Supported nitric acid on silica gel and polyvinyl pyrrolidone (PVP) as an efficient oxidizing agent for the oxidation of urazoles and bis-urazoles

Article abstract of DOI:10.1080/00397910902898619

A method for the oxidation of a good range of urazoles and bis-urazoles to the corresponding triazolinediones by supported nitric acid on silica gel (SiO2-HNO3) and/or polyvinyl pyrrolidone (PVP-HNO3) is described. Reactions have been carried out heterogeneously at room temperature in dichloromethane with good to excellent yields.

Full text of DOI:10.1080/00397910902898619

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Supported Nitric Acid on Silica
Gel and Polyvinyl Pyrrolidone
(PVP) as an Efficient Oxidizing
Agent for the Oxidation of
Urazoles and Bis-urazoles
Arash Ghorbani-Choghamarani a , Zahra Chenani a
Shadpour Mallakpour b
&
a Department of Chemistry, Faculty of Science , Ilam
University , Ilam, Iran
b Organic Polymer Chemistry Research Laboratory,
Department of Chemistry , Isfahan University of
Technology , Isfahan, Iran
Published online: 29 Oct 2009.
To cite this article: Arash Ghorbani-Choghamarani , Zahra Chenani & Shadpour
Mallakpour (2009) Supported Nitric Acid on Silica Gel and Polyvinyl Pyrrolidone
(PVP) as an Efficient Oxidizing Agent for the Oxidation of Urazoles and Bis-urazoles,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 39:23, 4264-4270, DOI: 10.1080/00397910902898619
To link to this article: http://dx.doi.org/10.1080/00397910902898619
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Synthetic Communications1, 39: 4264–4270, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902898619
Supported Nitric Acid on Silica Gel and Polyvinyl
Pyrrolidone (PVP) as an Efficient Oxidizing
Agent for the Oxidation of Urazoles and
Bis-urazoles
Arash Ghorbani-Choghamarani,1 Zahra Chenani,1 and
Shadpour Mallakpour2
1Department of Chemistry, Faculty of Science, Ilam University, Ilam, Iran
2Organic Polymer Chemistry Research Laboratory, Department of
Chemistry, Isfahan University of Technology, Isfahan, Iran
Abstract: A method for the oxidation of a good range of urazoles and
bis-urazoles to the corresponding triazolinediones by supported nitric acid on
silica gel (SiO2-HNO3) and=or polyvinyl pyrrolidone (PVP-HNO3) is described.
Reactions have been carried out heterogeneously at room temperature in
dichloromethane with good to excellent yields.
Keywords: Bis-urazoles, oxidation, polyvinyl pyrrolidone, triazolinediones,
urazoles
INTRODUCTION
In the past few years, supported reagents[1–4] have become increasingly
used in organic synthesis, mainly because the reactions are carried out
under mild conditions and the organic products are easily isolated. In
addition, the stability, low cost, heterogeneous nature of the reactions,
good yields of the products, short reaction times, and reusability are
other important advantages of these reagents.[5]
Received January 12, 2009.
Address correspondence to Arash Ghorbani-Choghamarani, Department of
Chemistry, Faculty of Science, Ilam University, P. O. Box 69315516, Ilam, Iran.
E-mail: arashghch58@yahoo.com
4264

Oxidation of Urazoles and Bis-urazoles
4265
4-Substituted-1,2,4-triazole-3,5-diones constitute an important class
of heterocyclic compounds, which can be prepared from the oxidation
of urazoles and bis-urazoles. They have been used both as substrate
and reagent in various organic transformations such as the reaction of
allylsilanes with triazolinedione,[6] oxidation of 1,4-dihydropyridines,[7]
oxidation of alcohols to aldehydes and ketones,[8] oxidative coupling of
thioles,[9] and [6 þ 2] cycloadditions.[10] High sensitivity and unusual
reactivity of 1,2,4-triazoline-3,5-diones make them of interest to organic
chemists but also make them difficult to prepare and purify.
There are several reports on the oxidation of urazoles and
bis-urazoles in the literature.[11–19] Despite these intensive efforts, this
transformation is not easy because these compounds are very sensitive
to the oxidizing agents and reaction conditions, and most of the oxidants
produce by-products, which either destroy the product or are difficult to
remove from the sensitive triazolinedione. In addition, most of oxidizing
systems require use of strong and toxic oxidants,[20] harsh condi-
tions,[17,18] long reaction times,[13–15] excess reagent,[13,19] or tedious
workup[15] or generate poor yields of products.[17]
EXPERIMENTAL
General
Chemicals were purchased from Fluka, Merck, and Aldrich chemical
companies. The oxidation products were characterized by comparison
1
of their spectral (IR, H NMR, and 13C NMR) and physical data with
authentic samples.
Preparation of SiO2-HNO3
In a 50-mL, round-bottomed flask, 2.82 g of HNO3 (65%, 3.2 mL) and
2.0 g of silica gel were stirred for 10 min, and a white solid (SiO2-
HNO3) was obtained.
Preparation of PVP-HNO3
In a 50-mL, round-bottomed flask, 2.82 g of HNO3 (65%, 3.2 mL) and
4.8 g of polyvinyl pyrrolidone (PVP) (4 g, 3.24 mmol) were stirred for
10 min, and a white solid (PVP-HNO3) was obtained.

