Oxidation of arylthiourea by cetyltrimethylammonium dichromate

Article abstract of DOI:10.1080/00397910903398684

With a view to investigate the oxidation behaviors of cetyltrimethylammonium dichromate on multifunctional groups, some arylthioureas were subjected to oxidation, both in neutral and in acidic conditions. In neutral conditions, the products were found to be a mixture of corresponding urea and isonitrile. In acidic conditions, the products were corresponding ureas only. A probable mechanism was proposed for the formation of the product, wherein the first step involves coupling of-NH₂ and-SH of one molecule to the-NH₂ and-SH of another molecule, respectively, which is followed by removal of nitrogen and sulfur. The microwave irradiation resulted in great yield of isonitrile than urea in neutral conditions. Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications to view the free supplemental file. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910903398684

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Oxidation of Arylthiourea by
Cetyltrimethylammonium Dichromate
Sandhyamayee Sahu a , Prangya Rani Sahoo a , Sabita Patel b & B. K.
Mishra a
a Center of Studies in Surface Science and Technology, Department
of Chemistry, Sambalpur University, Jyoti Vihar, India
b Department of Chemistry, National Institute of Technology,
Rourkela, India
Published online: 04 Oct 2010.
To cite this article: Sandhyamayee Sahu , Prangya Rani Sahoo , Sabita Patel & B. K. Mishra (2010)
Oxidation of Arylthiourea by Cetyltrimethylammonium Dichromate, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 40:21, 3268-3273,
DOI: 10.1080/00397910903398684
To link to this article: http://dx.doi.org/10.1080/00397910903398684
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Synthetic Communications1, 40: 3268–3273, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910903398684
OXIDATION OF ARYLTHIOUREA BY
CETYLTRIMETHYLAMMONIUM DICHROMATE
Sandhyamayee Sahu,1 Prangya Rani Sahoo,1 Sabita Patel,2
and B. K. Mishra1
1Center of Studies in Surface Science and Technology, Department of
Chemistry, Sambalpur University, Jyoti Vihar, India
2Department of Chemistry, National Institute of Technology, Rourkela, India
With a view to investigate the oxidation behaviors of cetyltrimethylammonium dichromate
on multifunctional groups, some arylthioureas were subjected to oxidation, both in neutral
and in acidic conditions. In neutral conditions, the products were found to be a mixture of
corresponding urea and isonitrile. In acidic conditions, the products were corresponding
ureas only. A probable mechanism was proposed for the formation of the product, wherein
the first step involves coupling of –NH2 and –SH of one molecule to the –NH2 and –SH
of another molecule, respectively, which is followed by removal of nitrogen and sulfur.
The microwave irradiation resulted in great yield of isonitrile than urea in neutral conditions.
Supplemental materials are available for this article. Go to the publisher’s online
edition of Synthetic Communications1 to view the free supplemental file.
Keywords: Cetyltrimethylammonium dichromate; isonitrile; thiourea
INTRODUCTION
Chromium(VI) oxidants with inorganic counterions are well established as
strong oxidants for many substrates, ranging from metal ions to naturally occurring
organic compounds. These oxidants are mostly water soluble and, because of their
high redox potential, are nonspecific. The thrust to make these oxidants lipopathic
and chemoselective has generated several onium ions as the counterion of the
chromates or dichromates. The pioneering work of Corey and coworkers gave birth
to the first reagent of this type (i.e., pyridinium chlorochromate)[1] and subsequently
many such other oxidants were established by various workers.[2] In our laboratory,
we have synthesized cetyltrimethylammonium dichromate (CTADC) and studied
the oxidation behavior of the oxidant toward various organic substrates.[3–7] In
the absence of acid, CTADC exhibits some bizarre reactions, leading to nonconven-
tional products.[3–5] The reagent is insoluble in water, which reduces contamination
of Cr(VI) in aqueous medium, and thus it can be used as a green reagent.
Received August 4, 2009.
Address correspondence to B. K. Mishra, Center of Studies in Surface Science and Technology,
Department of Chemistry, Sambalpur University, Jyoti Vihar 768019, India. E-mail: bijaym@
hotmail.com
3268

