Sulfated tungstate: An efficient catalyst for the Ritter reaction

Article abstract of DOI:10.1039/c0gc00759e

The use of sulfated tungstate as an efficient and reusable catalyst for the preparation of amides from alcohols and nitriles via the Ritter reaction pathway is discussed. The reaction proceeds under solvent free conditions. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0gc00759e

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Green Chemistry
Cite this: Green Chem., 2011, 13, 835
www.rsc.org/greenchem
COMMUNICATION
Sulfated tungstate: an efficient catalyst for the Ritter reaction†
Kamlesh V. Katkar, Pramod S. Chaudhari and Krishnacharya G. Akamanchi*
Received 1st November 2010, Accepted 3rd February 2011
DOI: 10.1039/c0gc00759e
The use of sulfated tungstate as an efficient and reusable
catalyst for the preparation of amides from alcohols and
nitriles via the Ritter reaction pathway is discussed. The
reaction proceeds under solvent free conditions.
Scheme
benzonitrile.
1 The Ritter reaction between 1-phenylethanol and
Recently, we have synthesized, characterized and demonstrated
the effectiveness of sulfated tungstate as a catalyst for amide
bond formation between carboxylic acids and amines.1 Its ease
of preparation, mild acidity, high stability and reusability has
caught our attention for the exploration of its potential to
catalyze many other useful reactions. The Ritter reaction is a
classical acid-catalyzed reaction between alcohols and nitriles
to form amides with 100% atom economy,2 and it finds wide ap-
plication in the synthesis of natural products,3 pharmaceuticals4
and heterocycles.5 Many synthetic methods are available in
the literature and can be broadly classified under two head-
ings, viz. homogeneous and heterogeneous catalysis. Homo-
geneous catalysis includes the use of sulfuric acid,6a–f trifluo-
romethane sulfonic anhydride,6g BF3·OEt2,6h triflic anhydride,6i,j
2,4-dinitrobenzenesulfonic acid,6k,l FeCl3·6H2O6m and I2/H2O.6n
More recently, I2/solvent free conditions6o have been also used
in a homogenous fashion. These methods suffer from a few
drawbacks, such as longer reaction times with lower yields, the
requirement for relatively large quantities of catalyst and a trace
amount of catalyst having to be recovered from the reaction mix-
ture. On the other hand, the use of heterogeneous acidic catalysts
like Nafion-H,7a heteropolyacids,7b,c Fe-Montmorillonite K10,7d
Fe(ClO4)3-SiO2,7e zeolites,7f P2O5-SiO2,7g PMA-SiO2,7h and SiO2-
OSO3H7i have shown promising results.
To understand the role of the catalyst, we studied the effect
of the wt% of catalyst on the yield of the product. The results
are depicted in Fig. 1. In a fixed reaction time of 4 h, a stepwise
increase in catalyst amount from 5 to 20 wt% showed an increase
in yield of the amide from 18 to 85%, respectively. The reaction
with 5 wt% catalyst conducted for a longer duration of 15 h gave
a 76% yield of the product. However, a further increase in the
reaction time led to the significant formation of benzamide as a
byproduct.
Fig. 1 Graph of isolated yield of N-(1-phenylethyl) benzamide vs. wt%
of sulfated tungstate (reaction conditions: 1-phenylethanol (1 g, 8.19
mmol), benzonitrile (0.844 g, 8.19 mmol), 100 ◦C, 4 h).
Herein, we report the application of sulfated tungstate for
the Ritter reaction. Initial experiments were conducted by
employing a 1 : 1 mole ratio of 1-phenylethanol and benzonitrile
under solvent free conditions in presence of 20 wt% of sulfated
tungstate at 100 ◦C. To our delight, the reaction proceeded well
to offer the expected product, N-(1-phenylethyl) benzamide, in
85% yield (Scheme 1). Further investigations were undertaken
using this model reaction.
