Imidazolium-based polymer supported gadolinium triflate as a heterogeneous recyclable Lewis acid catalyst for Michael additions

Article abstract of DOI:10.1016/j.tetlet.2008.03.108

A heterogeneous Lewis acid catalytic system has been developed by incorporating gadolinium triflate on to poly[styrene-co-(1-((4-vinylphenyl)methyl)-3-methylimidazolium) tetrafluoroborate] (1-Gd(OTf)₃), and the catalytic activity of this system has been examined for Michael additions of amines and thiols to α,β-unsaturated esters and acrylonitrile. The reactions proceed in moderate to excellent yields in the presence of catalytic system 1-Gd(OTf)₃. The catalytic system could be efficiently recycled and reused.

Full text of DOI:10.1016/j.tetlet.2008.03.108

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 3466–3470
Imidazolium-based polymer supported gadolinium triflate as a
heterogeneous recyclable Lewis acid catalyst for Michael additions
Ramesh Alleti a, Woon Su Oh a, Meher Perambuduru a, C. V. Ramana b, V. Prakash Reddy a,*
a Department of Chemistry, Missouri University of Science and Technology, 142 Schrenk Hall, Rolla, MO 65409-0010, USA
b Department Metallurgical and Materials Engineering, University of Texas at El Paso, El Paso, TX 79968, USA
Received 3 January 2008; revised 19 March 2008; accepted 20 March 2008
Available online 28 March 2008
Abstract
A heterogeneous Lewis acid catalytic system has been developed by incorporating gadolinium triflate on to poly[styrene-co-(1-((4-
vinylphenyl)methyl)-3-methylimidazolium) tetrafluoroborate] (1-Gd(OTf)3), and the catalytic activity of this system has been examined
for Michael additions of amines and thiols to a,b-unsaturated esters and acrylonitrile. The reactions proceed in moderate to excellent
yields in the presence of catalytic system 1-Gd(OTf)3. The catalytic system could be efficiently recycled and reused.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Gadolinium triflate; Michael additions; Heterogeneous catalysis; Acrylonitrile; Methyl acrylate
1. Introduction
reported a polystyrene supported Lewis acid catalyst
which was useful for various organic transformations.5
They also have reported Gd(OTf)3 as a water-tolerant
Lewis acid catalyst for aldol reactions of silyl enol ethers
with aldehydes in aqueous media.6 We have recently shown
that Gd(OTf)3 can be used as a mild and recyclable reagent
for acylation of alcohols and amines.7 As part of our ongo-
ing research on Lewis acid catalyzed reactions, we now
report a novel heterogeneous catalytic system which
involves incorporation of Gd(OTf)3 onto poly[styrene-
co-(1-((4-vinylphenyl)methyl)-3-methylimidazolium) tetra-
fluoroborate] (1-Gd- (OTf)3) as an active catalyst for the
Michael additions of various aliphatic and aromatic
amines, and thiols to a,b-unsaturated esters and
acrylonitrile.
One of the main themes of contemporary synthetic
organic chemistry is the development of atom-economic
and environmentally benign catalytic systems.1 In this con-
text, the development of heterogeneous catalysis is of prime
importance not only from the economic point of view, but
also due to the easy workup procedures involved in the
separation of products from catalyst. Imidazolium-based
supported catalysis is receiving increasing attention due
to the inherent advantages of the recyclability of the cata-
lyst system from the reaction mixture.2 Aluminum chloride
complexes of supported ionic liquids based on silica
support have been used in Friedel–Crafts alkylation
and acylation reactions.3 Recently, imidazolium salts
covalently attached to polystyrene supports have been
shown to be efficient catalysts for the nucleophilic
fluorination reactions.4 Kobayashi and Nagayama
2. Results and discussion
We have initially explored the catalytic system 1-
Gd(OTf)3 for Michael additions of amines to methyl acry-
late and acrylonitrile, and compared it to that of the poly-
styrene-based catalytic system, PS-Gd(OTf)3. The Michael
addition products of methyl acrylate are potentially useful
*
Corresponding author. Tel.: +1 573 341 4768; fax: +1 573 341 6033.
E-mail address: preddy@mst.edu (V. Prakash Reddy).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.03.108

