Room temperature suzuki reactions in aqueous media under air by palladium(II) complexes with pyrazole derived ligands

Article abstract of DOI:10.1071/CH09383

Two new palladium complexes with pyrazole derived ligands 2a-2b have been easily prepared and well characterized by elemental analysis, ¹H NMR, ¹³C NMR, and IR spectra. Their detailed structures are determined by a single-crystal X-r

Full text of DOI:10.1071/CH09383

Full Paper
CSIRO PUBLISHING
www.publish.csiro.au/journals/ajc
Aust. J. Chem. 2010, 63, 315–320
Room Temperature Suzuki Reactions in Aqueous Media
under Air by Palladium(II) Complexes with Pyrazole
Derived Ligands
Kai Xu,A Xin-Qi Hao,A Jun-Fang Gong,A,B Mao-Ping Song,A,B
and Yang-Jie WuA
ADepartment of Chemistry, Henan Key Laboratory of Chemical Biology and
Organic Chemistry, Zhengzhou University, Zhengzhou 450052, China.
BCorresponding authors. Email: gongjf@zzu.edu.cn; mpsong9350@zzu.edu.cn
Two new palladium complexes with pyrazole derived ligands 2a–2b have been easily prepared and well characterized
by elemental analysis, 1H NMR, 13C NMR, and IR spectra. Their detailed structures are determined by a single-crystal
X-ray analysis of 2a. The two compounds were successfully applied to the Suzuki coupling reactions of aryl bromides
with phenylboronic acid, in aqueous solution at room temperature under air, giving the desired coupled products in good
to excellent yields with catalyst loadings as low as 0.01–0.05 mol-%.
Manuscript received: 14 July 2009.
Manuscript accepted: 5 October 2009.
Introduction
RO
CH3
N
The palladium-catalyzed Suzuki reaction of aryl halides with
boronic acids is one of the most widely studied transition metal
catalystreactions for theformationof C–Cbonds.[1–5] Aplethora
of palladium catalysts, usually simple palladium salts or palla-
dium complexes with appropriate ligands have been successfully
used in this C–C coupling reaction under various reaction condi-
tions.Among them, phosphine ligand-based palladium catalysts;
especially those with bulky, electron-rich phosphine ligands;
haveprovedtobehighlyefficientfortheactivationofnotoriously
unreactive aryl chlorides.[6–9] However, the high price usually
associated with phosphines along with their negative environ-
mental effect and air-sensitivity, have encouraged researchers to
explore alternatives for phosphines. In this regard, Pd(ii) com-
plexes with nitrogen-containing ligands became attractive due to
their low cost and toxicity, easy accessibility, and insensitivity to
oxygen under aerobic conditions. Some recent examples of such
complexes are monodentate N-coordinated palladium-amine
complexes[10] and palladium-benzimidazole complexes[11,12] or
bidentate N-coordinated palladium complexes.[13,14] Particu-
larly, the benzimidazolium-pyrazole-palladium complexes are
found to be active catalysts for the Suzuki reaction of unacti-
vated aryl bromides with arylboronic acids at room temperature
under aerobic conditions with turnover frequencies (TOF) reach-
H3C
Cl
Pd
Cl
RO
Li2PdCl4
N
N
N
CH
3 CH3OH, rt, 5 h
N
N
CH3
OR
H3C
CH3
1
2
R ꢀ H (2a); yield: 90.4%
R ꢀ Ac (2b); yield: 92.5%
R ꢀ H (1a); R ꢀ Ac (1b)
Scheme 1.
quickly with a p-hydroxyacetophenone-oxime derived pallada-
cycle as a catalyst, than with acetophenone-oxime derivative
(5 h versus 4 days to afford similar turn over numbers), although
the two palladacycles promoted the reactions at comparable
speed in refluxing water.[21] Complex 2b with AcO-group in
the benzene ring was prepared for comparison with 2a.
Results and Discussion
Synthesis and Characterization of Complexes 2a–2b
The pyrazolyl-containing m-phenol derivative 1a was prepared
from commercially available 3-hydroxybenzaldehyde in three
steps by reduction of the aldehyde, bromination, and nucleo-
philic substitution with 3,5-dimethyl pyrazole. Acetylation of
hydroxy group in ligand 1a by acetyl chloride easily afforded
ligand 1b. Then 1a or 1b was reacted with Li2PdCl4 in MeOH
at room temperature for 5 h (Scheme 1). The resulting yellow
precipitate was collected and washed with CH2Cl2 to provide
the corresponding Pd(ii) complexes 2a or 2b in excellent yields.
