Efficient Suzuki reaction catalyzed by recyclable clay carbapalladacycle nanocomposite in ionic liquid media

Article abstract of DOI:10.1016/j.tetlet.2015.01.055

In this Letter, we report the efficient recyclability and recoverability of ionic tagged carbapalladacycle and its hybrid MMT clay-nanocomposite catalyst in ionic liquid media [TMBA]NTf₂ for the Suzuki reaction. It has several distinct advantages which include the use of low levels of catalyst concentration, formation of desired products in high yields and good selectivity using chloro, bromo and iodo substituted aryl halides, and negligible formation of homo coupling products.

Full text of DOI:10.1016/j.tetlet.2015.01.055

Tetrahedron Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Efficient Suzuki reaction catalyzed by recyclable clay
carbapalladacycle nanocomposite in ionic liquid media
⇑
Amanpreet Kaur, Vasundhara Singh
Department of Applied Sciences (Chemistry), PEC University of Technology, Chandigarh, India
a r t i c l e i n f o
a b s t r a c t
Article history:
In this Letter, we report the efficient recyclability and recoverability of ionic tagged carbapalladacycle and
its hybrid MMT clay-nanocomposite catalyst in ionic liquid media [TMBA]NTf2 for the Suzuki reaction. It
has several distinct advantages which include the use of low levels of catalyst concentration, formation of
desired products in high yields and good selectivity using chloro, bromo and iodo substituted aryl halides,
and negligible formation of homo coupling products.
Received 28 November 2014
Revised 6 January 2015
Accepted 7 January 2015
Available online xxxx
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Carbapalladacycle
Na+-MMT clay
Organic–inorganic hybrid material
Suzuki reaction
Ionic liquid
1. Introduction
mention a few. Among these, Montmorillonite clay, an inexpensive
and naturally occurring material, can act as an excellent host to
Palladium catalyzed Suzuki cross coupling reaction of aryl
halides with aryl boronic acids or esters is an important carbon–
carbon bond formation reaction for the synthesis of substituted
biaryls.1–4 The substituted biaryl moiety is present in many phar-
maceutically active compounds, herbicides, polymers, new materi-
als, liquid crystals and ligands5 to mention a few. Over the decades,
several homogenous palladium based catalysts have been devel-
oped and reported in the literature to promote the Suzuki reac-
tion.6,7 Among these, Najera et al.8 have developed oxime based
palladacycle catalysts having unique properties of being thermally
robust, air and moisture insensitive and avoid the use of phosphine
ligands which get oxidized during the reaction. However, despite
the various advantages associated with these catalysts, difficulties
are encountered with regard to their separation and recoverability
in the reaction process.
Further, based on the preference of the chemical industry and in
the context of green chemistry, the heterogenization of an active
homogenous catalyst is of interest for improved recyclability and
recoverability, wherein the catalyst has both an expensive metal
and ligand incorporated in it.9 In the past, this heterogenization
has been achieved by covalent or ionic immobilization on inor-
ganic support materials such as clays,10 silica,11 zeolites,12 poly-
mers13 and most recently, using ionic14,15 and florous tags16,17 to
active homogenous catalysts by ion exchange due to its smectite
structure and two dimensional lamellar form.18 This ionic immobi-
lization results in the formation of a stable organic–inorganic
hybrid catalytic system, particularly for metal and metal–ligand
complex based catalysts.19
The reaction medium used in any reaction is also of great signif-
icance and in particular, the use of benign reaction medium such as
water20,21 and ionic liquids22,23 has proven to be excellent substi-
tutes to the previously used high boiling solvents, such as DMF
in the Suzuki reaction.24
In continuation to our ongoing research program on the devel-
opment of new recyclable and recoverable heterogeneous catalysts
based on ionic liquids25 and solid supports,26,27 we had previously
developed a clay based carbapalladacycle nanocomposite catalyst
and successfully carried out the Heck and Sonogashira reaction.28
Encouraged by our previous results, we have expanded the appli-
cation of ammonium tagged oxime carbapalladacycle (1) and its
clay nanocomposite (2) as shown in Figure 1, for executing Suzuki
reaction in ionic liquid media. The reaction was also tried in aque-
ous medium but lower yields (65% for model reaction) were
obtained. Recyclability of the catalyst has also been carried out
upto seven cycles.
The IR, NMR, and mass spectral data of catalysts (1) and (2)
were comparable with those reported in the literature.
The optimization of reaction conditions for model Suzuki reac-
tion as shown in Scheme 1, has been done with respect to catalyst
⇑
Corresponding author. Tel.: +91 9888222214.
E-mail address: vasun7@yahoo.co.in (V. Singh).
http://dx.doi.org/10.1016/j.tetlet.2015.01.055
0040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
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A. Kaur, V. Singh / Tetrahedron Letters xxx (2015) xxx–xxx
Figure 1. Ammonium tagged oxime carbapalladacycle (1) and its clay nanocomposite (2).
