PdCl2-2,6-bis(1,5-diphenyl-1H-pyrazol-3-yl)pyridine catalyzed Suzuki-Miyaura cross-coupling

Article abstract of DOI:10.1016/j.catcom.2012.10.009

Six multidentate nitrogen ligands, 2,6-bis(1,5-diphenyl-1H-pyrazol-3-yl) pyridine (L1), 2,6-bis(1-methyl-5-phenyl-1H-pyrazol-3-yl)pyridine (L2), 2,6-bis(1,5-dimethyl-1H-pyrazol-3-yl)pyridine (L3), 2,6-bis(5-phenyl-1H-pyrazol- 3-yl)pyridine (L4), 2,6-bis(1-p-triflurophenyl-5-phenyl-1H-pyrazol-3-yl)pyridine (L5), and 2,6-bis(1-m-methylphenyl-5-phenyl-1H-pyrazol-3-yl)pyridine (L6) were synthesized and characterized. Their palladium complexes catalyzed Suzuki-Miyaura cross-coupling revealed that the catalytic activities of these complexes were in the order of PdCl₂-L5 > PdCl₂-L1 > PdCl₂-L6 > PdCl₂-L2 > PdCl₂-L3 > PdCl₂-L4. With a low Pd loading of 10^{- 5} mol%, PdCl ₂-L1 showed a high activity. The turnover numbers (TONs) were up to 58,000,000 for 4-bromobenzotrifluoride.

Full text of DOI:10.1016/j.catcom.2012.10.009

Catalysis Communications 29 (2012) 194–197
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
PdCl2-2,6-bis(1,5-diphenyl-1H-pyrazol-3-yl)pyridine catalyzed
Suzuki–Miyaura cross-coupling
Qin Yang a, Lei Wang b, Lei Lei a, Xue-Li Zheng a, Hai-yan Fu a, Mao-lin Yuan a, Hua Chen a, Rui-Xiang Li a,
⁎
a
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu, Sichuan 610064, PR China
b
College of Chemistry and Chemical Engineering, Xuchang University, Xuchang, Henan 461000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Six multidentate nitrogen ligands, 2,6-bis(1,5-diphenyl-1H-pyrazol-3-yl)pyridine (L1), 2,6-bis(1-methyl-5-
phenyl-1H-pyrazol-3-yl)pyridine (L2), 2,6-bis(1,5-dimethyl-1H-pyrazol-3-yl)pyridine (L3), 2,6-bis(5-phenyl-
1H-pyrazol-3-yl)pyridine (L4), 2,6-bis(1-p-triflurophenyl-5-phenyl-1H-pyrazol-3-yl)pyridine (L5), and 2,6-
bis(1-m-methylphenyl-5-phenyl-1H-pyrazol-3-yl)pyridine (L6) were synthesized and characterized. Their
palladium complexes catalyzed Suzuki–Miyaura cross-coupling revealed that the catalytic activities of these
complexes were in the order of PdCl2-L5>PdCl2-L1>PdCl2-L6>PdCl2-L2>PdCl2-L3>PdCl2-L4. With a low Pd
loading of 10−5 mol%, PdCl2-L1 showed a high activity. The turnover numbers (TONs) were up to 58,000,000
for 4-bromobenzotrifluoride.
Received 11 September 2012
Received in revised form 8 October 2012
Accepted 16 October 2012
Available online 23 October 2012
Keywords:
Suzuki reactions
Palladium
© 2012 Elsevier B.V. All rights reserved.
Nitrogen ligand
1. Introduction
2. Experiment section
The Suzuki–Miyaura cross-coupling is an extremely efficient method
for carbon–carbon bond formation [1–3]. Recently, multidentate nitrogen
ligands, which are easy to be synthesized, have been successfully
employed in this reaction [4–11] as the appropriate electron-donating
ability of nitrogen atoms in them facilitates the oxidative addition of sub-
strates [12–14]. However their activities were generally lower than the
phosphine analogues and a high catalyst loading had to be required. In
fact, lots of researches also revealed that the real active species was the
Pd nanoparticles in Pd catalyzed cross-coupling and various kinds of li-
gands played the role of stabilizing the nanoparticles in the reaction
systems [15–17]. No matter what the species are, Pd complexes or
nanoparticles, multidentate ligands are much more efficient in stabilizing
the active Pd species than the monodentate and bidentate ligands. For ex-
ample, the Pd system with the tetradentate phosphine Tedicyp showed
much higher efficiency than the monophosphine or biphosphine [18].
