Cucurbit[7]uril promoting PdCl2-catalyzed cross-coupling reaction of benzyl halides and arylboronic acids in aqueous media

Article abstract of DOI:10.1016/j.tetlet.2014.07.060

The research provides a novel approach for producing diarylmethane derivatives using CB[7]-NaCl-PdCl₂catalyzed Suzuki cross-coupling reaction of benzyl chloride derivatives and arylboronic acids in ethanol aqueous solution.

Full text of DOI:10.1016/j.tetlet.2014.07.060

Accepted Manuscript
Cucurbit[7]uril promoting PdCl2-catalyzed cross-coupling reaction of benzyl
halides and arylboronic acids in aqueous media
Guangkuan Zhao, Zhen Wang, Ruixin Wang, Jingya Li, Dapeng Zou, Yangjie
Wu
PII:
S0040-4039(14)01224-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.07.060
TETL 44903
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
21 March 2014
27 June 2014
17 July 2014
Please cite this article as: Zhao, G., Wang, Z., Wang, R., Li, J., Zou, D., Wu, Y., Cucurbit[7]uril promoting
PdCl2-catalyzed cross-coupling reaction of benzyl halides and arylboronic acids in aqueous media, Tetrahedron
Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.2014.07.060
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Cucurbit[7]uril promoting PdCl2-catalyzed
cross-coupling reaction of benzyl halides and
arylboronic acids in aqueous media
Leave this area blank for abstract info.
Guangkuan Zhaoa, Zhen Wanga, Ruixin Wanga, Jingya Lib, Dapeng Zoua, * and Yangjie Wua, *

1
Tetrahedron Letters
jo u rn al h ome p ag e : w ww .e lse vie r .c om
Cucurbit[7]uril promoting PdCl2-catalyzed cross-coupling reaction of benzyl halides
and arylboronic acids in aqueous media
Guangkuan Zhaoa, Zhen Wanga, Ruixin Wanga, Jingya Lib, Dapeng Zoua,∗ and Yangjie Wua,∗
a The College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, 450052, P. R. China.
b Tetranov Biopharm, LLC., No.75 Daxue Road , Zhengzhou, 450052, P. R. China.
ARTICLE INFO
ABSTRACT
Article history:
Received
The research provides a novel approach for producing diarylmethane derivatives using CB[7]-
NaCl-PdCl2 catalyzed Suzuki cross-coupling reaction of benzyl chloride derivatives and
arylboronic acids in ethanol aqueous solution.
Received in revised form
Accepted
2014 Elsevier Ltd. All rights reserved.
Available online
Keywords:
Diarymethane derivatives
Suzuki reaction
Cucurbit[7]uril
Supramolecular catalytic system
Introduction
carboxylate complexes could catalyze the reaction of benzyl
halides and arylboronic acids at room temperature to give high
yield of diarylmethanes. It is worth noting that Bandgar et al.10
described the coupling reactions between benzyl bromides and
arylboronic acids could be carried out in the presence of PdCl2 in
acetone aqueous solution without ligands, but the more available
benzyl chloride could not react with arylboronic acids under this
conditions.
Diarylmethane derivatives are one of the most important
building blocks in organic synthesis and drug discovery research.
Hundreds of biologically active compounds contain diarylmethyl
motifs (Figure 1), such as tofogliflozin applied in treatment of
type 2 diabetes1 and aryl-diketoacids2 used as inhibitor of HIV
integrase. Given these important applications, a variety of
synthetic methods toward this class of compounds are in demand.
Traditionally, diarylmethane derivatives have been obtained by
the following methods: (1) the reduction of diarylketones or
diarylcarbinols;3 (2) Friedel-Crafts alkylation using benzyl
halides or benzyl alcohols and aromatic compounds.4 However,
these methods proceed with low atom economy, require harsh
reaction conditions, and/or have limited selectivity. More
recently, Suzuki cross-coupling reaction of benzyl halides and
arylboronic acids that exhibits high regio and stereo-selectivity
and tolerates a wide range of functional groups has played more
and more important role in constructing diarylmethane building
blocks. In the process, phosphine-containing ligands5 were
widely used. Some of them could be used in aqueous media or
reused5d,5g,5i, but they are usually harmful to the environment.
Phosphine-free palladacyles were also applied to catalyze the
reaction of benzyl halides and arylboronic acids, such as oxime-
Figure 1. Bioactive compounds containing diarylmethyl motifs.
In recent years, host-guest chemistry studying the interaction
between macrocyclic compounds and metal ions which has
potential application in catalytic research has triggered more
attention. Efficient palladium-catalyzed systems based on 18-
crown-6,11 cryptand-2212 which are useful for the cross-coupling
of aryl bromides and arylboronic acids have been developed.
Although some efficient palladium-catalyzed systems based on
supramolecules have been reported, there are few reports
focusing on the catalytic action of cucurbit[n]uril.
carbapalladacycle,6
di(2-pyridyl)methylanmine-palladium,7
cyclopalladated ferrocenylimine.8 Although these catalysts
provided an alternative procedure to make diarylmethane
derivatives, they may need long time to be prepared. In addition,
Nolan9 reported that N-heterocyclic carbene (NHC)Pd-
———
∗ Corresponding author. Tel.: +86 371 6776 6865; fax: +86 371 6776 3390; e-mail: zdp@zzu.edu.cn, wyj@zzu.edu.cn

