Palladium-catalyzed ligand-free and aqueous Suzuki reaction for the construction of (hetero)aryl-substituted triphenylamine derivatives

Article abstract of DOI:10.1039/c2ra22275b

This paper reports an efficient synthesis of triphenylamine (TPA) derivatives via a palladium-catalyzed Suzuki reaction of (hetero)aryl halides with 4-(diphenylamino)phenylboronic acid (DPBA) in aqueous ethanol under aerobic and ligand-free conditions. Heteroaryl halides, namely pyridyl bromides, quinolyl bromides, pyrimidinyl bromides, 2-chloropyrazine and sulfur-containing heteroaryl bromides, could react smoothly with DPBA, affording good to excellent yields under mild conditions. In addition, aryl bromides were also successfully employed in the cross-couplings with DPBA and furnished the products in high yields at room temperature. The cross-coupling of 4-bromobenzonitrile with DPBA gave the desired product in a quantitative yield within 2 min, resulting in a TOF up to 5820 h^-1.

Full text of DOI:10.1039/c2ra22275b

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PAPER
Palladium-catalyzed ligand-free and aqueous Suzuki
reaction for the construction of (hetero)aryl-substituted
triphenylamine derivatives3
Cite this: RSC Advances, 2013, 3, 526
Chun Liu,* Xiaofeng Rao, Xinlong Song, Jieshan Qiu and Zilin Jin
This paper reports an efficient synthesis of triphenylamine (TPA) derivatives via a palladium-catalyzed
Suzuki reaction of (hetero)aryl halides with 4-(diphenylamino)phenylboronic acid (DPBA) in aqueous
ethanol under aerobic and ligand-free conditions. Heteroaryl halides, namely pyridyl bromides, quinolyl
bromides, pyrimidinyl bromides, 2-chloropyrazine and sulfur-containing heteroaryl bromides, could react
smoothly with DPBA, affording good to excellent yields under mild conditions. In addition, aryl bromides
were also successfully employed in the cross-couplings with DPBA and furnished the products in high
yields at room temperature. The cross-coupling of 4-bromobenzonitrile with DPBA gave the desired
Received 25th September 2012,
Accepted 31st October 2012
DOI: 10.1039/c2ra22275b
www.rsc.org/advances
product in a quantitative yield within 2 min, resulting in a TOF up to 5820 h21
.
intriguingly flexible reaction, it offers considerable potential in
the synthesis of pharmaceuticals, fine chemicals and advanced
functional materials. Generally, the Suzuki reaction is carried
out in the presence of a ligand.8 However, most ligands are air-
sensitive and difficult to prepare or rather expensive. In the
past few years, significant progress has been made in this area,
which enables this transformation to be performed under
mild conditions in the absence of a ligand.9 Very recently, we
reported a ligand-free and aerobic protocol for the synthesis of
N-heteroaryl-substituted 9-arylcarbazolyl derivatives in aqu-
eous ethanol.10 Herein, we report a fast and efficient Suzuki
reaction for the synthesis of TPA derivatives in aqueous
ethanol under aerobic and ligand-free conditions (Scheme 1).
Introduction
Triphenylamine (TPA) derivatives are receiving increasing
attention because of their intriguing physical properties as
well as being important pharmacophores in various biological
compounds.1 Moreover, due to good hole-transporting cap-
ability, strong electron-donating nature and high light-to-
electrical energy conversion efficiencies, TPA derivatives are
important structural motifs in organic light-emitting diodes
(OLEDs)2 and dye-sensitized solar cells (DSSCs).3 For example,
Zhou et al. synthesized a series of novel IrIII complexes with
triphenylamine dendrons and showed that the incorporation
of triphenylamine units into the IrIII complexes improved the
HI/HT properties and the morphological stability.4 Wang and
coworkers reported a sensitizer incorporating a lipophilic
alkoxy-substituted TPA electron donor, leading to a high
efficiency of 10.0–10.3% for DSSCs.5 However, these methods
for the synthesis of TPA derivatives, as described in the
literature, often require long reaction times, the use of harsh
conditions or toxic substrates, and the yields are generally
unpredictable because of side reactions such as dehalogena-
tion and homocoupling.6 Thus, it is still of great interest to
develop a facile and robust method to synthesize this
important type of compound.
Results and discussion
Optimization of reaction conditions
It is known that the nature of the base is an important factor
for determining the efficiency of the Suzuki reaction.
