Silica-supported cobalt(II) tetrasulfophthalocyanine catalyzed aerobic oxidation of thiols to disulfides under neutral conditions

Article abstract of DOI:10.1007/s00706-007-0789-9

A very simple and mild reaction is described for the aerobic oxidative coupling of thiols to disulfides by silica-supported cobalt(II) tetrasulfophthalocyanine as the catalyst in non-aqueous media under neutral conditions at room temperature. The catalyst can be reused for the oxidative coupling of several thiols without any significant loss of catalytic activity.

Full text of DOI:10.1007/s00706-007-0789-9

Monatsh Chem 139, 613–615 (2008)
DOI 10.1007/s00706-007-0789-9
Printed in The Netherlands
Silica-supported cobalt(II) tetrasulfophthalocyanine catalyzed aerobic
oxidation of thiols to disulfides under neutral conditions
Ahmad Shaabani, Naser Safari, Samad Shoghpour, Ali Hossein Rezayan
Department of Chemistry, Shahid Beheshti University, Tehran, Iran
Received 17 July 2007; Accepted 20 August 2007; published online 21 April 2008
#
Springer-Verlag 2008
Abstract A very simple and mild reaction is described
for the aerobic oxidative coupling of thiols to disul-
fides by silica-supported cobalt(II) tetrasulfophthalo-
cyanine as the catalyst in non-aqueous media under
neutral conditions at room temperature. The catalyst
can be reused for the oxidative coupling of several
thiols without any significant loss of catalytic activity.
To overcome these drawbacks, catalytic oxidation
using oxygen as co-oxidant has been developed by
several groups. One of the efficient oxidative cou-
plings of thiols to disulfides has been reported
by using transition metallophthalocyanines in basic
aqueous media [17–19]. However, not only the aque-
ous bases always cause a problem of spent caustic
materials, but the main drawbacks of these catalysts
are the poor solubility in common organic solvents
and the formation of ꢀ-oxo-dimers in the case of
water soluble tetrasulfonated phthalocyanines, which
decrease the reactivity in aqueous media [20]. Thus,
metallophthalocyanines have been immobilized on a
variety of solid supports for stability and easy cata-
lyst recovery. However, not only the efficiency of
these catalytic systems was lower than that obtained
under homogenous conditions but they are used with
H O or KHSO as activator of metallophthalocya-
Keywords Oxidation; Thiols; Disulfides; Cobalt(II) tetrasul-
fophthalocyanine.
Introduction
The oxidative coupling of thiols to disulfides is of
interest from the point of view of the synthetic, bio-
logical, and petroleum industry [1–5]. For selective
oxidation of thiols several methods have been de-
veloped. For example I =HI [6], Br [7], KMnO =
4
2
2
CuSO [8], H O in trifluoroethanol [9] and DMSO
4
2 2
2
2
4
[10], N-phenyltriazolinedione [11], manganese(III)
nines [21].
Schiff-base complex [12], bromine on hydrated silica
gel support [13], potassium dichromate [14], and
enzymatic [15] and electrochemical [16] methods
are used for this conversion. Most of these suffer
from drawbacks, such as the use of stoichiometric
amounts of reagents that generate undesirable waste
materials.
During the course of our studies on the develop-
ment of new routes for the oxidation of sulfur com-
pounds [22] and the synthesis of phthalocyanines [23],
we now describe aerobic oxidative coupling of thiols
Correspondence: Ahmad Shaabani, Department of Chemistry,
Shahid Beheshti University, P.O. Box 19396, 4716 Tehran,
Iran. E-mail: a-shaabani@cc.sbu.ac
Scheme 1