R. Alleti et al. / Tetrahedron Letters 49 (2008) 3466–3470
3467
intermediates for the preparation of b-amino esters, which
n
n
m
m
show biologically important properties and can be used as
precursors for the preparation of b-lactams.8 There are sev-
eral reported examples of the preparation of b-amino esters
by Lewis acid mediated Michael additions of amines to
a,b-unsaturated esters. However, some of these procedures
suffer from poor yields and are limited to aliphatic amines.9
Further, Michael addition of thiols to a,b-unsaturated
esters leads to b-thio esters which have important biologi-
cal applications.10
1) 1-Methylimidazole, DMF
2) NaBF4 / Acetone
BF4
+
N
Cl
2
1
N
CH3
Scheme 1. Preparation of poly[styrene-co-(1-((4-vinylphenyl)methyl)-3-
methylimidazolium) tetrafluoroborate].
Our control experiments revealed that under solvent-
free conditions, PS-Gd(OTf)3 and 1-Gd(OTf)3 gave similar
yields of the products for the Michael addition of N-methyl-
aniline to methyl acrylate. However, whereas the former
catalytic system involves a homogeneous reaction medium,
the latter provided a heterogeneous medium resulting in
relative ease in product separation from the catalytic sys-
tem. The workup of the reaction mixture using the latter
catalytic system is greatly simplified as it involves addition
of the solvent ethyl acetate, followed by filtration and
solvent evaporation. The catalyst system could thus be
recycled conveniently. Because of the ease of separation
of products from the catalyst system, and the ready recycla-
bility of the catalyst system, we have extended this reaction
to the Michael additions of a variety of secondary amines
and thiols to methyl(ethyl) acrylate, methyl methacrylate,
and acrylonitrile.
The catalytic system, 1-(Gd(OTf)3), could be recycled
and re-used for subsequent runs. Catalytic activity of the
catalyst is maintained for at least 3 cycles (Table 1, entries
1, 3, and 5). However, this catalytic system failed to give
Michael adducts with the weakly nucleophilic benzyl
carbamate, in accordance with earlier observations for
the related catalytic systems.12
In order to confirm the true heterogeneity of the cata-
lytic system (i.e., the absence of leaching of the Gd(OTf)3
into the reaction mixture), we have removed the catalyst
by decantation at 9% completion of the reaction, and let
the reaction proceed in the absence of the catalyst for
24 h, for the reaction shown in Table 1, entry 4. Analysis
of the reaction mixture after 24 h by GC/MS showed
12% completion of the reaction, indicating that there was
no leaching of the Gd(OTf)3 into the reaction mixture.
Further, we confirmed the absence of leaching of the
catalytic system by ICP-MS analysis for the Michael reac-
tion of morpholine with methyl acrylate (Table 1, entry 1),
We observed less than 0.1 ppm (corresponds to 1.513 Â
10À6 wt %) of the catalyst leaching for this reaction. How-
ever, we have observed relatively more leaching for the
reactions using acetonitrile as solvent (<600 ppm corres-
ponds to 5.68 Â 10À2 wt %), comparable with those of
the reported (polymer-micelle)-incarcerated catalytic sys-
tems.13 Based on these data, solvent-free reaction
conditions are more ideally suited for our polymer sup-
ported catalytic system. It is worth noting, however, that
some of the polymer-adsorbed Lewis acids involving AlCl3
show significant leaching of the catalyst during the
reaction.14
In summary, we have developed a novel heterogeneous
catalytic system by incorporating the Lewis acid catalyst
Gd(OTf)3 onto the imidazolium-based polymer 1, and
showed that it can be used as a convenient catalyst for
Michael additions of a variety of secondary amines to
a,b-unsaturated esters and acrylonitrile. The catalytic sys-
tem could be efficiently recycled and reused. Surface study
of the catalytic system using scanning electron-microscopy
(SEM) coupled with compositional mapping indicates that
Gd(OTf)3 is uniformly distributed on the polymer. Further
applications of the catalytic system for Lewis acid-cata-
lyzed organic reactions are currently under progress in
our laboratory.
Poly[styrene-co-4-chloromethylstyrene] (2),11 prepared
by AIBN initiated polymerization of styrene with 4-
vinylbenzyl chloride in chlorobenzene as the solvent, was
reacted with 1-methylimidazole in dimethylformamide to
obtain the poly[styrene-co-(1-((4-vinylphenyl)methyl)-3-
methylimidazolium) chloride]. Anion exchange4 of the
latter polymer using NaBF4 furnished polymer 1, which
was immobilized with catalytic amounts of Gd(OTf)3
(Scheme 1).
The 1-Gd(OTf)3 catalytic system was found to be conve-
nient for the Michael additions of a variety of amines and
thiols to a,b-unsaturated esters and acrylonitrile. The reac-
tions proceeding under mild conditions and high yields of
the adducts were obtained by simple workup procedures,
which involve separation of the catalyst system by filtration
and solvent evaporation. Most of the reactions were com-
plete in about 1 h at room temperature, using as low as
0.5 mol % of the catalyst based on Gd(OTf)3. Relatively
longer reaction times and larger amounts of catalyst (up
to 3 mol %) were required for the reactions of N-methyl-
aniline and p-methoxyaniline (Table 1, entries 4, 10, and
13). A relatively more nucleophilic primary aliphatic
amine, benzylamine, on the other hand, gave exclusively
the bis-addition product, N,N-bis[2-(methoxycarbonyl)-
ethyl]-benzylamine, as shown by GC/MS analysis.9c These
reactions are not synthetically useful for sterically crowded
b-substituted Michael acceptors, as reaction of methyl
3-methyl-2-butenoate with N-methyl benzylamine gave
only 19% of the Michael addition product, as shown by
GC/MS analysis.