The two new palladium complexes are stable towards moisture
and air. They were each characterized by elemental analysis,
1H NMR, 13C NMR, and IR spectra. The molecular structure
ing as high as 60000 h .
−1 [14] During our continuous research on
Suzuki reactions,[15–20] we have been interested in the develop-
ment of highly stable and also active palladium catalysts that
can be used in aqueous solution at room temperature under air.
For this purpose, we synthesized two new palladium complexes
withpyrazolederivedligands2a–2b(Scheme1)andinvestigated
their catalytic activity in room temperature Suzuki reactions.The
introduction of a hydroxy group in the benzene ring in complex
2a was enlightened by the literature results that room temper-
ature Suzuki couplings in MeOH/H2O proceeded much more
© CSIRO 2010
10.1071/CH09383
0004-9425/10/020315

316
K. Xu et al.
C(9)
C(10)
effects and OH· · · Cl hydrogen bonds, the crystal structure of
2a is extended into a 2D architecture (Fig. 2).
C(12)
C(11)
C(8)
Suzuki Coupling Reactions
N(2)
Many N-based compounds have been reported to be efficient
ligands for the Suzuki reactions, in most cases the reactions
were carried out in organic solvent at elevated temperature
(60–110◦C) and/or under an inert atmosphere.[12,17,24–26] Room
N(1)
C(7)
CI(1A)
C(1)
C(2)
Pd(1)
N(1A)
C(6)
C(5)
O(1)
C(3)
temperature Suzuki reactions under aerobic conditions could be
CI(1)
[10]
accomplished by using a simple amine/Pd(OAc)2
or glyoxal
bis(N-methyl-N-phenylhydrazone)/Pd(OAc)2 system[13] with a
Pd loading as high as 2 mol-%. The catalyst loadings could
be lowered to 0.01–0.5 mol-% with benzimidazolium-pyrazole-
palladium(ii) complexes[14] or N-phenylurea-palladium(ii)
complexes[27] as catalysts with the coupling of aryl bromides
and boronic acids at room temperature in alcohol-water media
under aerobic conditions.Very recently, phosphine free diamino-
diol based palladium catalysts were also demonstrated to be
effective in room temperature Suzuki reactions in MeOH
under aerobic conditions with typical catalyst loadings ranging
0.5–1 mol-%.[28] Based on the literature results and our own
experience with Suzuki reactions,[15–20] we tested the catalytic
activity of complexes 2a and 2b in the Suzuki reactions by
performing the reactions at room temperature in EtOH-water
media, under air in the presence of n-Bu4NBr (TBAB), with
K2CO3 as a base. It was believed that TBAB would act as a
phase-transfer catalyst under aqueous conditions and could also
stabilize catalytically active palladium nanoparticles, thereby
avoiding aggregation.[29] The results are shown inTable 1.At the
beginning 0.5 mol-% of complex 2a was used as the catalyst. It
was found that a variety of electronically and structurally diverse
arylbromidescoupledefficientlywithphenylboronicacidgiving
the corresponding biaryl products in excellent yields after 12 h.
These aryl bromides included electron-neutral para- and ortho-
bromotoluene, electron-rich para-, and ortho-bromoanisole as
well as electron-deficient para- and meta-bromonitrobenzene
(Table 1, entries 1, 5, 9, 17, 21, 24). Surprisingly, the electron-
deficient ortho-bromonitrobenzene only gave a 35% yield under
the same reaction conditions (entry 25). When a heteroaryl
bromide, such as 2-bromothiophene, was used as the coupling
partner, a moderate yield was obtained (66%, entry 26). Further
studies indicated that the catalyst loading could be lowered to
0.05 mol-% without obvious loss of activity except in the case
of 2-bromothiophene. The two complexes 2a and 2b exhibited
comparable activity under the same conditions, with complex
2b being slightly more active in most cases. Good yields could
be achieved in a shorter reaction time of 4.5 h with 0.05 mol-%
of 2b, or after 12 h with only 0.01 mol-% of 2b in the coupling
of para-bromoanisole with phenylboronic acid (85% and 76%
yields, respectively; Table 1, entries 15, 16). The effect of TBAB
in the same coupling reaction was also investigated. Excellent
yields could also be obtained in the presence of 0.5 mol-%
of 2b without the addition of TBAB (92%, entry 10). While
a moderate yield was obtained with 0.05 mol-% of 2b in the
absence of TBAB (79%, entry 14), indicating that TBAB could
accelerate the coupling especially at lower catalyst loadings.
