An efficient, simple and green Zn(Phen)2Cl2 complex catalyzed synthesis of 4-H-Benzo[b]pyrans in water at ambient temperature

Article abstract of DOI:10.1155/2011/684937

An efficient method for the synthesis of 4-H-benzo[b]pyrans has been developed by the use of Zn(Phen)₂Cl₂ (2 mol%) as an ecofriendly catalyst. The mixture of an appropriate aldehyde, malononitrile and dimedone in the presence of the

Full text of DOI:10.1155/2011/684937

ISSN: 0973-4945; CODEN ECJHAO
E-Journal of Chemistry
2011, 8(2), 529-534
http://www.e-journals.net
An Efficient, Simple and Green Zn(Phen)2Cl2
Complex Catalyzed Synthesis of 4-H-Benzo[b]pyrans
in Water at Ambient Temperature
E. MOSADDEGH*, A. HASSANKHANI§ and G. MANSOURI
*Department of Material Science
International Center for Science, High Technology &
Environmental Sciences, P O Box 76315-117, Kerman, Iran
§Department of Materials Science
International Center for Science, High Technology &
Environmental Sciences, P O Box 76315-117, Kerman, Iran
Young Researcher Club, Islamic Azad University of Kermanshah
P O Box 6718997551, Kermanshah, Iran
mosaddegh_e@yahoo.com
Received 16 June 2010; Accepted 1 September 2010
Abstract: An efficient method for the synthesis of 4-H-benzo[b]pyrans has been
developed by the use of Zn(Phen)2Cl2 (2 mol%) as an ecofriendly catalyst. The
mixture of an appropriate aldehyde, malononitrile and dimedone in the presence
of the Zn(Phen)2Cl2 as an inorganic catalyst in water at room temperature resulted
excellent yields of the corresponding products in 5 min. Very short experimental
time of the reaction, excellent yields, use of water as a green solvent, simple
work-up and uses no extra catalyst and purification are the advantages of this
method.
Keywords: Zn(Phen)2Cl2, 4-H-Benzo[b]pyrans, Malononitrile, Transition metal, Green chemistry.
Introduction
4-H-Benzo[b]pyrans and their derivatives are of considerable interest due to their
pharmacological activities1, such as spasmolytic, diuretic, anti coagulant, anti-cancer,
anti-ancaphylactia activity2-5. In addition, they have been shown to act as cognitive enhancers,
for the treatment of neurodegenerative disease, including Huntington's disease, Alzheimer's
disease, amyotrophic lateral Sclerosis, AIDS associated dementia and Down's syndrome as
well as for the treatment of Schizophrenia and myoclonus6,7. Also several biologically active

530
E. MOSADDEGH et al.
natural products contain these subunits8. A number of 2-amino-4H-pyrans are useful as
photoactive materials9, pigments10 potential biodegradable agrochemicals11. Thus a synthetic
interest has that included methodological improvements of the original reaction conditions.
The titled compounds are synthesized by reaction of malononitrile with dimedone in the presence of
an appropriate aldehyde. A variety of reagents such as hexadecyltrimethylammonium bromide
(HTMAB)12, NaBr13,14, rare earth perfluorooctanoate RE(PFO)3, (S)-proline15,16, Na2SeO4,
Ce(SO4)2.4H2O17, the use of microwave irradiation18, KF-basic alumina under ultrasound irradiation19,
tetramethylammonium hydroxide (TMAH)20, electrolysis21, tetrabutylammonium bromide
(TBABr)22 and amino-functionalized ionic liquid23 were employed to catalyze these reactions.
In spite of potential utility of aforementioned routes for the synthesis of 4-H-benzo
[b]pyran derivatives many of these methods involve expensive reagents, acidic conditions,
long reaction times, low yields, use of excess of reagents/catalyst and use of toxic organic
solvents and complicated work-up methods. The lack of facile and general synthetic
methodology under essentially natural, practical and mild reaction condition has promoted us
to develop an efficient, convenient and practical procedure for the one-pot green synthesis of
4-h-benzo[b]pyrans reaction under aqueous media. to the best of our knowledge a complex
catalyst has not been used in the synthesis of 4-H-benzo[b]pyrans.
Green chemistry emphasizes the development of environmentally benign chemical
processes and technologies24. According to the principle of safe chemistry, synthetic method
should be designed to use substances that exhibit little or no toxicity to human health and
environment25. In this regard and in continuation of our interest on application of
heterogeneous catalysts in organic reactions26, we wish to report herein, a new, simple, mild
and ecofriendly method for the synthesis of 4-H-benzo[b]pyrans via a one-pot
three-component tandem Knoevonagel-cyclo condensation reaction using Zn(Phen)2Cl2
(2 mol%) as environmentally friendly homogeneous complex catalyst with high catalytic
activity and reusability in very short reaction time in water base media (Scheme 1).
Scheme 1
Experimental
Melting points were determined on a gallenkamp melting point apparatus and are uncorrected.
NMR spectra were recorded at 500 (1H) and 125.77 (13C) MHz on Bruker DRX-500 avance
spectrometer at 500 and 125.77 MHz, respectively. All compounds were known in the
literature, the NMR and IR spectra of the products were in agreement with earlier data.
General procedure for the preparation of 4-H-Benzo[b]pyran4-H-Benzo[b]pyrans (4c)
In a typical general procedure, a mixture of aromatic aldehyde (1 mmol), malononitrile
(1 mmol) and dimedone (1 mmol) in H2O (3 mL) at room temperature, were stirred thoroughly
in the presence of catalytic amount of Zn(Phen)2Cl2 (10 mg, 2 mol%) about 5 min to afford the
4-H-benzo[b]pyrans in excellent yields. Completion of the reaction was confirmed by TLC.
Then the mixture was filtered. The solid product was separated by filtration from the soluble
catalyst after washing with H2O. Pure 4-H-benzo[b]pyran4-H-benzo[b]pyrans (4) was obtained
in 94% yield without any use of more purification. Structures of the products were confirmed
from physical and spectroscopic data (IR and 1H NMR) in comparison with the literature data.