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R. Alleti et al. / Tetrahedron Letters 49 (2008) 3466–3470
Table 1
Solvent-free 1-(Gd(OTf)3)-catalyzed Michael addition of amines and thiols to a,b-unsaturated esters and acrylonitrile
Entry
1
Amine/thiol
Substrated
Product
Yielda (%)
98 (92, 96)c
O
3a
O
NH
NH
N
OMe
OMe
O
O
2
3
3a
3a
95
N
O
Me
94 (96, 91)c
Ph
Ph
N
H
Ph
Ph
N
OMe
Me
O
H
N
4
3a
63b
N
OMe
Me
Me
O
5
6
Et2NH
Bn2NH
3a
3a
86 (91, 89)c
Et2N
OMe
O
82b
Bn2N
OMe
CN
N
7
8
3d
3d
3d
3d
90
O
NH
O
CN
N
90
NH
Me
CN
Ph
Ph
N
H
Ph
N
9
92
Me
H
N
Me
10
32b
N
Me
Ph
CN
CN
O
11
12
Et2NH
3d
3c
83
Et2N
99b
O
NH
N
OMe
O
O
NH2
HN
OMe
13
14
3a
3a
71b
OMe
OMe
O
PhSH
99b
PhS
OMe

R. Alleti et al. / Tetrahedron Letters 49 (2008) 3466–3470
3469
Table 1 (continued)
Entry
Amine/thiol
PhSH
Substrated
Product
Yielda (%)
98b
O
15
3b
PhS
OEt
O
16
3a
98b,e
O
NH
N
OMe
O
a
Isolated yields (0.5 mol % of the catalyst was used); reactions were carried out at room temperature for 1 h, with the exception for entries 4, 10, and 13
(24 h and 3 mol % of the catalyst).
b
Yields based on GC/MS.
Yields for 2nd and 3rd cycles.
3a = methyl acrylate; 3b = ethyl acrylate; 3c = methyl methacrylate; 3d = acrylonitrile.
Reaction was carried out in acetonitrile as solvent.
c
d
e
3. Experimental
References and notes
1. Green Chemistry: Designing Chemistry for the Environment; Anastas,
P. T., Williamson, T. C., Eds.; ACS Symp. Ser.: Washington, DC,
1996.
3.1. Procedure for the incorporation of Gd(OTf)3 onto
polymer 1
2. (a) Mehnert, C. P. Chem. Eur. J. 2005, 11, 50–56; Welton, T. Chem.
Rev. 1999, 99, 2071–2083; (b) Ionic Liquids in Synthesis; Wasserscheid,
P., Welton, T., Eds.; Wiley-VCH: Weinheim, Germany, 2003; (c)
Dupont, J.; de Souza, R. F.; Suarez, P. A. Z. Chem. Rev. 2002, 102,
3667–3691.
3. (a) Valkenberg, M. H.; DeCastro, C.; Holderich, W. F. Top. Catal.
2001, 14, 139–144; (b) DeCastro, C.; Sauvage, E.; Valkenberg, M. H.;
Holderich, W. F. J. Catal. 2000, 196, 86–94; (c) Valkenberg, M. H.;
deCastro, C.; Holderich, W. F. Appl. Catal., A 2001, 215, 185–190.
4. Kim, D. W.; Chi, D. Y. Angew. Chem., Int. Ed. 2004, 43, 483–
485.
Acetone (40 mL) was added to polymer 1 (1.0 g),
followed by Gd(OTf)3 (200 mg), and stirred at room
temperature for 24 h. Acetone was evaporated and the
polymer dried under vacuum for 4 h to give 1-Gd(OTf)3
as a powdery material.
3.2. General procedure for michael additions
To a mixture of the secondary amine (5.7 mmol) and
methyl acrylate (or acrylonitrile) (11.4 mmol), 1-Gd(OTf)3
(0.5 mol % based on gadolinium triflate) was added at room
temperature under solvent-free conditions and stirred for 1 h
(for reactions involving N-methylaniline reaction times of
over 24 h are required). After completion of the reaction,
ethyl acetate (10 mL) was added, filtered, and the solvent
evaporated under vacuum to obtain the products, essentially
pure by NMR (98%). 1H and 13C NMR, and GC/MS of the
products are in agreement with the literature data.15 The cat-
alytic system was dried for 3 h under vacuum and recycled
for subsequent runs. Relatively high conversions could be