The above results indicated that complexes 2a and 2b were
much more active than the related pyrazole-based bidentate
ligand-palladium complexes shown in Fig. 3.[30] It was reported
that ligand 3, in combination with Pd, did not catalyze the
Suzuki coupling reaction at all. The deposition of Pd black com-
menced after 1 h at room temperature or within a few minutes at
65–70◦C. Although ligands 4a and 4c could promote the
C(4)
Fig. 1. Molecular structure of complex 2a. Hydrogen atoms have
been omitted for clarity. Selected bond lengths [Å] and angles [◦] are
as follows: Pd(1)–N(1) 2.011(2), Pd(1)–N(1A) 2.011(2), Pd(1)–Cl(1)
2.3073(9), Pd(1)–Cl(1A) 2.3073(9), N(1)–N(2) 1.360(3), N(1)–C(10)
1.339(4), N(2)–C(8) 1.353(4), N(2)–C(7) 1.458(4) and N(1A)–Pd(1)–N(1)
N(1A)–Pd(1)–Cl(1A)
90.37(8), N(1A)–Pd(1)–Cl(1) 90.37(8), N(1)–Pd(1)–Cl(1) 89.63(8),
Cl(1A)–Pd(1)–Cl(1) 180.0.
180.000(1),
89.63(8),
N(1)–Pd(1)–Cl(1A)
of 2a was further confirmed by a single-crystal X-ray analysis.
Although X-ray analysis clearly indicates that complex 2a adopts
a trans-geometry in the solid state with the two chlorine atoms
being in a trans-position (see Fig. 1), the 1H NMR and 13C
NMR spectra of 2a as well as 2b are more complicated than
the expected trans-structure, suggesting the existence of other
isomers in solution. For example, the 1H NMR spectrum of 2a
exhibits two singlets at δ 6.14 and 5.88 ppm, for the benzyl–
CH2 protons. The signals for the pyrazole proton are observed
at δ 6.11 and 6.09 ppm as two singlets, and four singlets at δ 2.76,
2.54, 2.09, and 2.07 ppm are assigned to the –CH3 in the pyrazole
ring. For complex 2b, the corresponding singlets appear at 6.18
and 5.75 ppm, 5.92 and 5.89 ppm as well as 2.88, 2.55, 2.09,
and 2.05 ppm, respectively. The 13C NMR spectra of 2a and 2b
also exhibited more signals than expected. The complex NMR
spectra of 2a and 2b are most likely due to the presence of both
trans- and cis- structures in solution. Another possibility is that
all species are trans-, whereas the benzyl groups may be in a
syn- and anti- configuration.
Crystal Structure
The molecular structure of complex 2a is shown in Fig. 1.
