Sulphamic acid - An efficient and cost-effective solid acid catalyst for the pechmann reaction

Article abstract of DOI:10.1055/s-2004-830858

Sulphamic acid (H₂NSO₃H, SA) is used as an alternative to conventional acid catalysts in the Pechmann condensation of phenols with β-ketoester leading to the formation of substituted coumarin. The method is simple, cost-effective, solvent-free and gives good yields in a short reaction time.

Full text of DOI:10.1055/s-2004-830858

LETTER
1909
Sulphamic Acid – An Efficient and Cost-Effective Solid Acid Catalyst for the
Pechmann Reaction
S
P
ulphamic Ac
a
id–
a
Catal
n
yst for the
P
k
echmannRea
a
ction jkumar R. Singh, Devendrapratap U. Singh, Shriniwas D. Samant*
Organic Chemistry Research Laboratory, University Institute of Chemical Technology, N. M. Parekh Marg, Matunga, Mumbai-400 019,
India
Fax +91(22)24145614; E-mail: samantsd@udct.org
Received 12 May 2004
Sulphamic acid (H NSO H, SA), which is a common or-
2
3
Abstract: Sulphamic acid (H NSO H, SA) is used as an alternative
2
3
ganic acid with mild acidity, involatility and incorrositiv-
ity, is insoluble in common organic solvents. It is a very
stable white crystalline solid1 and it has already been
demonstrated that SA is comprised not of the amino sul-
to conventional acid catalysts in the Pechmann condensation of phe-
nols with b-ketoester leading to the formation of substituted cou-
marin. The method is simple, cost-effective, solvent-free and gives
good yields in a short reaction time.
4a
+
–
fonic acid, but rather of H NSO zwitterionic units by
Key words: sulphamic acid, Pechmann reaction, coumarins
3
3
1
4b,c
both X-ray and neutron diffraction techniques.
During
the last few years, SA has emerged as a promising solid
acid catalyst for acid catalyzed reactions, viz. acetaliza-
Coumarins and chromones occupy a unique place in the
1
5a
15b–15e
tion, esterification,
acetylation of alcohols and
realm of natural and synthetic organic chemistry. Mem-
phenols,1 nitrile formation, tetrahydropyranylation of
alcohols.1 Very recently, SA has been used as a
chemoselective catalyst for the transesterification of b-
5f
15g
bers of this group display a broad range of applications1,2
5h
as fragrances, pharmaceuticals, additives to foods and
cosmetics, agrochemicals, optical brightening agents, dis-
ketoesters1 and Beckmann rearrangement. The unique
catalytic feature and intrinsic zwitterionic property of SA
is very different from the conventional acidic catalyst,
which prompted us to explore the further application of
SA as an acidic catalyst in other carbon-carbon and car-
bon-heteroatom bond forming reactions.
5i
15j
3
persal fluorescent and tunable dye lasers and biological
activities like antihelminthic, hypnotic, insecticidal and
4
anticoagulant properties. These compounds can also be
used for the synthesis of other products such as fluorocou-
marins, chromenes, coumarones and 2-acyl resorcinols.5
The Pechmann reaction is a most commonly used method
6
The Pechmann reaction involves initial transesterification
of b-ketoester with phenol followed by intramolecular
cyclodehydration. In the present work we found that SA
alone can catalyze both the steps in the Pechmann
reaction.
for the preparation of coumarins. The reaction involves
condensation of very simple starting materials i.e. phenols
and b-keto esters in the presence of a variety of acidic con-
densing agents. Various reagents like H SO , P O , FeCl ,
2
4
2
5
3
ZnCl , POCl , AlCl , PPA, HCl, phosphoric acid, and tri-
2
3
3
7
fluoroacetic acid are used to effect this condensation.
Herein, we report an efficient SA-catalyzed solvent free
However, in the current context of environmental impact, Pechmann reaction.
these methods are not attractive as it requires catalyst in
8
excess, for example, sulfuric acid in 10–12 equivalents,
9
trifluoroacetic acid in 3–4 equivalents and P O is re-
EtO
O
2
5
OH
O
O
1
0
H2NSO3H
130 °C,
quired in 5-fold excess. Further, such reactions required
long reaction time and in some cases gave lower yields.
The present drive therefore is towards the development of
more effective, non-stoichiometric, preferably a heteroge-
+
R
R
O
R'
R'
neous catalyst. Recently, the solid acid catalysts11 have Scheme 1 Pechmann condensation using sulphamic acid (SA) as a
been tried for this reaction. The reaction has also been at- catalyst
1
2
13
tempted using microwave irradiation and ionic liquids.
However, in the case of ionic liquid method, it required
the use of HCl for the quenching of the reaction mixture,
thus making the process costly and environmentally haz-
ardous.
The reaction of resorcinol with ethyl acetoacetate was se-
lected as a model to test the feasibility of SA used as a cat-
alyst for Pechmann reaction. To optimize the molar
stoichiometry of SA required for the reaction, we carried
out several experiments at various temperatures and under
solvent-less condition. The best result was obtained with
0
.5:1:1 molar ratio of SA, resorcinol and ethyl acetoace-
tate, respectively, at 130 °C. To extend the scope of the
reaction and to generalize the procedure, we investigated
the reaction of a series of monohydric and polyhydric
SYNLETT 2004, No. 11, pp 1909–1912
Advanced online publication: 04.08.2004
DOI: 10.1055/s-2004-830858; Art ID: D11804ST
0
6
.0
9
.2
0
0
4
©
Georg Thieme Verlag Stuttgart · New York