To assess the standing of sulfated tungstate among other
acidic catalysts in the formation of N-(1-phenylethyl) benza-
mide, some comparative experiments were performed. For some
others, data is taken from the literature. Together they are shown
in Table 1. Comparative experiments performed with silica and
tungstic acid as catalysts did not show any reaction (entries 6
and 7, Table 1). Among the other acid catalysts, viz. FeCl3·6H2O,
I2/H2O, Fe-Montmorillonite K10, P2O5-SiO2 and PMA-SiO2,
sulfated tungstate was found to be the most efficient in terms
of the amount required, yield and turn over frequency (TOF)
(entries 1–5 and 8, Table 1).
Department of Pharmaceutical Sciences and Technology, University
Institute of Chemical Technology, Matunga, Mumbai, 400 019, India.
E-mail: kgap@rediffmail.com; Fax: +91 22-33611020; Tel: +91
22-33611111/2222
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c0gc00759e
With the established optimal conditions, the scope of the
reaction was probed. As highlighted in Table 2, a wide range
of substrate combinations, such as tert-butanol and structurally
modified secondary, and primary benzylic alcohols and different
This journal is
The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 835–838 | 835
©

View Online
Table 1 The effect of different acidic catalysts on the formation of N-(1-phenylethyl) benzamidea
Entry
Catalyst
wt%
Equivalent ratio alcohol : nitrile
Time/h
Yieldb (%)
Ref.
TOFc/h-1
1
2
3
4
5
6
7
8
9
Iodine
22d
10d
50d
60
1 : 1.1
1 : 1
1 : 2.5
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
1 : 1
4
1
14
3
7
6
6
4
10
85e
54
85
6n
6m
7d
7g
7h
1.04
5.40
0.72
1.1
0.14
0
0
37.1
0
FeCl3·6H2O
Fe-Montmorillonite K10
P2O5-SiO2
PMA-SiO2
Silica
Tungstic acid
Sulfated tungstate
Nil
68
42d
20
85
NRf
NRf
85
20
20
—
NRf
a This work (entries 6–9) reaction conditions: 1-phenylethanol (1 g, 8.19 mmol) and benzonitrile (0.844 g, 8.19 mmol) at 100 ◦C under solvent free
conditions. b Isolated yield of amide. c TOF = turn over number (TON)/hours. d For ready comparison, the mole ratio of catalyst was converted to
wt%. e Reaction carried out in water as the solvent. f NR = no reaction.
Table 2 Substrate scopea
Entry
1
R1
R2
Product
Time/h
3
Yieldb (%)
87
(CH3)3C
CH3
2
3
(CH3)3C
(CH3)3C
NCCH2
4
3
90
87
CH2 CH
4
5
(CH3)3C
(CH3)3C
Ph
3
3
95
90
2-Cl–C6H4
6
(CH3)3C
PhCH2
CH3
3
4
4
4
5
88
82
92
85
82
7
Ph(CH)CH3
Ph(CH)CH3
Ph(CH)CH3
Ph(CH)CH3
8
ClCH2
Ph
9
10
PhCH2
836 | Green Chem., 2011, 13, 835–838
This journal is
The Royal Society of Chemistry 2011
©

View Online
Table 2 (Contd.)
Entry
11
R1
R2
Product
Time/h
4
Yieldb (%)
Ph(CH)CH3
4-Cl–C6H4–CH2
88
12
13
Ph(CH)CH3
Ph(CH) Ph
4-C5H4N
4.5
6
88
CH3
88
14
15
16
Ph(CH) Ph
Ph(CH) Ph
Ph(CH) Ph
Ph
5
88
86
90
PhCH2
4-C5H4N
5
14
17
18
19
PhCH2
Ph
6
6
5
80
85
88
PhCH2
PhCH2
Ph
cyclo-C6H11
20
21
Ph
5
88
85
PhCH2
5.5
a Reaction conditions: alcohol (1.0 equiv), nitrile (1.0 equiv) and catalyst (20 wt%) at 100 ◦C under solvent free conditions. b Isolated yield. All
products are known and were identified by their melting point, IR and 1H-NMR spectra, according to the literature.
nitriles, were studied. In all cases, the reactions were clean and
high yielding.
