Silica sulfuric acid/NaNO2 as a novel heterogeneous system for the oxidation of urazoles under mild conditions

Article abstract of DOI:10.1081/SCC-120016329

Neat chlorosulfonic acid reacted with silica gel to give silica sulfuric acid (I) in which sulfuric acid functions immobilized on the surface of silica gel via covalent bonding. Urazoles and bis-urazoles can be readily converted to their corresponding triazolinediones with a combination of silica sulfuric acid (I), wet SiO₂ and sodium nitrite in dichloromethane at room temperature.

Full text of DOI:10.1081/SCC-120016329

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Silica Sulfuric Acid/NaNO2 as a Novel Heterogeneous
System for the Oxidation of Urazoles Under Mild
Conditions
Mohammad Ali Zolfigol a , Gholamabbas Chehardoli a & Shadpour E. Mallakpour b
a Chemistry Department, College of Science, Bu-Ali Sina University, Hamadan, Iran
b Organic Polymer Chemistry Research Laboratory, College of Chemistry, Isfahan University
of Technology, Isfahan, IR, Iran
Published online: 17 Aug 2006.
To cite this article: Mohammad Ali Zolfigol , Gholamabbas Chehardoli & Shadpour E. Mallakpour (2003): Silica Sulfuric Acid/
NaNO2 as a Novel Heterogeneous System for the Oxidation of Urazoles Under Mild Conditions, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 33:5, 833-841
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 5, pp. 833–841, 2003
Silica Sulfuric Acid/NaNO2 as a Novel
Heterogeneous System for the Oxidation
of Urazoles Under Mild Conditions
1
Mohammad Ali Zolfigol,1, Gholamabbas Chehardoli,
*
and Shadpour E. Mallakpour2
1Chemistry Department, College of Science,
Bu-Ali Sina University, Hamadan, Iran
2Organic Polymer Chemistry Research Laboratory,
College of Chemistry, Isfahan University of Technology,
Isfahan, IR, Iran
ABSTRACT
Neat chlorosulfonic acid reacted with silica gel to give silica sulfuric
acid (I) in which sulfuric acid functions immobilized on the surface
of silica gel via covalent bonding. Urazoles and bis-urazolescan be
readily converted to their corresponding triazolinediones with a
combination of silica sulfuric acid (I), wet SiO2 and sodium nitrite
in dichloromethane at room temperature.
*Correspondence: Mohammad Ali Zolfigol, Chemistry Department, College of
Science, Bu-Ali Sina University, Hamadan Zip Code 65174, Post Box No. 4135,
Iran; Fax: (þ98) 811 8272404; E-mail: zolfi@basu.ac.ir.
833
DOI: 10.1081/SCC-120016329
Copyright & 2003 by Marcel Dekker, Inc.
0039-7911 (Print); 1532-2432 (Online)
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834
Zolfigol,Chehardoli,and Mallakpour
Key Words: Silica sulfuric acid; Urazoles; Triazolinediones; Oxida-
tion; Heterogeneousconditions.
Industrially, a large excess of sulfuric acid is required (e.g. nitration,
nitrosation etc.) because the water by-produced slows the reaction down
by diluting the acid. At the end of the process, a large amount of ‘‘spent
acid’’ is obtained which, in batch processes, is usually neutralized and
disposed of, while, in continuous processes, has to be recycled by complex
techniques. Also, the separation of the products from the acid is often a
difficult and energy consuming process that habitually implies a basic
aqueous work-up. Moreover, sulfuric acid is corrosive and is dangerous
to transport and handle. So, any reduction in the amount of sulfuric acid
needed and/or any simplification in handling procedures would be highly
convenient in termsof risk reduction, economic advantage, and environ-
ment protection.[1] On the other hand, there is intense current research
and general interest in heterogeneous systems because of the perceived
opportunities such systems present for basic research and because of the
unquestioned importance such systems have in industry and in develop-
ing technologies.[2] In continuation of our studies on the application of
inorganic acidic salts we found that silica gel react with chlorosulfonic
acid to give silica sulfuric acid (I). It isinteresting to note that the reaction
is easy and clean not requiring any work-up procedure because the HCl
gas is evolved from the reaction vessel immediately (Sch. 1).
We believed that the silica sulfuric acid (I) is a superior proton source
to all of the reported acidic solid supports or acidic resins such as
polystyrene sulfonic acid and Nafion-H[3] for running reactionsunder
heterogeneous conditions. Therefore, we were interested in using this
inorganic acidic resin (I) asa new uslfuric acid function immobilized
on the surface of silica gel via covalent bonding for the in-situ generation
of HNO2 when used in conjunction with NaNO2, wet SiO2.
4-Substituted-1,2,4-triazole-3,5-diones (TADs) are notable for their
ability to participate in a wide range of concerted and stepwise condi-
tions.[4–10] Considerable attention has been paid to their addition to acti-
vated alkenes,[4–7] electrophilic aromatic substitution,[8] dehydrogenating
[10]
properties[9] and oxidation of alcoholsto aldehydesand ketone.s
Scheme 1.

