Zinc oxide nanoparticles: A highly efficient and readily recyclable catalyst for the synthesis of xanthenes

Article abstract of DOI:10.1016/j.cclet.2012.09.016

A novel and efficient route for the synthesis of 1,8- dioxooctahydroxanthenes and 14-aryl-14H-dibenzo[a,j]xanthenes is described through one-pot multi-component reaction of dimedone and 2-naphthol with various aryl aldehydes using ZnO nanoparticles under

Full text of DOI:10.1016/j.cclet.2012.09.016

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 1225–1229
www.elsevier.com/locate/cclet
Zinc oxide nanoparticles: A highly efficient and readily recyclable
catalyst for the synthesis of xanthenes
*
Javad Safaei-Ghomi , Mohammad Ali Ghasemzadeh
Department of Organic Chemistry, Faculty of Chemistry, University of Kashan, Kashan 51167, Islamic Republic of Iran
Received 2 July 2012
Available online 24 October 2012
Abstract
A novel and efficient route for the synthesis of 1,8-dioxooctahydroxanthenes and 14-aryl-14H-dibenzo[a,j]xanthenes is
described through one-pot multi-component reaction of dimedone and 2-naphthol with various aryl aldehydes using ZnO
nanoparticles under solvent-free conditions. This method provides a novel and improved pathway for the synthesis of xanthenes
in the terms of excellent yields, short reaction times, reusability and low catalyst loading.
# 2012 Javad Safaei-Ghomi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: ZnO nanoparticles; Xanthene; Solvent-free; Multi-component reaction; Heterogeneous catalysts
During the last decade, nanocrystalline metal oxides have received significant attention as efficient catalysts in
many organic reactions due to their high surface-to-volume ratio and coordination parts which provide a larger number
of active sites per unit area in comparison with their heterogeneous counter sites [1,2]. Nanocrystalline zinc oxide is
one of the extensively used surface materials for many chemical transformations, such as gas sensors [3], photoactivity
and flame-retardancy [4], solar cells [5], semiconductor [6] and antibacterial materials [7]. In last decade zinc oxide
nanoparticles were used as an active catalyst in many reactions including the Mannich reaction [8], the Knoevenagel
condensation [9] and synthesis of coumarins [10], b-phosphono malonates [11], benzimidazole [12], b-acetamido
ketones/esters [13], 4-amino-5-pyrimidinecarbonitriles [14], polyhydroquinoline [15] and the synthesis of 2,3-
disubstituted quinalolin-4(1H)-ones [16].
In recent years, great attention has been paid to the synthesis of xanthene derivatives because of their highly
absolute biological and physiological activities [17,18]. These compounds have found wide use in dyes [19], laser
technologies [20] and as pH-sensitive fluorescent materials [21].
There are several methods reported in the literature for the synthesis of 1,8-dioxooctahydroxanthenes involving the
condensation of two equivalent of dimedone (5,5-dimethyl-1,3-cyclohexane dione) with various aldehydes [22],
which catalysis by different catalysts such as alum [23], p-dodecylbenzenesulfonic acid [24], Fe3+-montmorilonite
[25], NaHSO4–SiO2 or silica chloride [26] and amberlyst-15 [27]. The classical method for the synthesis of 14-aryl-
14H-dibenzo[a,j]xanthenes is the condensation of 2-naphthol and aldehydes in the presence of various catalysts
including AcOH–H2SO4 [28], p-TSA [29], sulfamic acid–ionic liquid [30], iodine [31], heteropolyacid [32],
* Corresponding author.
E-mail address: safaei@kashanu.ac.ir (J. Safaei-Ghomi).
1001-8417/$ – see front matter # 2012 Javad Safaei-Ghomi. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.09.016