2
Tetrahedron
PdCl2, 146 mg NaCl and 20 mg CB[7]) were prepared (See ESI).
These catalysts are highly stable to air and moisture after being
exposed to air for several days with no decrease of their catalytic
activity. The results show that catalysts prepared in advance was
more effective in comparison with putting each compound into
reaction system separately. Among the three catalysts, Cat. 3
(See ESI) performed best and gave satisfactory yields.
Table 1. Effect of the amount of NaCl and CB[7] on the
Suzuki reactiona
Cat., 90 oC, Ar
Cl
NC
NC
+
B(OH)2
Figure 2. Structure of CB[n] (n =5-8, 10)
Base, Solvent
Yield(%)b
Cucurbit[n]uril (CB[n], n = 5-8, 10) is a family of macrocyclic
Entry
NaCl (equiv)
CB[7] mol%
molecules, which can accommodate many different chemical
species, forming host-guest inclusion complexes owing to its
hydrophobic cavities and polar carbonyl groups surrounding the
portals (Figure 2). In view of its specific structure and properties,
CB[n] was used to improve or catalyze some reactions,13
especially photodimerization reactions.14 Additionally, Cao15
reported a new type of palladium nanocatalyst based on CB[n],
which exhibits high catalytic activity towards Heck reaction and
Suzuki cross-coupling reactions of aryl halides and arylboronic
acids. Herein, we report that a new catalyst system of CB[7]-
NaCl-PdCl2 that could be used as an effective catalyst to produce
diarylmethane derivatives in Suzuki cross-coupling reaction in
aqueous media.
1
2
0
0.08
3
0
0
11
12
3
0
29
4
5
0
39
5
8
0
36
6
5
1.72
3.44
0.86
1.72
3.44
54
7
5
76
8
5
56c
66d
93e
9
5
10
5
Results and Discussion
a
Reaction conditions: 4-(chloromethyl)benzonitrile (0.5 mmol),
phenylboronic acid (0.6 mmol), PdCl2 (3 mol%), K2CO3 (0.75 mmol),
EtOH/H2O (5:1, 5 ml), 90 °C, under Ar atmosphere. b Isolated yields. c Using
the Cat. 1 (153.7 mg with 3 mol% PdCl2, 5 equiv NaCl and 0.86 mol%
CB[7]). Using the Cat. 2 (158.7 mg with 3 mol% PdCl2, 5 equiv NaCl and
1.72 mol% CB[7]). e Using the Cat. 3 (168.7 mg with 3 mol% PdCl2, 5 equiv
NaCl and 3.44 mol% CB[7]).
Bora et al.16a reported that the catalytic system based on PdCl2
and NaCl is effective in Suzuki cross-coupling reactions of aryl
bromides with arylboronic acids under mild conditions. Since
PdCl2 and the additive of NaCl could form water soluble
complex of Na2PdCl4 which actually catalyzed the Suzuki
reaction. In this paper, we were interested in the synthesis of
diarylmethane derivatives from benzyl chlorides and arylboronic
acids by using PdCl2/NaCl catalyst system in the presence of
CB[7].
d
To study the effect of base on the cross-coupling reaction of
4-(chloromethyl)benzonitrile with phenylboronic acid, different
bases were tested with Cat. 3 and K2CO3 was found to be the
best choice (Table 2, entries 1-5). Additional studies were carried
out to evaluate the effect of solvent at 90 °C in 5 mL of EtOH,