Therefore, the influence of various bases on the cross-coupling
reaction was first investigated. The cross-coupling of 2-bro-
mopyridine (0.25 mmol) with 4-(diphenylamino)phenylboro-
nic acid (DPBA) (0.375 mmol) was chosen as a model reaction.
The palladium-catalyzed Suzuki cross-coupling reaction has
been established as one of the most versatile and powerful
tools for the selective construction of aryl–aryl bonds.7 As an
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Linggong
Road 2, Dalian, 116024, China. E-mail: chunliu70@ yahoo.com;
Tel: (+86) 411-8498-6182
Scheme 1 The Suzuki reaction for the synthesis of TPA derivatives in aqueous
Electronic supplementary information (ESI) available: Experimental details
3
ethanol under ligand-free and aerobic conditions.
and characterization of cross-coupling products. See DOI: 10.1039/c2ra22275b
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Table 2 The effects of precatalysts and temperature on the Suzuki reactiona
As is evidenced from Table 1, the use of inorganic bases, such
as K2CO3, K3PO4?3H2O, or Na2CO3, delivered the desired
products in high yields (Table 1, entries 1–3). On the other
hand, organic bases, such as Et3N and 1,4-diazabicyclo-
[2,2,2]octance (DABCO), gave disappointing results (Table 1,
entries 6 and 7). We chose K2CO3 for the remainder of the
study as K2CO3 exhibited the highest activity among the bases
used.
The next investigation was carried out to study the influence
of different palladium species on the cross-coupling reaction,
and the results are illustrated in Table 2. Precatalysts with PdII
salts such as Pd(OAc)2 and PdCl2 exhibited high catalytic
activity (Table 2, entries 1 and 2). However, the activity was
decreased when PdII salt was replaced with zero-valent
palladium such as Pd2(dba)3 or Pd/C (Table 2, entries 3 and
4), which is consistent with both our recent results11 and Li’s
report.12 The effect of the catalyst loading on this reaction was
further evaluated. The yield decreased obviously when redu-
cing the catalyst loading from 1.5 mol% to 1.0 mol% or 0.5
mol% (Table 2, entries 5 and 6). The effect of the temperature
on the coupling reaction was also observed. The reaction
performed at 50 uC provided the expected cross-coupled
product in 35% yield, and the reaction was sluggish at room
temperature (Table 1, entries 7 and 8). Thus, the optimum
reaction conditions for this cross-coupling were found to be
1.5 mol% Pd(OAc)2, 2 equiv. of K2CO3, 80 uC in aqueous
ethanol under air.
Entry
Precatalyst
Temp. (uC)
Yield (%)b
1
2
3
4
5
6
7
8
PdCl2
80
80
80
80
80
80
50
82
97
43
Pd(OAc)2
Pd2(dba)3
5% Pd/C
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
55
79c
43d
35
room temperature
trace
a
Reaction conditions: 2-bromopyridine (0.25 mmol), DPBA (0.375
mmol), [Pd] (1.5 mol%), K2CO3 (0.5 mmol), EtOH/H2O (3 mL/1 mL),
b
c
d
10 min, in air. Isolated yields. Pd(OAc)2 (1.0 mol%). Pd(OAc)2
(0.5 mol%).
the presence of Pd(PPh3)4 for 24 h under nitrogen.4c Compared
with the reported methods, this protocol is the fastest and
simplest catalytic system for such a transformation.4c,6b,c,13
Various 2-pyridyl bromides bearing electron-donating groups,
such as methyl or methoxyl moieties, or electron-withdrawing
groups, such as fluoro, nitro or cyan moieties, performed with
high reactivity and delivered the desired products in high
yields (Table 3, entries 2–8). For example, the cross-coupling
between 2-bromo-4-methylpyridine and DPBA could be con-
ducted with 96% yield after 8 min (Table 3, entry 2). Electron-
poor 2-bromo-5-fluoropyridine resulted in 92% yield after 10
min (Table 3, entry 5). In particular, the cross-coupling of
2-bromo-5-(trifluoromethyl)pyridine and DPBA provided 95%
yield in 10 min, which is much more efficient than that
performed under nitrogen.14 Furthermore, 6-substituted-2-
bromopyridines were also successfully employed in the
cross-coupling reactions (Table 3, entries 9–14). For example,
2-bromo-6-methoxypyridine produced the expected biaryl 10 in
a 99% yield within 4 min, resulting in a TOF up to 990 h21
(Table 3, entry 10).
To further investigate the scope and limitations of this
methodology, we carried out cross-couplings of different
heteroaryl halides with DPBA under standard conditions.
