ZrOCl2.8H2O/SiO2: An efficient and recyclable catalyst for the preparation of coumarin derivatives by Pechmann condensation reaction

Article abstract of DOI:10.1016/j.catcom.2011.07.002

Silica gel supported zirconyl chloride octahydrate was found to be an efficient and recyclable catalyst for the synthesis of a series of biologically important molecules in high turnover numbers and rates. Several substituted coumarins can be prepared in high yield and purity by direct reaction of β-keto esters and phenol derivatives in the presence of a catalytic amount of ZrOCl₂.8H₂O/SiO₂ as Lewis acid and at ambient temperature under solvent-free conditions. This method which is called Pechmann condensation is a very easy and a rapid reaction for the synthesis of coumarin derivatives.

Full text of DOI:10.1016/j.catcom.2011.07.002

Catalysis Communications 14 (2011) 62–67
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Short Communication
ZrOCl2.8H2O/SiO2: An efficient and recyclable catalyst for the preparation of
coumarin derivatives by Pechmann condensation reaction
⁎
Bahador Karami , Mahtab Kiani
Department of Chemistry, Yasouj University, Yasouj, Zip Code: 75918-74831 P.O. Box 353, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 April 2011
Received in revised form 29 June 2011
Accepted 1 July 2011
Available online 12 July 2011
Silica gel supported zirconyl chloride octahydrate was found to be an efficient and recyclable catalyst for the
synthesis of a series of biologically important molecules in high turnover numbers and rates. Several
substituted coumarins can be prepared in high yield and purity by direct reaction of β-keto esters and phenol
derivatives in the presence of a catalytic amount of ZrOCl2.8H2O/SiO2 as Lewis acid and at ambient
temperature under solvent-free conditions. This method which is called Pechmann condensation is a very
easy and a rapid reaction for the synthesis of coumarin derivatives.
Keywords:
Coumarin
© 2011 Elsevier B.V. All rights reserved.
Recyclable catalysts
Silica gel
Zirconyl chloride octahydrate
Solvent-free conditions
1. Introduction
The importance of zirconium as a homogeneous catalyst has
already been cited in literature in the number of biologically and
Coumarins and their derivatives are very important organic
compounds. They are the structural unit of several natural products
[1]. Their applications range from pharmaceuticals [2], to optical
brighteners [3] and laser dyes [4]. Also coumarins and functionalized
coumarins have shown activity such as antimicrobials and chemother-
apeutics [5]. These properties have made coumarins interesting targets
for organic chemists. Various methods are known for the synthesis of
substituted coumarins in the literature including Pechmann, Perkin,
Knoevenagel, Claisen, Reformatsky, and Wittig reactions [6]. Most of the
procedures suffer from harsh reaction conditions such as the use of
stoichiometric amounts of mineral, toxic reagents, often under high
temperatures and longer reactions times, poor substituent's tolerance,
and low yields [7]. Thus, it is clearly evident that the need for the
development of new and flexible protocols is required. One of the most
widely used methods is the Pechmann reaction which involves the
condensation of a phenol with a ketoester [6].
In recent years, Lewis acids such as InCl3, AlCl3–nBPC, Yb(OTf)3,
ZrCl4, GaI3, Zr–SG and Sm(NO3)3 have been used to catalyze
Pechmann reactions [8,9]. Also, development of efficient and new
catalytic systems for various organic transformations is an active
ongoing research area and the scope for further improvement
towards milder reaction conditions. These protocols offer several
advantages such as catalysts recyclability, high yields of product, short
reaction time, and simple work up. These make as important
supplements to the existing methods.
pharmaceutically significant organic transformations [10–15]. The use
of zirconium salts in organic transformation such as conversion of
aromatic, aliphatic nitro compounds to primary amines [16a]
hydrodechlorination of dichlorodifluoromethane [16b] oxidation of
alcohols [16c] Michael reactions [16d-16f], and acylation
reactions [16g] has been recently reported. ZrOCl2.8H2O is relatively
nontoxic, inexpensive and insensitive to air. The most readily
available and important “Zirconyl” salt is ZrOCl2.8H2O which
crystallizes from dilute hydrochloric acid solutions and contains the
ion [Zr4.(OH)8 (H2O)16] 8+. Each Zr atom is in distorted square,
linked by pairs of hydroxy bridges and also bound to four water
molecules so that the Zr atom is co-ordinated with eight oxygen
atoms [17]. Our present work is directed to immobilize zirconium
complex onto the common solid support in order to combine the
properties such as catalyst selectivity and activity with the ease of
separation and catalyst reuse. The support material plays a critical role
in the performance of the resulting supported reagent catalyst. Silica
gel support was chosen due to its high surface area, excellent stability
(chemical and thermal), good accessibility, and ease of functionaliza-
tion of the surface groups [18,19]. The catalyst does not need
activation and is recycled many times under the same conditions
with fresh reactants to yield similar results without significant loss of
activity [17].
2. Experimental
Chemicals were purchased from Aldrich, Fluka and Merck chemical
⁎
Corresponding author. Tel./fax:+98 741 2223048.
E-mail address: karami@mail.yu.ac.ir (B. Karami).
companies and were freshly used after purification by standard
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.07.002

