Synthesis of coumarins catalyzed by eco-friendly W/ZrO2 solid acid catalyst

Article abstract of DOI:10.1081/SCC-100107007

Synthesis of substituted coumarins via reactions of resorcinol and substituted resorcinol with ethyl acetoacetate and ethyl α-methylacetoacetate (Pechmann reaction) are reported, in which the production of environmentally harmful waste streams is minimized by the use of a novel solid acid catalyst.

Full text of DOI:10.1081/SCC-100107007

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SYNTHESIS OF COUMARINS CATALYZED BY ECO-
FRIENDLY W/ZrO2 SOLID ACID CATALYST
Benjaram M. Reddy a , Vangala R. Reddy a & D. Giridhar a
a Inorganic and Physical Chemistry Division , Indian Institute of Chemical Technology ,
Hyderabad, 500 007, India
Published online: 16 Aug 2006.
To cite this article: Benjaram M. Reddy , Vangala R. Reddy & D. Giridhar (2001) SYNTHESIS OF COUMARINS CATALYZED BY
ECO-FRIENDLY W/ZrO2 SOLID ACID CATALYST, Synthetic Communications: An International Journal for Rapid Communication
of Synthetic Organic Chemistry, 31:23, 3603-3607, DOI: 10.1081/SCC-100107007
To link to this article: http://dx.doi.org/10.1081/SCC-100107007
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SYNTHETIC COMMUNICATIONS, 31(23), 3603–3607 (2001)
SYNTHESIS OF COUMARINS
CATALYZED BY ECO-FRIENDLY
W/ZrO2 SOLID ACID CATALYST
Benjaram M. Reddy,* Vangala R. Reddy,
and D. Giridhar
Inorganic and Physical Chemistry Division,
Indian Institute of Chemical Technology,
Hyderabad – 500 007, India
ABSTRACT
Synthesis of substituted coumarins via reactions of resorcinol
and substituted resorcinol with ethyl acetoacetate and ethyl
a-methylacetoacetate (Pechmann reaction) are reported, in
which the production of environmentally harmful waste
streams is minimized by the use of a novel solid acid catalyst.
Progress towards environmentally responsible processing is marked by
the elimination of waste and by-product generation and reduced dependence
on hazardous chemicals. The use of large quantities of concentrated sulfuric
acid as a condensing agent in the preparation of hydroxycoumarins by the
von Pechmann reaction (i.e., condensation of phenols with malic acid or
*Corresponding author.
3603
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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REDDY,REDDY,AND GIRIDHAR
with b-keto esters) is an established fact in the literature.1 Recent investiga-
tions for the Pechmann reaction employed various reagents such as zeolites,
clays and sulfonic acid resins resulting in simplified product recovery and
reduction of undesirable wasteful products.2–4 However, the zeolite and clay
based catalysts are highly susceptible to deactivation due to pore blocking
and hydration. Application of cationic exchange resins as condensing agents
results in better yields,5 however, most of the cationic exchange resins are
not resistant enough to high temperatures.
In this communication we report a new efficient ecofriendly W/ZrO2
solid acid catalyst for the synthesis of substituted coumarins from resorcinol
and substituted resorcinols with ethyl acetoacetate and ethyl-a-methylace-
toacetate in good yields. The reaction of resorcinol with ethyl acetoacetate
gives methylumbelliferone (4-methyl-7-hydroxycoumarin) a perfumery
ingredient for the insecticide Hymecromone.6 The tungstate modified zirco-
nia solid acid catalyst possesses many advantages such as easy to handle,
non-corrosive, low-cost, high activity, and high recycling capacity.
The sulfated zirconia solid superacid catalyst has attracted much
attention recently because of its ability to catalyze many acid catalyzed
reactions more effectively.7 However, the SO42ꢀ/ZrO2 catalyst gets deacti-
vated easily by loosing sulfate ions and thereby recycling of the catalyst is
highly restricted. Very recent investigations reveal that molybdenum or
tungsten doped ZrO2 is an alternative catalyst8–12 in reactions requiring
stronger acid sites (H0 ꢁ ꢀ 14.52).13 The major advantage of Mo-(or W-)
modified ZrO2 over sulfated catalyst is that they do not suffer from dopent
loss during thermal treatments and also undergoes significantly less deacti-
vation during catalytic runs. Although, solid superacids have been
attempted in petrochemical industries, their application in synthetic organic
chemistry remains unexplored.
The synthesis of substituted coumarins from resorcinol and substi-
tuted resorcinols with ethyl acetoacetate and ethyl-a-methylacetoacetate
are presented in Table 1. From entries 1–4 the reaction is carried out with
ethyl acetoacetate; and from entries 5–8 the reaction is carried out with
ethyl-a-methylacetoacetate. As can be noted from Table 1, the catalytic
activity of W/ZrO2 was found to be more promising. Different mechanisms
have been put forward to account for this reaction in the literature.5
The resorcinol as a result of resonance has a center of high electron density
at the position ortho to the hydroxyl group. Addition then takes place across
the double bond in the enolic form of the b-keto ester and results in good
yields. In case of orcinol, the position para to a methyl group is deactivated
for an electrophilic reagent. The activity of orcinol will therefore be less than
that of resorcinol. In case of pyrogallol, the third hydroxyl group deactivates
the position meta to it for an electrophilic reagent resulting in less yield.

