Efficient symmetrical bidentate dioxime ligand-accelerated homogeneous palladium-catalyzed Suzuki-Miyaura coupling reactions of aryl chlorides

Article abstract of DOI:10.1039/C6NJ02815B

A series of N,O-bidentate ligands were synthesized using the Vilsmeier-Haack reaction and oximation. 2,5-Dihydroxyterephthalaldehyde dioxime (L₈) as an efficient N,O-symmetrical bidentate ligand was prepared from hydroquinone. It was studied as a high activity ligand for palladium-catalyzed Suzuki-Miyaura cross-coupling reactions of aryl chlorides with arylboronic acids under mild conditions. The coupling reactions were performed in the presence of PdCl₂ as the catalyst, L₈ as the ligand, Na₂CO₃ as the base, PEG-400 as the PTC and in ethanol/water (1?:?1) as an environmentally benign solvent at 85 °C. Plentiful biaryls were obtained by the optimized reaction with good yields at a low palladium loading of 0.20 mol%.

Full text of DOI:10.1039/C6NJ02815B

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Efficient symmetrical bidentate dioxime ligand-
accelerated homogeneous palladium-catalyzed
Suzuki–Miyaura coupling reactions of aryl
chlorides†
Cite this:DOI: 10.1039/c6nj02815b
Jinyi Song,ab Hongyan Zhao,ab Yang Liu,ab Huatao Han,ab Zhuofei Li,ab
Wenyi Chu*ab and Zhizhong Sun*ab
A series of N,O-bidentate ligands were synthesized using the Vilsmeier–Haack reaction and oximation.
2,5-Dihydroxyterephthalaldehyde dioxime (L8) as an efficient N,O-symmetrical bidentate ligand was prepared
from hydroquinone. It was studied as a high activity ligand for palladium-catalyzed Suzuki–Miyaura cross-
coupling reactions of aryl chlorides with arylboronic acids under mild conditions. The coupling reactions
were performed in the presence of PdCl2 as the catalyst, L8 as the ligand, Na2CO3 as the base, PEG-400 as
the PTC and in ethanol/water (1 : 1) as an environmentally benign solvent at 85 1C. Plentiful biaryls were
obtained by the optimized reaction with good yields at a low palladium loading of 0.20 mol%.
Received (in Nottingham, UK)
8th September 2016,
Accepted 29th November 2016
DOI: 10.1039/c6nj02815b
www.rsc.org/njc
The palladium-catalyzed Suzuki–Miyaura reaction is one of the
most widely used ways to synthesize biaryls,1–3 which are structural
components of many natural products, materials and phar-
maceutical chemicals.4–6 Aryl bromides and iodides are constantly
used in Suzuki reactions, while aryl chlorides are rarely used
despite their low cost and greater usability, because the high
energy of the Caryl–Cl bond leads to reduced reaction activity.7,8
Therefore, it is necessary to develop an efficient catalytic system
to get aryl chlorides involved in the reaction. Generally, the
way to improve the efficiency of coupling reactions is by the
employment of ancillary ligands.9–11 Phosphine ligands12–14 are
commonly used in catalytic processes because of their high
activity in cyclometalated transformations. However, the clear
drawbacks of phosphines are the preparative difficulties, potential
toxicity, instability etc. From early literature reports, some bidentate
ligands could assist coupling reactions to give excellent yields.
Therefore, our focus will be on bidentate ligands instead of phos-
phines. N,N-Bidentate ligands such as 1,10-phenanthroline,15,16
ethylenediamine,17,18 2,20-bipyridine19,20 etc. and O,O-bidentate
ligands like dicarboxylic acids,21,22 b-diketones,23,24 pyrocatechol25
etc. have attracted considerable attention as competent ligands for
coupling reactions (see Scheme 1). Furthermore, N,O-bidentate
ligands like salicylaldoximes26,27 are also frequently used to enhance
Scheme 1 Common N,N- and O,O-bidentate ligands.
catalytic activity. In our research, by comparing N,N-bidentate
ligands with O,O-bidentate ligands, we found that the latter
showed higher activity in our catalytic reaction. Notably, it was
found that the catalytic activity of O,O-symmetrical bidentate
ligands was significantly higher than for asymmetric ligands.28
Therefore, N,O-symmetrical bidentate ligands should be effective
ligands in the Suzuki–Miyaura reaction.