pure water and EtOH/H2O mixture (Table 2, entries 5-10). Both
EtOH and EtOH/H2O (5:1) solution gave excellent results. The
yield of the reaction was increased slightly when some amount of
H2O was added to ethanol, which may due to that water could
improve the solubility of the catalytic system of CB[7]-NaCl-
PdCl2. For some water insoluble substrates, the use of EtOH/H2O
mixtures instead of pure water was found to be advantageous.18
Changing the solvent to dioxane/H2O, CH3CN/H2O, CH3OH/H2O,
acetone/H2O did not favour the reaction. The rate of reaction was
The
reaction
of
4-(chloromethyl)benzonitrile with
phenylboronic acid was chosen as a model reaction to explore the
optimized catalyst system under the conditions with EtOH/H2O
as solvent at 90 oC in an oil bath and K2CO3 as a base (as shown
in Table 1). In the absence of NaCl, only 11% of desired product
was obtained (Table 1, entry 1). The addition of NaCl to the
solution gives a slightly higher yield (Table 1, entry 2). The yield
of desired product was further improved by increasing the
amount of NaCl (Table 1, entries 2-4). When the amount of NaCl
was increased to 8 equiv, the yield of product was found slightly
decreased (Table 1, entry 5). So 5 equiv NaCl was chosen as
additive. Due to the fact that the carbonyl oxygens on the CB
portals are known to interact strongly with metal ions17 and our
interest in the modification and application of CB[n] derivatives,
we attempted to add CB[n] into the reaction system. To our
delight, the combination of PdCl2 and NaCl with CB[7] in the
solution gave the desired product in a slightly higher yield.
Increasing the amount of CB[7] from 1.72 mol% to 3.44 mol%,
the reaction yields improved from 54% to 76% (Table 1, entries
6-7). Both CB[6] and CB[8] performed worse than CB[7] in the
same condition which may due to their poor solubility in the
reaction system. Based on the data presented here, we believe
that PdCl2, CB[7] and NaCl are important factors for above
coupling reaction and CB[7] could obviously promoted this
reaction. In order to further simplify the operation and improve
the practicability of this catalytic system, three catalysts, Cat. 1
(2.7 mg PdCl2, 146 mg NaCl and 5 mg CB[7]), Cat. 2 (2.7 mg
PdCl2, 146 mg NaCl and 10 mg CB[7]) and Cat. 3 (2.7 mg
o
clearly accelerated when the oil bath is 90 C. Reducing the
reaction temperature from 90 oC to 60 oC lead the decrease of the
yield (Table 2, entries 15-18). Additionally, time optimization
revealed that the reaction gave excellent yield after 1 h. When the
reaction time was prolonged to 2 hours, the yield did not
dramatically increased (Table 2, entries 19-21). So the reaction
was performed in EtOH/H2O (5:1) in the presence of K2CO3 at
90 oC for 1 h.
Table 2. Solvent and base effects for Suzuki coupling
reactiona
T
(oC)
90
Time
(h)
3
Yield
(%)b
87
Entry
Base
Solvent
1
2
KOH
EtOH:H2O(5:1)
EtOH:H2O(5:1)
MeONa
90
3
56