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J. Safaei-Ghomi, M.A. Ghasemzadeh / Chinese Chemical Letters 23 (2012) 1225–1229
Scheme 1. ZnO nanoparticles catalyzed one-pot synthesis of xanthene derivatives.
amberlyst-15 [33], cyanuric chloride [34], LiBr [35], Yb(OTf)3 [36], Sc[N(SO2C8F17)2]3 [37], NaHSO4 [38], P2O5,
InCl3 [39], tetra-n-butylammonium bromide [40] and nano TiO2 [41].
Consequently, synthesis of 1,8-dioxooctahydroxanthene and 14-aryl-14H-dibenzo[a,j]xanthene derivatives with
the aim to develop new drug molecules has been an active area of the research. Herein we wish to report a novel and
effective method to the synthesis of xanthenes via multi-component coupling of dimedone and 2-naphthol with various
aryl aldehydes in the presence of ZnO nanoparticles (Scheme 1).
1. Experimental
Zinc oxide nanoparticles were prepared according to the procedure reported by Shen et al. [42]. In a typical
procedure, zinc acetate (9.10 g, 0.05 mol) and oxalic acid (5.4 g, 0.06 mol) were combined by grinding in an agate
mortar for 1 h at room temperature. Afterwards, the formed ZnC2O4Á2H2O nanoparticles were calcinated at 450 8C for
30 min to produce ZnO nanoparticles under thermal decomposition conditions.
The chemical purity of the samples as well as their stoichiometry was tested by EDAX studies. The EDAX spectra
given in Fig. 1a shows the presence of Zn and O as the only elementary components.
As shown in Fig. 1b the crystalline nature of the synthesized ZnO nanoparticles sample was further verified by X-
ray diffraction pattern (XRD). The crystallite size diameter (D) of the ZnO nanoparticles has been calculated by
Debye–Scherrer equation (D = Kl/b cos u). Crystallite size of ZnO has been found to be 24 nm.
In order to study the morphology and particle size of ZnO nanoparticles, SEM image of ZnO nanoparticles was
presented in Fig. 1c. These results show that hexagonal ZnO NPs were gained with particle size between 20 and 30 nm.
A mixture of 5,5-dimethyl-1,3-cyclohexanedione (0.28 g, 2 mmol), various aldehydes (1 mmol) and ZnO NPs
(0.01 g, 0.1 mmol, 10 mol%) was heated at 80 8C for 15–35 min. After completion of the reaction as indicated by
TLC, the reaction mixture was cooled to room temperature and solid obtained was dissolved in dichloromethane, the
catalyst was insoluble in CH2Cl2 and separated by a simple filtration. The solvent was evaporated and the residue was
recrystallized from ethanol to afford the pure 1,8-dioxooctahydroxanthenes.
A mixture of 2-naphthol (0.28 g, 2 mmol), aldehyde (1 mmol) and ZnO NPs (0.01 g, 0.1 mmol, 10 mol%) in a
round bottom flask was heated in the oil bath at 80 8C for 20–40 min. Upon completion, the work-up and purification
of the products were performed as mentioned in previous procedure.
3,4,6,7-Tetrahydro-9-(4-isopropylphenyl)-3,3,6,6-tetramethyl-2H-xanthene-1,8-(5H,9H)-dione (4e). Yellow solid;
mp: 203–206 8C; 1H NMR (CDCl3): d 1.00 (s, 6H, 2 Â CH3), 1.10 (s, 6H, 2 Â CH3), 1.18 (d, 6H, 2 Â CH3), 2.16–2.26
(m, 4H, 2 Â CH2), 2.46 (s, 4H, 2 Â CH2), 2.78–2.81 (m, 1H, CH), 4.73 (s, 1H, CH), 7.04–7.06 (d, 2H, J = 8.1 Hz, Ar–
H), 7.17–7.19 (d, 2H, J = 8.1 Hz, Ar–H). 13C NMR (CDCl3): d 23.9, 27.4, 29.2, 31.3, 32.2, 33.6, 40.8, 50.8, 115.8,
Fig. 1. (a) EDX image of ZnO NPs; (b) XRD pattern of ZnO NPs; and (c) SEM image of ZnO nanoparticles.