Similarly, the tridentate azetidine [11] and tetradentate N,O ligand [7]
gave higher TONs than other nitrogen-based ligands. Herein, we designed
a family of multidentate N-donor ligands and employed them in a
Suzuki–Miyaura reaction. Their combination with PdCl2 showed high ac-
tivities in the Suzuki reactions.
2.1. General information
All the chemicals received from commercial suppliers were used
without further purifications unless noted. 1H and 13C NMR spectra
were recorded using a 400 MHz spectrometer using TMS as the inter-
nal standard. A GC equipped with an FID detector and a 30 m×
0.32 mm×0.25 μm SE-30 column was used to detect the reactions.
Mass spectrometry was performed on a TOF mass analyzer using
electrospray ionization (ESI) mode. Single crystal X-ray diffraction
was carried at 293.15 K.
2.2. General procedure for the Suzuki reaction
Phenylbornic acid (183 mg, 1.5 mmol), KOH (84, 1.5 mmol) and aryl
halides (1 mmol) were added into the reaction tube with a magnetic bar,
and then dissolved in ethanol (3 mL). The EtOH solution of the desired
amount of catalyst (PdCl2: ligand=1:1.2), which was formed in situ at
50 °C for 0.5 h, was injected into the reaction mixture with a syringe.
After the resulting solution was stirred for the required time, it was cooled
to room temperature, quenched by 3 mL water, and extracted with ethyl
acetate (3×5 mL). The organic layer was collected and dried over MgSO4.
The crude products were purified by column chromatography (petro-
leum ether, ethyl acetate or hexane) on silica gel.
2.3. Synthesis of ligands
L1 and L4 were synthesized by the reported method [19,20]. L2,
L3, L5 and L6 were synthesized by a procedure similar to that of L1.
⁎
Corresponding author. Tel./fax: +86 28 85412904.
E-mail address: liruixiang@scu.edu.cn (R.-X. Li).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.catcom.2012.10.009

Q. Yang et al. / Catalysis Communications 29 (2012) 194–197
195
To get L3, 1,1-′(pyridine-2,6-diyl)bis(butane-1,3-dione) (123 mg,
0.5 mmol) which was prepared in the reported method was dissolved
in acetic acid (5 mL) in a two-necked round bottom flask. Then a
10 mL acetic acid solution of methyl hydrazine was slowly added
into the solution through a funnel at room temperature. The mixture
was then heated to 60 °C and stirred overnight, then cooled. A sodi-
um carbonate solution was slowly added into the cooled solution to
adjust it to neutral pH and then some white crystals were formed.
The crystals were filtered, washed with water, and dried under vacu-
um to give L3 as white solid. Isolated yield: 53% (141 mg). Mp: 208–
209 °C. 1H NMR (400 MHz, CDCl3) δ 7.74 (d, J=7.8 Hz, 2H, pyridine),
7.68–7.55 (m, 1H, pyridine), 6.75 (s, 2H, pyrazole), 3.78 (s, 6H,
N-CH3), 2.26 (s, 6H, C-CH3); 13C NMR (100 MHz, CDCl3) δ 11.2
(CH3), 36.3 (N-CH3), 104.3 (3-pyrazole), 117.9 (3-pyridine), 136.8
(2-pyrazole), 139.7 (4-pyridine), 150.6 (4-pyrazole), 151.9 (2-pyri-
dine). HRMS-ESI. calcd. for C15H18N5 (M+H)+ =268.1562, found
268.1564.
For L2, L5, and L6, methylhydrazine, 4-(trifluoromethyl)
phenylhydrazine and 3-(methylphenyl)hydrazinethe are treated with
3,3′-(pyridine-2,6-diyl)bis-(1-phenylpropane-1,3-dione), respectively,
under similar conditions. L5 (yield of 26%): white solid. Mp: 254–
256 °C. 1H NMR (400 MHz, CDCl3) δ 8.15 (d, J=8.0 Hz, 2H, pyridine),
7.92 (t, J=7.4 Hz, 1H, pyridine), 7.63 (d, J=8.0 Hz, 4H, phenyl),
7.58–7.44 (m, 6H, phenyl), 7.36 (d, J=11.2 Hz, 10H, phenyl). 13C
NMR (100 MHz, CDCl3): δ 153.2 (2-pyridine), 151.3 (3-pyrazole),
144.8 (5-pyrazole), 142.9 (4-pyridine), 137.4 (phenyl), 130.3 (phenyl),
129.1 (phenyl), 128.9 (phenyl), 128.8 (phenyl), 126.1 (phenyl), 126.1
(phenyl), 125.2 (phenyl), 124.98 (phenyl), 122.5 (3-pyridine), 108.0
(4-pyrazole). HRMS-ESI. calcd. for C37H24F6N5 (M+H)+=652.1936,
found 652.1953.