An efficient, Simple and Green Synthesis
531
General procedure for the synthesis of Zn(phen)2Cl2
To a 20 mL ethanolic solution of ZnCl2 (0.136 g, 1 mmol), 10 mL ethanolic solution of the
1,10-phenanthroline (0.360 g, 2 mmol) was added drop wise. The resulting white suspension was
stirred at 70 °C for 24 h. The suspension was cooled, filtered and then dried at room temperature.
White colored zinc complex, Zn(phen)2Cl2, was obtained in 89% yield. Structure of the zinc
complex was confirmed from spectroscopic data (IR and Anal. Calc) in comparison with those
reported literature27: Anal. Calc. For C24H16N4Cl2Zn: C,58.05; H, 3.22; N, 11.28. Found: C, 58.10;
H, 3.19; N, 11.26%; FT-IR (KBr, cm-1): 3046(br), 1622(m), 1579(m), 1515(s), 1427(s), 1345(m),
1223(m), 1143(s), 1092(m), 846(s), 725(s), 638(s), 577(m) and 418(m).
Results and Discussion
To show that Zn(Phen)2Cl2 is an efficient catalyst, we accomplished the reaction at room
temperature in the absence of catalyst for 4 h. The reaction just produced alkene (a) and no
benzo[b]pyran was formed (Figure 1.).
(a)
Figure 1
This prove the essential effect of Zn(Phen)2Cl2 as a mild and efficient catalyst in the
progressing of the reaction. In order to improve yield, we performed reaction using different
quantities of reagents. The best results was obtained with a 0.02:1:1:1.5 ratio of Zn(Phen)2Cl2
complex, aldehyde, dimedone and malononitrile. Also, we carried out a controlled reaction in
different solvents (Table 1).
Table 1. Effect of solvents in the synthesis of 4-H-benzo[b]pyran4-H-benzo[b]pyrans (4c)
using 2 mol% of Zn(Ph)2Cl2 complex at room temperature.
Entry
Solvent
H2O
Time
5 min
18 h
24 h
24 h
24 h
Yield, %
1
2
3
4
5
94
82
__
__
__
EtOH
CH2Cl2
Ethylacetate
1,2-Dichloroethane
As shown in Table 1, for this type of reaction, a polar solvent is generally needed to obtain
high yields with low mole percentages of catalyst in mild reaction conditions. The result
suggests that H2O at room temperature condition is the better reaction system than the organic
solvents media.
To investigate the versatility of the catalyst, the reaction of dimedone, malononitrile and
various aromatic aldehydes was carried out in H2O at room temperature using 2 mol% of
Zn(Phen)2Cl2 (Table 2).
4-H-Benzo[b]pyrans derivatives containing electron-withdrawing groups such as nitro
and halide groups or electron-donating groups such as hydroxyl and alkoxy groups (Table 2)
were formed in a short experimental time (5 min) with excellent yields (90%-100%).
It is also noticed that the condensation using Zn(Ph)2Cl2 complex proceeds rapidly and is
superior to the reported procedures. This claim is justified through the representative
examples, illustrated in Table 3.