J. Safaei-Ghomi, M.A. Ghasemzadeh / Chinese Chemical Letters 23 (2012) 1225–1229
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126.1, 128.1, 141.3, 146.5, 162.1, 196.4. FT-IR (KBr, cmÀ1): 3071, 2961, 1665 (C O), 1624 (C C, Ar), 1359, 1198
(C–O). Anal. calcd. for C26H32O3: C 79.56, H 8.22. Found C 79.39, H 8.36. MS (EI) (m/z): 392 (M+).
14-(4-Cyanophenyl)-14H-dibenzo[a,j]xanthene (5n). White crystal; m.p. = 218–219 8C; 1H NMR (CDCl3): d 6.61
(s, 1H, CH) 7.43–7.45 (d, 2H, J = 8 Hz, Ar–H), 7.51–7.53 (d, 2H, J = 7.8 Hz, Ar–H), 7.59–7.61(t, 2H, Ar), 7.68–7.70
(d, 2H, J = 8 Hz, Ar–H), 7.84–7.87 (m, 4H, Ar), 8.00–8.02 (d, 2H, J = 7.8 Hz, Ar–H), 8.28–8.30 (d, 2H, Ar). 13C NMR
(CDCl3): d 36.9, 95.1, 115.9,116.2, 117.1, 120.2, 122.5, 124.8, 126.1, 128.1, 129.7, 132.6, 134.1, 135.9, 148.3, 159.1.
FT-IR (KBr, cmÀ1): 3041, 2218 (CBN), 1622 (C C, Ar), 1583, 1242 (C–O), 809. Anal. calcd. for C28H17NO: C 87.71,
H 4.47, N 3.65. Found C 87.58, H 4.59. N 3.78. MS (EI) (m/z): 383 (M+).
2. Results and discussion
In order to determine the optimized reaction conditions, we selected reaction of 4-bromo-benzaldehyde and
dimedone as model study in this research. The reaction conditions were optimized on the basis of the catalysts,
solvents and different temperatures for the synthesis 1,8-dioxooctahydroxanthenes (Table 1).
Afterwards, to test the efficiency of the catalytic activity, we choose to focus our initial studies using various
nanocatalysts such as MgO, NiO, Mn3O4 and ZnO under solvent-free conditions (Table 1). As a result of these
experiments we found that ZnO NPs (Table 1, entry 5) is the most effective catalyst for these condensation reactions.
Duringoptimizationofreactionconditionsthemodelreactionwasperformedindifferentsolventsandalsoundersolvent-
free conditions. In according to Table 1 no product was obtained in the absence of the catalyst and in the presence of the
catalystatroomtemperature(Table1, entries1and2), indicatingthecatalystandhightemperaturearenecessaryforthese
reactions. We found that the best results were obtained when the reaction was carried out at 80 8C under solvent-free
conditions. Also the resulting of Table 1 shows the optimum amount of the catalyst was 0.01 g (0.1 mmol, 10 mol%) of
ZnO NPs which increasing of this amount did not show any significant change in yield and time of reaction.
In the continuations of this research, we encouraged testing reaction of 2-naphthol and aldehydes. Thus, we used
the optimized reaction conditions in the presence of ZnO NPs to produce 14-aryl-14H-dibenzo[a,j]xanthene
derivatives. A series of experiments were carried out and as a result of these we prepared a number of 14-aryl-14H-
dibenzo[a,j]xanthenes in high yields and short reaction times.
In the present work, we successfully synthesized ZnO nanoparticles and used it in synthesis of 1,8-
dioxooctahydroxanthene and 14-aryl-14H-dibenzo[a,j]xanthene derivatives (Scheme 1). Application of this catalytic
system effectively gives desired products in excellent yields (Table 2). As seen from Table 2, aromatic aldehydes with
electron-withdrawing groups such as NO2 and Cl reacted faster than those with electron-releasing groups such as
isopropyl and methoxy as expected; also sterically hindered aromatic aldehydes required longer reaction times.
A plausible mechanism for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes using ZnO NPs has been shown in
Scheme 2. We suppose that zinc oxide nanoparticles behave as the Lewis acid and coordinate with carbonyl and
hydroxyl groups to promote cyclization reaction, so interaction of nanocatalyst with reactants leading to speed up the
rate of reaction. Finally the product was obtained and the ZnO NPs being released for further reactions. Moreover the
Table 1
Optimization of model reaction by catalyst amount of ZnO NPs, solvent and temperature.a
Entry
Catalyst (mol%)
Solvent
T (8C)
Time (min)
Yield (%)b
1
2
None
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
Solvent-free
CH2Cl2
80
300
100
60
None
30
MgO NPs (20%)
NiO NPs (20%)
Mn3O4 NPs (20%)
ZnO NPs (20%)
ZnO NPs (15%)
ZnO NPs (10%)
ZnO NPs (5%)
ZnO NPs (10%)
ZnO NPs (10%)
ZnO NPs (10%)
rt
3
80
45
4
80
40
60
5
80
17
92
6
80
17
92
7
80
17
92
8
80
20
85
8
Reflux
Reflux
Reflux
150
120
180
45
9
CH3CN
60
10
EtOH
50
a
Reaction conditions: dimedone (2 mmol) and 4-bromo-benzaldehyde (1 mmol).
Isolated yields.
b

