Oxidation of thiols to bisulfides using silica chloride as a heterogeneous catalyst

Article abstract of DOI:10.1246/cl.2006.1048

Silica chloride is used as a selective and effective heterogeneous catalyst for the rapid conversion of thiols to disulfides with quantitative yields in a very short period of time. Copyright

Full text of DOI:10.1246/cl.2006.1048

1
048
Chemistry Letters Vol.35, No.9 (2006)
Oxidation of Thiols to Disulfides
Using Silica Chloride as a Heterogeneous Catalyst
Ã
Manisha Sathe, Ramarao Ghorpade, and Mahabir Parshad Kaushik
Process Technology Development Division, Defence R & D Establishment,
Jhansi Road, Gwalior-474002 (MP), India
(Received July 18, 2006; CL-060808; E-mail: mpkaushik@rediffmail.com)
Silica chloride is used as a selective and effective heteroge-
neous catalyst for the rapid conversion of thiols to disulfides with
quantitative yields in a very short period of time.
dialkyl/diaryl disulfide in 10 min with excellent yields, Table 1.
We have found that this procedure is suitable for rapid and
efficient oxidation of thiols and dithiols to the disulfides. Filtra-
tion is only required to remove the catalyst and evaporation of
the filtrate afforded pure products. Several examples of silica
chloride assisted couplings of aliphatic, aromatic, and heteroar-
omatic thiols into disulfides are presented in Table 1. In case of
2-aminobenzenethiol (Entry 12) the amino hydrochloride salt
was obtained which was basified with cold saturated sodium
bicarbonate solution followed by the extraction with diethyl
ether to afford bis (2-aminophenyl) disulfide. A simple filtration
at the end of the reaction followed by removal of the solvent
from the filtrate allows product isolation in quantitative yields.
The extent of chlorination of the silica surfaces was deter-
mined by suspending 1 g of silica chloride in 25 mL of boiled
distilled water and titrating with 0.1 M (9.3 mL) NaOH using
Disulfides are versatile building blocks for the synthesis of
1
various organosulfur compounds and they play an important
2
role in biological and chemical processes. Disulfides are also
3
used in the sulfenylation of enolates and other anions. Several
4
methods have been reported in the literature for the synthesis
of disulfides from thiols, which make use of various oxidizing
5
6
agents ranging from molecular oxygen to halogens, metal
7
8
oxides, oxo- and peroxosalts. However, most of the reported
methods involve the use of strong oxidants that are capable of
reacting with other oxidizable functionalities such as aldehyde
and amino groups present in the substrate. These methods also
suffer from one or other disadvantages. Therefore, efforts were
made to develop new reagents/catalyst with high activity, selec-
tivity, and broad substrate generality, which can lead to practi-
cal, efficient, and environmentally friendly chemical synthesis
of the disulfides.
Table 1.
Entry
1
Substrate
C H SH
Producta
(C H S-)
Yield/%b
Recently, we have initiated work in the field of solid sup-
ported reagents for various chemical transformations in organic
2
5
2
5
2
98
99
98
2
3
(CH ) CHSH
3
2
3 2 2
((CH ) CHS-)
9
synthesis. Silica gel is one of the extensively used surface
C H SH
4 9 2
(C H S-)
4
9
material for different chemical transformations in organic chem-
istry.10 One such modified silica is silica chloride (SiO2–Cl),
which has been reported to be an efficient reagent for the synthe-
4
5
6
C5H11SH
C6H5SH
(C5H11S-)2
(C6H5S-)2
97
99
96
97
6
C H11SH
6
(C H11S-)2
1
1
sis of many organic compounds. The efficiency of silica chlo-
ride, under operationally simple conditions has prompted us to
explore the possibility of this catalyst for the synthesis of disul-
fides. Herein, we report a rapid, efficient, economic, environ-
mentally benign, and easy to scale-up method for the effective
conversion of alkane/arenethiols to their corresponding disul-
fides at room temperature using silica chloride as heterogeneous
catalyst Scheme 1.
7
8
4-CH3-C6H4SH
4-(CH3-C6H4S-)2
(C H -CH S-)
C H -CH SH
6
5
2
6
5
2
2
99
97
9
4-NO2-C6H4SH
(4-NO2-C6H4S-)2
(2-Cl-C6H4S-)2
1
1
0
1
2-Cl-C6H4SH
CH3
98
99
98
92
CH3
S
3
H C
SH
S
CH3
CH3
3
H C
The developed method has allowed us to obtain quantitative
2
12
2-NH2-C6H4SH
(2-NH2-C6H4S-)2
1
yields of the products in reduced reaction times. The room
ꢀ
temperature (20–25 C) reaction of alkane/arenethiols in the
presence of silica chloride and air afforded the corresponding
S
1
3
SHCH2CH2CH2SH
CH2
S
S
SHCH2CH2CH2CH2SH
97
1
1
4
5
(CH2)2
SiO -Cl (5 mol %),O
S
2
2
R
SH
R
S
S
R
CH Cl , 0 °C 10 min
2
2
CHO OHC
CHO
SH
R = Alkyl/ Aryl
98
95
S
S
S
S
SH SiO -Cl (5 mol %), O
S
S
2
2
(
CH )n
(CH2)n
16
2
SHCH2CH2SH
SH CH Cl , 0 °C 10 min
2
2
S
S
n = 1, 2
a
All compounds have been characterized by IR, NMR, and MS
spectroscopy. Isolated yield.
b
Scheme 1.
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.9 (2006)
1049
phenolphthalein as an indicator. The amount of immobilized Cl
has been found to be 0.93 mol equiv. per gram of SiO2. Our stud-
ies have shown that thionyl chloride is a satisfactory chlorinating
agent for silica, if used undiluted. When diluted with dry ben-
zene, a lesser numbers of silanol groups were replaced by chlo-
rine. The extent of reaction with thionyl chloride gives values for
active silanols per unit area of silica surface, comparable to other
methods, for determining available activities. It was also ob-
served that oxidation of thiol does not take place in the absence
of silica chloride and in the presence of silica gel. Advantage of
using silica chloride was that it can be prepared by the readily
available materials and also can be removed easily from the
reaction mixture.
In conclusion, an efficient and simple method for the synthe-
sis of dialkyl/diaryl disulfide has been described using silica
chloride as a heterogeneous catalyst. The present procedure is
attractive because of its simplicity, general applicability, and ex-
cellent yields of the products. It does not require any oxidizing
agents. Therefore, the present method is a useful alternative of
the existing methodologies.
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We thank Shri K. Sekhar, Director DRDE. Gwalior for his
keen interest and encouragement in the present study.
1
2 To a solution of alkane/arenethiol (1 mmol) in dichloro-
ꢀ
References and Notes
methane, silica chloride (5 mol %) was added at 0 C and
1
2
3
4
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Published on the web (Advance View) August 12, 2006; doi:10.1246/cl.2006.1048