1
910
P. R. Singh et al.
LETTER
phenols with a variety of b-keto esters to obtain the corre- 13, and 16) required longer reaction time due to presence
1
6
sponding coumarins (Table 1). An interesting feature of another phenyl group. Phenol, catechol, cresol and p-
observed was that 3-methoxy phenol (Table 1, entry 2) hydroquinone failed to undergo reaction. In general, we
showed no detectable demethylation under the given con- observed that the reaction proceeded faster than conven-
ditions. However, reaction by conventional method using tional ones and the yields were comparable.
AlCl in nitrobenzene showed significant demethylation
3
1
3a
at high temperature. Naphthols (Table 1, entries 5, 8,
a
Table 1 Synthesis of Coumarins from Phenols and b-Keto Esters Catalyzed by Sulphamic Acid
Entry
1
Substituents (phenols)
b-keto ester
Product
Time (min)
20
Yield (%)b
94
HO
OH
O
O
O
O
O
O
HO
O
O
OEt
OEt
OEt
2
3
MeO
HO
OH
MeO
HO
O
O
20
20
88
96
OH
OH
O
O
O
O
OH
OH
OH
OH
4
5
O
O
O
O
25
55
82
68
HO
HO
OEt
OEt
OH
O
O
6
O
O
20
85
HO
HO
OH
OH
HO
O
O
OEt
7
8
O
O
O
O
O
O
O
20
80
88
62
OEt
OEt
OH
OH
O
OH
OH
9
OH
O
O
HO
O
O
O
45
45
40
80
82
82
Ph
Ph
Ph
OEt
OEt
OEt
Ph
O
1
1
0
1
HO
HO
OH
OH
O
O
O
O
HO
OH
OH Ph
O
HO
O
Ph
Synlett 2004, No. 11, 1909–1912 © Thieme Stuttgart · New York

LETTER
Sulphamic Acid – A Catalyst for the Pechmann Reaction
1911
a
Table 1 Synthesis of Coumarins from Phenols and b-Keto Esters Catalyzed by Sulphamic Acid (continued)
Entry
Substituents (phenols)
b-keto ester
Product
Time (min)
50
Yield (%)b
1
2
HO
OH
O
O
HO
O
O
80
OEt
OEt
1
3
OH
O
O
O
60
66
O
1
1
1
4
5
6
HO
HO
OH
OH
O
O
O
O
O
O
HO
HO
O
O
O
O
25
25
80
86
88
62
OEt
OEt
OEt
OH
OH
OH
O
O
O
1
7
HO
OH
O
O
O
55
70
OEt
OH
a
Phenol: 10 mmol; b-ketoester: 10 mmol; SA: 5 mmol; reaction temperature: 130 °C; solventless.
Isolated and unoptimized yields.
b
In conclusion, we have demonstrated an efficient and sim-
ple alternative for the preparation of substituted cou-
marins via the Pechmann condensation using SA.
Prominent among the advantages of this new method are
operational simplicity, good yield in short reaction times,
solvent-free condition, very inexpensive, easily available
catalyst and easy workup procedure employed.
(7) (a) Appel, H. J. Chem. Soc. 1935, 1031. (b) Woods, L. L.;
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(
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(
(
b) Robertson, A.; Sandrock, W. F.; Henry, C. B. J. Chem.
Acknowledgment
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(
11) (a) John, E. V. O.; Israelstam, S. S. J. Org. Chem. 1961, 26,
PRS and DUS are grateful to G. D. Gokhale Charitable Trust for
fellowships.
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Synlett 2004, No. 11, 1909–1912 © Thieme Stuttgart · New York

1
912
P. R. Singh et al.
LETTER
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of phenol (10 mmol), and b-keto ester (10 mmol), the
sulphamic acid (5 mmol) was added and the reaction mixture
was stirred at 130 °C in a pre-heated oil-bath. After
completion of the reaction at the desired time as indicated in
Table 1, the reaction mixture was poured into water and the
resultant product was filtered. The products were further
purified by column chromatography. The compounds are
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Synlett 2004, No. 11, 1909–1912 © Thieme Stuttgart · New York