Oxidative coupling of thiols to disulfides in solution with tripropylammonium halochromates, (C3H7) 3NH[CrO3X], (X=F, Cl) adsorbed on alumina

Article abstract of DOI:10.1080/10426500802081137

A mild and efficient method for the oxidative coupling of thiols by tripropylammonium fluorochromate and tripropylammonium chlorochromate absorbed on alumina in solution is reported. Alumina absorbed tripropylammonium fluorochromate and tripropylammonium chlorochromate are efficient and new reagents, which are easily prepared and oxidize thiols to the corresponding disulfides quickly. The reactions are clean and readily controlled to stop at the disulfide stage without the formation of common over-oxidized side products. Thus the advantages of the easy procedure and work up, short reaction times, and excellent yields make this a viable alternative method.

Full text of DOI:10.1080/10426500802081137

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Oxidative Coupling of Thiols
to Disulfides in Solution
with Tripropylammonium
Halochromates,
(C3H7)3NH[CrO3X], (X = F, Cl)
Adsorbed on Alumina
Ali HassaniJoshaghani a , Shahriare Ghammamy b
,
Shahryar Bagi b , Ali Moghimi c & Zahra Javanshir d
a Department of Chemical Engineering, Faculty of
Engineering , Islamic Azad University , Arak Branch,
Arak , Iran
b Department of Chemistry, Faculty of Science ,
Imam Khomeini International University , Qazvin ,
Iran
c Department of Chemistry, Faculty of Science ,
Islamic Azad University , Varamin , Iran
d Departments of Chemistry, Faculty of Science ,
Islamic Azad University , Ardabil Branch, Ardabil ,
Iran
Published online: 19 Dec 2008.
To cite this article: Ali HassaniJoshaghani , Shahriare Ghammamy , Shahryar Bagi ,
Ali Moghimi & Zahra Javanshir (2008) Oxidative Coupling of Thiols to Disulfides in
Solution with Tripropylammonium Halochromates, (C3H7)3NH[CrO3X], (X = F, Cl)
Adsorbed on Alumina, Phosphorus, Sulfur, and Silicon and the Related Elements,
184:1, 164-170, DOI: 10.1080/10426500802081137
To link to this article: http://dx.doi.org/10.1080/10426500802081137

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Phosphorus, Sulfur, and Silicon, 184:164–170, 2009
Copyright © Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500802081137
Oxidative Coupling of Thiols to Disulfides in Solution
with Tripropylammonium Halochromates,
(C3H7)3NH[CrO3X], (X=F, Cl) Adsorbed on Alumina
Ali HassaniJoshaghani,1 Shahriare Ghammamy,2
Shahryar Bagi,2 Ali Moghimi,3 and Zahra Javanshir4
1Department of Chemical Engineering, Faculty of Engineering,
Islamic Azad University, Arak Branch, Arak, Iran
2Department of Chemistry, Faculty of Science, Imam Khomeini
International University, Qazvin, Iran
3Department of Chemistry, Faculty of Science, Islamic Azad University,
Varamin, Iran
4Departments of Chemistry, Faculty of Science, Islamic Azad University,
Ardabil Branch, Ardabil, Iran
A mild and efficient method for the oxidative coupling of thiols by tripropylammo-
nium fluorochromate and tripropylammonium chlorochromate absorbed on alu-
mina in solution is reported. Alumina absorbed tripropylammonium fluorochro-
mate and tripropylammonium chlorochromate are efficient and new reagents,
which are easily prepared and oxidize thiols to the corresponding disulfides quickly.
The reactions are clean and readily controlled to stop at the disulfide stage without
the formation of common over-oxidized side products. Thus the advantages of the
easy procedure and work up, short reaction times, and excellent yields make this a
viable alternative method.
Keywords Alumina; heterogeneous oxidants; thiols; tripropylammonium chlorochro-
mate(VI); tripropylammonium fluorochromate(VI)
INTRODUCTION
The oxidation of organic substrates in aprotic solvents, under mild and
neutral conditions, is important in modern organic synthesis. There-
fore, the search for new oxidizing agents is of interest to synthetic
Received 2 December 2007; accepted 25 February 2008.
The author thanks Dr. A. R. Mahjoub, Mr. M. K. Mohammady, and Mrs. S.
Khorsandtabar for valuable discussion.
Address correspondence to Shahriare Ghammamy, Department of Chemistry, Faculty
of Science, Imam Khomeini International University, Qazvin, Iran. E-mail: shghamami@
ikiu.ac.ir, shahryar bagi@yahoo.com
164

