Oxidation of some organic thiols with tetramethylammonium fluorochromate(VI)

Article abstract of DOI:10.1134/S1070363207020156

Tetramethylammonium fluorochromate oxidizes alcohols to the corresponding aldehydes and ketones without polymerization of double bonds, overoxidation, and other side reactions. This reagent is effective for oxidation of thiols to disulfides.

Full text of DOI:10.1134/S1070363207020156

ISSN 1070-3632, Russian Journal of General Chemistry, 2007, Vol. 77, No. 2, pp. 282 284.
Pleiades Publishing, Ltd., 2007.
Original Russian Text
pp. 310 312.
H. Imanieh, S. Ghamami, M.K. Mohammadi, A. Jangjoo, 2007, published in Zhurnal Obshchei Khimii, 2007, Vol. 77, No. 2,
Oxidation of Some Organic Thiols with Tetramethylammonium
Fluorochromate(VI)
H. Imanieha, S. Ghamamia, M. K. Mohammadib, and A. Jangjooc
a Department of Chemistry, Faculty of Science, Imam Khomeini International University, Qazvin, Iran
b Department of Chemistry, Faculty of Science, Islamic Azad University of Sciences & Researching, P.O.
Box 14155/775, Tehran, Iran; e-mail: m_k_mohammadi@yahoo.com
c Firouzabad Branch, Islamic Azad University, Firouzabad, Iran
Received February 20, 2006
Abstract Tetramethylammonium fluorochromate oxidizes alcohols to the corresponding aldehydes and
ketones without polymerization of double bonds, overoxidation, and other side reactions. This reagent is
effective for oxidation of thiols to disulfides.
DOI: 10.1134/S1070363207020156
The oxidation of thiols to disulfides [1] or to sul-
fonic acids [2] in the absence and presence of catalysts
has been the subject of several recent studies. Atten-
tion has been focused primarily on the range of pro-
ducts that can be obtained depending on the reaction
conditions. In general, the reaction seems to lead to
the almost exclusive formation of disulfides when
oxidation occurs in aqueous solution, and to a variety
of disulfides and sulfenic, sulfinic, and sulfonic acids
when carried out in nonaqueous solutions [3]. In both
cases, the reaction has been suggested [4] to involve
initial formation of a disulfide, followed by its
hydrolysis to yield sulfur-containing acids. Much
attention was given recently to chromium-based oxi-
dizing agents. Some of which have become quite
popular and effective, e.g., the Collins reagent [5],
chromium trioxide 3,5-dimethylpyrazole complex [6],
pyridinium chlorochromate [7], pyridinium fluoro-
chromate [8], and quinolinium bromochromate [9].
As a part of our effort to develop [10] new methodo-
logies in economically viable and environmentally
benign conditions, we report here a very simple oxi-
dative protocol using tetramethylammonium fluoro-
chromate at room temperature in acetonitrile as a
solvent. Tetramethylammonium fluorochromate has a
number of advantages over the conventional oxidation
reagents in various organic solvents.
As seen from the table, saturated aliphatic acyclic
and allylic thiols underwent clean oxidation with
tetramethylammonium fluorochromate to the corres-
ponding disulfides at ambient temperature within a
reasonable period of time in the presence of aceto-
nitrile as a solvent in good yield and with high prod-
uct purity. This can be attributed mainly to the im-
mobilization of the Cr-containing by-products in the
solvent [12]. The overall reaction can be represented
as
R S H + (CH3)4N+CrO3F
R S S R + (CH3)4N+CrO2F .
Tetramethylammonium fluorochromate undergoes
two-electron reduction. This is in agreement with our
previous observations with other halochromates.
No overoxidation was noted even with an excess
of the reagent. The reaction of aromatic thiols was
essentially insensitive to substitution of the aromatic
ring, but possibly depended on the solvent used. The
reaction conditions have the advantages of leaving
unchanged a chiral amino acid group.
The notable feature of our procedure is elimination
of several side reactions typical of various conven-
tional oxidative methods using high-valence metals
as oxidants [13]. 1,2-Diols and -hydroxy ketones
have been reported to undergo oxidative cleavage of
the C C bond with tetramethylammonium fluoro-
chromate in dichloromethane, but under solvent-
free conditions these substrates cleanly produce the
desired oxidation products, namely, 1,2-diketones in
reasonable yield without appreciable formation of by-
products. Allylic primary thiols smoothly underwent
A number of aliphatic, aromatic, and allylic thiols
were subjected to oxidative transformation with tetra-
methylammonium fluorochromate in a solvent. Quite
interestingly, both aliphatic [11] and allylic primary
thiols gave the corresponding disulfides under mild
conditions without detectable (by 300 MHz 1H NMR)
amounts of the sulfones or sulfonic acids.
282