Herein, we designed and synthesized a variety of salicylaldoxime
derivative ligands mainly via the Vilsmeier–Haack reaction29
and oximation (see Scheme 2). Among the synthesized ligands,
L8, as a novel ligand, demonstrated the highest activity due to
its electron-rich character and the existence of multiple bonding
sites which are considered to increase the steric congestion
around a metal centre. Moreover, the synthetic method for L8
a School of Chemistry and Materials Science, Heilongjiang University,
Harbin 150080, P. R. China. E-mail: wenyichu@hlju.edu.cn;
Fax: +86-451-86609135; Tel: +86-451-86609135
b Key Laboratory of Chemical Engineering Process & Technology for High-efficiency
Conversion, College of Heilongjiang Province, Harbin 150080, P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c6nj02815b
Scheme 2 Synthesis of salicylaldoxime derivative ligands.
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Table 1 Evaluation of the catalysts and ligands in the Suzuki reactiona
was relatively more mild and economical than that for L9.
Compared with phosphine ligands which are sensitive to water
and air, L8 as an oxime derivative was described as being stable
and showed high efficiency in reaction processes. It is note-
worthy to mention that when the reaction was catalyzed by PdCl2
it was heterogeneous with low to moderate yields and reaction
efficiency, while when using the novel ligand these issues could
be avoided. We report the Suzuki–Miyaura coupling reaction of
aryl chlorides with arylboronic acids over a neotype homogeneous
catalytic system based upon L8 coordinated to PdCl2, making the
method more attractive than previous methods.
The dioxime L8, as an efficient symmetrical bidentate ligand,
was acquired in moderate yield by the formylation of hydroquinone
using Vilsmeier–Haack reagent and oximation (Scheme 2).
The complex structure of L8 and PdCl2 was confirmed by
elemental analysis, FT-IR, 1H and 13C NMR spectroscopy and
mass spectral data. For the relevant data associated with the other
synthesized salicylaldoxime derivative ligands and complexes, see
the ESI†. The elemental analyses and the molecular ion peaks of
the complexes were consistent with the proposed formulation.
From the FT-IR data of the complex of L8, it can be seen that the
Catalyst
(quantity [mol%])
Ligand
(quantity [mol%])
Entry
Yieldb (%)
1
2
3
4
5
6
7
8
PdCl2 (0.20)
PdCl2 (0.20)
PdCl2 (0.20)
PdCl2 (0.20)
PdCl2 (0.20)
PdCl2 (0.20)
PdCl2 (0.20)
PdCl2 (0.20)
PdCl2 (0.20)
PdCl2 (0.20)
PdCl2 (0.20)
PdCl2 (0.20)
Pd(OAc)2 (0.10)
Pd(OAc)2 (0.10)
Pd(OAc)2 (0.20)
Pd(OAc)2 (0.20)
Pd(OAc)2 (0.40)
Pd(OAc)2 (0.40)
PdCl2 (0.05)
PdCl2 (0.10)
PdCl2 (0.40)
PdCl2 (0.50)
—
—
20
22
30
43
45
51
46
50
80
75
55
61
16
43
33
81
45
80
28
48
80
79
0
L1 (2.0)
L2 (2.0)
L3 (2.0)
L4 (2.0)
L5 (2.0)
L6 (2.0)
L7 (2.0)
L8 (2.0)
L9 (2.0)
L8 (1.0)
L8 (4.0)
—
L8 (2.0)
—
L8 (2.0)
—
L8 (2.0)
L8 (2.0)
L8 (2.0)
L8 (2.0)
L8 (2.0)
L8 (2.0)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
n
O–H band appears at 3451 cmꢀ1, which is higher than that of the
ligand (3337 cmꢀ1) due to chemical changes. Compared with L8
(1651 cmꢀ1), the nCQN band appears at 1624 cmꢀ1 owing to the
donation of electrons from the nitrogen atom into the empty
orbitals of the metal, resulting in a red shift. The Ar–CQN signal
in the 1H NMR spectrum appears at d 8.41 ppm, and so compared
to L8 (d 8.24 ppm) a downfield shift of 0.17 ppm was observed.