The results are illustrated in Table 4. The coupling between
3-bromopyridine and DPBA provided 77% yield in 30 min
(Table 4, entry 1). Using 5-bromo-2-methoxypyridine instead of
3-bromopyridine afforded the target product 16 in 83% for 10
min (Table 4, entry 2), which was much efficient than our
previously reported method.13a Noticeably, 2-bromoquinoline
and 3-bromoquinoline underwent cross-coupling smoothly
and afforded the desired products in 97% and 85% yields,
respectively (Table 4, entries 3 and 4). However, 8-bromoquino-
line was less active and delivered the product in 41% yield
even over a prolonged period (Table 4, entry 5). The cross-
coupling of 5-bromopyrimidine and DPBA provided a 91%
yield after 15 min (Table 4, entry 7). Due to the electronic
effects of the nitrogen atoms of 2-bromopyrimidine, the
reactivity decreased (Table 4, entry 8). In addition, deactivated
2-chloropyrazine is also a good coupling partner in the
catalytic system (Table 4, entry 9). Furthermore, 2,6-dibromo-
pyridine delivered the double coupling product 27 in 57%
yield using a 3 mol% Pd(OAc)2 loading (Table 4, entry 13). It is
noteworthy that this catalytic system is also suitable for sulfur-
Scope and limitations of substrates
With the optimized conditions in hand, we further explored
the generality of the cross-coupling between DPBA with a
variety of 2-pyridyl bromides using 1.5 mol% Pd(OAc)2 in
EtOH/H2O, and the results are shown in Table 3. The coupling
of 2-bromopyridine with DPBA exclusively gave the desired
product 1 in 97% yield after 10 min (Table 3, entry 1), showing
high efficiency. In 2006, a procedure was reported for the
construction of N,N-diphenyl-4-(pyridin-2-yl)aniline (product
1), which was prepared from 2-fluoropyridine in about 36%
yield via two synthetic steps.6b Later, the Stille cross-coupling
was used to synthesize the product 1 from 2-(tributylstannyl)-
pyridine and (4-bromophenyl)diphenylamine in 75% yield in
Table 1 The effect of base on the Suzuki reactiona
Entry
Base
Time (min)
Yield (%)b
1
2
3
4
5
6
7
K3PO4?3H2O
K2CO3
Na2CO3
NaHCO3
EtONa
Et3N
DABCO
10
10
10
10
10
10
10
91
97
88
76
59
34
40
a
Reaction conditions: 2-bromopyridine (0.25 mmol), DPBA (0.375
mmol), Pd(OAc)2 (1.5 mol%), base (0.5 mmol), EtOH/H2O (3 mL/1
b
mL), under air. Isolated yield.
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Table 3 The Suzuki reaction of DPBA with pyridyl bromidesa
containing heteroaryl bromides. The reaction between 2-bro-
mothiophene and DPBA could be conducted with a 98% yield
after 30 min and 3-bromothiophene produced the expected
biaryl 25 in 81% yield within 60 min, respectively (Table 4,
entries 10 and 11). Moreover, 2-bromothiazole coupling with
DPBA afforded the target product in 62% yield after 4 h
(Table 4, entry 12), which was comparable with the result in
the presence of Pd(PPh3)4 under nitrogen in toluene at 110 uC
for 24 h.15
Time
Yield
(%)b
Entry
1
R
Product
(min)
H
10
97
1
In addition to heteroaryl halides, aryl bromides as one of the
cross-coupling partners also exhibited high reactivity and
afforded excellent yields. As shown in Table 4, the electronic
nature of the substituent group has a small influence on the
reactivity of the cross-coupling reactions (Table 4, entries 14–
17). The coupling of 4-bromobenzonitrile with DPBA gave the
desired product 28 in a quantitative isolated yield after 2 min,
resulting in a TOF up to 5820 h21 (Table 4, entry 14). To our
delight, ortho-substituted substrate 2-bromobenzonitrile
exhibited almost the same reactivity, and the coupling was
completed in 4 min (Table 4, entry 15). A quantitative yield of
2
4-CH3
8
96
2
3
4
5
6
7
3
4
5
6
7
8
5-CH3
5-OCH3
5-F
20
25
10
10
10
30
94
87
92
95
91
90
product
32
was
obtained
by
using
4-bromo-
N,N-diphenylaniline as
a
coupling partner in 10 min
(Table 4, entry 18). To the best of our knowledge, product 32
is commonly used as hole transport material in OLEDs,16 and
the present approach is the most efficient one for such a
transformation.17 Thus, this work provides a simple, fast and
practical method for the synthesis of aryl-substistuted TPA
derivatives, which are the potential building blocks for the
construction of various important intermediates and advanced
functional materials.
