 18,
this catalytic method was also suitable for sterically hindered
materials such as 2-bromobenzaldehyde and 2-iodoanisole, which
indicated the wide substrate compatibility of this system. Addition-
ally, in order to evaluate the feasibility of this protocol for less active
substrates, we further investigated the coupling between aryl
chlorides and phenylboronic acid. However, a low 12% yield of
4-acetylbiphenyl was obtained when 4-chloroacetophenone was
subjected to the system, and prolonged reaction time and/or
increased catalyst loading led to no obvious improvement (entry 19).
With the aforementioned promising results, we were encour-
aged to evaluate the extension of arylboronic acids[17] as sub-
strates in this system. To our delight, all substituted arylboronic
acids tested underwent the coupling reactions efficiently with
aryl iodines and aryl bromines in neat water, providing the
2
0.2
3
0.1
98
4
0.05
0.02
0.01
0.001
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
-
98
5
98 (89)
80
6
7
20
8
98 (94)
85
9
10
11
12
13
14
15
16d
17e
26
98 (97)
18
K3PO4
NEt3
80
96
-
25
K2CO3
K2CO3
65
-
27
aReaction
conditions:
3-nitrobromobenzene
(1.0mmol),
phenylboronic acid (1.5mmol), Pd(L-proline)2, base (1.0mmol),
H2O (3.0ml), refluxed for 5 h, unless otherwise stated.
bIsolated yield based on 3-nitrobromobenzene used.
cValues in parentheses are the isolated yields when the reactions
were carried out for 3 h.
dThe reaction was catalyzed by 0.02 mol% Pd(OAc)2 for 3 h.
eReuse of the aqueous phase of entry 16 for 3 h.
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Appl. Organometal. Chem. (2014)

Pd(L-proline)2-catalyzed Suzuki–Miyaura reaction
Table 2. Suzuki–Miyaura coupling reaction between different aryl
Table 3. Suzuki–Miyaura coupling reaction between different aryl
halides and phenylboronic acida
halides and substituted arylboronic acida
Entry
X
R
n
Time (h) Yieldb (%)
1
I
H
0.01
0.01
0.02
0.01
0.02
0.02
0.01
0.01
0.01
0.02
0.02
0.02
0.01
0.01
0.01
0.1
2
3
97
93
96
96
95
96
96
98
94
92
91
98
96
96
90
90
92
95
12
Entry
X
R1
R2
n
Time (h) Yieldb (%)
2
I
4-OH
1
I
H
F
OMe
OMe
OMe
OMe
COMe
COMe
COMe
COMe
COMe
COMe
OH
0.02
0.04
0.04
0.04
0.04
0.2
4
6
94
94
94
92
90
83
84
89
96
91
87
35
90
3
I
4-OMe
4-F
12
5
2
I
4
I
3
I
COMe
COMe
H
8
5
I
4-NO2
3-NO2
4-COMe
H
6
4
Br
I
8
6
I
6
5
8
7
I
3
6
I
OMe
COMe
COMe
CH2OH
CHO
H
20
20
20
20
20
12
15
4
8
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
I
2
7
I
0.2
9
4-CH2OH
4-Me
4
8
Br
Br
Br
Br
Br
0.2
10
11c
12
13
14
15
16
17
18
19
15
8
9
0.2
4-Cl
10
11
12
13
0.2
3-NO2
4-CHO
4-COMe
4-COOH
4-NH2
2-CHO
2-OMe
4-COMe
5
0.02
0.04
0.2
4
CHO
OH
4
5
12
12
12
12
aReaction conditions: aryl halide (1.0 mmol), arylboronic acid
(1.5 mmol), cat. (0.02–0.2 mol%), K2CO3 (1.0 mmol), H2O (3.0 ml),
refluxed.
bIsolated yield based on aryl halide used.
0.2
0.2
Cl
0.2
aReaction conditions: aryl halide (1.0 mmol), phenylboronic acid
(1.5 mmol), cat. (0.01–0.2 mol%), K2CO3 (1.0 mmol), H2O (3.0 ml),
refluxed.
bIsolated yield based on aryl halide used.
cOnly 4-chlorobiphenyl was obtained.
Table 4. Reusability of the catalyst Pd(L-proline)2 with 3-
nitrobromobenzene and phenylboronic acid as model substratesa
Cycle
1
2
3
4
5
6
Time (h)
3
3
3
4
5
5
Yieldb (%)
98
97
97
95
93
94
desired products in good to excellent yields (Table 3). Functional
groups on arylboronic acid such as methoxy, acetyl and hydroxyl
were well tolerated (entries 1–13). Notably, the coupling
reactions between aryl halides and 4-methoxyphenylboronic acid
took place smoothly in the presence of 0.02–0.04 mol%
palladium catalysts (entries 1–4), whereas, in the cases of
electron-deficient 4-acetylphenylboronic acid, lower reactivity
was observed. With treatment using higher catalyst loading or
longer reaction time, satisfactory yields could be achieved
(entries 5–10). Furthermore, arylboronic acid bearing active
substitutes such as 4-hydroxylphenylboronic acid could also be
used in this coupling system in neat water (entries 11–13).
