ZrOCl2·8H2O as an efficient catalyst for the synthesis of dibenzo [b,i]xanthene-tetraones and fluorescent hydroxyl naphthalene-1,4-diones

Article abstract of DOI:10.1007/s11164-013-1413-8

Abstract An efficient and environmentally benign protocol for the synthesis of dibenzo [b,i]xanthene-tetraones and fluorescent hydroxyl naphthalene-1,4-diones in the presence of a catalytic amount of ZrOCl₂·8H₂O as an inexpensive and eco-friendly catalyst with high catalytic activity under solvent-free conditions is reported. Graphical abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-013-1413-8

Res Chem Intermed
DOI 10.1007/s11164-013-1413-8
ZrOCl2ꢀ8H2O as an efficient catalyst for the synthesis
of dibenzo [b,i]xanthene-tetraones and fluorescent
hydroxyl naphthalene-1,4-diones
•
Hamid Reza Tavakoli Sayed Mojtaba Moosavi
Ayoob Bazgir
•
Received: 15 April 2013 / Accepted: 18 September 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract An efficient and environmentally benign protocol for the synthesis of
dibenzo [b,i]xanthene-tetraones and fluorescent hydroxyl naphthalene-1,4-diones in the
presence of a catalytic amount of ZrOCl2ꢀ8H2O as an inexpensive and eco-friendly
catalyst with high catalytic activity under solvent-free conditions is reported.
Keywords Dibenzoxanthene ꢀ Hydroxyl naphthalene-1,4-diones ꢀ ZrOCl2 ꢀ
Solvent-free
Introduction
The use of zirconium(IV) salts as an efficient Lewis acid for various transformations
has been well documented in the literature, because of their easy availability,
moisture stability, and low toxicity [1–3]. Among the various types of Zr(IV) salts,
ZrOCl2ꢀ8H2O in particular has the advantages of moisture stability, readily
availability, and easy handling [2]. Also, the low toxicity of ZrOCl2ꢀ8H2O is evident
from the LD50 [LD50 (ZrOCl2ꢀ8H2O, oral rat) = 3,500 mg/kg] [3]. Therefore, the
application of ZrOCl2ꢀ8H2O in organic synthesis is of renewed interest.
It has been well known for many years that dyes have been most widely used in
dyeing, as well as in high technology as such as lasers, liquid crystalline displays,
electro-optical devices, and ink-jet printers [4, 5]. Xanthene derivatives occupy an
important position among different families of dyes, owing to a number of reasons
related to their photochemical and photophysical properties [6]. Rose Bengal, Eosin,
and other xanthene dyes are the most frequently employed dye-sensitizers when a
H. R. Tavakoli ꢀ A. Bazgir (&)
Department of Chemistry, Shahid Beheshti University, G.C., P. O. Box 19396-4716, Tehran, Iran
e-mail: a_bazgir@sbu.ac.ir
H. R. Tavakoli ꢀ S. M. Moosavi
Department of Chemistry, Imam Hossein University, Tehran, Iran
123

H. R. Tavakoli et al.
quantitative interpretation of the photodynamic effect is required [7]. In recent
years, a number of analyte sensors have been designed using these scaffolds via
synthesis of new xanthene-based dyes [8]. Furthermore, xanthenes have been
reported to possess diverse biological and therapeutic properties such as antibac-
terial, antiviral, and anti-inflammatory activities, as well as being used in
photodynamic therapy [9, 10].
Fluorescent heterocyclic compounds are of interest in many disciplines as,
e.g., emitters for electroluminescence devices [11], molecular probes for
biochemical research [12], in traditional textile and polymer fields [13],
fluorescent whitening agents [14], and photo-conducting materials [15]. Quinone
derivatives have been reported as fluorescence compounds [16–19]. On the other
hand, molecules with the quinone structure constitute one of the most interesting
classes of compounds in organic chemistry, due to their biological properties,
their industrial applications, and their potential as intermediates in the synthesis
of heterocycles [20].
As part of our research aimed at developing new methods for organic
transformations [21–24], and due to the unique advantages of ZrOCl2ꢀ8H2O, the
aim of our research described here was to develop the synthesis of dibenzo
[b,i]xanthene-tetraones and fluorescent hydroxyl naphthalene-1,4-diones employing
ZrOCl2ꢀ8H2O as an efficient and mild Lewis acid catalyst.
Experimental
General procedure for the synthesis of 3
A mixture of 2-hydroxynaphthalene-1,4-dione (2 mmol), aldehydes (1 mmol), and
ZrOCl2ꢀ8H2O (20 mol %) was stirred at 100 °C for an appropriate time (the
progress of the reaction was monitored by TLC). After completion, the precipitated
reaction mixture was washed with H2O (5 ml) and EtOH (5 ml) to afford the solid
pure product 3. The catalyst was separated from the precipitated reaction mixture
simply by washing with water and EtOH, and reused after washing with CH2Cl2 and
drying at 60 °C.
General procedure for the synthesis of 5
A mixture of 2-hydroxynaphthalene-1,4-dione (1 mmol), aldehydes (1 mmol),
amines (1 mmol) and ZrOCl2ꢀ8H2O (20 mol %) was stirred at 100 °C for an
appropriate time (the progress of the reaction was monitored by TLC). After
completion, the precipitated reaction mixture was washed with H2O (5 ml) and
EtOH (5 ml) to afford the solid pure product 5. The catalyst was separated from the
precipitated reaction mixture simply by washing with water and EtOH, and reused
after washing with CH2Cl2 and drying at 60 °C.
All the products are known and were fully characterized by a comparison with
authentic samples (melting point) and IR spectra [22, 24].
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ZrOCl2ꢀ8H2O as an efficient catalyst
Table 1 Screening of the reaction conditions
O
O
Ph
O
O
O
OH
ZrOCl2.8H2O
PhCHO
O
O
1
2a
3a
Entry
Solvent
ZrOCl2ꢀ8H2O
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
EtOH (Reflux)
CHCl3(Reflux)
CH3CN(Reflux)
S.F. (100 °C)
S.F. (100 °C)
S.F. (100 °C)
S.F. (80 °C)
20 mol%
20 mol%
20 mol%
20 mol%
25 mol%
15 mol%
20 mol%
0
10
10
10
6
45
Trace
\30
84
6
83
6
75
6
65
S.F. (100 °C)
6
\30
S.F. Solvent-free
Table 2 Synthesis of dibenzoxanthene-tetraones 3
O
O
O
Ar
O
O
OH
ZrOCl2.8H2O (20 mol%)
ArCHO
2a-g
Neat, 100 oC
O
O
1
3a-g
Product 3
Ar
Time (h)
Yield (%)
a
b
c
d
e
f
C6H5
6
84 (82.80)a
4-Cl-C6H4
4-Br-C6H4
4-F-C6H4
6
81
79
82
79
80
90
6
6
4-Me-C6H4
2-Cl-C6H4
3-NO2-C6H4
7
6.5
6
g
a
Isolated yield after recycling of catalyst
Results and discussion
Initially, the reaction of 2-hydroxynaphthalene-1,4-dione 1 (2 mmol) and benzal-
dehyde 2a (1 mmol) as a simple model substrate in the presence of ZrOCl2ꢀ8H2O in
different solvents and under solvent-free conditions was investigated to optimize the
123