5-CF3
5-NO2
5-CN
8
9
9
6-CH3
4
4
98
99
Conclusions
In summary, a very fast and highly efficient method for the
synthesis of (hetero)aryl-substituted TPA derivatives has been
developed. Operational simplicity, short reaction time and
good yield are the key advantages of the present protocol. This
aerobic and water-involved protocol is in accordance with the
concept of green chemistry and is of great interest for the
synthesis of various intermediates and advanced functional
materials. Further investigations including photophysical
properties of these TPA derivatives and synthetic applications
of this methodology are currently under-way in our laboratory.
10
6-OCH3
10
11
11
12
6-F
8
89
92
6-CHO
10
12
13
13
14
6-CN
10
6
90
95
Experimental section
General remarks
Unless otherwise noted, all the reactions were carried out in
air. All N-heteroaryl halides were purchased from Alfa Aesar or
Avocado. 4-(Diphenylamino)phenylboronic acid (DPBA) was
purchased from Trusyn Chem-Tech Co., Ltd, China. Other
chemicals were purchased from commercial sources and used
without further purification. NMR spectra were recorded on a
Brucker Advance II 400 spectrometer using TMS as internal
6-COCH3
14
a
Reaction conditions: pyridyl bromide (0.25 mmol), DPBA (0.375
mmol), Pd(OAc)2 (1.5 mol%), K2CO3 (0.5 mmol), EtOH/H2O (3 mL/1
mL), 80 uC, under air. The reaction was monitored by TLC. Isolated
b
1
standard (400 MHz for H NMR and 100 MHz for 13C NMR).
yields.
Mass spectroscopy data of the products were collected with a
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Table 4 The Suzuki reactions of DPBA with heteroaryl halides or aryl bromidesa
Entry
1
Ar–X
Product
Time (min)
30
Yield (%)b
77
15
16
2
3
10
8
83
97
17
18
4
5
20
85
41
120
19
6
10
95
20
21
22
23
24
25
26
7
8
15
60
91
71
84
98
81
62
57c
9
15
10
11
12
13
30
60
240
60
27
28
14
15
2
4
97d
95d
29
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Table 4 (Continued)
Entry
16
Ar–X
Product
Time (min)
10
Yield (%)b
91d
30
17
18
10
10
92d
86d
31
32
a
Reaction conditions: heteroaryl halide (0.25 mmol), DPBA (0.375 mmol), Pd(OAc)2 (1.5 mol%), K2CO3 (0.5 mmol), EtOH/H2O (3 mL/1 mL), 80
b
c
uC, under air. The reaction was monitored by TLC. Isolated yields. Reaction conditions: 2,6-dibromopyridine (0.25 mmol), DPBA (0.75
d
mmol), Pd(OAc)2 (3.0 mol%), K2CO3 (1.0 mmol), EtOH/H2O (3 mL/1 mL), 80 uC, under air. Reaction conditions: aryl bromide (0.25 mmol),
DPBA (0.375 mmol), Pd(OAc)2 (0.5 mol%), K2CO3 (0.5 mmol), EtOH/H2O (3 mL/1 mL), room temperature, under air.
MS-EI instrument. All products were isolated by short
chromatography on a silica gel (200–300 mesh) column using
petroleum ether (60–90 uC), unless otherwise noted.
Compounds described in the literature were characterized by
1H NMR spectra compared to reported data.
4-(5-Methoxypyridin-2-yl)-N,N-diphenylaniline (4)
1
Yield: 87%; purple solid, mp 120–121 uC. H NMR (400 MHz,
CDCl3, 25 uC) d = 8.35 (d, J = 4.4 Hz, 1H), 7.81–7.79 (m, 2H),
7.60 (d, J = 8.0 Hz, 1H), 7.28–7.23 (m, 5H), 7.14–7.11 (m, 6H),
7.05–7.01 (m, 2H), 3.89 (s, 3H) ppm. 13C NMR (100 MHz,
CDCl3, 25 uC) d = 154.47, 149.89, 148.01, 147.61, 136.83,
133.11, 129.29, 127.25, 124.55, 123.63, 123.05, 121.54, 120.22,
55.71 ppm. MS (EI): m/z = 352.1585 [M]+.
General procedure for the Suzuki cross-coupling of DPBA with
heteroaryl halides
A mixture of heteroaryl halide (0.25 mmol), DPBA (0.375
mmol), K2CO3 (0.5 mmol), Pd(OAc)2 (1.5 mol%), ethanol (3
mL) and distilled water (1 mL) was stirred at 80 uC in air for
indicated time. The reaction mixture was added to brine (15
mL) and extracted four times with ethyl acetate (4 6 15 mL).
