Communication
DOI: 10.1002/bkcs.11018
BULLETIN OF THE
A. Taher et al.
KOREAN CHEMICAL SOCIETY
Microwave-accelerated Suzuki Coupling Reaction Catalyzed
by Heterogeneous NHC-Pd
*
Abu Taher, Hye-Young Yoo, Phil-In Oh, Sanggon Moon, and Myung-Jong Jin
Department of Chemistry and Chemical Engineering, Inha University, Incheon 402-751, Korea.
*E-mail: mjjin@inha.ac.kr
Received May 19, 2016, Accepted October 31, 2016
Keywords: Microwave irradiation, Heterogeneous catalysis, Palladium, Suzuki reaction
The Suzuki coupling of aryl halides with arylboronic acids is
one of the most useful palladium-catalyzed cross-coupling reac-
tions for the preparation of substituted biphenyls which are
important compounds as precursors to pharmaceuticals, ligands,
and polymers.1 Despite the great efficiency of homogeneous
palladium catalysts, there are some practical problems in their
separation and reuse after completion of the reaction.2 Heteroge-
neous catalysis seems to be an attractive solution to the pro-
blems because the catalysts can be easily separated from the
reaction mixture and reused continuously.3 Therefore, successful
development of homogeneous catalysts has been often followed
by attempts to heterogenize the catalysts.4 Although considera-
ble efforts have been devoted to the development of efficient
heterogeneous catalysts, most of the catalysts suffer from lower
catalytic activities than homogeneous analogs. In order to over-
come this barrier, microwave irradiation can be used as an effec-
tive tool in the catalysis. Microwave irradiation heating has
several advantages over conventional radiant heating, such as
high heating rate, uniform heating, and less energy requirement,
resulting in a rapid temperature rise of the entire reaction mix-
ture.5 Therefore, the microwave irradiation has emerged as a
powerful technique towards sustainable chemistry because it
offers efficient, safe, and clean eco-friendly processes.6
We have previously reported that silica gel-supported N-
heterocyclic carbene (NHC)-Pd complex 1 can serve as an
active catalyst for the Suzuki reaction.7 The Pd complex
(Pd loading: 0.34 mmol/g) was obtained from immobilization
of 1-(3-triethoxysilylpropyl)-3-methylimidazolium chloride on
silica gel, followed by reaction of Pd(OAc)2 (Scheme 1). It
would be of interest to explore the heterogeneous catalysis
under microwave irradiation. Herein, we describe the micro-
wave irradiation-accelerated Suzuki reaction mediated by heter-
ogeneous Pd complex 1.
whereas the usual thermal heating gave very poor yield for
24 h (Eq. (2)).
1 (0.1 mol %)
Me
+
Me
X
(HO)2B
(1:1)
DMF-H2O
K2CO 85 o
C
3,
GC yield
99
93 %
X = Br, 0.1 Pd mol%, 85 o
C,
micro:
thermal:
2 min
40 min
(1)
(2)
%
X = Cl, 0.1 Pd mol%, 115 oC, micro:
thermal:
6
80
%
min
<
24 h
5 %
We performed the reaction of various aryl halides with
phenylboronic acid in order to investigate the scope and gen-
erality of this process (Table 1).8 As expected, the reactions
of bromobenzene, electron-poor 4-bromoacetoph-enone, 1-
bromo-4-nitrobenzene, and 1-bromo-3-nitrobenzene were
completed within 2 min at 85ꢀC (entries 1–4). Electron-rich
2-bromotoluene, 2-bromoanisole, and 4-bromoanisole were
also coupled very well (entries 5–7) in excellent yields.
Besides, this catalysis worked well in the coupling with dif-
ferent arylboronic acids under the same conditions. (entries
8 and 9). It should be noted that sterically hindered 2-bromo-
1,3-dimethylbenzene could undergo the reaction to give a
similar result, although somewhat high temperature was
required (entry 10). Catalyst 1 showed high activities for all
of aryl bromides no matter if there are electronically neutral,
electron-donating or electron-withdrawing substituents. From
an economic point of view, the use of aryl chlorides is highly
desirable compared with aryl iodides and bromides. There-
fore, we tested the coupling of several aryl chlorides at 120ꢀC.
Chlorobenzene, deactivated 2-chlorotoluene and 4-
chloroanisole were successfully coupled with phenylboronic
acid (entries 11–13). In contrast, 1-chloro-4-nitrobenzene pos-
sessing electron-withdrawing group as well as 4-chlorotoluene
was slowly activated under thermal heating.7 We next
extended the scope of this system to the coupling of heteroaryl
halides. It is known that the reaction is more difficult because
In the initial studies, we attempted the Suzuki coupling of 4-
bromotoluene with phenylboronic acid under conventional and
microwave irradiation, respectively. The reactions were con-
ducted in the presence of 0.1 mol % 1 at 85ꢀC. By employing
microwave irradiation, the reaction time was shortened from
40 min to 2 min in reaching almost full conversion (eq. 1). It is
well-known that aryl chlorides are much more difficult to acti-
vate than other substrates. Remarkably, the coupling of phenyl-
boronic acid with unreactive 4-chlorotoluene proceeded faster
and achieved high yield upon 6 min microwave irradiation,
Scheme 1. (i) silica gel, toluene, reflux, 12 h; (ii) Pd(OAc)2,
DMSO, 60ꢀC, 4 h.
Bull. Korean Chem. Soc. 2016
© 2016 Korean Chemical Society, Seoul & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Wiley Online Library
1
Taher, Abu