The Pd atom has a square-planar coordination geometry, which
is bonded to two chlorine atoms and two nitrogen atoms
from two ligands of pyrazolyl-containing m-phenol deriva-
tive 1a. Each of the chlorine and nitrogen atoms are in the
trans-position, with bond angles of N(1A)–Pd(1)–N(1) being
180.000(1)◦ and Cl(1A)–Pd(1)–Cl(1) being 180.0◦. All the
bond distances and angles around Pd(ii) centre are similar
to those observed in the related monodentate N-coordinated
palladium-benzimidazole[12,22] or pyrazole complexes.[23]
In complex 2a, chlorine atom forms hydrogen bond with
the adjacent –OH group of aryl ring (Cl1· · ·H1E = 2.356 Å,
Cl1· · ·H1E-O1 = 167.7◦). Intermolecular π-stacking interac-
tions involving the neighbouring benzene rings are also evident
in the crystal structure. The interplane distance and angle are
∼3.5107 Å and 0◦, respectively. Owing to the π-π stacking

Room Temperature Suzuki Reactions
317
b
O(1)
CI(1)
c
N(2)
N(1)
Pd(1)
N(1)
N(2)
CI(1)
O(1)
H(1E)
O(1)
H(1E)
CI(1)
CI(1)
O(1)
C(2)
C(1)
N(2)
N(1)
Pd(1)
N(2)
N(1)
Pd(1)
C(3)
C(4)
C(5)
C(6)
C(1)
C(4)
N(1)
N(2)
C(6)
N(1)
N(2)
C(5)
C(3)
C(2)
O(1)
CI(1)
O(1)
CI(1)
O(1)
CI(1)
N(2)
Pd
CI
N
N(1)
Pd(1)
N(1)
N(2)
O
C
O(1)
CI(1)
H
Fig. 2. 2D lamellar structure of complex 2a formed by hydrogen bonds and π–π interactions. Non-hydrogen bonding H atoms have been omitted for clarity.
couplingof para-bromoanisolewithphenylboronicacidat45◦C,
affording the coupled product in moderate yields (around 70%),
in the presence of 1 mol-% of Pd2(dba)3, neither of them were
effective at room temperature. The same coupling was observed
at room temperature when ligand 4b or 4d with 1 mol-% of
Pd2(dba)3 were used. In this case, the catalyst loading was obvi-
ouslyhighandtheyieldsoftheproductsweremoderate(77–78%
after 10 h).[30]
Li2PdCl4 (0.1 mol L−1 in CH3OH) was obtained by stirring
PdCl2 and two equivalent of anhydrous LiCl in CH3OH at
room temperature for 20 h. All other chemicals were used as
purchased. Melting points were measured using a WC-1 micro-
scopic apparatus and were uncorrected. Elemental analyses were
determined with aThermo Flash EA 1112 elemental analyzer. IR
spectra were collected on a Bruker VECTOR22 spectrophoto-
1
meter in KBr pellets. H and 13C NMR spectra were recorded
The coupling of non-activated and activated aryl chlorides
with phenylboronic acid, as catalyzed by 2, was also investi-
gated. Unfortunately, formation of the coupled products was not
observed in any appreciable amounts with a catalyst loading of
1 mol-%, under the same reaction conditions as those applied
to the aryl bromides at room temperature (data not shown in
Table 1).
In conclusion, we have demonstrated that easily accessible
and stable palladium complexes 2 are highly active catalysts for
the Suzuki coupling of aryl bromides with phenylboronic acid
in aqueous media at room temperature under air. The present
catalytic process is easy to handle and the reaction conditions
are mild, affording the desired coupled products in high yields.
on a Bruker DPX-400 spectrometer, with TMS as an internal
standard.
General Method for the Synthesis of Palladium(II)
Complexes 2a–2b
A mixture of pyrazolyl-containing m-phenol derivative 1a
(202 mg, 1.0 mmol), acetyl chloride (85 µL, 1.2 mmol), and
sodium hydride (48 mg, 2.0 mmol) in 10 mL of CH2Cl2 was
stirred at room temperature for 12 h. Then the reaction was
quenched with water. The aqueous layer was extracted with
dichloromethane, and the organic layers were dried over MgSO4,
filtered, and evaporated. The crude product was purified by
preparativeTLC on silica gel plates eluting with CH2Cl2/AcOEt
(1:1) to afford 1b as an oil (150 mg, 61.5% yield).
Experimental
General
1b: δH (400 MHz, CDCl3) 7.31 (t, J = 7.9 Hz, 1H, Ar–
H), 6.98 (dd, J = 1.9, 7.9 Hz, 1H, Ar–H), 6.93 (d, J = 7.9 Hz,
1H, Ar–H), 6.79 (s, 1H, Ar–H), 5.85 (s, 1H, Pz–H), 5.21 (s,
3,5-dimethylpyrazole[31] and pyrazolyl-containing m-phenol
derivative 1a[18] were prepared according to literature methods.