OCH3
Cl
Cl
Catalyst (1/2)
2 equiv K2CO3
B(OH)2
OCH3
+
[TMBA]NTf2
80oC
I
Scheme 1. General scheme for the model Suzuki reaction using catalyst (1) and (2).
concentration, temperature, reaction time, base and ionic liquid/
aqueous media used in the reaction.
Further, it is known that nature of base is also a very important
factor for determining the efficiency of the Suzuki cross-coupling
reaction.29–31 Therefore, the influence of various organic and inor-
ganic bases was investigated for Suzuki coupling reaction of 4-
iodoanisole and 3-chlorophenylboronic acid in [TMBA]NTf2 and
the results are summarized in Table 2. The results show that Na2-
CO3 (Table 2, entry 1) is the best choice as compared to the other
bases.
We also explored the possibility of carrying out the model
Suzuki reaction with catalyst (1) in water as the reaction media
under optimized reaction conditions. The desired biaryl product
was obtained in a lower GC–MS yield of 65.87% and the recover-
ability of the catalyst from the aqueous media also posed a
problem.
Under the optimized conditions, as determined for catalysts (1)
(Pd content 2.39 mmol/1 g of clay as determined by AAS), the reac-
tion was also carried out with catalyst (2) (Pd content 1.178 mmol/
1 g of clay as determined by AAS) and both the catalysts were fur-
ther employed for the Suzuki reaction with various phenylboronic
acids using 4-iodoanisole and 4-bromoanisole as illustrated in
Initial assessment of the catalyst (1) was done by optimization
of catalyst loading, time, and temperature of reaction between 4-
iodoanisole and 3-chlorophenylboronic acid as the model reaction
with Na2CO3 and [TMBA]NTf2 [tetramethyl butyl ammonium bis
(trifluoromethanesulfonyl) imide] as the base and solvent respec-
tively as shown in Table 1. In the reaction carried out over a time
period of 12 h, the catalyst concentration was varied from
0.05 mol % to 0.006 mol % (entries 1–4). The lowest concentration
of catalyst at which maximum yield was obtained was found to
be 0.012 mol % (entry 3). The effect of time of reaction was also
studied by reducing it to 6 h for catalyst concentration of
0.025 mol % (entry 5) and 0.012 mol % (entry 6), but a lower yield
of 73% and 55%, respectively of the desired product was obtained.
Further lowering of reaction temperature to 60 °C resulted in a
decrease in yield to 62% (entry 7). Accordingly, a precatalyst load-
ing of 0.012 mol % of Pd was used at 80 °C for 12 h for our subse-
quent experiments as the optimized reaction conditions.
Table 1
Table 2
Effect of catalyst loading, time, and temperature on Suzuki reactiona
Effect of base on Suzuki reactiona
Entry (mol % of Pd)
Catalyst
Time (h)
Temp (°C)
Yieldb (%)
Entry
Base
Yieldb (%)
1
2
3
4
5
6
7
0.05
12
12
12
12
6
80
80
80
80
80
80
60
98
97
96
47
73
55
62
1
2
3
4
5
6
Na2CO3
96
30
67
44
75
83
0.025
0.012
0.006
0.025
0.012
0.012
CH3COONaÁ3H2O
Et3N
Morpholine
K2CO3
Na3PO4Á12H2O
6
12
a
Reaction condition: 4-iodoanisole (0.1 g, 1 mmol), 3-chlorophenylboronic acid
a
Reaction condition: 4-iodoanisole (0.1 g, 1 mmol), 3-chlorophenylboronic acid
(0.1 g, 1.5 mmol), catalyst (1) (0.012 mol % of Pd), Base (2 mmol), TMBA]NTf2
(0.1 g, 1.5 mmol), catalyst (1), Na2CO3 (2 mmol), TMBA]NTf2 (0.5 ml).
(0.5 ml), 80 °C, 12 h.
b
b
Isolated yield.
Isolated yield.
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A. Kaur, V. Singh / Tetrahedron Letters xxx (2015) xxx–xxx
3
Table 3
Optimized results of Suzuki reaction with 4-iodoanisole/4-bromoanisole at 80 °C in 12 h using catalyst (1) and (2) in [TMBA] NTf2
a
Entry
1
Boronic acid
Catalyst
Yieldb (%)
TON
TOF (hÀ1
)
3-Chlorophenylboronic acid
(1)
(2)
(1)
(2)
(1)
(2)
(1)
(2)
96/84
95/83
92/82
93/80
87/89
89/89
90/84
91/83
8000/7000
7916/6916
7666/6833
7750/6666
7250/7416
7416/7416
7500/7000
7583/6916
667/583
660/576
639/569
646/556
604/618
618/618
625/583
632/576
2
3
4
3-Nitrophenylboronic acid
4-Methoxyphenylboronic acid
m-Tolyphenylboronic acid
a
Reaction condition: 4-iodoanisole/4-bromoanisole (0.1 g, 1 mmol), boronic acid (1.5 mmol), catalyst (0.012 mol % of Pd), Na2CO3 (2 mmol), TMBA]NTf2 (0.5 ml), 80 °C,
12 h;
b
Isolated yield.