3269

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S. SAHU ET AL.
To explore the chemoselectivity of CTADC, a series of substituted
phenylthioureas have been synthesized to be used as the substrates (Table 1).[8]
These were characterized from their infrared (IR) spectral data and melting points,
which were compared with those of authentic samples. The oxidation was carried
out both in neutral and acidic conditions.
RESULTS AND DISCUSSION
CTADC was found to be a dehydrogenating agent in the oxidation of amine,
thiol, and cholesterol, and thus the products of phenylthiourea are the corresponding
diazo compound, disulfide, and benzothiazole (Scheme 1).
When phenylthiourea was refluxed with CTADC in acetonitrile without any
acid for more than 12 h, the appearance of an insoluble green material in the reaction
mixture indicated the formation of Cr(III). On washing the residue by ether with
mechanical agitation, no organic compound was obtained in the ether medium.
The product was isolated by removal of acetonitrile under low pressure as a pasty
mass with malodor. The thin-layer chromatography (TLC) on silica sheets exhibited
two spots, which necessitated a separation of these two species. Among these, the
major product (60% of the yield) was phenyl urea, which was confirmed from its
IR and NMR spectral characteristics. The minor product (40% of the yield) was a
liquid retaining the malodor. The elemental analysis does not show the presence
of sulfur in the product. The IR spectra exhibit characteristic bands at
2126–2130 cmÀ1 for the isonitrile group. The NMR peaks are in the aromatic region
only. Thus, the product was characterized as phenyl isonitrile.
When the reaction was undertaken in the presence of acetic acid (20%) and the
pasty mass was subjected to column chromatography, a white solid mass was
obtained, which was characterized as phenyl urea. No trace of the corresponding
isonitrile was detected in the product. The same product was also obtained when
phenylthiourea was oxidized in aqueous medium by potassium dichromate in
sulfuric acid by a standard method.[9]
To generalize the reaction, substituted phenyl thioureas were subjected to
oxidation in neutral conditions as well as in the presence of acetic acid. In all the
Scheme 1.

OXIDATION OF ARYLTHIOUREA BY CTADC
3271
Scheme 2.
cases, the products were found to be corresponding ureas and isonitriles in neutral
conditions and corresponding ureas only in the acidic conditions (Scheme 2).
Formation of urea as the oxidation product of corresponding thiourea is
not new.[10] Tajbakhsh et al.[11] selectively obtained phenyl urea from a mixture of
phenylthiourea and diphenyl thioketone by using quinolinium fluorochromate in
acetonitrile. Further, Puri et al.[12] reported the formation of corresponding disulfide
from the oxidation of thiourea by K2Cr2O7 in the presence of bromoacetic
acid. For the formation of isonitrile, a probable mechanism has been proposed
wherein the first step involves the coupling of –NH2 and –SH of one molecule
with the –NH2 and –SH of another molecule, respectively, followed by removal of
nitrogen and sulfur (Scheme 3).
To optimize the oxidation reaction in neutral conditions, the phenylthioureas
were subjected to oxidation by CTADC under microwave irradiation. Surprisingly,
the reaction, which had required around 12 h of reflux to yield the product, now
needed only seconds to get the products with greater yield of isonitrile (see the
Supplementary Materials, available online). The application of microwaves offers
a very quick and clean method for the oxidation reaction. The reaction time and
the yield of the products are given in Table 1.
Scheme 3.

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S. SAHU ET AL.
In conclusion, dichromate with amphiphilic counterion, such as cetyltrimethy-
lammonium ion, becomes mild and on oxidation of arylthiourea yields arylisonitrile
in nonpolar organic solvent and arylurea in the presence of acetic acid.
EXPERIMENTAL
General Method for Oxidation of Arylthioureas with
CTADC in Acetonitrile
A solution of (0.002 mol) of arylthiourea and CTADC (0.00066 mol) in
acetonitrile was refluxed for 12–15 h. The reaction was monitored by TLC. After
completion of the reaction, the green precipitate was filtered off, and the filtrate
was reduced to a paste under low pressure. The products were separated from the
mixture by column chromatography using a mixture of ethyl acetate and toluene
in different proportions.
For the reaction in the presence of acetic acid, it was mixed in the reaction
system with acetonitrile to obtain a 20% solution.
General Method for Oxidation of Arylthioureas with
CTADC in Microwave Conditions
A mixture of arylthiourea and CTADC in a 3:1 molar ratio was thoroughly
ground in a mortar. The mixture was irradiated at 800 W until the reaction
mixture turned green. The reaction mixture was cooled to room temperature, and
the products were separated by column chromatography on silica gel eluted with a
toluene–ethyl acetate mixture.
ACKNOWLEDGMENTS
B. K. M. thanks the University Grants Commission, New Delhi, for financial
assistance through a research project. S. S. thanks the Council of Scientific and
Industrial Research, New Delhi, for a senior research fellowship.
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