Pechmann reaction in chloroaluminate ionic liquid

Article abstract of DOI:10.1055/s-2002-19332

Substituted coumarins were synthesized in good yields via the Pechmann reaction in chloroaluminate ionic liquid. 1-Butylpyridinium chloroaluminate ionic liquid was used as a solvent cum catalyst for this condensation reaction.

Full text of DOI:10.1055/s-2002-19332

152
LETTER
Pechmann Reaction in Chloroaluminate Ionic Liquid
P
echmannReactio
m
ninChloroalumina
i
te Ionic
t
Liquid C. Khandekar, Bhushan M. Khadilkar*
Applied Organic Chemistry Laboratory, University Department of Chemical Technology, University of Mumbai, Matunga,
Mumbai-400 019, India
Fax +91(22)4145619; E-mail: bhushank@vsnl.com
Received 6 September 2001
ic melts are reportedly classified as basic (X = 0–0.5),
neutral (X = 0.5), acidic (X = 0.5–0.67) and very acidic
(X = 0.67–0.75), where X is the mole fraction of AlCl3
present. All reactions were carried out in the acidic melt
with X = 0.67.17,18 Under optimized conditions, to our sur-
prise the same reaction proceeded even in the basic melt
(X = 0.39), but with slightly lower yield. The reactions
were carried out at atmospheric pressure in an oil bath pre-
heated to 130 °C. From the Table we can see that the re-
actions proceeded faster than the conventional ones and
the yields are comparative with the other methods report-
ed.
Abstract: Substituted coumarins were synthesized in good yields
via the Pechmann reaction in chloroaluminate ionic liquid. 1-Bu-
tylpyridinium chloroaluminate ionic liquid was used as a solvent
cum catalyst for this condensation reaction.
Key words: ionic liquid, Pechmann reaction, coumarins
Coumarins are an important group of naturally occurring
compounds widely distributed in the plant kingdom.
Members of this group display a broad range of
applications1,2 as fragrances, pharmaceuticals, additives
to food and cosmetics, agrochemicals, optical brightening
agents, dispersed fluorescent and tunable dye lasers,3 and
biological activities like anthelmintic, hypnotic, insecti-
cidal, and anticoagulant4 properties. Coumarins also act as
intermediates for the synthesis of fluorocoumarins,
chromenes, coumarones, and 2-acylresorcinols.5
Coumarins can be synthesized by various methods includ-
ing the Perkin,6 Pechmann,7 Knoevenagel,8 Refor-
matsky,9 and the Wittig10 reactions. Among these
Pechmann reaction is the most widely used method for the
preparation of coumarins since it proceeds from very sim-
ple starting materials and gives good yields of various
substituted coumarins. Various reagents like H2SO4, P2O5,
FeCl3, ZnCl2, POCl3, AlCl3, HCl, phosphoric acid, triflu-
oroacetic acid are known to effect this condensation.11 In
all these methods, the mixture of phenol, -ketoester and
the acidic catalyst was allowed to stand for long time or
Scheme
In conclusion, we have demonstrated an efficient and sim-
ple alternative for the preparation of substituted cou-
marins via the Pechmann reaction using 1-
butylpyridinium chloroaluminate ionic liquid. Prominent
among the advantages of this new method are operational
simplicity, good yields in short reaction times and easy
work-up procedures employed.
was heated above 150 °C. Recently, cation exchange Acknowledgement
resins12 and solid acid catalysts13 have been used. Howev-
The authors are thankful to AICTE (New Delhi) for financial assi-
stance.
er such reactions require, either temperature of about
150 °C or higher or long reaction times and in some cases
give lower yields. More recently, there have been reports
on the use of microwaves14 for accelerated synthesis of
different coumarins.
References
(1) Kennedy, R. O.; Thornes, R. D. Coumarins: Biology,
Applications and Mode of Action; Wiley and Sons:
Chichester, 1997.
(2) Murray, R. D. H.; Medez, J.; Brown, S. A. The Natural
Coumarins: Occurrence, Chemistry, and Biochemistry;
Wiley: New York, 1982.
In recent years there is a growing interest in the use of ion-
ic liquids for organic synthesis.15 In continuation of our
studies in chloroaluminate ionic liquids,16 we report for
the first time, the synthesis of some important coumarins
including 7-hydroxy-4-methylcoumarin (a starting mater-
ial for the preparation of the insecticide “Hymero-
cromone”) in 1-butylpyridinium chloroaluminate ionic
liquid, in good yields (Scheme). The chloroaluminate ion-
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1437-2096,E;2002,0,01,0152,0154,ftx,en;G19701ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214
(9) Shirner, R. L. Org React. 1942, 1, 1.

LETTER
Pechmann Reaction in Chloroaluminate Ionic Liquid
153
Table Results of the Pechmann Reaction in Chloroaluminate Ionic Liquid
Entry Phenol (1)
Ketoester (2)
Product (3)
Time (min)
35
Yield [%]a mp °C (Lit.)
1
90b
84c
75d
70e
182–184
(185)14a
ethyl acetoacetate
2
3
25
45
94b
87c
284–285
(280)14a
ethyl acetoacetate
ethyl acetoacetate
82b
74c
153–154
(155)14a
4
5
6
40
55
50
80b
71c
244–246
(248)7
ethyl acetoacetate
ethyl acetoacetate
34b
72b
163–164
(164–165)7
293–294
(decomp)
ethyl acetoacetate
(299–302 deomp)11d
7
8
55
50
60
70
74b
70b
240–242
ethyl benzoylacetate
ethyl benzoylacetate
(242–244)11a
244–246
(246–247)11b
9
88b
74c
244–246
(247)11c
2-carbethoxy cyclopen-
tanone
10
71b
62c
215–217
(218)11c
2-carbethoxy cyclopen-
tanone
a All yields refer to pure isolated products.
b Using 8.73 mmol of melt with X= 0.67.
c Using 3.5 mmol of melt with X= 0.67.
d Using 8.73 mmol of melt with X = 0.5.
e Using 8.73 mmol of melt with X= 0.39.
Synlett 2002, No. 1, 152–154 ISSN 0936-5214 © Thieme Stuttgart · New York

154
A. C. Khandekar, B. M. Khadilkar
LETTER
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(17) Ionic melt was prepared following the procedure described
previously, ref.16b
.
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(18) Experimental Procedure: A mixture of phenol (10 mmol),
ketoester (10 mmol) and the ionic liquid (8.7 mmol) was
stirred at 130 °C in a pre-heated oil bath. After completion of
the reaction at the desired time as indicated in the table, the
reaction mixture was quenched by adding 6 M HCl. The
solid product obtained was filtered off and recrystallized
from appropriate solvents. The compounds are all well
known and in agreement with spectral and physical data.
Synlett 2002, No. 1, 152–154 ISSN 0936-5214 © Thieme Stuttgart · New York