318
K. Xu et al.
Table 1. Suzuki coupling reactions of aryl bromides with phenylboronic acid catalyzed by complexes 2a–2b
Reaction conditions: aryl bromide (0.5 mmol), PhB(OH)2 (0.6 mmol), K2CO3 (1.0 mmol), n-Bu4NBr (0.5 mmol), C2H5OH/H2O 2 mL (v/v = 1:1), at room
temperature under air for 12 h
Entry
Aryl bromide
Cat. [mol-%]
Product
YieldA [%]
Turn over numbersB
1
2
3
4
2a (0.5)
2a (0.1)
2a (0.05)
2b (0.05)
>99
>99
94
198
990
1880
1980
Me
Me
Br
>99
Me
Br
Me
5
6
7
8
2a (0.5)
2a (0.1)
2a (0.05)
2b (0.05)
2a (0.5)
2b (0.5)
2a (0.1)
2a (0.05)
2b (0.05)
2b (0.05)
2b (0.05)
2b (0.01)
2a (0.5)
2a (0.1)
2a (0.05)
2b (0.05)
2a (0.5)
2a (0.05)
2b (0.05)
2a (0.5)
>99
92
87
>99
>99
92
>99
>99
>99
79
198
920
1740
1980
198
9
MeO
Br
MeO
10C
11
12
13
14C
15D
16
17
18
19
20
21
22
23
24
184
990
1980
1980
1580
1700
7600
194
85
76
97
97
OMe
Br
OMe
970
93
1860
1980
198
1920
1840
192
>99
>99
96
92
96
O2N
Br
O2N
Br
O2N
O2N
25
2a (0.5)
35
70
NO2
Br
NO2
26
27
2a (0.5)
2a (0.1)
66
43
132
430
Br
S
S
28
29
2a (0.05)
2b (0.05)
39
45
780
900
AIsolated yields based on aryl bromide.
BTON = mol of product/mol of the catalyst.
Cn-Bu4NBr was not added.
DReaction time 4.5 h.
H3C
CH3 H3C
CH3
H3C
Bu-t
H3C
CH3
H3C
Bu-t
N N
N N
N N
N N
N N
NMe2
CHꢀNPh
CHꢀNPh
CHꢀNAr
4b Ar ꢀ 2,6-(i-Pr)2C6H3
Fig. 3. Pyrazole-based bidentate ligands for the palladium-catalyzed Suzuki reaction reported in ref. [30].
CHꢀNAr
4d Ar ꢀ 2,6-(i-Pr)2C6H3
4c
3
4a
2H, CH2), 2.27 (s, 3H, Ac), 2.24 (s, 3H, CH3), 2.15 (s, 3H,
CH3). δC (100 MHz, CDCl3): δ 169.3, 150.9, 147.7, 139.2,
139.1, 129.7, 123.9, 120.7, 119.7, 105.6, 52.1, 21.1, 13.5, 11.1.
νmax(KBr)/cm−1 2927, 1764, 1601, 1554, 1451, 1376, 1308,
1205, 1142, 1018, 786, 704.
To a stirred solution of ligand 1a (101 mg, 0.5 mmol) or
1b (122 mg, 0.5 mmol) in CH3OH (10 mL) was added drop-
wise a solution of Li2PdCl4 in CH3OH (0.1 mol L−1, 5 mL) at
room temperature. The reaction mixture was stirred at rt for 5 h.
The resultant yellow precipitate was collected and washed with

Room Temperature Suzuki Reactions
319
CH2Cl2 to give 2a (263 mg, 90.4% yield) or 2b (308 mg, 92.5%
yield).
26.00◦, 4187reflections, 2380unique(Rint = 0.0284), complete-
ness to θ 26.00◦ 97.4%, max. and min. transmission 0.8582
and 0.8272, data/restraints/parameters 2380/0/156, goodness-
of-fit on F2 1.097, final R indices [I >2σ(I)]: R1 = 0.0350,
wR2 = 0.0898, R indices (all data): R1 = 0.0412, wR2 = 0.0931,
extinction coefficient 0.0184(15), largest diff. peak and hole
0.460 and −0.476 e Å−3. CCDC reference number 748824.
2a: m.p. >270◦C. δH (400 MHz, DMSO-d6): The complex
exists as a mixture of isomers in solution with a ratio of ∼1.1:1.