a
Reaction conditions: 4-chlorotoluene (1.0 mmol, 1.0 equiv.), phenylboronic
acid (1.2 equiv.), Na2CO3 (2.0 equiv.), ethanol:water = 1:1 (8 mL), PEG-400
b
(0.01 equiv.), 85 1C, 5.0 h. Isolated yield.
Meanwhile, an Ar–OH signal had disappeared, supporting the data indicated that 2.0 mol% (compared with the molar quantity
deprotonation of an OH group of the ligand, and at the same time of 4-chlorotoluene) was the best (Table 1, entries 9, 11 and 12).
the protonation of a nitrogen in an N–OH group. A similar Use of L8 in excessive amounts can cause an insufficient amount
downfield shift was also observed in the 13C NMR spectrum of of the ligand to form the Pd(0) complex, as excess oxime could
the complex. It was confirmed that the N and O atoms mutually lead to the formation of oxime-ethers, as is seen in similar
coordinated with PdCl2. The catalytic activities of the synthesized reaction systems.30 Moreover, Pd(II) is propitious to be reduced
ligands and PdCl2 were verified using the Suzuki–Miyaura cross- to Pd(0) in the presence of oximes.
coupling reaction. Initially, the reaction of 4-chlorotoluene
The catalysts of coupling reactions are principally Pd-based.
with phenylboronic acid was selected as the model to evaluate Comparing Pd salts in the same quantities (Table 1, entries 1
the ligands and catalysts. The results of the experiments are and 15), PdCl2 catalyzed the reaction without a ligand to give
summarized in Table 1.
only a 20% yield, a catalytic effect in arrears of Pd(OAc)2 in the
The experiments indicated that the yield of the reaction was absence of a ligand. However, the addition of L8 to PdCl2
very low without a ligand (Table 1, entry 1). Phenanthroline (L1) catalyzed the reaction, and the yield could reach up to 80%
as an N,N-bidentate ligand showed some activity, resulting in a (Table 1, entry 9) which far surpassed the result of Pd(OAc)2. In
22% yield (Table 1, entry 2). It could be seen that the catalytic the presence of L8 and Pd(OAc)2, the highest yield that could be
effect of phenanthroline on the reaction was far less than that reached was 81%, which was close to that achieved by PdCl2
of the O,O-bidentate ligands L2 and L3, while when L3 was the when combined with L8 (Table 1, entries 14, 16 and 18). We
O,O-symmetrical bidentate ligand it showed better activity than preferred to use PdCl2 as the catalyst as it is relatively inexpensive
the O,O-asymmetrical bidentate ligand L2 (Table 1, entries 3 which makes the reaction more cost-effective. From the catalyst
and 4). Salicylaldoxime (L4) as an N,O-bidentate ligand showed quantity data (Table 1, entries 9 and 19–22), the preliminary results
similar activity to L3 (Table 1, entries 4 and 5). When comparing showed that a 0.20 mol% palladium loading favored the reaction.
similar N,O-asymmetrical bidentate ligands (L5, L6 and L7), Decreasing the amount of the catalyst was accompanied by a drop
the yields exhibited small fluctuations (Table 1, entries 6–8). in the yield (Table 1, entries 9, 19 and 20), while continuing to
The addition of N,O-symmetrical bidentate ligands L8 and L9 increase the amount of the catalyst had no significant effect on the
significantly increased the yield of the reaction to 80% and 75% yield of the reaction (Table 1, entries 9, 21 and 22). The reaction
(Table 1, entries 9 and 10). The results proved that the N,O- without a catalyst did not occur at all (Table 1, entry 23).