Oxidative Coupling of Thiols
165
organic chemists. Many such reagents have been developed in recent
years with some success.1Disulfides are one of the most important
organic sulfur compounds, possessing an exclusive chemistry both in
biochemistry2and in synthetic areas.3 Disulfides are also key interme-
diates in a wide variety of organic synthetic routes.4−6 Sweetening of
catalyst poisons thiols to low volatile disulfides in oil industries,7−8 and
also industrial applications of disulfides in vulcanization of rubbers and
elastomers are two of the most common application fields of disulfides.
These applications led us to investigate the introduction and applica-
tions of a new member of this category of reagents in oxidation of thiols
to the corresponding disulfides. Many stoichiometric reagents such as
manganese dioxide,9 dichromates,10 halosilane-chromium trioxide,11
diethyl azodicarboxylate,12 nickel peroxide,13 chromium peroxide,14
diaryl telluroxide,15 tetrabutylammonium ceric(IV) nitrate,16 sodium
perborate,17 silver trifluoromethane sulphonate,18 and permangenate19
have been developed for this transformation. In this respect, we wish to
report that alumina-absorbed tripropylammonium fluorochromate and
tripropylammonium chlorochromate are able to oxidize thiols to their
disulfides efficiently under different reaction conditions.
RESULTS AND DISCUSSION
The existing reagents suffer from either one or more of the following
disadvantages such as availability of the reagent, cumbersome work-up
procedure, high cost of the reagent, over-oxidation, or oxidation of other
functional groups in the presence of thiol group. As a result, there is
still a need for the development of general, efficient, and new reagents
to synthesize disulfides from the corresponding thiols under mild reac-
tion conditions. These reactions are not only of interest from ecological
viewpoint, but also in many cases offer considerable synthetic advan-
tages in terms of the yield, selectivity, and simplicity of the reaction
procedure. The results obtained with tripropylammonium fluorochro-
mate(VI) (TriPAFC) and tripropylammonium chlorochromate(VI)
(TriPACC) absorbed on alumina are very satisfactory and show the
new reagents as valuable additions to the existing oxidizing agents.
Different thiols were subjected to oxidations with TriPAFC and
TriPACC absorbed on Al2O3in dichloromethane (Scheme 1). These oxi-
dations take place under mild and completely heterogeneous conditions
giving excellent yields (Table I).
Oxidations may also occur using only TriPAFC and TriPACC, in the
absence of Al2O3, but considerable improvements are observed in the
presence of the absorbent. This implies that the Al2O3 may act as a

166
A. HassaniJoshaghani et al.
TriPAHC/Al2O3
RSSR
RSH
TriPAHC: (C3H7)3NH[CrO3F] or (C3H7)3NH[CrO3Cl]
SCHEME 1
reaction medium, providing an effective heterogeneous surface area
for the oxidation and at the same time making the work-up much more
convenient.20
The nature of the solvent does not appear to be particularly criti-
cal. Hydrocarbons, benzene, ethers, and chlorinated hydrocarbons are
equally effective, the practical choice being oriented by the solubility of
the products and the desired reaction temperature. The chromium(VI)
contents are easily determined iodometrically.
The IR spectra of TriPAFC and TriPACC are similar to that of other
fluoro- and chlorochromates.21 TriPAFC and TriPACC are soluble in
water, dimethylformamide, acetonitrile, acetone, and dichloromethane;
they are only sparingly soluble in benzene, carbon tetrachloride, chlo-
roform, and hexane.
TABLE I Oxidative Coupling of Thiols via TriPACC and TriPACC on
Alumina
TriPAFC
TriPACC
Time Yield Time Yield
Substrate
Product
(min)
100
(%)
78
(min)
135
(%)
85
CH -CH-S-S-CH-CH
1
CH -CH-SH
3
3
3
CH
CH
CH
3
3
3
2
3
n-C15H11-SH
n-C8H17-SH
C5H11-S-S-C5H11
C8H17-S-S-C8H17
92
95
86
90
100
105
84
90
4
106
78
123
67
5
6
HOOC-CH2−SH
HOOC-CH2-S-S-CH2-COOH
100
116
80
87
140
130
89
90
7
8
120
132
84
81
145
153
85
87

