An efficient aqueous microwave-assisted Suzuki-Miyaura cross-coupling reaction in the thiazole series

Article abstract of DOI:10.1039/b916123f

A new simple, rapid and high yielding synthesis of various 5-substituted thiazoles by Suzuki-Miyaura cross-coupling reaction is described using microwave irradiation in aqueous medium without organic co-solvent in the presence of tetrabutylammonium bromid

Full text of DOI:10.1039/b916123f

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www.rsc.org/greenchem | Green Chemistry
An efficient aqueous microwave-assisted Suzuki–Miyaura cross-coupling
reaction in the thiazole series†
Anita Cohen, Maxime D. Crozet, Pascal Rathelot and Patrice Vanelle*
Received 25th May 2009, Accepted 10th August 2009
First published as an Advance Article on the web 3rd September 2009
DOI: 10.1039/b916123f
A new simple, rapid and high yielding synthesis of various
5-substituted thiazoles by Suzuki–Miyaura cross-coupling
reaction is described using microwave irradiation in aque-
ous medium without organic co-solvent in the presence of
tetrabutylammonium bromide.
but surprisingly very few investigations were conducted in the
thiazole series.
The thiazole nucleus is commonly found in the chemical
structure of natural compounds, such as the coenzyme derived
from vitamin B1 (thiamin), and various synthetic compounds
exhibiting a wide variety of biological activity,16 as gastric anti-
secretory agents (famotidin, nizatidin) or anti-infectious agents
(ritonavir, nitazoxanide, tenonitrozole, aminitrozole, etc.), while
others have found applications as liquid crystals17 and cosmetic
sunscreens.18
The classical method for the synthesis of thiazoles is the
Hantzsch process.19 This method gives excellent yields for
simple thiazoles; however, for some substituted examples low
yields have been reported.18,20 In order to synthesize aryl-
substituted thiazoles, only few examples are described, partic-
ularly using standard conditions of heating in pure organic
solvent (toluene,21,22 DME,9,23,24 xylene,24 dioxane,25 THF6),
or blended with an hydrophilic co-solvent (DME/water,26,27
benzene/methanol6,28). These processes afforded the synthesis
of thiazoles especially 2-aryl-substituted with various yields
(from 17% to 92% according to the different reported examples).
Indeed, thanks to the most electron-deficient 2-position, stud-
ies report regiocontrolled Suzuki–Miyaura reaction and thus
show that the C-2 position is the most reactive one.6,21,24,26–29
Nevertheless, cross-coupling in the C-5 position is also possible
under standard Suzuki–Miyaura conditions but with prolonged
reaction times and an excess of arylboronic acid.21
Introduction
Nowadays, the development of the field of green chemistry
through organic reactions conducted in water has become one
of the most exciting research endeavours for organic chemists in
order to prepare new biologically active molecules.1 Therefore,
within the last years, many faster, cleaner and eco-friendlier
chemical processes have been developed.2,3,4
Two substantial areas of current green research are the use
of water as a solvent for organic synthesis in addition to
microwave irradiation for efficient heating of reaction mixtures.5
Actually, water is an attractive alternative to traditional organic
solvents because it is inexpensive, non-flammable, non-toxic,
and environmentally sustainable by alleviating the problem
of pollution by organic solvents. Furthermore, water is an
attractive solvent for metal-catalyzed reactions, such as Suzuki–
Miyaura couplings.6 The Suzuki reaction has gained prominence
in recent years7 because of the availability of functionally
substituted boronic acids which are environmentally safer than
most other organometallics and the importance of biaryl or aryl-
heteroaryl pharmacophores in a variety of biologically active
molecules (losartan8 or many other compounds described as
potential antiemetics9 or antimalarials10 for example).
Thus, we report herein a practical, inexpensive, rapid
and green microwave-promoted method for the preparation
of 4-arylsulfonyl-5-aryl-2-methylthiazole and 4-arylsulfonyl-5-
styryl-2-methylthiazole derivatives in water.