B. Karami, M. Kiani / Catalysis Communications 14 (2011) 62–67
63
Fig. 1. XRD pattern of ZrOCl2.8H2O/SiO2.
procedures (distillation and recrystallization). The products were
isolated and identified by comparing their physical and spectral data
with those in the literature [20–28]. IR spectra were recorded on FT-IR
JASCO-680 using KBr disks and the 1H-NMR spectra were obtained on a
Brucker instrument 400 MHz model as CDCl3 solutions, and the
chemical shifts were expressed in δ units with Me4Si (TMS) as the
internal standard.
Specific detailed data for each of the compounds are given below:
(1a) 7-Hydroxy-4-methyl-2H-chromen-2-one Colorless prisms, mp
185–187 °C; Yield 94%. 1H-NMR (400 MHz, DMSO-d6): δ
10.138 (s, 1H), 7.373 (m, 5H), 6.722 (s, 1H), 6.470 (s, 1H),
5.957 (s, 1H), 2.292 (s, 3H).13C-NMR (100 MHz, DMSO-d6):δ
160.09, 156.05, 155.96, 155.51, 143.93, 139.75, 128.34, 127.92,
127.75, 113.88, 112.52, 108.19, 105.44, 21.65. IR (KBr, cm−1):
(3000–3400), 3050, 2900, 1690, 1615, 1590, 1500, 1080.
2.1. Preparation of catalyst
(4a) 5-Hydroxy-4,7-dimethyl-2H-chromen-2-one Colorless prisms,
mp 257–260 °C; Yield 89%.1HNMR (400 MHz, DMSO-d6): δ
10.52 (s, 1H), 6.62 (d, 1H, J=1.2 Hz), 6.57 (d, 1H, J=1.2 Hz),
The grafted silica gel (1 g) was stirred with zirconium (IV)
oxychloride (0.161 g, 0.5 mmol) in chloroform (10 ml) which was
heated under reflux condition for 2 h. The mixture was then filtered,
washed thoroughly with chloroform (3×10 ml), and the obtained
catalyst was dried.
6.03 (d, 1H, J=1.2 Hz), 2.54 (d, 3H, J=1.2 Hz), 2.27 (s, 3H).13
C
NMR (100 MHz, DMSO-d6): δ 160.29, 156.90, 155.28, 154.04,
143.18, 112.37, 112.3, 108.15, 106.96, 23.91, 21.56. IR (KBr,
cm−1):3393 (OH), 1655 (C=O).
(5a) 5-Hydroxy-7-methyl-4-phenyl-2H-chromen-2-one Colorless
prisms, mp 211–213 °C; Yield 80%. 1HNMR (400 MHz, DMSO-
d6): δ 10.14 (s, 1H), 7.35 (m, 5H,), 6.72 (s, 1H), 6.47 (s, 1H), 5.96
(s, 1H), 3.01 (s, 3H).13 C NMR (100 MHz, DMSO-d6): δ160.09,
156.05, 155.96, 155.51, 143.93, 139.75, 128.34, 127.92, 127.75,
113.88, 112.52, 108.19, 105.44, 21.65. IR (KBr, cm−1): 3180 (OH),
1680 (C=O).
2.2. Catalyst characterization
X-Ray Diffraction (XRD) pattern was obtained by Philips X Pert Pro
X diffractometer operated with a Ni-filtered Cu Kα radiation source.
X-Ray Fluorescence was recorded by X-Ray Fluorescence Analyzer,
Bruker, S4 PIONEER, Germany.
2.3. General procedure
In a typical experimental procedure, β-ketoester (1 mmol) was
added to a mixture of substituted phenol (1 mmol) and ZrOCl2.8H2O/
SiO2 (0.27 g, 10 mol%) in a screw-cap vial. The reaction mixture was
stirred in a preheated oil bath (90 °C). After the completion of the
reaction, the solid product was suspended in water (20 ml). The
resulting crude product wasfiltered off and recrystallized from hot EtOH
to give a white crystalline solid [6,20–29].
2.3.1. Preparation of 7-Hydroxy-4-methyl-2H-chromen-2-one (1a)
The compound 1a was prepared according to the general
procedure using ethyl acetoacetate (0.13 g, 1 mmol), resorcinol
(0.11 g, 1 mmol) and ZrOCl2.8H2O/SiO2 (0.27 g, 10 mol%). The
reaction progress was monitored by TLC. After the completion of the
reaction, the solid product was suspended in water (20 ml). The
resulting crude product was filtered off and recrystallized from hot
EtOH to give a white crystalline solid.
Fig. 2. XRF data of ZrOCl2.8H2O/SiO2.