Oxidative Coupling of Thiols
167
EXPERIMENTAL
CrO3 (Merck, Pennsylvania, USA) was used without further purifi-
cation. Solvents were purified by standard methods. Infrared spectra
were recorded as KBr disks on a Shimadzu model 420 spectropho-
tometer. The UV/visible measurements were made on an Uvicon model
1
922 spectrometer. H, 13C, and 19F NMR (for TriPAFC) were carried
out on a Bruker AVANCE DRX 500 spectrometer at 500, 125, 470.66,
MHz respectively. All the chemical shifts are quoted in ppm using the
high-frequency positive convention; 1H and 13C NMR spectra were ref-
erenced to external SiMe4 and 19F NMR spectra to external CFCl3.
Chromium was estimated iodometrically. Elemental analyses for car-
bon, hydrogen, and nitrogen were obtained from the Microanalytical
Laboratories, Department of Chemistry, OIRC, Tehran, Iran.
Synthesis of Tripropylammonium FluorochromateTriPAFC,
(C3H7)3NH[CrO3F]
A 1g (10 mmol) sample of chromium (VI) oxide (CrO3), and 0.9 mL
(20 mmol) of 40% hydrofluoric acid were added to 20 mL of water
in a 100 mL polyethylene beaker with stirring. After 5–7 min, the
homogeneous solution was cooled to ca. 3–5◦C. To the resultant clear
orange solution, tripropylamine (1.4 mL, 10 mmol) was added dropwise
with stirring over a period of 0.5 h and stirring was continued for
0.5 h at −4◦C. The precipitated yellowish-orange solid was isolated by
filtration on a polyethylene funnel, washed with petroleum ether (3 ×
60 mL) and dried in vacuum for 2 h at room temperature. Yield: (98%);
mp 142◦C, I.R. (KBr): 904 cm−1 ν1(A1) or ν(CrO3), 647 cm−1 ν2(A1) or
ν(Cr-F), 949 cm−1 ν4(E) or ν(CrO3). Electronic absorption at 22321 cm−1
,
1
1
1
1
corresponded to A2 → E (ε = 140 M−1cm−1); 28725 m−1 to E→ E
1
1
(ε = 667 M−1cm−1) and 35971 cm−1 to A1 → E(ε = 1287 M−1cm−1).
UV/Visible, 13C NMR, 1H NMR and 19F NMR were all consistent with
the TriPAFC structure.22 The above procedure can be scaled up to
larger quantities, if desired. The pH of 0.01 M solution of TriPAFC in
water was 3.3.
Synthesis of Tripropylammonium Chlorochromate,
(C3H7)3NH[CrO3Cl]
Chromium(VI) oxide (1.0 g, 10 mmol) was dissolved in water in a beaker
and hydrochloric acid (0.25 mL, 15 mmol) was added with stirring at
0◦C. To the resultant clear orange solution, tripropylamine (1.4 mL,
10 mmol) was added dropwise with stirring over a period of 0.5 h and

