Oxidation of urazoles to triazolinediones with benzyltriphenylphosphonium peroxymonosulfate under solvent-free conditions

Article abstract of DOI:10.1246/cl.2001.164

Benzyltriphenylphosphonium peroxymonosulfate (PhCH₂Ph₃PHSO₅) in the presence of A1Cl₃ was used as an effective oxidizing reagent for the oxidation of urazoles to their corresponding triazolinediones in high yields under solvent-free conditions.

Full text of DOI:10.1246/cl.2001.164

164
Chemistry Letters 2001
Oxidation of Urazoles to Triazolinediones with Benzyltriphenylphosphonium Peroxymonosulfate
under Solvent-Free Conditions
Abdol Reza Hajipour*, Shadpour E. Mallakpour,* and Hadi Adibi
Pharmaceutical Research Laboratory, College of Chemistry, Isfahan University of Technology, Isfahan 84156, IR Iran
(Received November 10, 2000; CL-001021)
Benzyltriphenylphosphonium peroxymonosulfate
(PhCH2Ph3PHSO5) in the presence of AlCl3 was used as an
effective oxidizing reagent for the oxidation of urazoles to their
corresponding triazolinediones in high yields under solvent-free
conditions.
lyst the reaction does not proceed at all even with a higher
molar ratio of the oxidant (1.2 mmol). For this aim, the catalyt-
ic effects of several Lewis acids such as ZnCl2, FeCl6·6H2O,
BiCl3 and AlCl3 were thoroughly investigated. Surprisingly,
only AlCl3 was found to be an effective catalyst during this
investigation. The process in its entirely involves a simple mix-
ing of benzyltriphenylphosphonium peroxymonosulfate
(PhCH2Ph3PHSO5) (1) and urazoles (2) in the presence of
AlCl3 (1 molar ratio) in a mortar and grinding the mixture for
the time specified in Table 1 at room temperature. The yields
of the reactions are high and the reaction times are exceedingly
short (10–15 min). We tried the reaction of urazole (2a) with
reagent (1) (1 mmol) in acetonitrile in the absence of Lewis
acid at room temperature, and observed that the reaction did not
complete after 60 min.
4-Substituted-1,2,4-triazoline-3,5-diones are notable for
their ability to participate in a wide range of reactivity and reac-
tion types, e. g. [4 + 2]1,2 and [2 + 2]3 cycloadditions, ene reac-
tions,4 electrophilic aromatic substitution,5 dehydrogenating
properties6 and oxidation of alcohols to aldehydes and ketones.7
The unusual reactivity that makes 1,2,4-triazoline-3,5-diones
(3) of interest also makes them hard to prepare and purify. For
example: 4-phenyl-1,2,4-triazoline-3,5-dione (3a) is an
extremely reactive dienophile and enophile which is at least
1000 times more reactive than tetracyanoethylene in the
Diels–Alder reaction with 2-chlorobutadiene and 2000 times
more reactive than maleic anhydride.1 All known methods of
synthesis of these compounds require oxidation of the corre-
sponding 1,2,4-triazolidine-3,5-diones (2), more commonly
known as urazoles.
In recent years, a wide variety of reagents have been
reported for this transformation,8–11 but as far as we know this
transformation is not so easy and is a tricky step because these
compounds are very sensitive to the oxidizing agents and reac-
tion conditions. However, most of the reported reagents pro-
duce by-products, which either destroy, or are difficult to
remove from the sensitive triazolinediones.
Oxone® (2KHSO5·KHSO4·K2SO4) is an inexpensive,
water-soluble and stable oxidizing reagent that is commercially
available. In continuation of our ongoing program to develop
environmentally benign methods using solid supports,12 here
we wish to report the oxidation of urazoles (2) to their corre-
sponding triazolinediones (3) with benzyltriphenylphosphoni-
um peroxymonosulfate (1) in the presence of AlCl3 under sol-
vent-free conditions. This reagent is readily prepared by the
dropwise addition of an aqueous solution of oxone®
(2KHSO5·KHSO4·K2SO4), to an aqueous solution of benzyl-
triphenylphosphonium chloride at room temperature.13
Filtration and drying of the precipitates resulted in a white pow-
der, which could be stored for months without losing its oxida-
–
tion ability. The amounts of HSO5 in this reagent have been
In summary, in this study we introduced a new methodolo-
gy for the oxidation of different types of urazoles under sol-
vent-free conditions. This method is superior to previously
reported methods in terms of high yields, purity of products,
facile work-up, cheaper and safer reagent and short reaction
time.
Urazoles were synthesized according to reported proce-
dures.1,4,5,10,11 The reagent (1) was prepared according to our
previously reported procedures.13 Yields refer to isolated pure
determined by an iodometric titration method.14 The measure-
ments are consistent with almost 99% by weight of active oxi-
dizing agent. This reagent is quite soluble in methylene chlo-
ride, chloroform, acetone, and acetonitrile and insoluble in non-
polar solvents such as carbon tetrachloride, n-hexane, and ether.
At first, we decided to investigate the oxidation of 4-
phenylurazole (2a) with this reagent under solvent-free condi-
tions without catalyst, and observed that in the absence of cata-
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
165
products. The oxidation products were characterized by com-
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1
parison of their spectral (IR, UV, H-NMR) and physical data
5
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with the authentic samples.8–11 All H-NMR spectra were
recorded at 90 MHz in CDCl3 relative to TMS (0.00 ppm) and
IR spectra were recorded on Shimadzu 435 IR spectrometer.
All reactions were carried out under solvent-free conditions in a
hood with strong ventilation.
6
7
Oxidation of 4-phenylurazole (2a) with reagent (1) to 4-
phenyl-1,2,4-triazoline-3,5-dione (3a): A typical procedure: A
mixture of 4-phenylurazole (1 mmol, 0.177 g) (2a), oxidizing
agent (1) (1 mmol, 0.47 g) and AlCl3 (1 mmol, 0.13 g) in a
mortar was ground with a pestle for the time specified in Table
1 until a deep-red color appeared. When TLC showed complete
disappearance of urazole, the mixture was extracted with
CH2Cl2. Evaporation of the solvent gave 4-phenyl-1,2,4-triazo-
line-3,5-dione (3a). The yield was 0.17 g (94%) of crystalline
red solid (3a), mp 170–174 oC (dec.) [Lit10 mp 170–178 oC].
8
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The authors are thankful of the Isfahan University of
Technology (IUT), IR Iran for financial support.
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