The reaction was also smooth in case of malononitrile with
2 equivalents of tert-butanol and gave 90% of N1,N3-di-tert
butylmalonamide (entry 2, Table 2). It is noteworthy that
the synthesis of N-tert-butylacrylamide from acrylonitrile was
achieved in 87% yield without the polymerization of acrylonitrile
(entry 3, Table 2). Reactions with hindered aromatic nitriles,
as well as with other differently-substituted nitriles, led to the
corresponding amides in very good yields (entries 5 and 11,
This journal is
The Royal Society of Chemistry 2011
Green Chem., 2011, 13, 835–838 | 837
©

View Online
Table 3 Catalyst reusability study in the synthesis of N-(1-phenylethyl)
It can be seen that sulfated tungstate is considerably cleaner
and more competent than the other catalysts. In particular, the
waste produced (E-factor) during the reaction is very much
less compared to PMA-SiO2. However, the E-factor ignores
recyclability factors such as recycled solvents and the reused
catalyst, which obviously increases the accuracy.
benzamidea
Run no.
Yield of N-(1-phenylethyl) benzamide (%)
Fresh
85
83
82
82
80
First recycle
Second recycle
Third recycle
Fourth recycle
Conclusions
a Reaction conditions: 1-phenylethanol (1 g, 8.19 mmol), benzonitrile
(0.844 g, 8.19 mmol) at 100 ◦C under solvent free conditions for 4 h.
In conclusion, the present work describes sulfated tungstate as a
very promising catalyst for the Ritter reaction between alcohols
and nitriles. The catalyst is reusable and effective for a wide
range of substrates under solvent free conditions, and hence has
potential for large scale applications. Further studies, including
applications of this method to synthesize bioactive molecules,
are currently in progress in our laboratory.
Table 2). In the case of chloroacetonitrile, a beautiful solid
product was observed during the reaction sequence (entry 8,
Table 2). Heterocyclic nitriles such as 4-cyanopyridine gave good
yields under optimized conditions (entries 12 and 16 Table 2).
In a case where both substrates were solid, a small amount (2–3
mL) of n-propanol was added to the reaction mixture in order to
maintain fluidity and facilitate the surface contact of the catalyst
and substrates, and the product was obtained in 90% yield (entry
16, Table 2). In the case of a terpene such as borneol, good yields
were obtained with both benzonitrile and benzyl cyanide in short
reaction times, as compared to another known method (entries
20 and 21, Table 2).6o
It is important to note that the catalyst was simply recovered
from the reaction mixture by adding ethyl acetate followed by
filtration without any acidic or basic work-up, even after a fourth
reuse. For a reusability study, the recovered catalyst was heated
in an oven at 100 ◦C for 1 h to remove the solvent and moisture.
From Table 3, it is shown that the catalyst was reusable four times
without any significant loss of activity.
In order to prove that the reaction is heterogeneous, a standard
leaching experiment was conducted by the hot filtration method.
The model reaction proceeded for 30 min in the presence of the
catalyst at 100 ◦C. The hot reaction mixture was then filtered
under suction to remove the catalyst. The filtered reaction
mixture was then heated without the catalyst for 12 h. There
was no formation of N-(1-phenylethyl) benzamide, even after
12 h, indicating that no homogeneous catalyst was involved.
To measure the “green-ness” of the reaction, we used different
parameters of green chemistry. Table 4 shows a comparison of
sulfated tungstate with different acid catalysts.
Acknowledgements
The authors thank the University Grant Commission (UGC)
of India and the Centre for Green Technology, University of
Mumbai, India for financial support.
Notes and references
1 P. S. Chaudhari, S. D. Salim, R. V. Sawant and K. G. Akamanchi,
Green Chem., 2010, 12, 1707.
2 L. I. Krimen and D. L. Cota, Org. React., 1969, 17, 213.
3 L. Kurti, B. Czako, in Strategic Applications of Named Reactions
in Organic Synthesis: Background and Detailed Mechanisms, Elsevier
Science, Amsterdam, 1st edn, 2005, pp. 383.
4 S. J. Chang, Org. Process Res. Dev., 1999, 3, 232.
5 (a) E. J. Tillmanns and J. J. Ritter, J. Org. Chem., 1957, 22, 839; (b) R.
Oda, M. Okano, S. Tokiura and F. Misurni, Bull. Chem. Soc. Jpn.,
1962, 35, 1219; (c) A. I. Meyers and J. J. Ritter, J. Org. Chem., 1958,
23, 1918; (d) A. I. Meyers, J. Org. Chem., 1960, 25, 1147; (e) V. A.