532
E. MOSADDEGH et al.
Table 2. Synthesis of 4 catalyzed by Zn(Phen)2Cl2 at ambient temperature in water as solvent
Product
4a
Ar
C6H5
yield (%)a
mp, oC
222-224
207-209
208-209
178-179
207-209
231-233
212-214
209-210
188-190
122-124
Mp., reported[Ref
226-228[19]
203-205[18]
207-209[17]
180-182[16]
208-211[19]
236-238[19]
214-216[19]
200-202[17]
186-187[17]
122-125[28]
]
90
91
94
96
97
93
99
100
93
90
4-Br- C6H4
4-Cl- C6H4
4b
4c
2,4-Cl2-C6H3
3-O2N- C6H4
3-OH- C6H4
3-H3C-C6H4
2-Cl- C6H4
4d
4e
4f
4g
4h
4i
3-H3CO- C6H4
4-H3CO- C6H4
4j
aYields refer to the isolated products
Table 3. Synthesis of 2-amino-4-(4-chlorophenyl)-3-cyano-7,7-dimethyl-5-oxo-4H-5,6,7,8-
tetrahydrobenzopyran using different reagents and reaction conditions
Entry
Reagent
NaBr, 0.4 mmol
Na2SeO4, 0.1 g
TMAH, 10 mol%
___
Conditions
MW(Power: 60), 70 oC
EtOH/H2O, reflux
H2O, r.t
Time
10 min
3h
yield(%)ref
90[12]
1
2
3
4
5
90[16]
83[20]
95[17]
91[22]
0.5-2 h
4-5 h
2 h
electrolysis (10 mA)
H2O, reflux
TBABr, 10 mol%
Ce(SO4)2.4H2O,
2.5 mol%
6
7
EtOH/H2O, 45oC
30 min
90[17]
Zn(Phen)2Cl2
H2O, r.t
5 min
94Present work
aYields refer to the isolated products; r.t: room temperature
Reusability of Zn(Phen)2Cl2
The reusability of the catalysts is one of the most important benefits and makes them useful
for commercial applications. Thus the recovery and reusability of Zn(Phen)2Cl2 was
investigated. Fortunately, the precipitated material in the reaction shown in Scheme 1 was
successfully recycled three times. The catalyst could be recycled in two ways.
Method A
In the first method, the reaction mixture was filtered and washed with H2O. The soluble
catalyst was easily reused after distillation of solvent, washing with CHCl3 and drying at room
temperature. The recycled catalyst could be examined in next run in the reaction between
4-chlorobenzaldehyde, malononitrile and dimedone.
Method B
In the second method, the reaction mixture was filtered and the solid product was washed with
H2O. Zn(Phen)2Cl2 complex is soluble in water. The remaining solution including soluble
catalyst could be reused as such for subsequent experiments (up to 3 cycles). In the next run,
4-chlorobenzaldehyde, malononitrile and dimedone was added to the filtered solution to
afford the comparable yields of the product as shown in Figure 2.

An efficient, Simple and Green Synthesis
533
Run No.
Figure 2. The reusability of the catalyst in the synthesis of 4-H-benzo[b]pyran4-H- benzo
[b]pyrans (4c).
It was interesting to note that the Zn(Ph)2Cl2 could be reused in three cycles via this
method without any loss of its activity, more purification, distillation of solvent and dry
catalyst. Thus the remained mixture of the reaction could be used several times to produce the
4-H-benzo[b]pyran derivatives with simple work-up and not using cumbersome apparatus for
recycling of the catalyst. A little increase in reaction time is attributable to the mechanical loss
of some of the 10 mg of catalyst used, increase in volume of solvent or some combination of
these factors. Also, a little increase was observed in the product yields. It could be because of
the soluble and remaining products from the previous run. So, this improved procedure could
be used to obtain the products with more efficiency. The presented procedure is continual,
very simple and clean, environmentally friendly due to use water as green solvent, cost
effective, more efficient and easier work-up than reported methods. Also, the product could be
obtained continual in the same solvent environment.
Conclusion
In conclusion, the present method is an operationally simple and environmentally friendly
procedure for the synthesis of compound 4 using catalytic amount of Zn(Phen)2Cl2 at
ambient temperature. In addition recyclability and moderate Lewis acidity of the catalyst,
safe conditions, cost effective, excellent yields of products without any use of more
purification, very short reaction time and use of water as a green solvent make this
methodology a valid contribution to the existing processes in the field of
4-H-benzo[b]pyrans derivatives synthesis. To the best of our knowledge, this procedure
provides the first example of complex catalyzed the efficient synthetic method for
4-H-benzo[b]pyrans derivatives.
Acknowledgment
We thank the Khoramshahr Marine Science and Technology University, for partial financial
support of this investigation.
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