The solvent was concentrated under vacuum, and the product
was isolated by short-column chromatography on silica gel
(200–300 mesh).
6-(4-(Diphenylamino)phenyl)nicotinonitrile (8)
1
Yield: 90%; yellow solid, mp 144–145 uC. H NMR (400 MHz,
CDCl3, 25 uC) d = 8.87 (s, 1H), 7.92–7.90 (m, 3H), 7.74 (d, J = 7.6
Hz, 1H), 7.30 (t, J = 8.0 Hz, 4.4 Hz, 4H), 7.16–7.11 (m, 8H) ppm.
13C NMR (100 MHz, CDCl3, 25 uC) d = 159.97, 152.46, 150.30,
146.94, 139.57, 130.01, 129.51, 128.31, 125.42, 124.05,121.88,
118.90, 117.32, 106.63 ppm. MS (EI): m/z = 347.1426 [M]+.
General procedure for the Suzuki cross-coupling of DPBA with
aryl bromides
6-(4-(Diphenylamino)phenyl)picolinaldehyde (12)
1
Yield: 92%; yellow solid, mp 139–140 uC. H NMR (400 MHz,
A mixture of aryl bromide (0.25 mmol), DPBA (0.375 mmol),
K2CO3 (0.5 mmol), Pd(OAc)2 (0.5 mol%), ethanol (3 mL) and
distilled water (1 mL) was stirred at room temperature in air
for indicated time. The reaction mixture was added to brine
(15 mL) and extracted four times with ethyl acetate (4 6 15
mL). The solvent was concentrated under vacuum, and the
product was isolated by short-column chromatography on
silica gel (200–300 mesh).
CDCl3, 25 uC) d = 10.14 (s, 1H), 7.97–7.94 (m, 2H), 7.89–7.83
(m, 3H), 7.31–7.27 (m, 4H), 7.19–7.14 (m, 6H), 7.10–7.06 (m,
2H) ppm. 13C NMR (100 MHz, CDCl3, 25 uC) d = 194.08, 157.60,
152.70, 149.40, 147.31, 137.65, 131.49, 129.43, 127.87, 125.01,
123.75, 123.59, 122.85, 119.06 ppm. MS (EI): m/z = 350.1421
[M]+.
N,N-Diphenyl-4-(quinolin-8-yl)aniline (19)
1
Yield: 41%; white solid, mp 152–153 uC. H NMR (400 MHz,
4-(4-Methylpyridin-2-yl)-N,N-diphenylaniline (2)
CDCl3, 25 uC) d = 8.97–8.96 (m, 1H), 8.21–8.18 (dd, J = 7.6 Hz,
1.6 Hz, 1H), 7.80–7.74 (m, 2H), 7.64 (d, J = 8.0 Hz, 2H), 7.59 (d,
J = 7.6 Hz, 1H), 7.43–7.40 (m, 1H), 7.30–7.26 (m, 4H), 7.21–7.17
(m, 6H), 7.03 (d, J = 8.4 Hz, 2H) ppm. 13C NMR (100 MHz,
CDCl3, 25 uC) d = 150.08, 147.81, 147.05, 146.04, 140.34,
136.36, 133.35, 131.47, 130.14, 129.25, 128.85, 127.15, 126.37,
124.73, 122.92, 122.84, 120.93 ppm. MS (EI): m/z = 372.1631
[M]+.
Yield: 96%; colorless oil. 1H NMR (400 MHz, CDCl3, 25 uC) d =
8.50 (d, J = 2.4 Hz, 1H), 7.84 (d, J = 7.6 Hz, 2H), 7.49 (s, 1H),
7.28–7.25 (m, 4H), 7.14–7.12 (m, 6H), 7.06–7.02 (m, 3H), 2.40
(s, 3H) ppm. 13C NMR (100 MHz, CDCl3, 25 uC) d = 157.16,
149.50, 148.72, 147.76, 147.70, 133.51, 129.46, 127.91, 124.83,
123.49, 123.31, 122.73, 121.04, 21.47 ppm. MS (EI): m/z =
336.1632 [M]+.
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Acknowledgements
The authors thank the financial support from the National
Natural Science Foundation of China (21276043, 21076034,
20836002), the Fundamental Research Funds for the Central
Universities (DUT11LK15), and the Ministry of Education (the
Program for New Century Excellent Talents in University).
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