Glushkov and Y. V. Shklyaev, Mendeleev Commun., 1998, 8, 17.
6 (a) J. J. Ritter and P. P. Minieri, J. Am. Chem. Soc., 1948, 70, 4045;
(b) F. R. Benson and J. J. Ritter, J. Am. Chem. Soc., 1949, 71, 4128;
(c) H. Plaut and J. J. Ritter, J. Am. Chem. Soc., 1951, 73, 4076;
(d) J. J. Ritter, and N. Y. Yonkers, U.S. Patent 2,573,673, 1951;
(e) K. L. Reddy, Tetrahedron Lett., 2003, 44, 1453; (f) J. C. Baum,
J. E. Milne, J. A. Murry and O. R. Thiel, J. Org. Chem., 2009, 74,
2207; (g) A. G. Martinez, R. M. Alvarez, E. T. Vilar, A. G. Fraile,
M. Hanack and L. R. Subramanian, Tetrahedron Lett., 1989, 30,
581; (h) H. Firouzabadi, A. R. Sardarian and H. Badparva, Synth.
Commun., 1994, 24, 601; (i) I. L. Baraznenok, V. G. Nenajdenko
and E. S. Balenkova, Tetrahedron, 2000, 56, 3077; (j) V. Justribo
and M. I. Colombo, Tetrahedron Lett., 2003, 44, 8023; (k) R. Sanz,
A. Martinez, V. Guilarte, J. M. Alvarez-Guiterrez and F. Rodriguez,
Eur. J. Org. Chem., 2007, 4642; (l) M. Barbero, S. Bazzi, S. Cadamuro
and S. Dughera, Eur. J. Org. Chem., 2009, 430; (m) B. Anxionnat, A.
Gue´rinot, S. Reymond and J. Cossy, Tetrahedron Lett., 2009, 50, 3470;
(n) P. Theerthagiri, A. Lalitha and P. N. Arunachalam, Tetrahedron
Lett., 2010, 51, 2813; (o) Y. Kasashima, A. Uzawa, K. Hashimoto,
Y. Yokoyama, T. Mino, M. Sakamoto and T. Fujita, J. Oleo Science,
2010, 59, 607.
Table 4 A comparison of “green-ness” with various catalysts for the
preparation of N-(1-phenylethyl) benzamidea
Catalyst
E-factorb
Mass intensity
Yieldc (%)
Iodine
20.98
3.59
8.03
16.59
52.39
0.85
32.67
33.10
83.30
60.27
172.80
10.06
85
54
85
68
85
85
7 (a) G. A. Olah, T. Yamato and S. Lyer, Mater. Chem. Phys., 1987,
17, 21; (b) Okuhara, X. Chen and H. Matsuda, Appl. Catal., A, 2000,
200, 109; (c) H. Firouzabadi, N. Iranpoor and A. Khoshnood, Catal.
Commun., 2008, 9, 529; (d) M. M. Lakourag, B. Movassagh and J.
Fasihi, Synth. Commun., 2000, 30, 821; (e) P. Salehi and A. R. Motlagh,
Synth. Commun., 2000, 30, 671; (f) X. Chen and T. Okuhara, J. Catal.,
2002, 207, 194; (g) F. Tamaddon, M. Khoobi and E. Keshavarz,
Tetrahedron Lett., 2007, 48, 3643; (h) J. S. Yadav, B. V. S. Reddy, T.
Pandurangam, Y. J. Reddy and M. K. Gupta, Catal. Commun., 2008,
9, 1297; (i) B. B. F. Mirjalilia and B. Sadeghi, Iran. J. Org. Chem., 2009,
2, 76.
FeCl3·6H2O
Fe-Montmorillonite K10
P2O5-SiO2
PMA-SiO2
Sulfated tungstate
a Data is taken from the references mentioned Table 1. b The E-factor
shown does not account for the waste produced in the synthesis of the
catalyst. c Isolated yield.
838 | Green Chem., 2011, 13, 835–838
This journal is
The Royal Society of Chemistry 2011
©