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Table 1. W/ZrO2 Solid Acid Catalyzed Synthesis of Substituted Coumarins

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REDDY,REDDY,AND GIRIDHAR
Similarly, the decrease in the yield, in case of phloroglucinol can be
attributed due to steric hindrance caused by third hydroxyl group. The
substitution of hydrogen atom by methyl in case of b-keto ester is less
active because of its lesser extent of the enolization.
Preparation of the Catalyst
To make the W/ZrO2 solid acid catalyst, Zr(OH)4 was prepared
first from zirconium oxychloride by hydrolysis with dilute ammonia. In a
typical experiment the required quantity of ZrOCl2.8H2O (Loba Chemie,
GR grade) was dissolved in doubly distilled water and the pH of the result-
ing solution was 2. To this clear solution dilute NH4OH was added drop
wise from a burette with constant stirring until the pH of solution reached 8.
The obtained precipitate was washed several times until free from chloride
ions and dried at 393 K for 48 h. To make 13 wt.% W/ZrO2 catalyst, the
requisite quantity of ammonium tungstate (J. T. Baker, England; GR grade)
was dissolved in 1 M oxalic acid solution. To this clear solution the desired
amount of finely powdered oven dried precipitate was added and the excess
water was evaporated on a water bath. The resulting sample was oven
dried again at 393 K for 48 h and calcined at 873 K for 6 h in a flow of
oxygen. The X-ray powder diffraction analysis of the sample revealed the
presence of XRD lines due to ZrO2 tetragonal phase. The preferential
formation and subsequent stabilization of the tetragonal zirconia was
mainly attributed to the observed high acidity and activity of promoted
zirconia solid acid catalysts.8,11,14
General Procedure for Substituted Coumarins Synthesis
In a typical reaction procedure (Table 1) a mixture of 1 : 1 molar
amounts of reactants with the catalyst (0.3 g) in solvent toluene (20 ml)
was refluxed for 6 h. After completion of the reaction the catalyst was
filtered off when the reaction mixture is in hot condition and the wet catalyst
was used for recycling. No appreciable change in the activity was noticed.
The product was recovered from the filtrate. The isolated yields were
obtained after recrystallization from aqueous methanol. NMR and mass
spectrometric techniques were utilized along with their melting points for
analyzing the products. In the controlled experiments, no reaction was
observed in the absence of W-ZrO2 catalyst in the reaction mixture.

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ACKNOWLEDGMENTS
VRR thanks Council of Scientific and Industrial Research, New Delhi
for the award of a Senior Research Fellowship.
REFERENCES
1. von Pechmann, H. Ber. 1884, 17, 929; von Pechmann, H.; Duisberg, C.
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¨
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13. An acid that is stronger than H0 ¼ ꢀ 12, which corresponds to the acid
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Received in Japan October 25, 2000

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