Nevertheless, only a few examples of Suzuki–Miyaura cross-
coupling reactions using water in conjunction with microwave
irradiation are reported in the literature.2,11,12
In view of our general interest in microwave-assisted organic
reactions12,13 and cleaner chemical processes in order to synthe-
size new potentially bioactive compounds,14 we examined the
Suzuki–Miyaura reaction in the thiazole series. Indeed, this cou-
pling reaction has found extensive use in heterocyclic synthesis15
Results and discussion
We present here the results from our investigations concern-
ing the Suzuki–Miyaura reaction in neat water, catalyzed by
0.05 equivalent (eq) of palladium catalyst, in presence of 1 eq of
tetrabutylammonium bromide (TBAB) used as phase transfer
agent, 5 eq of Na2CO3 used as base and microwave heating,
in thiazole series substituted in the C-2 and C-4 positions
(Scheme 1).
The required starting material for the Suzuki cross-coupling
reaction is the 5-bromo-2-methyl-4-(tosylmethyl)thiazole 4
which was prepared in 59% overall yield by sequential con-
densation between 1,3-dichloroacetone with thioacetamide,30
cyclization using ZnCl2 in methanol,31 bromination with
N-bromosuccinimide (NBS) in CH3CN as proposed by Gueiffier
Laboratoire de Pharmaco-Chimie Radicalaire, LPCR, Faculte´ de
Pharmacie, Universite´s d’Aix-Marseille I, II, III-CNRS 6264:
Laboratoire Chimie Provence, 27 Boulevard Jean Moulin,
13385, Marseille Cedex 05, France.
E-mail: patrice.vanelle@pharmacie.univ-mrs.fr;
Fax: +33-4-91-79-46-77; Tel: +33-4-91-83-55-80
† Electronic supplementary information (ESI) available: Experimental
and characterisation details and crystallographic data. CCDC reference
numbers 727244. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b916123f
1736 | Green Chem., 2009, 11, 1736–1742
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Unfortunately, with 1.3 equivalent of arylboronic acid we
did not observe complete conversion of compound 4. As the
starting sulfone 4 is a relatively strong C–H acid, we decided to
confirm the position of the aryl group by X-ray crystallography
for compound 7 (Fig. 1) which gave clean crystals, in order to
verify the reactivity of the bromine atom in C-5 position and to
eliminate the possible formation of arylated sulfones as proposed
by Beletskaya.39 The other structures were assigned by analogy
and spectral comparison.
Scheme 1
in imidazo[1,2-a]-pyridine series32 and p-toluenesulfinic acid
sodium salt in water under microwave irradiation (Scheme 2).
Fig. 1 X-Ray structure of the compound 7.
Scheme 2
The last reaction which furnished compound 4 (with 92%
yield) was adapted to microwave experimental conditions in
water in order to avoid using DMSO as solvent, generally used
to synthesize arylsulfonyl derivatives33 through SRN1 experimen-
tal conditions. The 4-tosylmethyl group was chosen because
sulfones are well-known to be employed in many synthetic
methodologies34 and it can be used further for the preparation
of various 4-substituted heterocycles.35
Owing to the fact that the bromoheterocyclic sulfone 4 is
a strongly C–H acidic sulfone, its expected solubility in the
basic medium of the Suzuki–Miyaura experimental conditions
offers the opportunity to develop Suzuki–Miyaura reactions in
aqueous water.
After confirmation of the desired reactivity, our research was
directed by the appropriate arylboronic amount needed to lead
the reaction to completion.