7.60–7.34 (m, 10H, phenyl), 7.14 (s, 2H, pyrazole), 3.98 (s, 6H, N-CH3);
13C NMR (100 MHz, CDCl3) δ 37.8(\CH3), 105.2 (4-pyrazole), 118.4
(3-pyridine), 128.5 (3-pyridine), 128.7 (phenyl), 128.8 (phenyl), 130.7
(phenyl), 137.1 (4-pyridine), 145.1 (5-pyrazole), 151.0 (3-pyrazole),
151.8 (2-pyridine). HRMS-ESI. calcd. for
C =
25H22N5 (M+H)+
392.1875, found 392.1879. The structure of L2 was further identified
by its single crystal structure. The single crystal was obtained by evapo-
ration of a CDCl3 solvent. The exact structure was shown in Fig. 1.
3. Result and discussion
It is well known that a ligand plays an important role in modifying
the efficiency of a catalyst in a homogeneous catalysis [8]. With the
six ligands in hand, the effects of the ligand structures and reaction
conditions on the Suzuki–Miyaura coupling were investigated in detail.
We chose the coupling of phenylboronic acid and 4-bromoanisole
as the model reaction. The effect of solvents on the reaction was first-
ly investigated at a low reaction temperature of 50 °C with L1 as the
model ligand and KOH as the base (Table 1, entries 1–6). The reaction
gave the highest yield in ethanol. The other solvents, whether they
are protonic or aprotic solvents, gave lower yields. Subsequently,
the effect of bases on the reaction was explored. The organic base
and the weak inorganic base NaHCO3 were less efficient for the reac-
tion. By increasing the basicity of the bases, the conversion of the
substrate was improved. The strong base KOH gave the highest con-
version (Table 1, entries 7–10). Even if performed at room tempera-
ture, the reaction could proceed smoothly and give a yield of 58% in
half an hour, which is higher than most of the N-donor catalyst sys-
tems at room temperature [7,16,21,22]. If the reaction temperature
was elevated to 70 °C, the desired product can be obtained with a
quantitative yield in 0.5 h. In order to shorten the reaction time in
the high ratio of substrate to catalyst, 70 °C was chosen as the reac-
tion temperature in the following reactions. KOH and ethanol were
used as the optimum base and solvent, respectively.
L6 (yield of 17%): white solid. Mp: 189–192 °C. 1H NMR (400 MHz,
CDCl3) δ 8.01 (d, J=7.8 Hz, 2H,3-pyridine), 7.74 (t, J=7.8 Hz, 1H,
2-pyridine), 7.26 (d, J=6.8 Hz, 13H, phenyl), 7.14 (t, J=7.7 Hz, 3H,
phenyl), 7.07 (d, J=7.5 Hz, 2H, phenyl), 7.00 (d, J=7.8 Hz, 2H,
4-pyrazole), 2.29 (s, 6H,-CH3). 13C NMR (100 MHz, CDCl3): δ 144.5
(2-pyridine), 140.1 (3-pyrazole), 139.1 (5-pyrazole), 130.7 (4-pyridine),
128.8 (phenyl), 128.6 (phenyl), 128.4 (phenyl), 128.2 (phenyl), 126.0
(phenyl), 122.6 (phenyl), 119.1 (phenyl), 106.9 (4-pyrazole), 21.3
(\CH3). HRMS-ESI. calcd. for C37H29N5 (M+H)+=544.2510, found
544.2537.
To detect the effect of ligand structures on the reaction, the six li-
gands were screened under the optimum conditions. The result was
shown in Scheme 1. Obviously, the existence of a phenyl ring, when-
ever it was on a nitrogen or carbon atom, was more favorable to im-
prove the catalyst activity than the ligand with the methyl group.
However, L3 and L4 gave unsatisfactory yields. It is supposed that
methyl as an electron-donating substituent on the pyrazole resulted
in the relatively low efficiency of L3. The hydrogen atom on the
L2 (yield of 67%). Mp: 216–218 °C. 1H NMR (400 MHz, CDCl3) δ 7.94
(d, J=7.8 Hz, 2H, 3-pyridine), 7.80 (t, J=7.8 Hz, 1H, 4-pyridine),
Fig. 1. ORTEP view of the molecular structure of L2.