OXIDATION OF SOME ORGANIC THIOLS
283
Oxidation of certain aromatic, aliphatic, and allylic thiols (oxidant/substrate ratio 1:1)
Substrate
Time, min
Product
Yield, %
HS (CH2)7 CH3
CH2=CH CH SH
cyclo-C6H11SH
C6H5SH
160
85
CH3 (CH2)7 S S (CH2)7 CH3
CH2=CH CH2 S S CH2 H=CH2
(cyclo-C6H11)2S2
80
87
80
85
82
240
120
75
(C6H5)2S2
4-H3CC6H4SH
(4-H3CC6H4)2S2
oxidation to the corresponding di- , -unsaturated
disulfides in a fairly good yield and without poly-
merization of the olefinic linkage. Another very im-
portant advantage of our procedure is that the isolated
double bonds remain unaffected under the reaction
conditions. Furthermore, no isomerization of the iso-
lated double bonds to the more stable conjugated
systems in the oxidation products was observed.
EXPERIMENTAL
CrO3 (Merck) was used without additional purifica-
tion. Solvents were purified by standard methods. The
IR spectra were recorded on a Shimadzu model
420 spectrophotometer using KBr pellets. The UV/
visible measurements were made on an Uvicon model
1
922 spectrometer. The H, 13C, and X19F NMR spec-
tra were taken on a Bruker AVANCE DRX 500 spec-
trometer at 500, 125, and 470.66 MHz, respectively,
with the following external references: SiMe4 for H
Thus, the suggested method for oxidation under
mild conditions decreases the dispersion of offensive
materials in the environment and ensures the maximal
use of renewable resources. From this standpoint, this
method can be considered as a relatively green techno-
logy having more advantages and wider applicability
compared to the conventional oxidation reagents.
1
and 13C, and CFCl3 for 19F. The chromium content
was determined by iodometric titration. To determine
chromium in its reduced species, they were oxidized
with an acidic K2S2O8 solution. The C, H, and N
content was determined at the Microanalytical Labo-
ratories, Department of Chemistry, OIRC, Tehran.
Melting points were measured on an Electrothermal
9100 melting point apparatus. The substrates are
commercially available. Most of the products are also
reported and the references are given whenever neces-
sary. The thiols were either commercial products or
were prepared by known methods [14] and purified
by distillation under reduced pressure or crystalliza-
tion. Their purity was checked by comparing their
boiling or melting points with published values.
The yields of the products are, in general, good. In
some cases lower yields were obtained because of the
loss of the volatile products during their isolation.
Sometimes the products were contaminated (as de-
1
tected by H NMR) with the starting materials after
the initial isolation, and they were further purified by
filtration chromatography over a small bed of silica
gel or neutral silica using hexane as eluent. Some un-
identified by-products were formed (in an amount of
1
Preparation of tetramethylammonium fluoro-
chromate. The reagent, (CH3)4N+CrO3F , can be
readily prepared in excellent yield from the reaction
of CrO3 with tetramethylammonium fluoride in aceto-
nitrile in a molar ratio of 1 : 1. The entire procedure
had been described in [15].
up to 10% by H NMR) in a few cases; they were
removed by column chromatography.
Thus, tetramethylammonium fluorochromate in a
solvent selectively oxidizes primary aliphatic,
aromatic, and allylic thiols to the corresponding di-
sulfides without isomerization and polymerization of
double bonds, overoxidation, and other side reactions,
keeping intact the acid-sensitive functionalities. The
important advantages of this procedure include opera-
tional simplicity (ease of set-up and work-up), good
yield of the oxidized products and their high purity
(due to immobilization of the chromium by-products
on the surface of silica), mild reaction conditions,
good selectivity, and versatility (applicability to a
variety of substitution patterns).
Oxidation of thiols (general procedure). A thiol
(5 mmol) was added dropwise to a solution of 0.96 g
of tetramethylammonium chromate in acetonitrile (the
reagent was preliminarily evaporared with 10 ml of
acetonitrile to obtain a white friable powder) in the
cold (0 58 C) with stirring and then kept at room
temperature until the reaction was complete (as
monitored by TLC).
The product was isolated almost pure by filtration
chromatography of the solid reaction mixture through
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 77 No. 2 2007

284
IMANIEH et al.
a small bed of neutral silica using diethyl ether as
eluent; then the solvent was evaporated under reduced
pressure. It was further purified, if required, by
column chromatography on silica gel or neutral silica
to obtain the corresponding products (see table). For
solid thiols, the ether solution was added, with the
subsequent immediate evaporation of the solvent, after
which the process was performed as described above.
3. Vyas, S. and Sharma, P.K., Proc. Indian Acad. Sci.,
Chem. Sci., 2002, p. 137.
4. Fraile, J.M., Garcia, J.I., Lazaro, B., and Mayoral, J.A.,
Chem. Commun., 1998, p. 1807.
5. Collins, J.C., Hess, W.W., and Frank, F.J., Tetrahedron
Lett., 1968, p. 3363.
6. Bora, U., Chaudhuri, W.W., Dey, D., Kalita, D.,
Kharmawphlang, W., and Mandal, G.C., Tetrahedron,
2001, p. 2445.
Most products are known compounds, and they
were identified by their IR and 1H NMR spectra.
Typical oxidation procedure. Tetramethylam-
monium fluorochromate (3.86 g) was dissolved in
30 ml of acetonitrile in a 100-ml two-necked round-
bottomed flask equipped with a reflux condenser and
a mechanical stirrer. A solution of octanethiol
(20 mmol) in acetonitrile (3 ml) was added, and the
mixture was vigorously stirred at room temperature.
The reaction progress was monitored by GLC at 1-h
intervals. After the reaction completion, the mixture
was stirred for an additional 30 min, diluted with dry
diethyl ether (40 ml), and filtered through a short
column with silica gel (2 cm). The combined filtrate
on evaporation gave a crude product which was
distilled through a short Vigreux column. Yield of
dioctyl disulfide [bp 178 180 C (30 mm Hg)] 79%.
7. Piancatelli, A., Scettri, A., and D’Auria, M., Synthesis,
1982, p. 245.
8. Bhattacharjee, M.N., Chaudhuri, M.K., Dastagup-
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11. Vyas, S. and Sharma, P.K., Proc. Indian Acad. Sci.,
Chem. Sci., 2002, vol. 114, p. 137.
12. Verma, R.S. and Saini, R.K., Tetrahedron Lett., 1998,
vol. 39, p. 1481.
13. Fatiadi, A.J., Organic Synthesis by Oxidation with
Metal Compounds, Mijs, M.J. and Jonge, C.R.H.I. de,
Eds., New York: Plenum, 1986, p. 119.
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