168
A. HassaniJoshaghani et al.
stirring was continued for 0.5 h at –4◦C. The precipitated orange solid
was isolated by filtration, washed with anhydrous ether (3 × 60 mL),
and dried in vacuum for 2 h at room temperature. Yield: (95%); mp
135◦C. C9H22ClCrNO3: Calc. C, 38.64; H, 7.871; N, 5.008 Found: C,
37.93; H, 7.75; N, 4.88. I.R. (KBr): 901 cm−1 ν1(A1) or ν(CrO3), 432
cm−1ν2(A1) or ν(Cr-Cl), 949 cm−1 ν4(E) or ν(CrO3). Electronic absorp-
1
tion at 22123 cm−1, corresponded to 1A2 → E (ε = 351 M−1cm−1); 28169
1
1
1
1
m−1 to E→ E (ε = 793 M−1cm−1); 35087 cm−1 to A2 → A1(ε = 1031
1
M
−1cm−1) and 41493 cm−1 to 1A1 → E(ε = 219 M−1cm−1). UV/Visible,
13C NMR and H NMR were all consistent with the TriPACC struc-
ture. 1H-NMR (500MHZ, CD3CN): δ = 0.77 (t, 3H, CH3), δ = 1.93 ppm
(m, 2H, CH2 ), δ = 3. 24 ppm (t, 2H, CH2 ), 13C-NMR (125 MHZ,
CD3CN): δ 11.05, 20.12, 56.45. The above procedure can be scaled up
to larger quantities, if desired. The pH of 0.01 M solution TriPACC in
water was 2.9.
1
General Procedure for Alumina Supporting of TriPAFC
and TriPACC
For supporting of the TriPAFC and TriPACC on alumina, the above
synthetic methods were repeated, but before the tripropylamine addi-
tion step, alumina (mesh 65–250) was added. The supported reagents
were separated and washed by cold water and acetone. The capacity
of the supported reagents was determined by stirring overnight 0.5 g
of the supported reagents with 10 mL of 2 N aqueous potassium hy-
droxide, filtering off and titrating iodometrically the obtained chromate
solution. The determined average capacity of the dried supported for
the above reagents were 1–1.5 mmol of TriPAFC per 1 g of alumina and
1.3–1.6 mmol of TriPACC per 1 g of alumina. The supported reagents
so obtained did not noticeably lose their activities either on storing in
air at room temperature for several weeks or on refluxing for 5 h in
benzene or hexane.
General Procedure for Oxidation of Organic Substrates
with Alumina-Absorbed TriPAFC and TriPACC
To a stirred suspension of tripropylammonium halochromate absorbed
on alumina in dichloromethane (generally 5 cm3, 10 mmol), a solu-
tion of the substrate in the minimum amount of dichloromethane was
added dropwise, the molar ratio of substrate to the oxidant being 1:2
(Table I). The completion of the reaction was followed by TLC using
ether:petroleum ether (60:40) as eluant. The mixture was diluted with

Oxidative Coupling of Thiols
169
ether 10 mL (1:1 v/v) and filtered through a short column of alumina
to give a clear solution. The solution was evaporated, and the resid-
ual product was purified by distillation, recrystallization, or column
chromatography. Analysis of the reaction mixture for the correspond-
ing disulfide compound was accomplished by the procedure reported in
earlier articles 9–13.
CONCLUSIONS
New reagents tripropylammonium fluorochromate(VI) (TriPAFC) and
tripropylammonium chlorochromate(VI) (TriPACC) absorbed on alu-
mina are easily synthesized. They prove to be low cost, readily avail-
able oxidizing reagents for a variety of thiols. Theirs advantages include
higher yields, shorter reaction times, lower thiol:oxidant molar ratios,
application to pH sensitive molecules, and ease of separation of prod-
ucts. Moreover, during the reaction, the color of the oxidants changes
from orange to brown, thus providing a visual means for ascertaining
the progress of the oxidation. Many functional groups are inert towards
these oxidizing agents, including thiols, sulfides, and phenols, enhanc-
ing the usefulness of the oxidants and the oxidation conditions for the
synthesis of highly functionalized molecules.
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