Microwave-assisted Suzuki coupling on a KF-alumina surface: Synthesis of polyaryls

Article abstract of DOI:10.1016/S0040-4039(03)00731-7

A range of conjugated polyaryls has been synthesized through one-pot microwave assisted palladium-catalyzed consecutive Suzuki coupling reactions on a KF-alumina surface with notable features including rapid reaction times, solvent-free conditions, high yields, atom economic and air-insensitive reactions.

Full text of DOI:10.1016/S0040-4039(03)00731-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 3817–3820
Microwave-assisted Suzuki coupling on a KF–alumina surface:
synthesis of polyaryls
Basudeb Basu,* Pralay Das, Md. Mosharef H. Bhuiyan and Satadru Jha
Department of Chemistry, University of North Bengal, Darjeeling 734 430, India
Received 6 January 2003; revised 5 March 2003; accepted 14 March 2003
Abstract—A range of conjugated polyaryls has been synthesized through one-pot microwave assisted palladium-catalyzed
consecutive Suzuki coupling reactions on a KF–alumina surface with notable features including rapid reaction times, solvent-free
conditions, high yields, atom economic and air-insensitive reactions. © 2003 Elsevier Science Ltd. All rights reserved.
Biaryls and higher homologues are an important class
of conjugated polyaromatic compounds, originating
from benzene as unique building blocks. On the one
hand, multinuclear aromatics develop the concept of
aromaticity and supramolecular benzene chemistry,1
and on the other hand, lead to wide applications in
material and biological sciences.2 Polyaryls find several
applications as liquid crystals,2a laser-dyes2b and con-
ducting polymers.2c,d For example, the terphenyls (o-,
m-, p-isomers) are used industrially as heat storage and
transfer agents and as textile dye carriers whilst the
p-isomer has found application as a laser dye. 9,10-
Diphenylanthracene is used as a fluorescer in a peroxy-
oxalate chemiluminescence system.3 In addition, biaryls
and higher homologues are often present as subunits in
numerous biologically active natural products, pharma-
ceuticals and agrochemicals.4 The synthesis of polyaro-
matic hydrocarbons (PAHs) and their derivatives has
therefore been of considerable interest to organic and
physical chemists for many years.
the methods are not being recognized from the view-
point of green chemistry. Recent literature reports
confirm that palladium-catalyzed Suzuki coupling reac-
tions of aryl halides with aryl boronic acids (or esters)
have become convenient and widely used synthetic
methods for regioselective arylꢀaryl bond formation.5,6
The reasons are manifold: ease of access of the
organoboronic species, broad range of functional group
tolerance, low toxicity of the inorganic residues, etc.
However, although the Suzuki coupling reaction is one
of the most useful methods yet developed for the syn-
theses of both symmetrical and unsymmetrical biaryls,
there are still improvements and further applications
that could be made to render it more effective. Surpris-
ingly, little attempt has been made to investigate con-
secutive cross-couplings in a one-pot reaction. A few
sporadic examples of bis-Suzuki couplings are known in
the literature5e,7 and therefore much new research
should be extended towards the scope and the amelio-
ration of the economical and ecological parameters of
the Suzuki reaction. We envisaged that a multi-Suzuki
coupling of di-, tri- or tetra-haloaromatics with aryl
boronic acids could lead to terphenyls and higher
homologues in a one-pot reaction. Kabalka et al.8a and
Villemin et al.8b have reported that Suzuki cross-cou-
pling reactions can be efficiently performed on the
surface of KF–alumina with microwave irradiation. In
conjunction with our interest in hetero cross-coupling
reactions on KF–alumina surfaces,8c we investigated
microwave-assisted palladium-catalyzed coupling of
polyhaloaromatics with arylboronic acids on KF–alu-
mina and other inorganic oxides. In this letter, we
report our results, which expand the scope of the
Suzuki reaction on a solvent-free inorganic surface
A variety of methods are available for the synthesis of
biaryls via coupling of benzene rings promoted by
different reagents.5 The transition metal-catalyzed
cross-coupling
of
Grignard
reagents
with
dihalobenzenes5b,c and other methods5d produce the
desired terphenyls in poor yields. However, most of the
classical methods are often complicated by harsh reac-
tion conditions, lack of selectivity and generality, or the
requirement of expensive reagents. Moreover, many of
Keywords: polyaromatic hydrocarbons; Suzuki coupling; palladium;
KF–alumina; microwave assisted reactions.
leading to the synthesis of
polyaromatics.
a large variety of
* Corresponding author: Fax: +91-353-2581546; e-mail: basu
nbu@indiatimes.com
–
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)00731-7