To demonstrate the additional advantage of the aqueous
Suzuki-Miyaura reaction system, we turned our attention to the
reuse of the catalytic system, and 3-nitrobromobenzene and
phenylboronic acid were selected as model substrates. The
results of the recycling experiment are displayed in Table 4. After
the reaction, the product was simply extracted from the aqueous
phase with CH2Cl2 four times. After the extraction, the aqueous
phase was charged with fresh substrates and base, and stirred
under standard conditions. To our delight, the catalyst could be
reused at least six times with no significant decrease in its cata-
lytic activity, demonstrating the potential practical application
of this system.
aReaction
conditions:
3-nitrobromobenzene
(1.0 mmol),
phenylboronic acid (1.5 mmol), cat. (0.02 mol%), K2CO3
(1.0 mmol), H2O (3.0 mL), refluxed.
bIsolated yield based on 3-nitrobromobenzene used (average of
two runs).
Conclusions
We have developed an efficient, aqueous Suzuki–Miyaura
coupling reaction system using a water-soluble palladium cata-
lyst with L-proline as an efficient N,O-ligand. Under the optimized
conditions, a wide range of coupling reactions between aryl
halides and arylboronic acids proceeded smoothly in neat water,
providing the desired biaryl products with good to excellent
yields. Furthermore, the catalytic system could be recycled at
least six times without significant loss in activity. The use of green
reagents, such as L-proline as the ligand and water as the solvent
made the new system attractive for environmentally sustainable
processes. Meanwhile, this catalytic system provides a potential
new avenue for the development of water-soluble and readily avail-
able palladium catalysts for Suzuki–Miyaura coupling reactions.
Appl. Organometal. Chem. (2014)
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

G. Zhang et al.
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Experimental
Preparation of the Pd(L-Proline)2 Complex
Prior to the synthesis experiment, all reagents and solvents
were dehydrated so that free from moisture. Then, L-proline
(0.460 g, 4.0 mmol) was dissolved in 5.0 ml CH3OH in a 50 ml
round-bottom flask equipped with a magnetic stirrer. 0.5 ml
Et3N was added and the mixture stirred for 1 h. Subsequently,
Pd(OAc)2 (0.449 g, 2.0 mmol), which had been dissolved in a
further 5.0 ml CH3OH, was introduced. The resulting mixture
was stirred at room temperature under N2 for 12 h, during which
period the coordinated complex precipitated. After that, the solid
was filtered and washed with CH3OH (2 × 2 ml). Pd(L-proline)2
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was obtained as
a light-yellow powder (0.566 g, 62.3%).
1H NMR (500 MHz, D2O): δ (ppm) 1.59–1.69 (m, 2H, C(4)―H and
C(4′)―H), 1.84–1.92(m, 2H, C(4)―H and C(4′)―H), 1.94–2.01
(m, 2H, C(3)―H & C(3′)―H), 2.12–2.19 (m, 2H, C(3)―H & C(3′)
―H), 3.03–3.13 (m, 4H, C(5)―H & C(5′)―H), 3.85(t, J = 8.2 Hz, 2H,
C(2)―H and C(2′)―H); 13C NMR (125 MHz, D2O): δ (ppm) 24.6
(C4), 24.8(C4′), 29.4 (C3), 30.1(d, J = 46.7 Hz, C3′), 51.1 (C5), 52.7
(C5′), 63.6 (C2), 65.0 (C2′), 186.5 (COO), 188.1(COO′). FT-IR (KBr,
cmÀ1): 3434 (m, ν(N―H) ); 1622 (s, νas(COO) ), 1352(s, νs(COO) ),
1315 (m, ν(CO) ), 542(w, ν(Pd―O) ), 466 (w, ν(Pd―N)). HRMS-
ESI: calcd for C10H16N2O4Pd · H+ 335.0223; found 335.0223.
General Procedure for the Suzuki–Miyaura Coupling Reaction
Between 3-Nitrobromobenzene and Phenylboronic acid
3-Nitrobromobenzene (1.0 mmol, 0.2020 g), phenylboronic acid
(1.5 mmol, 0.1829 g) and K2CO3 (1.0 mmol, 0.1382 g) were placed
in a sealed tube, and 3.0 ml water containing an appropriate
amount of catalyst was introduced. The reaction was stirred un-
der reflux. After the reaction, the aqueous phase was extracted
with CH2Cl2 four times (4 × 1.5 ml). The combined organic layers
were dried over anhydrous Na2SO4, concentrated under vacuum
and purified by column chromatography (n-hexane/ethyl acetate,
20:1) to afford the desired product: 3-nitrobiphenyl (0.1952 g,
98%). 1H NMR (500 MHz, CDCl3): δ (ppm) 7.45(tt, J = 6.5, 1.5 Hz,
1H, C(4′)―H), 7.48–7.53 (m, 2H, C(3′)-H and C(5′)―H), 7.58–7.64
(m, 3H, C(2′)―H & C(6′)―H and C(5)―H), 7.91(dq, J = 7.5, 1.0 Hz,
1H, C(6)―H), 8.19(dq, J = 8.0, 1.0 Hz, 1H, C(4)―H), 8.44
(t, J = 2.0 Hz, 1H, C(2)―H); 13C NMR (125 MHz, CDCl3): δ (ppm)
121.8(C4), 122.0 (C2), 127.1 (C4′), 128.5 (C2′ and C6′), 129.2 (C3′
and C5′), 129.7 (C5), 133.0 (C6), 138.6 (C1), 142.8 (C1′), 148.7 (C3).
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Acknowledgments
We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (No. 20702051), the
Natural Science Foundation of Zhejiang Province (LY13B020017)
and the Key Innovation Team of Science and Technology in
Zhejiang Province (No. 2010R50018).
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