6
14
A. Shaabani et al.
Table 1 Selective oxidation of thiols (1 mmol) to disulfides with cobalt(II) tetrasulfophthalocyanine at room temperature
ꢀ
ꢀ
Product
R
Yield=%
mp= C or bp= C
Found
Reported
69–72b
a
2
2
2
2
2
2
2
2
2
2
2
a
b
c
d
e
f
g
h
i
PhCH2
Ph
4-MePh
3-ClPh
4-ClPh
4-MeOPh
2-furylmethyl
2-naphthyl
cyclohexyl
98, 98, 98, 98,98
a
70
61
45
210–213
72
43
b
98, 98
97, 97
59–60
45–46
213–219
e
73–74
43–44
a
c
d
98
97
98, 98
a
f
g
h
f
95
96
98
92
95
109–112
136
126
71–73
111–113
112–115
c
139
125–130
i
k
l
CH3ðCH2Þ7
ꢁ
ꢁ
74–75
114–115
CH3ðCH2Þ3
a
The same catalyst was used for each of the five runs
b
c
d
e
f
g
h
i
Ref. [24], Ref. [25], Ref. [26], Ref. [27], Ref. [28], Ref. [29], Ref. [30], Ref. [31]
to disulfides using silica-supported cobalt(II) tetra-
sulfophthalocyanine as a reusable catalyst in non-
aqueous media under neutral conditions at room
temperature (Scheme 1).
ing material, efficiently catalyzed the selectively syn-
thesis of disulfide by aerobic oxidation of thiols in
non-aqueous media under neutral conditions at room
temperature. The reaction is clean and it allows eas-
ier product separation. The catalyst can be reused in
further reactions or in continuous operations without
loss of catalytic activity.
Results and discussion
Cobalt(II) tetrasulfophthalocyanine-silica is readily
prepared by the addition of silica to an aqueous so-
lution of cobalt(II) tetrasulfophthalocyanine at room
temperature. The slurry is poured into a chromato-
graphy column, and the reaction is done by perco-
lating of a solution of thiols in EtOAc through the
column. The pure product is obtained by evapora-
tion of the solvent. As indicated in Table 1, various
alkyl and aryl thiols are converted into the corre-
sponding disulfides in very high yields. This is a
highly useful reaction for the oxidation of thiols,
which are meager impurities in petroleum. A fur-
ther set of experiments was carried out to examine
the reusability of the silica-supported cobalt(II) tet-
rasulfophthalocyanine catalyst. After the oxidative
coupling of benzylthiol (5 times, entry 2a, Table 1),
the same column was used for the oxidations of
benzenethiol (98% entry 2b, Table 1), 4-methylben-
zenethiol (97%, entry 2c, Table 1), and 4-methox-
ybenzenethiol (98%, Entry 2f, Table 1) proving the
high efficiency of the silica-supported cobalt(II) tet-
rasulfophthalocyanine catalyst.
Experimental
Melting points were measured on an Electrothermal 9100
1
apparatus. IR and H NMR spectra were recorded on a
Shimadzu IR-470 spectrometer and a BRUKER DRX-300
1
AVANCE spectrometer at 300.13 MHz. H NMR spectra were
obtained using CDCl solutions.
3
Cobalt(II) tetrasulfophthalocyanine was prepared according
to Ref. [23c].
Typical experimental procedure for the preparation
of 1,2-dibenzyldisulfide
Preparation of silica-supported cobalt(II) tetrasulfophthalocya-
nine (CoTSPc=SiO ) was accomplished by dissolving 30 mg
2
3
cobalt(II) tetrasulfophthalocyanine in 25cm water and adding
1 g silica. The suspension is stirred magnetically at room tem-
perature for 30min. The slurry is poured into a column of
1
0
5cm length and 2.15cm diameter. Then, the solution of
3
.124 g benzylthiol (1mmol) in 2 cm EtOAc was added to
the column, it was eluated with EtOAc:n-hexane (15:1) through
the column containing the silica-supported cobalt(II) tetrasul-
3
fophthalocyanine at a convenient flow rate (0.5cm =min), and
the effluent solution of product was collected. After evapo-
ration of solvent under reduced pressure, the product 2a
(0.242g, 98%) is obtained with high purity. All of the products
In conclusion, we have shown that the silica-sup-
ported cobalt(II) tetrasulfophthalocyanine, which can
be easily prepared from commercially available start-
1
are known and they were characterized by IR and H NMR
spectra and melting point which were compared with those
obtained from authentic samples.

Oxidation of thiols to disulfides
615
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