3
3
Cs2CO3
AcONa
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
EtOH:H2O(5:1)
EtOH:H2O(5:1)
EtOH:H2O(5:1)
H2O
90
90
90
90
90
90
90
90
90
90
90
90
30
60
90
110
90
90
90
3
3
83
55
93
43
48
64
91
88
20
72
15
22
37
75
95
96
81
93
94
Entry
1
R1
4-CN
2-CN
4-NO2
4-Cl
R2
H
Time(h)
Yield % b
94
4
1
1
1
1
1
1
1
1
1
1
1
1
1
3
3
3
3
3
3
3
5
3
2
H
88
6
3
3
H
45
7
EtOH:H2O(1:4)
EtOH:H2O(1:1)
EtOH:H2O(4:1)
EtOH
3
4
H
86
8
3
5
4-COOCH3
4-CH3
2-CH3
3-CH3
2,6-CH3
4-OCH3
H
H
50
9
3
6
H
82
10
11
12
13
14
15
16
17
18
19
20
21
3
7
H
77
MeCN:H2O(5:1)
MeOH:H2O(5:1)
Acetone:H2O(5:1)
Dioxane:H2O(5:1)
EtOH:H2O(5:1)
EtOH:H2O(5:1)
EtOH:H2O(5:1)
EtOH:H2O(5:1)
EtOH:H2O(5:1)
EtOH:H2O(5:1)
EtOH:H2O(5:1)
3
8
H
76
3
9
H
67
3
10
11
12
13
14
15
16
17
18
19
H
16
77c
82
3
H
3
H
4-OCH3
4-CF3
4-OCH3
2-OCH3
4-CH3
2-CH3
4-CF3
4-Cl
2-Cl
3
H
61
3
4-CN
4-CN
4-CN
4-CN
4-CN
4-CN
4-CN
89
3
44
0.5
1
87
81
2
67
a
Reaction conditions: 4-(chloromethyl)benzonitrile (0.5 mmol),
phenylboronic acid (0.6 mmol), Cat. 3 (168.7 mg), base (0.75 mmol), solvent
(5 mL), under Ar atmosphere, 90 °C. b Isolated yields.
70
20
Tracec
a
Reaction conditions: benzyl chloride (0.5 mmol), arylboronic acid (0.6
mmol), K2CO3 (0.75 mmol), EtOH/H2O (5:1, 5 ml), Cat. 3 (168.7 mg), 90
°C, under Ar atmosphere. b Isolated yields. c GC-MS yield.
To evaluate the scope and limitations of the catalyst system on
the Suzuki coupling reaction, a wide array of benzyl chloride
derivatives were coupled to different arylboronic acids. The
results were shown in Table 3. It was found that most reactions
took place smoothly to provide the desired products in moderate
to good yields. Generally, the electronic properties of benzyl
chloride influenced the coupling reactions significantly. The
benzyl chlorides which contain electron-withdrawing substituent
groups can be coupled with arylboronic acid to give moderate to
good yields. Whereas, the benzyl chlorides with electron-
donating groups produce lower yields (Table 3, entry 1 vs 6,
entry 2 vs 7). Notably, in contrast with benzyl chloride, reversed
electronic effect was observed when substituted arylboronic acid
were reacted with 4-(chloromethyl)benzonitrile, where electron-
rich arylboronic acid gave higher yield than that of electron-
dificient arylboronic acid (Table 3, entries 12-20). The benzyl
chlorides and arylboronic acid with substituents at ortho-position
gave lower yields than those at para-position (Table 3, entry 6 vs
7, entry 14 vs 15, entry 16 vs 17), demonstrating the steric
hindrance likely has a negative effect on the coupling reaction. 1-
(Chloromethyl)-4-nitrobenzene gave a lower yield due to that it
tends to give the reduction by-product of 1-methyl-4-
nitrobenzene in this reaction condition (Table 3, entry 3). Gotten
the alcoholysis byproduct (detected by GC-MS) of ethyl 4-
benzylbenzoate facilely in the condition, methyl 4-
(chloromethyl)benzoate reacted with phenylboronic acid to
obtain the desired product in 50% yield (Table 3, entry 5). 1-
(Chloromethyl)-4-methoxybenzene gave only 16% of the target
product because it was easily destroyed in the reaction condition
to give 1-(ethoxymethyl)-4-methoxybenzene as detected by GC-
MS (Table 3, entry 10).
On the basis of the above results and the literature report,16 we
presumed that the possible reason of the catalyst could
significantly improve the yield of the reaction is that the oxygen
atoms pointing outward the cavity of CB[7] can bind sodium ions
kinetically first (Table 1, entries 1 vs 5).19 Then palladium
chloride and sodium chloride formed the complex Na2PdCl4
resulting in higher density of Pd(II) near the portal of CB[7].20
The cross-coupling reaction of benzyl chlorides and
phenylboronic acids were then accelerated in the portal of CB[7].
Conclusion
In summary, we have demonstrated that the pre-prepared
catalytic system based on CB[7]-NaCl-PdCl2 exhibited excellent
catalytic activity in the cross-coupling reaction of various benzyl
chlorides with a variety of arylboronic acids in aqueous solution.
The catalytic system is air-stable after being exposed to air for
several days and no decrease in activity was found in the
coupling reaction. Thereby, the present method provides an
effective tool for the synthesis of diarylmethane derivatives.
Besides, the results will be a significant addition to the research
of supramolecular chemistry based on macrocyclic compounds.
Acknowledgments
We are grateful to the Natural Science Foundation of China
(20772114, 21172200), the Innovation Fund for Outstanding
Scholar of Henan Province (0621001100) and Research Program
of Fundamental and Advanced Technology of Henan Province
(122300413203) for financial support.
Table 3. Suzuki coupling reaction catalyzed by CB[7]-NaCl-
a
PdCl2
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