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Silica Sulfuric Acid/NaNO2
835
The unusual reactivity which makes 1,2,4-triazoline-3,5-diones (2,4) of
interest also renders them hard to prepare and purify.
Although a variety of reagentsare capable of effecting these oxida-
tions,[11–14] asfar aswe know, thistransformation isnot so easy and isa
difficult step as these compounds are very sensitive to the oxidizing agents
and reaction conditions. However, most of the reported reagents produce
by-productswhich either detsroy, or are difficult to remove from, the
sensitive triazolinedione. Another major drawback to the older procedures
is their use of reagents which are either highly toxic or present serious
disposal problems (or both).[11] Very recently, we among many others
have demonstrated that the heterogeneous reagent systems have many
advantages such as simple experimental procedures, mild reaction
conditionsand minimization of chemical watse ascompared to the
liquid phase counterparts.[15,16] Thus, in this article we wish to report
a simple, cheap and convenient method for the effective oxidation of
urazolesand bis-urazolesto their correpsonding triazolinediones( 2,4)
under mild and heterogeneousconditions(Schs. 2 and 3).
Different typesof urazoles( 1) and bis-urazoles( 3) were subjected to
the oxidation reaction in the presence of silica sulfuric acid (I), NaNO2
(II) and wet SiO2 (50% w/w) in dichloromethane (Schs. 2 and 3).
The oxidation reactionswere performed under mild and completely
heterogeneouscondition,s at room temperature with good to excellent
yields(Table 1).
1
R1
R2
a
b
c
d
e
f
H
H
Me
Et
Na
H
n-Pr
n-Bu
H
Cyclohexyl
Ph
H
g
h
i
H
4-Cl-C6H4
4-NO2-C6H6
3,4-Cl2-C6H3
H
H
Scheme 2.

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836
Zolfigol,Chehardoli,and Mallakpour
Scheme 3.
Table 1. Oxidation of urazoles( 1) and bis-urazoles( 3) to their corresponding
triazolinediones( 2,4) with a combination of silica sulfuric acid (I), NaNO2 (II)
and wet SiO2 (50% w/w) in dichloromethane at room temperature.
(Reagent/substrate)a
Urazole
or (bis)
Time
(h)
Yieldb
(%)
Product
I
II
1a
1b
1c
1d
1e
1f
2a[11]
2b[17]
2c[14]
2d[18]
2e[11]
2f [13]
2g[19]
2h[11]
2i[18]
4a[8]
1.5
1.5
2.5
1.5
1.5
1.5
1.5
1.5
1.5
6
1.5
1.5
2.5
1.5
1.5
1.5
1.5
1.5
1.5
4
1.5
1.5
1.5
1.5
2
100c
100c
98
95
99
2
96
83
1g
1h
1i
1.5
2
87
98
2.5
4.5
4.5
3a
3b
98
75
4b[8]
4
4
aWet SiO2: substrate (mono) (0.4 g: 1 mmol) and wet SiO2: substrate (bis)
(1 g: 1 mmol).
bIsolated yields.
cConversion.

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Silica Sulfuric Acid/NaNO2
837
The present oxidation reaction can be readily carried out only by
placing, I, NaNO2 (II), urazoles( 1) or bis-urazoles( 3), wet SiO2
(50% w/w) and CH2Cl2 as the best solvent in a reaction vessel and effi-
ciently stirring the resulting heterogeneous mixture at room temperature.
The triazolinediones( 2) and bis-triazolinediones( 4) productsare
obtained by simple filtration and evaporation of the solvent. The results
and reaction conditionsare tabulated in Table 1. Asmentioned above the
oxidation reactionsare heterogeneousbecause urazolesand
bis-urazoles
[(1,3) white solids] are insoluble in dichloromethane whereas all of the
triazolinedionesand bis-triazolinediones[(2 ,4), red and pink, respectively]
are very soluble in dichloromethane. Therefore, the oxidation reaction
hasbeen performed at the surface of wet SiO via in situ generation of
2
HNO2 in small amounts.
Although the reaction occurswithout wet SiO 2, the reaction duration
isvery long and iscompleted after several days. Therefore, we think that
the presence of the wet SiO2 providesan effective heterogeneoussurface
area for the in situ generation of HNO2. It also simplities work-up.
Furthermore, the oxidation reaction doesnot occur in the abesnce of
silica sulfuric acid (I).
Similarly, this new system acts as N2O4 because a number of
reactionsare known in which nitrogen tetroxide ðN2O4 , NOþNO3ÀÞ
[4,13,14,18]
actsasa nitroasting agent.
Therefore, on the basis of our
observations, the previously reported results about the applications of
N2O4,[4,13,14,19] and product which are obtained the following mechanism
could be proposed (Sch. 4).
In conclusion, silica sulfuric acid (I) is a superior proton source to all
of the reported acidic solid supports or acidic resins in convenience, cost
Scheme 4.