symmetrical bidentate dioxime L8 is a valid ligand for the
The other reaction conditions including the base, solvent and
coupling reaction. Then, the amount of L8 was selected; the temperature were selected using the data in Table 2. The solvent
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Table 2 Evaluation of the conditions in the Suzuki reactiona
Table 3 Suzuki reactionsa of various aryl chlorides with arylboronic acids
using L8
Entry Solvent (V/V)
Base
PTC
T/1C Yieldb (%)
Entry
R1
R2
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
Glycerin
EtOH
H2O
EtOH : H2O = 1 : 1 Na2CO3
EtOH : H2O = 1 : 2 Na2CO3
EtOH : H2O = 2 : 1 Na2CO3
DMF
DMSO
THF
Toluene
Dioxane
EtOH : H2O = 1 : 1 Et3N
EtOH : H2O = 1 : 1 Pyridine
EtOH : H2O = 1 : 1 K3PO4
Na2CO3
Na2CO3
Na2CO3
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
85
85
85
85
85
85
31
56
20
80
72
75
82
42
30
43
31
25
20
47
38
59
61
55
68
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
H
H
4-CF3
4-CF3
4-CF3
4-CF3
4-CF3
0.5
3.0
3.0
3.0
5.0
5.5
7.0
8.0
8.0
8.0
10.0
8.0
8.0
5.0
5.0
5.0
5.0
5.0
10.0
9.5
7.0
5.0
7.0
6.0
6.5
6.0
5.5
6.5
8.0
8.5
8.0
8.0
10.0
9.0
6.0
90
88
85
83
80
78
65
60
62
70
75
70
72
78
76
73
74
75
70
77
65
71
70
68
65
67
68
64
78
77
79
76
75
74
70
4-NO2
4-CHO
4-CN
4-Me
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
PEG-400 115
4-OMe
3-OMe
2-OMe
2-NO2
3,4-OMe
3,4-OBn
4-Me
4-OMe
4-NO2
4-CHO
4-CN
4-OMe
4-CN
3,4-OBn
3,4-OBn
2-NO2
4-NO2
4-Me
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
PEG-400
85
85
85
85
85
85
85
85
85
85
85
75
9
10
11
12
13
14
15
16
17
18
19
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35c
EtOH : H2O = 1 : 1 NaHCO3 PEG-400
EtOH : H2O = 1 : 1 NaOH
EtOH : H2O = 1 : 1 K2CO3
EtOH : H2O = 1 : 1 Na2CO3
EtOH : H2O = 1 : 1 Na2CO3
PEG-400
PEG-400
—
4-Me
PEG-400
4-OMe
4-OMe
4-Phenyl
3,5-F
2,5-Me
3,4-OMe
2-Me
2-Me
2-Me
2-Me
2-Me
1-Naphthalene
1-Naphthalene
1-Naphthalene
2-Naphthalene
2-Naphthalene
2-Naphthalene
H
a
Reaction conditions: 4-chlorotoluene (1.0 mmol, 1.0 equiv.), phenyl-
boronic acid (1.2 equiv.), PdCl2 (0.20 mol%), L8 (2.0 mol%), base (2.0 equiv.),
solvent (8 mL), PTC (0.01 equiv.), 5.0 h. b Isolated yield.