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J. Safaei-Ghomi, M.A. Ghasemzadeh / Chinese Chemical Letters 23 (2012) 1225–1229
Table 2
Synthesis of 1,8-dioxooctahydroxanthenes (4) and 14-aryl-14H-dibenzo[a,j]xanthenes (5) catalyzed by ZnO NPs.
Entry
Aldehyde
Products 4(a–o)
M.p. (8C) Ref.
Products 5(a–o)
M.p. (8C) Ref.
Time (min)/yield (%)a
Time (min)/yield (%)a
1
2
C6H5
22/91
32/87
23/88
30/86
35/82
15/96
17/92
15/97
19/91
24/91
17/93
34/84
20/93
19/95
32/86
201–203 [23]
214–215 [23]
248–250 [23]
241–243 [23]
203–205b
28/87
31/83
28/84
37/80
40/78
20/94
23/92
20/95
28/90
25/86
24/92
35/86
25/92
28/94
37/86
181–183 [39]
227–229 [39]
135–136 [41]
203–204 [39]
152–154 [39]
287–289 [39]
296–297 [39]
310–312 [39]
211–212 [29]
211–212 [41]
239–240 [39]
213–215 [41]
252–254b
4-CH3C6H4
4-HOC6H4
4-MeOC6H4
4-(CH3)2CHC6H4
4-ClC6H4
3
4
5
6
227–229 [23]
240–241 [23]
225–226 [23]
166–168 [23]
183–184 [23]
225–226 [23]
246–248 [22]
211–213b
7
4-BrC6H4
8
4-NO2C6H4
3-NO2C6H4
3-ClC6H4
9
10
11
12
13
14
15
4-FC6H4
2-NO2C6H4
4-CHOC6H4
4-NCC6H4
4-MeSC6H4
216–217b
256–257b
218–219b
264–266b
a
Isolated yield.
b
New products.
Scheme 2. The proposed mechanism for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes catalyzed by ZnO NPs.
influence of catalytic behavior of ZnO NPs for the synthesis of 1,8-dioxooctahydroxanthenes is the same as above-
mentioned mechanism for the preparation of 14-aryl-14H-dibenzo[a,j]xanthenes.
3. Conclusion
In conclusion, we have demonstrated an efficient, one-pot method for the synthesis of 1,8-dioxooctahydroxanthene
and 14-aryl-14H-dibenzo[a,j]xanthene derivatives using zinc oxide nanoparticles as a mild, green and eco-
environmentally catalyst under solvent-free conditions. The simplicity, easy workup, as well as safety and reusability
of catalyst are advantages of this procedure over the previously reported ones.
Acknowledgments
The authors are grateful to University of Kashan for supporting this work by Grant 159196/VII. Also we thank Iran
National Science Foundation for supporting this work.

J. Safaei-Ghomi, M.A. Ghasemzadeh / Chinese Chemical Letters 23 (2012) 1225–1229
1229
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