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Efficient synthesis of 4-heteroaryl-substituted triphenylamine derivatives via a ligand-free Suzuki reaction.

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Full Paper
Received: 15 August 2011
Revised: 13 September 2011
Accepted: 23 September 2011
Published online in Wiley Online Library: 29 November 2011
(wileyonlinelibrary.com) DOI 10.1002/aoc.1851
Efficient synthesis of 4-heteroaryl-substituted
triphenylamine derivatives via a ligand-free
Suzuki reaction
Chun Liu*, Yonghua Wu, Na Han and Jieshan Qiu
This paper reports an efficient synthesis of 4-heteroaryl-substituted triphenylamine derivatives via the palladium-catalyzed
Suzuki reaction of heteroaryl halides with 4-(diphenylamino)phenylboronic acid in ethylene glycol under ligand-free and aerobic conditions. These derivatives are important structural motifs for use in dye-sensitized solar cells and organic electroluminescence materials. Copyright © 2011 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: palladium; ligand-free; heteroaryl compound; Suzuki reaction; triphenylamine derivatives
Introduction
862
The synthesis of aryl-substituted compounds has attracted interest because of the importance of this pharmacophore in various
biologically active compounds[1] as well as intriguing physical
properties.[2] These compounds have found extensive use in the
synthesis of natural products, pharmaceuticals, and advanced
functional materials,[3] such as dye-sensitized solar cells (DSSCs)[4]
and organic electroluminescence materials.[5] Triphenylamine
(TPA) derivatives are an example of such compounds and have
found many important applications.[6] The combination of a
TPA unit with linear p-conjugated systems could be expected to
lead to amorphous materials with isotropic optical and chargetransport properties.[7] 4-Aryl-substituted triphenylamine derivatives have also attracted considerable attention due to their wide
range of applications in DSSCs and organic light-emitting diodes
(OLEDs). For example, Shen et al. synthesized a series of novel organic dyes based on a TPA moiety as a donor and oligothiophene
as a p-conjugated unit, which provided the highest solar energy
conversion efficiency () of up to 7.26%.[8]
Numerous approaches to the synthesis of 4-aryl-substituted
triphenylamine derivatives have been reported but these often
require long reaction times, the use of harsh conditions or toxic substrates or give low yields.[9] Therefore, it is of great interest to develop
facile synthetic methods for this important type of compounds.
Among the carbon–carbon cross-coupling reactions, the palladium-catalyzed Suzuki reaction is one of the most powerful methods for the construction of an aryl–aryl unit from aryl halides and
aryl boronic acids,[10] owing to the broad functional group tolerance and the innocuous nature of boronic acids, which are generally non-toxic and stable to air, heat and moisture.[11] Consequently,
it is possible to synthesize 4-aryl-substitued TPA derivatives via the
Suzuki reaction. Very recently, we reported a ligand-free and aerobic protocol for the synthesis of 4-aryl-substituted TPA derivatives
via the Suzuki reaction of aryl halides with 4-(diphenylamino)phenylboronic acid in aqueous i-PrOH.[12] This protocol can efficiently
activate 2-N-heteroaryl halides; however, it is less efficient for 3-N-
Appl. Organometal. Chem. 2011, 25, 862–866
heteroaryl halides. In a recently published communication, we described a general method for the construction of aryl-substituted
N-heteroarenes in ethylene glycol (EG).[13] In the present paper,
we report the synthesis of 4-aryl-substituted TPA derivatives in EG
via the palladium-catalyzed ligand-free and aerobic Suzuki reaction
of 2-, 3- or 5-N-heteroaryl halides with 4-(diphenylamino)phenyl
boronic acid.
Results and Discussion
The coupling between 2-bromopyridine and 4-(diphenylamino)
phenylboronic acid was chosen as a model reaction for optimizing the reaction conditions. Results of this preliminary survey
are shown in Table 1. In an initial experiment, we discovered that
the reaction could be performed under air thus, improving its operational simplicity. First, we explored the effect of temperature
on the model reaction (Table 1, entries 1–3). The yields of the
target product decreased to some extent when the temperature
went down. An excellent yield was obtained at 80 C (Table 1,
entry 3). Changing the pre-catalyst was also investigated (Table 1,
entries 3 and 4). Zero-valent 5% Pd/C provided a low yield of the
product. We further screened bases for the model reaction (Table 1,
entries 3, 5–7). It was clear from the results that inorganic bases
(Table 1, entries 3 and 5) were much better than organic ones
(Table 1, entries 6 and 7). Among them, K3PO47H2O is suitable for
activating this reaction system. So far, heteroaryl halides have rarely
been employed in the ligand-free Suzuki reactions.[14] However, the
Suzuki reaction between 2-bromopyridine and 4-(diphenylamino)
* Correspondence to: Chun Liu, State Key Laboratory of Fine Chemicals, Dalian
University of Technology, Linggong Road 2, Dalian 116024, People’s Republic
of China. E-mail: chunliu70@yahoo.com
State Key Laboratory of Fine Chemicals, Dalian University of Technology,
Dalian 116024, People’s Republic of China
Copyright © 2011 John Wiley & Sons, Ltd.