H
has previously been exhibited to be critical for the reaction. Aprotic
and protic solvents were selected. When using protic solvents as the
reaction medium (Table 2, entries 1–3), a 56% yield was obtained
for the coupling reaction in EtOH. When comparing the use of
ethanol and water in different ratios (1 : 1, 1 : 2 and 2 : 1), yields
of 80%, 72% and 75% were achieved, respectively (Table 2,
entries 4–6). When using aprotic solvents as the reaction medium
(Table 2, entries 7–11), DMF was useful for the reaction, giving an
82% yield, but the disadvantage was the high temperatures
required. Above all, ethanol and water (1 : 1) was the most suitable
solvent system for the reaction. The base also plays a vital role in
the reaction. When organic bases were selected including Et3N
and pyridine (Table 2, entries 12 and 13), the results showed that
the reaction progressed in poor yield.
4-OMe
4-CHO
4-NO2
2-NO2
H
4-OMe
4-CHO
H
3,4-OBn
2-OBn, 4-OMe
2,6-Cl pyridine
a
Reaction conditions: aryl chloride (1.0 mmol, 1.0 equiv.), arylboronic
acid (1.2 equiv.), PdCl2 (0.20 mol%), L8 (2.0 mol%), PEG-400 (0.01 equiv.),
Na2CO3 (2.0 equiv.), ethanol : water = 1 : 1 (8 mL). The progress of the
b
c
reactions was monitored by TLC. Isolated yield. Reaction conditions:
aryl chloride (1.0 mmol, 1.0 equiv.), arylboronic acid (2.4 equiv.), PdCl2
(0.20 mol%), L8 (2.0 mol%), PEG-400 (0.01 equiv.), Na2CO3 (4.0 equiv.),
ethanol : water = 1 : 1 (8 mL).
Conversely, inorganic bases were found to be more helpful
in the reaction. In particular, the use of Na2CO3 improved the
reaction process to give the highest yield in ethanol/water in Table 3, the effects of the aryl chlorides were firstly investigated.
solvent (80%, Table 2, entry 4). When experiments were carried Aryl chlorides with either electron-withdrawing or electron-donating
out in the presence of PEG-400 as a phase transfer catalyst groups could perform the reaction under the optimum conditions.
(PTC) the yield was significantly increased to 80%. When no Moreover, the yields correlated with the properties and positions
phase transfer catalyst was present, the yield only reached 55% of the substituents. Chlorobenzene was used as the substrate to
(Table 2, entries 4 and 18). PEG-400 likely acts as a stabilizer for generate biphenyl in 90% yield within 0.5 h (Table 3, entry 1).
some low ligated palladium species involved in the catalysis, para-Substituted aryl chlorides with electron-withdrawing groups
thereby improving the yields of the coupling products. The such as nitro, formyl, and cyano groups (Table 3, entries 2–4)
temperatures selected were 75 1C and 85 1C (reflux temperature) brought about overall superior yields of 88%, 85% and 83%
and gave isolated yields of 68% and 80% (Table 2, entries 19 within 3 hours, respectively.
and 4). Therefore, the elevated temperature could accelerate the
rate of the reaction, so 85 1C was a reasonable choice.
When the substituents were electron-donating groups like
methyl and methoxy groups (Table 3, entries 5 and 6), the reaction
With the optimum reaction system conditions, we expanded also proceeded easily in approximately 80% yield though over
the substrate scope to a variety of aryl chlorides and arylboronic 5 hours. The reason for this is that an electron-withdrawing
acids varying their electronic and steric characteristic in the group is conducive to an oxidative addition reaction, which
Suzuki–Miyaura reaction, these are compiled in Table 3. As shown is the rate-determining step in the Suzuki–Miyaura reaction.
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Apart from the electronic features, the impact of steric hindrance
was also investigated (Table 3, entries 2 and 6–9). ortho-Substituted
chlorides were compared with meta- and para-substituted chlorides,
and the former gave a 60% yield (Table 3, entry 8). When the aryl
chlorides had multiple substituents (Table 3, entries 10 and 11), the
reaction yield exceeded 70% over 8 hours.