64
B. Karami, M. Kiani / Catalysis Communications 14 (2011) 62–67
Fig. 3. FT-IR spectra of ZrOCl2.8H2O, SiO2 and ZrOCl2.8H2O/SiO2.
Scheme 1. Synthesis of coumarin derivatives by Pechmann condensation using β-ketoesters and phenol derivatives in the presence of ZrOCl2.8H2O/SiO2.
(7a) 5,7-Dihydroxy-4-methyl-2H-chromen-2-one White solid, mp
(14a) 7,8-Dihydroxy-4-methyl-2H-chromen-2-one White solid, mp
240–242 °C; Yield 89%. 1H NMR (400 MHz, DMSO-d6): δ 10.1
(s, 1H), 9.35(s,1H), 7.07 (d, 1H, J=8.8 Hz), 6.81 (d, 1H,
J=8.4 Hz), 6.12 (d, 1H, J=1.2 Hz), 2.4 (d, 3H, J=1.2 Hz). 13 C
NMR (100 MHz, DMSO-d6): δ 160.71, 154.35, 149.81, 143.47,
132.60, 115.88, 113.23, 112.56, 110.60, 18.63. IR (KBr, cm−1):
3231 (OH), 1668 (C=O).
(15a) 7-amino-4-methyl-2H-chromen-2-one Light yellow solid, mp
223–226 °C; Yield 98%. 1H NMR (400 MHz, DMSO-d6): δ 7.4 (d,
1H, J=8.8 Hz), 6.56 (d, 1H, J=7.2 Hz), 6.39 (s, 1H), 6.09 (s,
2H), 5.89 (s, 1H), 2.39 (s, 3H). 13 C NMR (100 MHz, DMSO-d6):
δ 160.90, 152.80, 151.00, 148.00, 127.60, 113.00, 112.50,
111.10, 106.70, 21.2. IR (KBr, cm−1): 3439 (N–H),1684 (C=O).
282–285 °C; Yield 98%. 1H-NMR (400 MHz, DMSO-d6): δ 10.51
(1H, s), 10.28 (1H, s), 6.24 (1H, s), 6.15 (1H, s), 5.83 (1H, s),
2.49 (3H, s).13 C-NMR (100 MHz, DMSO-d6): δ 16.51, 160.55,
158.39, 156.96, 155.43, 109.22, 102.55, 99.45, 94.98, 23.88. IR
(KBr cm−1): 3400, 3070, 2940, 1670, 1600, 1480, 1080.
(10a) : 7-Methoxy-4-methyl-2H-chromen-2-one Colorless prisms, mp
162–164 °C; Yield 94%.1H NMR (400 MHz, CDCl3): δ 7.50 (d, 1H,
J=8.8 Hz), 6.87 (dd, 1H, J=8.8, 2.4 Hz), 6.83 (d, 1H, J=2.4 Hz),
6.14 (d, 1H, J=1.2 Hz), 3.88 (s, 3H), 2.40 (d, 3H, J=1.2 Hz). 13
C
NMR (100 MHz, CDCl3): δ 162.83, 160.59, 155.24, 153.86, 126.88,
113.56, 112.54, 111.58, 101.16, 56.36, 18.58. IR (KBr, cm-1): 3400
(OH), 1705 (C=O).
Fig. 4. Effect of ZrOCl2.8H2O/SiO2 as catalyst in synthesis of coumarin.
Fig. 5. Effect of temperature in synthesis of coumarin.