9.45 (s, 1H, Ar–OH), 9.37 (s, 1H, Ar–OH), 7.14 (t, J = 7.8 Hz,
1H, Ar–H), 7.01 (t, J = 7.8 Hz, 1H, Ar–H), 6.85 (d, J = 7.6 Hz,
1H, Ar–H), 6.73–6.64 (m, 4H, Ar–H), 6.50 (s, 1H, Ar–H), 6.14
(s, 2H, CH2), 6.11 (s, 1H, Pz–H), 6.09 (s, 1H, Pz–H), 5.88 (s,
2H, CH2), 2.76 (s, 3H, CH3), 2.54 (s, 3H, CH3), 2.09 (s, 3H,
CH3), 2.07 (s, 3H, CH3). δC (100 MHz, DMSO-d6): δ 157.3,
148.8, 148.7, 143.5, 137.0, 136.9, 129.4, 129.2, 117.6, 117.5,
114.4, 114.3, 113.9, 113.7, 107.6, 107.5, 53.0, 52.8, 14.6, 14.4,
11.3. νmax(KBr)/cm−1 3349, 2958, 1725, 1620, 1591, 1553,
1476, 1402, 1307, 1215, 1152, 1071, 1036, 957, 867, 801,
767, 685, 610. Found: C 48.90, H 4.84, N 9.48%. Calcd for
C24H28Cl2N4O2Pd: C 49.54, H 4.85, N 9.63%.
Accessory Publication
1
The Accessory Publication contains H NMR and 13C NMR
spectra of ligand 1b as well as those of palladium complexes 2a
and 2b. It is available from the Journal’s website.
Acknowledgements
2b: m.p. 192–193◦C. δH (400 MHz, CDCl3): The complex
exists as a mixture of isomers in solution with a ratio of ∼1.2:1.
The ratio changed to ∼2:1 after the sample solution was placed
at room temperature for 3–4 days. 7.39 (t, J = 8.0 Hz, 1H, Ar–
H), 7.28–7.26 (m, 1H, Ar–H), 7.24–7.18 (m, 2H, Ar–H), 7.06 (d,
J = 8.0 Hz, 1H, Ar–H), 6.98–6.93 (m, 3H, Ar–H), 6.18 (s, 2H,
CH2), 5.92 (s, 1H, Pz–H), 5.89 (s, 1H, Pz–H), 5.75 (s, 2H, CH2),
2.88 (s, 3H, CH3), 2.55 (s, 3H, CH3), 2.28 (s, 3H, Ac), 2.27 (s,
3H, Ac), 2.09 (s, 3H, CH3), 2.05 (s, 3H, CH3). δC (100 MHz,
CDCl3): δ 169.5, 151.0, 150.9, 150.5, 150.4, 143.7, 137.0, 136.9,
129.9, 129.5, 124.7, 124.3, 121.3, 121.0, 120.6, 120.3, 108.3,
108.1, 53.4, 52.8, 21.2, 15.2, 14.9, 12.0, 11.9. νmax(KBr)/cm−1
2925, 1764, 1591, 1558, 1443, 1373, 1318, 1205, 1144, 1014,
952, 891, 817, 791, 720, 692. Found: C 50.43, H 4.84, N 8.30%.
Calcd for C28H32Cl2N4O4Pd: C 50.50, H 4.84, N 8.41%.
We are grateful to the National Natural Science Foundation of China
(20872133) and the Innovation Fund for Outstanding Scholar of Henan
Province (074200510005) for financial support of this work.
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General Procedure for the Suzuki Reactions
A tube was charged with aryl bromide (0.5 mmol), phenyl-
boronic acid (0.6 mmol), K2CO3 (1.0 mmol), the catalyst 2
(0.0025 mmol, 0.5 mol-%) or a catalyst solution in EtOH
(0.00005 or 0.00025 mmol mL−1), n-Bu4NBr (0.5 mmol), and
EtOH-H2O (2 mL, v/v = 1:1) under air. The reaction mixture
was stirred at room temperature for 12 h (time not optimized).
Water was subsequently added, and the aqueous phase was
extracted with dichloromethane. The combined organic layers
were washed with water, dried over MgSO4, filtered, and evap-
orated. The products were isolated by flash chromatography on
silica gel (the purified products were identified by comparison of
melting points with the literature values or by 1H NMR spectra).
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