For arylboronic acids with electron-withdrawing or electron-
donating groups, such as trifluoromethyl, methoxy and phenyl
groups (Table 3, entries 12–20), the reactions could afford the
corresponding biaryls in moderate to good yields. Phenylboronic
acid could give better yields than substituted phenylboronic
acids (Table 3, entries 2–6 and 12–16). Likewise, when the aryl-
boronic acids had multiple substituents (Table 3, entries 21–23),
the reactions also occurred. For instance, from the reaction
with 2,5-dimethyl phenylboronic acid the product was isolated
in 71% yield after 5 hours (Table 3, entry 22). Encouraged by the
successful results of the reaction of the aryl chlorides with
phenylboronic acid, further study was conducted with some
sterically hindered arylboronic acids. Under the L8/PdCl2 catalytic
system, a higher yield was obtained for p-tolylboronic acid relative
to o-tolylboronic acid (65%, Table 3, entry 25), and aryl chlorides
with both electron donating and electron withdrawing groups
furnished the product in moderate to good yields (Table 3, entries
25–27). It is noteworthy to mention that 2-nitrochlorobenzene
coupled with o-tolylboronic acid in 64% yield (Table 3, entry 28).
The steric hindrance of the arylboronic acid had little effect on
the reaction. 1-Naphthyl and 2-naphthyl boronic acids also
participated with excellent reactivity (Table 3, entries 29–34).
4-Bromoanisole coupled with 1-naphthalene boronic acid in
almost 77% yield. Similarly, an electron-poor aryl chloride also
provided the intended product with 1-naphthalene boronic acid in
high yields. 2-Naphthalene boronic acid was also investigated and
gave the relative products in moderate yields (Table 3, entries 33
and 34). It should be emphasized that 2,6-dichloropyridine as an
aryl chloride demonstrated compatibility as a heteroaromatic aryl
chloride and a multiply chloro substituted aromatic in the coupling
reaction process (Table 3, entry 35). To our great delight, five novel
biaryl compounds were synthesized with good yields using the
catalyst system (Table 3, entries 19–21, 33 and 34).
Scheme 3 Proposed mechanism.
L8 facilitated the coupling reaction, not only as an efficient
ligand for palladium coordination, but also to provide a homo-
geneous system to promote the integrality of the reaction. Moreover,
the reaction can be performed in good yields at a low palladium
loading of 0.20 mol%. Furthermore, five novel biaryls were synthe-
sized under the catalyst system. Extension of the application of the
synthesized ligands to other reactions is still underway.
Experimental
In a 20 mL reaction flask, an aryl halide (1.0 mmol, 1.0 equiv.),
an aryl boronic acid (1.2 equiv.), Na2CO3 (2.0 equiv.), PEG-400
(0.01 equiv.), the ligand (2.0 mol%) and PdCl2 (0.20 mol%) were
charged and dissolved in 8 mL of ethanol aqueous solution
(Vethanol : Vwater = 1 : 1). The reaction mixture was stirred at 85 1C
and monitored by TLC. At the end of the reaction, the reaction
mixture was poured into water and the aqueous layer was
extracted with ethyl acetate 3 times (3 ꢁ 10 mL), and the organic
extracts were then dried over anhydrous sodium sulfate. After
filtration and removal of the solvent, the residue was purified by
column chromatography to give the biaryl products. The purity
of the products matched with authentic samples.
Based on literature reports,11,31,32 we propose that the
mechanism of the Suzuki–Miyaura reaction could be shown
as in Scheme 3. Initially, Pd(0) is formed by the reduction of
Pd(II) in the Suzuki reaction system, Pd(0) is then stabilized by
the ligand L8 and the complex A as the effective catalyst is
formed. Then an oxidative addition occurs between the
complex A and the aryl chloride to produce the intermediate B,
which undergoes transmetallation with the arylboronic acid to
afford the intermediate C in the presence of Na2CO3. Finally, a
reductive elimination provides the corresponding biaryl products
and the regeneration of the complex A, thereby resuming the
catalytic cycle.
This work was supported by a fund from the Natural Science
Foundation of Heilongjiang Province of China (No. B201208).
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