B. Karami, M. Kiani / Catalysis Communications 14 (2011) 62–67
65
Table 1
Synthesis of coumarins via von Pechmann condensation of phenols with β-ketoesters induced by ZrOCl2.8H2O/SiO2.
Entry
1
Substituted phenol
β-Ketoester
Producta
Time (min)
40
Mp (°C)
Yields (%)b
O
O
185–1876
94
CH3
O
H3C
ph
OC2H5
HO
HO
HO
OH
OH
OH
HO
O
(1a)
2
80
252–25523
88
O
O
Ph
O
OC2H5
HO
HO
O
(2a)
3
4
O
O
10
35
179–18224
257–2606
91
89
CH2Cl
ClH2C
OC2H5
O
O
(3a)
O
O
CH3
CH3
CH3
OH CH3
H3C
ph
OC2H5
HO
HO
HO
OH
OH
OH
H3C
H3C
O
O
(4a)
5
6
70
8
211–213
80
81
O
O
OH Ph
OC2H5
O
O
(5a)
O
O
l
162–16529
CH3 CH2C
ClH2C
OC2H5
HO
HO
HO
HO
O
O
(6a)
7
OH
OH
OH
OH
O
O
10
40
5
282–2856
241–24424
185–18824
162–16420
98
94
98
94
OH CH3
H3C
ph
OC2H5
HO
HO
HO
OH
OH
OH
O
O
(7a)
8
OH Ph
O
O
OC2H5
O
O
(8a)
9
O
O
OH CH2Cl
ClH2C
OC2H5
O
O
(9a)
10
O
O
35
CH3
H3C
OC2H5
O
O
OCH3
H3CO
(10a)
11
12
OH
O
O
30
45
179–18229
153–15521
96
75
CH2Cl
ClH2C
OC2H5
O
O
OCH3
H3CO
(11a)
OH
O
O
O
O
H3C
OC2H5
O
O
CH3
(12a)
13
O
O
45
163–16526
99
OH
ClH2C
OC2H5
CH2Cl
(13a)
(continued on next page)