Efficient synthesis of 4-heteroaryl-substituted TPA derivatives
Table 1. Screening of optimum conditions for Suzuki coupling of 2-bromopyridine with 4-(diphenylamino)phenylboronic acida
Entry
Catalyst
Base
Temperature ( C)
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd/C
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
K3PO47H2O
K3PO47H2O
K3PO47H2O
K3PO47H2O
NaOH
CH3ONa
Et3N
25
50
80
80
80
80
80
30
30
30
30
30
30
30
12
51
92
16
86
36
49
a
Reaction conditions: 2-bromopyridine (0.25 mmol), 4-(diphenylamino)phenylboronic acid (0.375 mmol), base (0.5 mmol), catalyst amount (1 mol%),
ethylene glycol (2 ml), under air. The reactions were monitored by TLC.
b
Isolated yields.
Appl. Organometal. Chem. 2011, 25, 862–866
Experimental
General Information
Unless stated otherwise, all the reactions were carried out under
air. All heteroaryl halides and arylboronic acids were purchased
from Alfa Aesar, Avocado, Heysham, UK. All other chemicals were
purchased from commercial sources and used without further purification. 1H NMR spectra were recorded on a Varian Inova 400
spectrometer. 13 C NMR spectra were recorded at 100 MHz using
TMS as internal standard. Mass spectroscopic data of the products
were collected on a MS-EI instrument or a Bruker Apex IV FTMS. All
products were isolated by short chromatography on a silica gel
(200–300 mesh) column using petroleum ether (60–90 C), unless
otherwise noted. Compounds described in the literature were
characterized by 1H NMR spectra compared to reported data.
Typical Procedure for the Suzuki Reaction of N- or S-Heteroaryl
Halides with TPA
A mixture of heteroaryl halide (0.25 mmol), 4-(diphenylamino)
phenylboronic acid (0.375 mmol), Pd(OAc)2 (1.0 mol%, 0.56 mg),
K3PO47H2O (0.5 mmol, 169.2 mg) and ethylene glycol (2 ml) was
stirred at 80 C for indicated time. The mixture was added to
brine (10 ml) and extracted four times with diethyl ether
(4 10 ml). The organic solution was concentrated in vacuo and
the product was isolated by short chromatography on a silica
gel (200–300 mesh) column. NMR spectra of all products as well
as detailed descriptions of experimental procedures are available
in the Supporting Information.
4-(6-Methoxypyridin-3-yl)-N,N-diphenylaniline (Table 2, entry 10)
White solid (m.p. 108–110 C). 1H NMR (400 MHz, CDCl3): d 8.37 (d,
J = 4.0 Hz, 1H, Py), 7.76 (dd, J = 12.0, 4.0 Hz, 1H, Py), 7.40–7.37 (m,
2H, C6H4), 7.29–7.25 (m, 4H, Ar-H), 7.14–7.12 (m, 6H, Ar-H), 7.05–
7.02 (m, 2H, C6H4), 6.80 (s, J = 8.0 Hz, 1H, Py), 3.98 (s, 3H, OCH3),
ppm. 13 C NMR (100 MHz, CDCl3): d 163.3 (CPy), 147.6 (Cph), 147.3
(Cph), 144.5 (CPy), 137.1 (CPy), 131.7 (Cph), 129.7 (Cph), 129.3 (Cph),
127.3 (Cph), 124.5 (Cph), 124.0 (Cph), 123.1(CPy), 110.8 (CPy), 53.6
(OCH3), ppm. ESI-MS: 352.1581 (M)+.
N,N-Diphenyl-4-(pyrimidin-2-yl)aniline (Table 2, entry 17)
Green solid (m.p.: 168–169 C). 1H NMR (400 MHz, CDCl3): d 8.74
(d, J = 4.8 Hz, 2H, Pyr), 8.28 (d, J = 8.8 Hz, 2H, C6H4), 7.30–7.26 (m,
4H, Ar-H), 7.16–7.13 (m, 5H, C6H4, Ar-H and Pyr), 7.11–7.05 (m,
4H, Ar-H), ppm. 13 C NMR (100 MHz, CDCl3): d 164.5 (Cpyr), 157.1
Copyright © 2011 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc
863
phenylboronic acid in EG afforded the cross-coupled product in
high isolated yield in the absence of a ligand at 80 C under air.