Table 3. The yield of products with 4-bromoanisole was slightly
lower and in the range of 80 to 89%. GC–MS analysis of the product
mixture indicated the absence of formation of Glaser type homo-
coupling products.
The Suzuki reaction was extended to the coupling of chloroben-
zene with various phenylboronic acids as shown in Table 4. How-
ever, the Suzuki coupling of chlorobenzene was difficult to proceed
under optimized conditions and gave very poor yield of the desired
products. No change in yield was observed even after increasing
the time of reaction to 18 h, temperature to 100 °C and catalyst
concentration to 0.025 mol % of Pd.
The absence of catalytically active species in the solution lea-
ched from the solid was studied for the model reaction using the
hot filtration method by filtering the reaction mixture when the
reaction had proceeded to nearly 50% completion. The clear filtrate
solution was allowed to react for an additional 48 h and the iso-
lated product was analyzed by GC–MS analysis. No further pro-
gress of reaction was observed, which confirmed the absence of
leaching of palladium.
Figure 2. Recycling experiments of Suzuki reaction of 4-iodoanisole and 3-
chlorophenylboronic acid.
For recycling studies, seven consecutive reactions were carried
out with the same catalyst at the same substrate-to-catalyst ratio
on the model reaction under optimized conditions, as shown in
Scheme 1. Both catalysts exhibit decreasing activities but the
immobilized clay nanocomposite performs better even after seven
cycles for the Suzuki reaction, as there was no significant decrease
in the final conversion of the product formed. No change in selec-
tivity was noticed upto five cycles while in sixth and seventh
cycles, 2% decrease in selectivity was observed. The results for
the recycling studies are as shown in Figure 2. No precipitation
of palladium black was observed indicating the stability of the cat-
alyst (1) in ionic liquid media and catalyst (2) due to immobiliza-
tion in the interlayers of clay and in ionic liquid media.
Time(h-1
)
Figure 3. Kinetic curves of Suzuki reaction of 4-iodoanisole and 3-chlorophenyl-
boronic acid.
heterogeneous phases having the unique properties of being air
and moisture insensitive, thermally stable, are used in low concen-
tration, and are both recyclable and recoverable in nature. The
ionic liquid used as reaction medium was critical for immobiliza-
tion of the catalysts and was recyclable also. The inert ammonium
Kinetics of the reaction was studied using catalyst (1) and (2)
under optimized conditions and the results are as shown in Figure 3
indicating comparable behavior of both the catalysts.
In conclusion, we have efficiently executed the Suzuki coupling
reaction using (1) and (2) catalysts both in homogeneous as well as
Table 4
a
Optimized results of Suzuki reaction with chlorobenzene at 80 °C using catalyst (1) and (2) in [TMBA] NTf2
Entry
1
Boronic acid
Catalyst
Time (h)
Yieldb (%)
TON
TOF (hÀ1
)
3-Chlorophenylboronic acid
(1)
(2)
(1)
(2)
(1)
(2)
(1)
(2)
12
18
12
12
12
18
12
18
60
55
24
26
58
52
51
50
5000
4483
2000
2166
4833
4333
4250
4166
417
249
167
180
403
241
354
231
2
3
4
3-Nitrophenylboronic phenylboronic
4-Methoxyphenylboronic acid
m-Tolyphenylboronic acid
a
Reaction condition: chlorobenzene (0.1 g, 1 mmol), boronic acid (1.5 mmol), catalyst (0.012 mol % of Pd), Na2CO3 (2 mmol), TMBA]NTf2 (0.5 ml), 80 °C, 12–18 h.
Isolated yield.
b
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A. Kaur, V. Singh / Tetrahedron Letters xxx (2015) xxx–xxx
tag did not interfere in the course of the reaction and neat products
were obtained in high yields and purity. Further studies for opti-
mizing reaction conditions in aqueous media using these catalysts
are underway.
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The authors are thankful to PEC University of Technology, Chan-
digarh and TEQIP II (World Bank funded project) for fellowship and
financial support, RSIC, Chandigarh and NIPER, Mohali for the ana-
lytical data.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.01.
055.
Please cite this article in press as: Kaur, A.; Singh, V. Tetrahedron Lett. (2015), http://dx.doi.org/10.1016/j.tetlet.2015.01.055