Finally, the optimum quantity of arylboronic acid was
found to be 3.5 equivalents for complete consumption of
starting materials without any prolonged reaction times. The
excess of arylboronic acid is then recovered after column
chromatography. As recommended by Stefaniak et al. for a
green work-up procedure, we used ethyl acetate for extraction
as “preferred” solvent and ethyl acetate/cyclohexane (5/5) for
column chromatography eluent as “usable” mixture.40 In order
to optimize the method already developed in our laboratories for
imidazo[1,2-a] pyridine series,12 we tried using a lower catalyst
loading. Thus, we succeeded in reducing the loading from
10 mol% to 5 mol% without any reduction of yield (Method
A: Table 1, entries 1–8 and 11–13). However, when no reaction
was observed or poor yields were obtained, we decided to use
5 mol% of Pd(OAc)2 as catalyst (Method B: Table 1, entries
9–10 and 15–16). Indeed, Badone and co-workers reported that,
when using water, addition of Pd(OAc)2 and 1 equivalent TBAB
to the reaction mixture greatly accelerates the Suzuki reaction.37
Unfortunately, diphenylboronic acid reacted poorly under
these last conditions. Thus, we finally investigated the use of
[PPh3]2PdCl2 (Method C: Table 1, entry 14) which is known to
give good results under microwave irradiation conditions with
aryl chlorides in DMF.41 Consequently, after a 10 h reaction
time, no starting material remained by TLC, affording, after
flash chromatography, the thiazole derivative 18 in 96% yield.
Finally, in order to estimate the usefulness of the microwave
heating, we performed, as representative examples, the synthesis
of compounds 5, 18 and 19 under conventional heating. The
desired products were obtained with lower or similar yields
but with longer reaction times (compound 5: 2.5 h with 80%,
compound 18: 18 h with 82%, compound 19: 23 h with 85%).
Furthermore, based on the first reports regarding the Suzuki–
Miyaura reactions using microwave promotion, we decided to
use Pd(PPh3)4 (method A) as catalyst.36
Because of the easy formation of the anion 4, a large amount
of base was employed. The formation of sulfonyl anion in basic
medium could allow a better solubility of the reagent in water,
which would allow the cross-coupling reaction to proceed in
aqueous medium. Moreover, in order to facilitate solvation of
the organic reagents, a tetraalkylammonium salt was added as
phase transfer agent. As the Suzuki reaction involves many ionic
intermediates, tetrabutylammonium bromide is appropriate.
According to Leadbeater and co-workers, the ammonium ion
also plays a catalytic role activating the boronic acid by the
formation of a boronate complex [ArB(OH)3]-[Bu4N]+.11c,37,38
The first method which was tested to carry out cross-coupling
reaction, used 10 mol% of Pd(PPh3)4 as catalyst, 5 equivalents
of Na2CO3 as base, 1 equivalent of tetrabutylammonium
bromide (TBAB) as phase transfer agent and 1.3 equivalent
of arylboronic acid under microwave irradiation (300 W) for
1 h at 100 ◦C. The disappearance of starting materials was
monitored by TLC. No organic solvent or co-solvent were used
or investigated.
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Table 1 Microwave-mediated Suzuki coupling reaction of 5-bromo-2-methyl-4-(tosylmethyl)thiazole and arylboronic acids in water
Experimental conditions
Time [method]
Entry
1
Arylboronic acid
Product
Compound number
Yield%
97
5
1.5 h [Aa]
1 h [Aa]
1 h [Aa]
1 h [Aa]
1.5 h [Aa]
2
3
4
5
6
7
8
9
95
89
96
81
6
10
2 h [Aa]
91
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Table 1 (Contd.)
Experimental conditions
Time [method]
Entry
7
Arylboronic acid
Product
Compound number
Yield%
92
11
1 h [Aa]
8
12
1.5 h [Aa]
98
9
13
14
1 h [Bb]
84
74
10
2 h [Bb]
11
15
2.5 h [Aa]
74
12
16
2 h [Aa]
73
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Table 1 (Contd.)