To investigate the universality of this catalytic system, we performed the Suzuki reaction of different heteroaryl halides with 4(diphenylamino)phenyl boronic acid under optimized conditions
The results are shown in Table 2. The cross-coupling of 2-bromopyridine and TPA furnished the products in high yield in 30 min
(Table 2, entry 1), which is more efficient than that performed under nitrogen protection.[15] The electronic effect of the substituents on the 2-bromopyridine has no significant influence on the
reaction activity. Various 2-halogenated pyridines bearing electron-withdrawing groups, such as nitro, fluoro and acetyl moieties (Table 2, entries 2–5), and electron-donating groups, such
as methoxyl, methyl moieties (Table 2, entries 6–8), coupled with
4-(diphenylamino)phenylboronic acid in high yields.
Interestingly, the reaction of 3-bromopyridine provided 90%
yield in 2 h (Table 2, entry 9), which was much efficient than that
carried out in an aqueous medium.[12] The reaction of 2-methoxy5-bromopyridine performed smoothly even with a relatively low
catalyst loading (Table 2, entry 10).
Moreover, 5-bromopyrimidine and 5-bromoindole underwent
cross-coupling smoothly and afforded the desired products in
99% and 96% yields, respectively (Table 2, entries 11 and 12).
Noticeably, the reaction of 2-chloropyrazine gave the desired
coupling product in 78% yield (Table 2, entry 13). The reaction
of 2-bromoquinoline gave 89% yield in 16 min (Table 2, entry 14).
The coupling between 3-bromoquinoline and 4-(diphenylamino)
phenylboronic acid smoothly produced the biaryl in 91% yield
after 35 min (Table 2, entry 15). It is noteworthy that the coupling
reaction of 2-bromothiophene and 4-(diphenylamino)phenyl
boronic acid provided 95% yield in 30 min (Table 2, entry 16), the
product of which is widely used to construct advanced functional
materials, such as DSSCs and sensing devices. The present method
is more efficient than the previous reports.[16] On the other hand,
the electronic effects of the nitrogen atoms of 2-bromopyrimidine
adversely affected the reaction. The coupling reaction of 2bromopyrimidine with 4-(diphenylamino)phenylboronic acid gave
rise to a 55% yield in the presence of 2.0 mol% Pd(OAc)2 for 1.5 h
(Table 2, entry 17).
In conclusion, we have developed a general, simple and highly
efficient method for the Pd(OAc)2-catalyzed Suzuki reaction of
N- or S-heteroaryl halides with 4-(diphenylamino)phenylboronic
acid for the synthesis of 4-aryl-substituted TPA derivatives. The
formation of desired products proceeded well under ligand-free
and aerobic conditions with high efficiency and good functional
group tolerance.
C. Liu et al.
Table 2. Suzuki reaction of heteroaryl halides with 4-(diphenylamino)phenylboronic acida
Entry
Heteroaryl-X
Product
Time (min)
Yieldb (%)
Reference
1
30
92
9d
2
60
86
12
3
50
88
12
4
30
95c
12
5
15
95
12
6
5
84
12
7
120
80
12
8
40
92
12
9
120
90
9d
10
12
89d
11
30
99
12
12
40
96
12
13
75
78
12
864
wileyonlinelibrary.com/journal/aoc
Copyright © 2011 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2011, 25, 862–866
Efficient synthesis of 4-heteroaryl-substituted TPA derivatives
Table 2. (Continued)
Time (min)
Yieldb (%)
Reference
14
16
89
12
15
35
91
12
16
30
95
16(a)
17
90
55c
Entry
Heteroaryl-X
Product
Reaction conditions: heteroaryl halides (0.25 mmol), 4-(diphenylamino)phenylboronic acid (0.375 mmol), Pd(OAc)2 (1.0 mol%), K3PO47H2O
(0.5 mmol), ethylene glycol (2 ml), 80 C, under air. The reaction was monitored by TLC.
b
Isolated yields.
c
Pd(OAc)2 (2.0 mol%).
d
Pd(OAc)2 (0.2 mol% ).
a
(Cpyr), 150.3 (Cph), 147.3 (Cph), 130.9 (Cph), 129.4 (Cph), 129.2 (Cph),
125.2 (Cph), 123.6 (Cph), 122.1 (Cph), 118.2 (Cpyr), ppm. ESI-MS:
324.1491 (M + H)+.
SUPPORTING INFORMATION
Supporting information may be found in the online version of
this article.
Acknowledgments
The authors thank the National Natural Science Foundation of
China (21076034, 20923006), the Fundamental Research Funds
for the Central Universities (DUT11LK15), and the Ministry of
Education (Program for New Century Excellent Talents in University) for financial support.
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Appl. Organometal. Chem. 2011, 25, 862–866
Copyright © 2011 John Wiley & Sons, Ltd.
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