achieved in the second and third cycles (Table 1, entries 1,
3, and 5). In a controlled experiment in the absence of the
catalyst, for the reaction shown in Table 1, entry 4, no prod-
uct was detected even after 24 h at room temperature.
5. Kobayashi, S.; Nagayama, S. J. Am. Chem. Soc. 1998, 120, 2985–
2986.
6. (a) Kobayashi, S.; Hamada, T.; Nagayama, S.; Manabe, K. Org. Lett.
2001, 3, 165–167; (b) Kobayashi, S.; Hachiya, I. J. Org. Chem. 1994,
59, 3590–3596.
7. (a) Alleti, R.; Perambuduru, M.; Samantha, S.; Reddy, V. P. J. Mol.
Catal. A: Chem. 2005, 226, 57–59; (b) Alleti, R.; Oh, W. S.;
Perambuduru, M.; Afrasiabi, Z.; Sinn, E.; Reddy, V. P. Green Chem.
2005, 7, 203–206.
8. (a) Enantioselective Synthesis of b-Amino Acids; Juaristi, E., Ed.;
Wiley-VCH: New York, 1997; (b) Palomo, C.; Aizpurua, J. M.;
Ganboa, I.; Oiarbide, M. Curr. Med. Chem. 2004, 11, 1837–1872; (c)
France, S.; Weatherwax, A.; Taggi Andrew, E.; Lectka, T. Acc. Chem.
Res. 2004, 37, 592–600.
9. (a) Choudary, B. M.; Sridhar, C.; Sateesh, M.; Sreedhar, B. J. Mol.
Catal. A: Chem. 2004, 212, 237–243; (b) Ben Ayed, T.; Amri, H.; El,
G.; Villieras, J. Tetrahedron 1995, 51, 9633–9642; (c) Shaikh, N. S.;
Deshpande, V. H.; Bedekar, A. V. Tetrahedron 2001, 57, 9045–9048;
(d) Matsubara, S.; Yoshioka, M.; Utimoto, K. Chem. Lett. 1994, 827–
830.
10. (a) Chem. Thiol Group; Fluharty, A. L., Ed.; Wiley: Chechester, 1974;
Part 2; pp 589–668; (b) Chirotechnology: Industrial Synthesis of
Optically Active Compounds; Sheldon, R. A., Ed.; Dekker: New York,
1993; (c) Fujita, E.; Nagao, Y. Bioorg. Chem. 1977, 6, 287–309.
11. Lee, Y.-S.; Byoun, Y.-S. Bull. Korean Chem. Soc. 2002, 23, 1833–
1835.
12. Kobayashi, S.; Kakumoto, K.; Sugiura, M. Org. Lett. 2002, 4, 1319–
1322.
13. (a) Lattanzi, A.; Leadbeater, N. E. Org. Lett. 2002, 4, 1519–1521; (b)
Takeuchi, M.; Akiyama, R.; Kobayashi, S. J. Am. Chem. Soc. 2005,
127, 13096–13097; (c) Nishio, R.; Sugiura, M.; Kobayashi, S. Org.
Lett. 2005, 7, 4831–4834.
Acknowledgments
Support of our work by the donors of the American
Chemical Society Petroleum Research Fund (PRF No.
39643-AC) is gratefully acknowledged. We thank Mani-
kantan B. Nair for the measurement of polymer molecular
weights, Honglan Shi, Srinivas R. Gorla, Ranjith Kolli,
and Professor Shubhender Kapila for ICP-MS analysis,
and Professors Frank D. Blum and Ekkehard Sinn for
helpful discussions.

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R. Alleti et al. / Tetrahedron Letters 49 (2008) 3466–3470
14. (a) Neckers, D. C.; Kooistra, D. A.; Green, G. W. J. Am. Chem. Soc.
1972, 94, 9284–9285; (b) Srirattnai, K.; Damronglerd, S.; Omi, S.;
Roengsumran, S.; Petsom, A.; Ma, G.-H. Tetrahedron Lett. 2002, 43,
4555–4557.
15. (a) Azizi, N.; Saidi, M. R. Tetrahedron 2004, 60, 383–387; (b) Liu, M.;
Sibi, M. P. Tetrahedron 2002, 58, 7991–8035; (c) Bartoli, G.; Bosco,
M.; Marcantoni, E.; Petrini, M.; Sambri, L.; Torregiani, E. J. Org.
Chem. 2001, 66, 9052–9055; (d) Moghaddam, F. M.; Mohammadi,
M.; Hosseinnia, A. Synth. Commun. 2000, 30, 643–650; (e) Becht, J.-
M.; Wagner, A.; Mioskowski, C. J. Org. Chem. 2003, 68, 5758–
5761.