66
B. Karami, M. Kiani / Catalysis Communications 14 (2011) 62–67
Table 1 (continued)
Entry
14
Substituted phenol
β-Ketoester
Producta
Time (min)
Mp (°C)
Yields (%)b
89
O
O
CH3
O
40
240–24227
H3C
OC2H5
HO
OH
OH
HO
O
OH
(14a)
CH3
15
OH
O
O
35
223–22622
98
H3C
OC2H5
NH2
O
O
(15a)
H2N
a Products were characterized by their IR and NMR spectra. b) Yields refer to isolated products.
3. Results and discussion
were treated with ZrOCl2.8H2O/SiO2 (10 mol %) at 90 °C for 40 min to
give the corresponding coumarin 1a in 94% yield. Coumarin 1a was
characterized by FT-IR, NMR and other spectral data. By similar
method, resorcinol 1 was treated with ethyl 4-chloroacetoacetate,
ethyl benzoylacetate (Table 1, entries 2 and 3) to furnish coumarins 2a
and 3a, respectively.
3.1. Characterizations
The XRD pattern of ZrOCl2.8H2O/SiO2 is shown in Fig. 1. It was
reported that high degree mixing of Zr–Si in oxide form often led to
the absence of XRD pattern for ZrOCl2.8H2O. The broad peak around
22° (2θ) from the smaller inset could be attributed to insertion of Zr
into the framework of amorphous SiO2. The XRF data of ZrOCl2.8H2O/
SiO2 is shown in Fig. 2 and indicates the presence of Cl and ZrO in the
supported ZrOCl2.8H2O/SiO2 matrix.
The FT-IR spectra of ZrOCl2.8H2O, SiO2 and ZrOCl2.8H2O/SiO2 are
shown in Fig. 3. The spectrum of ZrOCl2.8H2O/SiO2 shows the
characteristic bonds of ZrOCl2.8H2O and SiO2.The wave numbers in
800, 1020, 1150, 1610, 2040 and 3400 cm−1 of ZrOCl2 were observed
in catalyst spectra.
According to the above results, other phenolic substrates were
subjected to the Pechmann reaction using ZrOCl2.8H2O/SiO2. In all
cases, the reactions proceeded successfully to afford the correspond-
ing coumarins in good to excellent yields. The electron-donating
substituents in the meta position to the phenolic –OH facilitated the
cyclization. The reactivity of phloroglucinol with ethyl acetoacetate
was observed to be higher than pyrrogallol due to two hydroxyl
groups at the meta-positions in phloroglucinol compared to one
hydroxy group in pyrrogallol. Presence of m-hydroxy group strongly
activates the substrates due to resonance effect. m-methoxyphenol
(entry 10) showed no detectable demethylation under the reaction
conditions. Likewise, 1-naphthol (entry 12) requires a slightly higher
temperature and longer reaction time. These results show that besides
the catalyst acidity and the reactivity of phenolic substrates, the
structural feature of catalyst could also play significant roles in
manipulating the course of this reaction.
3.2. Pechmann condensation of aromatic alcohols
To study the efficiency of ZrOCl2.8H2O/SiO2 as a catalyst for
Pechmann condensation, the reaction of β-ketoesters and phenol
derivatives was selected as a model. The expected coumarins were
obtained as pure products in high yield under solvent-free conditions
(Scheme 1).
In order to further optimize the reaction conditions, we also
studied the effect of catalyst amount on these condensation reactions.
As can be observed from Fig. 4, good yields are achieved by using only
10 mol% of ZrOCl2.8H2O. It can also be extracted from Fig. 5 that, the
best temperature was 90 º C. There was no change in the reaction
profile when more than 20 mol% of ZrOCl2.8H2O was used, and very
slow conversion was observed when smaller amount of ZrOCl2.8H2O
was utilized. When similar reactions were carried out in the absence
of the catalyst, approximately 5–10% conversion was obtained after
2 days. The moderate temperature (90 °C) was also necessary for high
conversions.
From Table 1, it is observed that for most of the substrates, the
reaction time is reduced drastically even at ambient conditions in
contrast to reported methods, with an excellent yield of the
coumarins. The reactions are remarkably clean, and no chromato-
graphic separation is necessary to get the spectra-pure compounds.
3.3. Catalyst recycle study
In order to check the stability of the catalyst, the catalyst was
recycled (fresh+3 cycles) using ZrOCl2.8H2O/SiO2 in Pechmann con-
densation. The results are presented in Table 2. After workup of the
reaction mixture, the catalyst was separated by filtration, washed
Furthermore, to optimize the reaction conditions, equimolar
quantities of resorcinol 1 (Table 1, entry 1) and ethyl acetoacetate
Table 3
Comparison of the results for the preparation of 7-Hydroxy-4-methyl-2H-chromen-2-
one with other catalysts.
Table 2
Catalyst
mol % Time (min) Temperature(°C)
Yield %) [Ref.]
Reusability of the ZrOCl2.8H2O/SiO2.
Sc(OTf)3
CAN
ZrCl4
10
10
2
10
20
60
10
8
42
12
80
110
70
99 [26]
94 [27]
95 [30]
Product
Recovery
Time (min)
Yields (%)
—
1
2
3
4
5
6
7
8
10
15
18
20
22
23
25
26
30
98
83
82
78
78
76
76
76
75
GaI3
Reflux (under CH2Cl2) 96 [31]
70
OH CH3
Ionic liquid
anhyd.FeCl3
Sm(NO3)3.6H2O
InCl3
Bi(NO3)3.5H2O
ZrOCl2.8H2O/SiO2 10
80 [32]
10
10
5
20
30
15
10
80
65
80
80
98 [33]
98 [34]
94 [35]
98
HO
O
O
⁎
⁎
Results of this work.

B. Karami, M. Kiani / Catalysis Communications 14 (2011) 62–67
67
Scheme 2. The suggested mechanism for the Pechmann condensation of phenols with β-ketoesters in the presence of ZrOCl2.8H2O/SiO2 catalyst.
with chloroform and dried. The recovered catalyst after each reaction
was reused in other reaction.
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4. Conclusions
In conclusion, the present procedure of the synthesis of coumarins by
the ZrOCl2.8H2O/SiO2 catalyzed condensation of phenol and β-ketoesters
provides an efficient and much improved modification of the von
Pechmann reaction. In addition to its simplicity and mild reaction
conditions, this method has the ability to tolerate a wide variety of
substitutions in both components. Thus, this method will offer easy
access to substituted coumarins with varied substitution patterns in high
yields. It is believed that this procedure will find important applications
in the synthesis of coumarins.
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Acknowledgements
The authors gratefully acknowledge Yasouj University of Iran for
supporting this work.