Experimental conditions
Time [method]
Entry
13
Arylboronic acid
Product
Compound number
Yield%
88
17
2 h [Aa]
14
18
10 h [Cc]
96
15
16
19
20
4.5 h [Bb]
87
93
6 h [Bb]
a Method A: 5-bromo-2-methyl-4-(tosylmethyl)thiazole (1) (1 eq), arylboronic acid (3.5 eq), Pd(PPh3)4 (0.05 eq), Na2CO3 (5 eq), TBAB (1 eq.),
H2O. An initial microwave irradiation of 300 W was used, the temperature being ramped from room temperature (rt) to 100 ◦C where it was then
held for a time t. b Method B: 5-bromo-2-methyl-4-(tosylmethyl)thiazole (1) (1 eq), arylboronic acid (3.5 eq), Pd(OAc)2 (0.05 eq), Na2CO3 (5 eq),
TBAB (1 eq.), H2O. An initial microwave irradiation of 500 W was used, the temperature being ramped from rt to 100 ◦C where it was then held for a
time t. c Method C: 5-bromo-2-methyl-4-(tosylmethyl)thiazole (1) (1 eq), arylboronic acid (3.5 eq), [(C6H5)3P]2PdCl2 (0.05 eq), Na2CO3 (5 eq), TBAB
(1 eq.), H2O. An initial microwave irradiation of 500 W was used, the temperature being ramped from rt to 100 ◦C where it was then held for a time t.
The synthetic utility of this developed method could now be
exploited by a variety of substituents at the 4-position allowing
access to a large series of substituted thiazoles. Moreover,
Conclusion
these experimental conditions allowed the synthesis by reaction
with (E)-styrylboronic acid and (E)-4-methylstyrylboronic acid
compounds 19 and 20 with excellent yields, which constitutes an
interesting alternative to the Heck reaction, which is well-known
to be idiosyncratic in 5-bromothiazole series.24 The extension
of this green approach to other heterocycles and palladium-
catalyzed coupling reactions involving bioactive compounds is
in progress in our laboratory.
In summary, we have shown that by using microwave irradiation
and water as a solvent, it was possible to realize Suzuki–
Miyaura cross-couplings and synthesize a series of 5-substituted
thiazoles more efficiently, more rapidly and more ecologically
than was described in the literature under standard conditions.
Furthermore, the presence of a tosylmethyl group at the
4-position makes the 5-position less electron-deficient, and thus
leads to a poor theoretical reactivity of this last position.
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General procedure
In a 100 mL round-bottom flask were placed 5-bromo-2-methyl-
4-(tosylmethyl)thiazole (300 mg, 0.87 mmol), arylboronic or
styrylboronic acid (3.5 eq, 3.05 mmol). palladium catalyst
(0.05 eq, 4.35 mmol), tetrabutylammonium bromide (1 eq, 0.87
mmol), sodium carbonate (5 eq, 4.33 mmol) and water (20
mL). The vessel was then placed into the synthesis multimode
microwave oven cavity (ETHOS Synth Lab station) and carried
out under microwave irradiation at 100 ◦C for appropriate time
at the appropriate power. The reaction mixture was stirred
continuously during the reaction. The disappearance of starting
materials was monitored by TLC. After being cooled down, the
mixture was then extracted with EtOAc (8 ¥ 15 mL). The organic
layers were dried over anhydrous sodium sulfate and removed
under vacuum. Purification by chromatographic column, elut-
ing with EtOAc/cyclohexane (5/5), and recrystallization from
propan-2-ol gave the corresponding required product.
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For example: 2-methyl-5-phenyl-4-(tosylmethyl)thiazole 5
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Yellow needles; mp 166 ◦C (from propan-2-ol); (Found: C,
62.9; H, 5.0; N, 4.05. C18H17NO2S2 requires C, 62.9; H, 5.0; N,
4.1%). dH(200 MHz; CDCl3) 2.43 (3 H, s, CH3), 2.67 (3 H, s,
CH3), 4.51 (2 H, s, CH2), 7.22-7.36 (7 H, m, H ar), 7.62 (2 H, d,
J 8.2, H ar); dC(50 MHz, CDCl3) 18.7, 21.7, 56.0, 128.6, 128.9,
129.1, 129.5, 129.6, 136.0, 136.6, 139.4, 144.8, 165.3.
Acknowledgements
This work was supported by the CNRS and the Universities
1
of Aix-Marseille. The authors thank V. Remusat for H and
13C spectra recording, M. Giorgi for the X-ray diffraction and
structure analysis. A. Cohen thanks the Assistance Publique-
Hoˆpitaux de Marseille for hospital appointment.
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