4266
A. Ghorbani-Choghamarani, Z. Chenani, and S. Mallakpour
Oxidation of 4-Naphthylurazole 1b to
4-Naphthyl-1,2,4-triazoline-3,5-dione 2b by PVP-HNO3
A suspension of 4-naphthylurazole 1b (0.227 g, 1 mmol) and PVP-HNO3
(0.25 g) in dichloromethane (DCM, 5 mL) was stirred at room tempera-
ture for 110 min and then filtered. The residue was washed with CH2Cl2
(20 mL). Anhydrous Na2SO4 (1.5 g) was added to the filtrate and filtered
off after 20 min. Finally, CH2Cl2 was removed, and 4-naphthyl-1,2,4-
triazoline-3,5-dione 2b was obtained in 99% yield (0.222 g).
RESULTS AND DISCUSSION
In continuation of our recent investigations on the application of new
reagents on the functionalization of organic compounds,[21,22] we became
interested in introducing an efficient supported reagent for the selective
oxidation of urazoles and bis-urazoles to the corresponding triazoline-
diones. Therefore, we used SiO2-HNO3 and PVP-HNO3 to prepare a
mild oxidizing reagent.
Scheme 1. Efficient oxidation of urazoles and bis-urazoles.

Oxidation of Urazoles and Bis-urazoles
4267
Recently, we have used PVP-HNO3 and SiO2-HNO3 for the oxidation
of sulfides to sulfoxides.[21] To investigate the scope and limitation of these
supported reagents, we decided to apply them in diffrent oxidation reac-
tion. Herein, we disclose a new heterogeneous protocol for the oxidation
of a wide variety of urazoles and bis-urazoles 1 to the corresponding
triazolidione 2 using SiO2-HNO3 and=or PVP-HNO3 in DCM with good
to excellent yields (see Scheme 1 and Table 1).
Table 1. Oxidation of urazole derivatives to the corresponding triazolinediones
using PVP-HNO3 I and=or SiO2-HNO3 II in dichloromethane at room temperature
Substrate=Reagentsa
Entry Substrate Product
I
II
Time (min) Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
1a
1a
1b
1b
1c
1c
1d
1d
1e
1e
1f
2a
2a
2b
2b
2c
2c
2d
2d
2e
2e
2f
0.25
—
0.25
—
0.25
—
0.25
—
0.25
—
0.4
—
0.25
—
0.4
—
0.25
—
0.25
—
0.25
—
0.5
—
—
0.2
—
0.2
—
0.2
—
0.2
—
0.2
—
0.32
—
0.2
—
0.32
—
0.2
—
0.2
—
0.2
—
0.4
—
60
35
110
10
55
5
75
99
99
99
99
99
98
95
97
98
99
91
100c
100c
90
85
96
94
99
99
99
99
98
99
81
84
65
10
70
50
150
35
100
15
90
35
120
15
35
10
120
5
1f
2f
1g
1g
1h
1h
1i
2g
2g
2h
2h
2i
1i
2i
1j
2j
1j
2j
1k
1k
1l
2k
2k
2l
55
15
65
75
1l
2l
1m
1m
2m
2m
0.5
—
0.4
aI and II refer to grams of PVP-HNO3 and SiO2-HNO3.
bIsolated yields.
cConversion.

4268
A. Ghorbani-Choghamarani, Z. Chenani, and S. Mallakpour
Scheme 2. Chemoselective oxidation of 4-methoxyphenyl urazole to
4-methoxyphenyl-1,2,4-triazoline-3,5-dione.
As is evident from Table 1, the reaction proceeds more efficiently and
rapidly with SiO2-HNO3 than with PVP-HNO3.
The oxidation reactions were carried out under completely heteroge-
neous conditions by mixing urazole or bis-urazole 1 with supported nitric
acid on silica or PVP in DCM. Reactants and reagents are insoluble in
DCM but red or pink product 2 is soluble in the reaction solvent; there-
fore, triazolinediones can be readily obtained by simple filtration and
evaporation of CH2Cl2.
To investigate chemoselectivity and mildness of this procedure, a
urazole containing an activated aromatic moiety (4-methoxyphenyl
urazole) was subjected to oxidation reaction with supported nitric acid
on silica gel and PVP, and surprisingly, no nitrated product was
obsereved (Table 1, entries 5 and 6; Scheme 2).
The mechanism of this transformation initiates with auto-ionization
of nitric acid, which generates in situ nitronium ion ðNOþ2 Þ. Then nucleo-
philic attack of nitrogen of urazole on the NOþ2 followed by elimination
of HNO2 gives the corresponding triazolinedione. In the next step, HNO2
might generate NOþ, which oxidize another molecule of urazole.
In summary, herein we report an efficient and heterogenous method
for the selective oxidation of urazoles and bis-urazoles under mild condi-
tions. This method offers the advantage of shorter reaction times, high
selectivity, nontoxicity, cost-effective reagent or catalyst, and easy
workup.
ACKNOWLEDGMENT
We are grateful to Ilam University, Ilam, Iran, for research facilities and
financial support of this research project.

Oxidation of Urazoles and Bis-urazoles
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