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B. Basu et al. / Tetrahedron Letters 44 (2003) 3817–3820
Table 1. Solid-supported [KF–Al2O3] polyarylations of polyhaloaromatics with phenyl boronic acid using catalytic Pd(OAc)2
under microwave irradiation

B. Basu et al. / Tetrahedron Letters 44 (2003) 3817–3820
3819
In summary, we have demonstrated that it is possible
to perform palladium-catalyzed Suzuki couplings of
polyhaloaromatics with phenyl boronic acid on a sur-
face of KF–alumina with the aid of microwave irradi-
ation from a domestic microwave oven. The method is
fast, operationally simple, and allows rapid access to a
variety of polyaromatic hydrocarbons.
We initially focused our attention on the cross-cou-
pling of dibromoaromatics with phenylboronic acids
on KF– alumina using microwave irradiation. As can
be seen from the results presented in Table 1, the
dibromobenzenes afforded the corresponding ter-
phenyls as the major products (entries 1–3). Varying
amounts of mono-substituted biaryls (15–20%) were
detected on TLC, which could be easily separated
from the terphenyls by column chromatography.
While comparing the reaction rate between dibromo-
and diiodoarenes (entries 2 and 4), the latter afforded
the corresponding terphenyl 4 in a slightly better yield.
However, different conditions were required to accom-
plish the desired bis-coupling. 2,4-Dibromoanisole
(entry 5) and 4-bromo-2-iodoanisole (entry 6) afforded
the terphenyl 9 in almost comparable yields. Thus, no
significant reactivity difference between bromo- and
iodo-substituents was observed. Among the other
dibromoarenes, 2,6-dibromopyridine (entry 7) and
9,10-dibromoanthracene (entry 10) yielded the desired
products 12 and 18, respectively, in excellent yields.
The biphenyl systems (entries 8 and 9) produced the
corresponding products 14 and 16 in yields of 50–60%.
In the case of tribromoaromatics (entries 11–13), the
corresponding tris-coupled compounds 20, 22 and 24
were isolated as the major products (Table 1). Small
amounts of mono- or bis-coupled products were
detected by TLC. The 1,4-dibromo-2,5-diiodobenzene
25 yielded the tetraphenylbenzene 26 in 55% yield
(entry 14). Villemin and co-workers obtained their best
results on Suzuki couplings with aryl iodides using
mono-mode microwave irradiation.8b Our conditions
using KF–alumina (1:4) and irradiation from a domes-
tic microwave oven, however, enabled polyarylations
in good to excellent yields. We also examined such
poly-Suzuki couplings on other inorganic surfaces
(Table 2). The results were best on the surface of
KF–alumina (1:4). The reactions were carried out in
air. All the reactants were intimately mixed with the
inorganic surface before being placed in a domestic
microwave oven (Kenstar; Model OM-9925E) and
irradiated at the appropriate power (W) and time
(Table 1). In many Suzuki cross-couplings, a crucial
role is played by the ligands,9 which complex with
palladium salts. The above solvent-free conditions,
however, required no such ancillary ligands, which is
advantageous in view of atom economy in the reac-
tion.
Representative procedure for a multi-Suzuki coupling:
1,3-Dibromobenzene 3 (236 mg, 1.0 mmol), phenyl
boronic acid (305 mg, 2.5 mmol) and palladium ace-
tate (10 mg, 0.04 mmol) were intimately mixed with
1.5 g of KF–alumina (prepared according to Ref. 8c)
and the mixture was irradiated in
a domestic
microwave oven at 80 W for 15 min. The solid mix-
ture was then placed on a silica gel column and eluted
with petroleum ether:EtOAc (99:1) to furnish m-ter-
1
phenyl 4 (190 mg, 83%); mp 87–88°C (lit.10 89°C); H
NMR (CDCl3, 400 MHz): l 7.30–7.84 (m, 14H); 13C
NMR (CDCl3, 100 MHz): l 126.1, 127.2, 127.4, 128.8,
129.2, 141.1, 141.7.
Acknowledgements
We gratefully acknowledge financial assistance from
the Department of Science and Technology, New
Delhi (Grant No. SP/S1/G13/97). P.D. is a Junior
Research Fellow under this project.
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Entry Substrate (A)
Surface (B)
Conditions
Yield
(%)
1
2
3
4
1,3-Dibromobenzene KF–Al2O3 (1:4) 80 W/15 min 83
1,3-Dibromobenzene Al2O3
1,3-Dibromobenzene MgO
1,3-Dibromobenzene MgO–K2CO3
(3:2)
80 W/15 min 20–25
80 W/15 min 25
80 W/15 min 30

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