196
Q. Yang et al. / Catalysis Communications 29 (2012) 194–197
Table 1
The effect of the base and solvent on the Suzuki–Miyaura reaction
.
Entry
Base
Temperature (°C)
Solvent
Yield (%)
1
2
3
4
5
6
7
8
KOH
KOH
KOH
KOH
KOH
KOH
K2CO3
NaHCO3
Et3N
Pyridine
KOH
50
50
50
50
50
50
50
50
50
50
25
70
EtOH
90
80
63
TRACE
TRACE
25
58
13
MeOH
i-PrOH
Dioxane
DMF
Toluene
EtOH
EtOH
EtOH
EtOH
EtOH
9
27
Trace
58
10
11
12
KOH
EtOH
100
Reaction conditions: 4-bromoanisole (1 mmol), phenylboronic acid (1.5 mmol), KOH
(1.5 mmol), EtOH (3 mL), PdCl2 (10−4 mmol), L1 (1.2×10−4 mmol), 0.5 h, GC yield.
Fig. 2. The yield vs. time.
nitrogen atom of the pyrazole ring (L4) could be dissociated under
the reaction condition, so L4 was transformed into an anion with a
high electron density. These results indicated that methyl substituted
N and high electron density in the pyrazole ring leads to the low ac-
tivity of the Pd species and the conjugation between the N atom
and the phenyl ring may cause the electron delocalization, which
could promote the activity of the Pd species. It is well known that a
bulky substituent near a coordinate atom would facilitate the reduc-
tive elimination in the catalytic cycle and the strong basicity of the co-
ordination atom is favorable to stabilize active species and improve
the catalyst activity. In this system, the conjugation structure of li-
gands, which may cause the electron delocalization, seemed to facili-
tate the active Pd species in the catalytic cycle. In order to support the
suggestion, L5 with an electron-drawing substituent in the phenyl
ring and L6 with an electron-donating substituent in the same phenyl
ring were synthesized. It was found that the introduction of the
trifluoromethyl group on the phenyl ring resulted in the increase of
the yield from 78.1% to 95.2%, while the introduction of the methyl
group caused the yield to decrease from 78.1% to 65.5%. The results
further supported that the weak basicity of the nitrogen atom and
electron delocalization were favorable in this system.
To clarify whether the weak basicity of the nitrogen atom, which is
caused by electron delocalization, will promote the catalyst activity or
extend the lifetime of Pd species in the catalyst system, the relation-
ship between conversion and time (L1 and L3 as ligands) were inves-
tigated and shown in Fig. 2. The curves showed that the activity of the
Pd species was dramatically decreased with the existence of the
methyl group (L3) and the yield was near 32% after 24 h, while the
catalytic activity with L1 as ligand can last up to 60 h in the existence
of the phenyl group. It could be seen that the electron delocalization
improved both the activity and lifetime of the active species. It may
be due to the possibility that the electron delocalization of a ligand
can keep the stability of a low-valence complex [23,24].
Owing to the low cost of the starting materials and easy synthesis of
L1, we continued to use L1 as the ligand in the following research. The
coupling results of various aryl halides were shown in Table 2. When
the loading of Pd was 10−3 mol%, both the electron-deficient and
electron-rich aryl bromides gave the desired products in excellent yields
in 2 h (Table 2, entries 1–5). In particular, the electron-deficient aryl bro-
mides formed the corresponding biphenyl derivatives in almost quantita-
tive yields. Accordingly, trifluoromethyl substituted aryl bromides were
selected to react with phenylboronic acid in a lower catalyst loading of
10−4 mol%. 3-Bromobenzotrifluoride and 4-bromobenzotrifluoride also
gave products in high yields of 86% (Table 2, entry 10) and 83%
(Table 2, entry 6) in 20 h, respectively. When it was performed at room
temperature, 4-bromobenzotrifluoride could still give a moderate yield
of 75% (Table 2, entry 8). It is identified that this catalytic system was
highly efficient even at room temperature. However, it seemed that the
hindrance of ortho-substitutions retarded the reaction process. When
the methyl group or methoxyl group was introduced in the ortho-
position of bromine, the reaction rate obviously slowed down. The load-
ing of PdCl2 had to be increased to 0.05 mol% and the reaction time had
to be prolonged to 20 h, and good conversion of ortho-substituted aryl
halides could be obtained (Table 2, entries 12–13). The results are similar
to the reported nitrogen-based catalyst systems [6,9]. However, it has
been reported that heteroaromatic compounds were the deactivated sub-
strates due to their strong coordination ability, which can poison the
catalyst [25]. A high catalyst loading (>0.1 mol%) was necessary for
most of the phosphine-free catalyst systems [4,7,26,27]. In this case, for
Scheme 1. The effect of the ligands for the Suzuki reaction.