H. R. Tavakoli et al.
reaction conditions. It was found that the reaction under solvent-free conditions at
100 °C after 6 h resulted in a higher isolated yield (Table 1). Similarly, the molar
ratio of ZrOCl2ꢀ8H2O was studied with the optimum amount being 20 mol % (entry
4). When this reaction was carried out without ZrOCl2ꢀ8H2O, the yield of the
expected product was a trace (entry 8).
Using the optimized conditions, the generality of this reaction was examined
using several types of aromatic aldehydes 3a–h. In all cases, the reactions gave the
corresponding products in good isolated yields (Table 2).
Another advantage of this approach could be related to the heterogeneous
catalytic process under solvent-free conditions and the reusability of catalyst. We
found that the catalyst could be separated from the reaction mixture simply by
washing with water and reusing after washing with CH2Cl2 and drying at 60 °C.
The reusability of the catalyst was checked by the reaction of 2-hydroxynaphtha-
lene-1,4-dione 1 and benzaldehyde 2a under optimized reaction conditions. The
results showed that the catalyst can be used effectively three times without any loss
of its activity (Table 2, entry 1). Therefore, the recyclability of catalyst makes the
process economical and potentially viable for commercial applications.
To further explore the potential of ZrOCl2ꢀ8H2O, we investigated the reaction of
2-hydroxynaphthalene-1,4-dione 1 and aldehydes 2 with aromatic amines 4 and
obtained fluorescent hydroxyl naphthalene-1,4-dione derivatives 5 in good isolated
yields under the same reaction conditions (Table 3).
The proposed mechanism involves the attack of in situ-generated enolate I on the
in situ-generated aldimine II as shown in Scheme 1 [25]. ZrOCl2, used in catalytic
amounts, is recycled in the reaction.
Table 3 Synthesis of hydroxyl naphthalene-1,4-diones 5
O
O
Ar
O
Ar'
OH
ArCHO + Ar'NH2
ZrOCl2.8H2O (20 mol%)
Neat, 100 oC
N
H
OH
O
1
2
4
5
Product 5
Ar
C6H5
Ar0
Time (h)
Yield (%)
a
b
c
d
e
f
C6H5
7
7
7
7
9
8
9
9
7
7
88
80
81
85
85
84
83
86
79
87
4-Me–C6H4
4-OH–C6H4
4-NO2–C6H4
C6H5
C6H5
C6H5
C6H5
4-Me–C6H4
4-Me–C6H4
4-Me–C6H4
4-Me–C6H4
4-Me–C6H4
4-Me–C6H4
4-MeO–C6H4
4-Me–C6H4
4-OH–C6H4
4-NO2–C6H4
2-Cl–C6H4
g
h
i
j
123

ZrOCl2ꢀ8H2O as an efficient catalyst
O
O
Cl
Cl
Zr
Cl
Cl
O
Zr
OH
Cl
Cl
+
Ar'NH2
N
Ar
Ar
Zr
O
H
Ar'
O
II
O
O
Ar
Ar'
I
N
Cl
Cl
H
O
O
Zr
OH
O
H
O
O
Cl
Cl
Zr
O
O
HO
O
Scheme 1 Proposed mechanism for the synthesis of 5
Conclusion
In conclusion, we have demonstrated that ZrOCl2ꢀ8H2O can be used as a green and
reusable catalyst for the efficient synthesis of dibenzo [b,i]xanthene-tetraones and
fluorescent hydroxyl naphthalene-1,4-diones under solvent-free conditions. More-
over, the cheapness, easy availability of the reagent, and easy and clean workup
make this method attractive for organic chemists.
Acknowledgment We are grateful for financial support from the Research Council of Shahid Beheshti,
University.
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