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838
Zolfigol,Chehardoli,and Mallakpour
and ease of production procedure, is insoluble in all of the organic
solvents and is an excellent proton source for organic reactions under
heterogeneousconditions. In addition, practical and efficient oxidation of
urazoles was achieved by the present methodology. The low cost and
availability of the reagents, simple procedure and work-up make this
method attractive for large-scale operations. Moreover, The new element
here isthat the reaction isheterogeneous. Thiscould be worthwhile in an
industrial setting.
EXPERIMENTAL SECTION
General
Chemicalswere purchaesd from Fluka, Merck, Riedel-dehaen AG
and Aldrich chemical companies. Yields refer to isolated pure products.
The oxidation productswere characterized by compariosn of their
1
spectral (IR, UV, H-NMR, and 13C-NMR) and physical data with the
authentic samples. All urazoles and bis-urazoleswere ysntheizsed
according to our previously reported procedures.[4,7,8,13,14]
Preparation of Silica Sulfuric Acid (I)
A 500 mL suction flask was used. It was equipped with a constant-
pressure dropping funnel containing of chlorosulfonic acid (23.3 g, 0.2 mol)
and gas inlet tube for conducting of HCl gas over an adsorbing solution
i.e., water. Into it were charged 60.0 g of silica gel. Chlorosulfonic acid
wasadded dropwise over a period of 30 min at room temperature. HCl
gas to evolved from reaction vessel immediately (Sch. 1). After the addi-
tion wascompleted the mixture washsaken for 30 min. A white oslid
(silica sulfuric acid) 76.0 g (ꢀ100%) wasobtained.
Oxidation of 4-Phenyl Urazole (1f) to 4-Phenyl-1,2,4-
triazoline-3,5-dione (2f). A Typical Procedure
A suspension of compound 1f (0.177 g, 1 mmol), silica sulfuric acid
[I (0.57 g, 1.5 mmol)], wet SiO2 (50% w/w) (0.4 g) and NaNO2 (0.103 g,
1.5 mmol) in dichloromethane (10 mL) wasstirred at room temperature
for 2 h and then filtered. The residue was washed with CH2Cl2 (2 ꢁ 5 mL)

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Silica Sulfuric Acid/NaNO2
839
and dried over anhydrousNa 2SO4 (3 g). After 20 min, the resulting
mixture wasfiltered. Dichloromethane wasremoved by water bath
(40–50ꢂC)[20] and simple distillation. The yield was 0.168 g (96%) of
crystalline red solid (2f), m.p. 168–175ꢂC, [lit.[13] m.p. 170–178ꢂC].
Oxidation of 4,40-(4,40-Diphenylmethylene)-bis-urazole (3b)
to bis( p-3,5-Dioxo-1,2,4-triazoline-4-ylphenyl)methane (4b).
A Typical Procedure
A mixture of 3b (0.366 g, 1 mmol), I (1.52 g, 4 mmol)], wet SiO2
(50% w/w, 1 g) was crushed to a fine powder with a mortar and pestle.
The resulting mixture was added to the dichloromethane (60 mL) and the
suspension was vigorously stirred. NaNO2 (0.276 g, 4 mmol) wasadded
in four portions(each addition after 30 min) to the reaction mixture.
After the final addition the red suspension was allowed to stir for another
120 min and wasfiltered. The reidsue waswahsed with CH
2Cl2
(2 ꢁ 10 mL). The filtrate wasdried over anhydrousNa 2SO4 (6 g) and
wasfiltered again. Dichloromethane wasremoved by simple distillation.
The yield was 0.271 g (75%) of a crystalline pink solid (4b) m.p.
ꢂ
182–185ꢂC (dec.) [lit.[8] m.p. 185>dec. C].
ACKNOWLEDGMENT
Financial support for this work by the research affairs of Bu-Ali Sina
University, Hamadan, Iran.
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Silica Sulfuric Acid/NaNO2
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Received in the UK December 3, 2001

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