Q. Yang et al. / Catalysis Communications 29 (2012) 194–197
197
Table 2
Suzuki coupling of a variety aryl halidesa.
Entry
Substrate
S/C
t (h)
Yield (%)
Entry
Substrate
S/C
t (h)
Yield(%)
1
2
3
4
5
6
7
8
9
4-Bromoanisole
4-Bromotoluene
105
105
105
105
105
106
107
105
105
2
2
2
2
2
20
120
12
2
84 (1a)
95 (2a)
96 (3a)
97 (4a)
78 (5a)
83 (6a)
86 (6a)
75b (6a)
92 (7a)
10
11
12
13
14
15
16
17
18
3-Bromobenzotrifluoride
3-Bromoanisole
2-Bromotoluene
2-Bromoanisole
3-Bromopyridine
3-Bromoquinoline
4-Chloronitrobenzene
4-Chlorobenzotrifluoride
4-Chloroacetophenone
106
20
2
20
20
2
20
20
20
20
86 (8a)
93 (9a)
105
4-Bromobenzaldehyde
4-Bromonitrobenzene
4-Bromobenzonitrile
4-Bromobenzotrifluoride
4-Bromobenzotrifluoride
4-Bromobenzotrifluoride
4-Bromoacetophenone
2000
2000
104
98 (10a)
83 (11a)
80 (12a)
86 (13a)
81c (4a)
80c (6a)
81c (7a)
105
200
200
200
a
Reaction conditions: PdCl2/L1 as catalyst, aryl halide (1 mmol), phenylboronic acid (1.5 mmol), KOH (1.5 mmol), EtOH (3 mL), reaction temperature was 70 °C, isolated yield
of two runs.
b
c
At room temperature.
100 °C in DMF.
3-bromoquinoline, a moderate satisfactory yield of 63% could be achieved
with a low Pd loading of 0.001 mol% in 2 h and the yield could increase to
86% if the reaction time was prolonged to 20 h. But for 3-bromopyridine,
a poor yield of 25% was given in 2 h. By increasing PdCl2 loading to
0.01 mol%, the yield was up to 80% in 2 h (Table 2, entries 14–15). As it
is well known the Suzuki cross-coupling of aryl chlorides is not as easy
as aryl bromides. Generally, the high catalyst loading and harsh condi-
tions are required in order to get their corresponding coupling products
[12,28]. After the reaction temperature was elevated to 100 °C, the cata-
lytic system could catalyze the coupling of electron-deficient aryl chlo-
rides and phenylboronic acid and gave the satisfactory yields in the
presence of 0.5 mol% catalyst (Table 2, entries 16–18). It just paralleled
to the reported phosphine-free catalytic systems for the Suzuki coupling
of aryl chlorides [6,7,9,29]. Encouraged by the high efficiency of this
optimal catalytic system, the ratio of PdCl2 to the substrate was decreased
to 10−7 mol%, and the couplings of phenylboronic acid and
4-trifluoromethyl bromobenzene were further tested. As shown in
Table 2, entry 7, it afforded the corresponding substituted biphenyl in
the yield of 86% and the TON is high at up to 8,600,000 and when the
catalyst loading was decreased to 10−8 mol%, a high TON of up to
58,000,000 was obtained. This result demonstrated that the catalyst
system is not only of a high activity, but also has a long lifetime. To
our best knowledge, although it was not as efficient as the tetradentate
phosphines Tedicyp [30] and a-Cytep [31], the PdCl2/L1 system provid-
ed the highest TON in a Pd catalyzed Suzuki–Miyaura reaction with
multidentate nitrogen ligands [32], for the reported highest TON was
only up to 548,400 [7].
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://
dx.doi.org/10.1016/j.catcom.2012.10.009.
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4. Conclusion
A family of polydentate nitrogen ligands with different substitu-
ents was synthesized and identified. The complexes generated by
combining ligand L1 or L5 with PdCl2 in situ were proved to be highly
efficient for the Suzuki reactions of various aryl halides under aerobic
conditions. Specially, PdCl2/L1 could give a conversion of 58% even if
the loading of Pd was as low as 10−8 mol%. We supposed that the
high TON may be attributed to the long lifetime of the Pd species in
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This work was supported by the National Natural Science Founda-
tion of China (no. 21202104), and National Science Talents Fund for Re-
search Training and Research Ability Improvement Project (j1103015).