Cobalt phthalocyanine mediated aerobic oxidation of thiols: A simple and convenient preparation of disulphides

Article abstract of DOI:10.1081/SCC-120003604

Cobalt phthalocyaninetetrasulphonamide/tetrasodium salt of cobalt tetrasulphophthalocyanine catalyzed oxidation with molecular oxygen form a simple, efficient and environmentally acceptable synthetic tool for the oxidation of thiols to disulphides in excellent yields, under mild conditions and without any side reactions.

Full text of DOI:10.1081/SCC-120003604

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COBALT PHTHALOCYANINE MEDIATED AEROBIC
OXIDATION OF THIOLS: A SIMPLE AND CONVENIENT
PREPARATION OF DISULPHIDES
T. Venkateshwar Rao a , K. Narasimha Rao a , Suman L. Jain a & Bir Sain a
a Chemical and Biosciences Division, Indian Institute of Petroleum, Dehradun, 248 005, India
Version of record first published: 16 Aug 2006.
To cite this article: T. Venkateshwar Rao , K. Narasimha Rao , Suman L. Jain & Bir Sain (2002): COBALT PHTHALOCYANINE
MEDIATED AEROBIC OXIDATION OF THIOLS: A SIMPLE AND CONVENIENT PREPARATION OF DISULPHIDES, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 32:8, 1151-1157
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SYNTHETIC COMMUNICATIONS, 32(8), 1151–1157 (2002)
COBALT PHTHALOCYANINE MEDIATED
AEROBIC OXIDATION OF THIOLS:
A SIMPLE AND CONVENIENT
PREPARATION OF DISULPHIDES
*
T. Venkateshwar Rao, K. Narasimha Rao,
Suman L. Jain, and Bir Sainy
Chemical and Biosciences Division, Indian Institute
of Petroleum, Dehradun-248 005, India
ABSTRACT
Cobalt phthalocyaninetetrasulphonamide/tetrasodium salt of
cobalt tetrasulphophthalocyanine catalyzed oxidation with
molecular oxygen form a simple, efficient and environmentally
acceptable synthetic tool for the oxidation of thiols to disul-
phides in excellent yields, under mild conditions and without
anyside reactions.
Selective oxidative conversion of thiols to disulphides is an important
reaction from a synthetic as well as biological viewpoint1,2 and many
stoichiometric reagents like manganese dioxide,3 dichromates,4 chloro-
chromates,5 halosilane-chromium trioxide,6 diethyl azodicarboxylate,7
*Current address: Catalyst Division, Indian Institute of Chemical Technology,
Hyderabad–500 071, India.
yCorresponding author.
1151
Copyright & 2002 byMarcel Dekker, Inc.
www.dekker.com

1152
RAO ET AL.
nickel peroxide,8 chromium peroxide,9 diaryl telluroxide,10 tetrabutyl
ammonium cerium (IV) nitrate,11 sodium perborate,12 silver trifluoro-
methane sulphonate13 and permanganate14 have been reported to be effec-
tive oxidants for this transformation.
The increasing environmental concern surrounding the use of toxic
and dangerous oxidants has impelled organic chemists to develop catalytic
oxidation methodologies using molecular oxygen as primary oxidants.15 In
this context catalytic oxidation of thiols to disulphides by molecular oxygen
using basic alumina,16 intercalated [MoVI O2 {O2 CC(S)} (Ph2)2]2À in ZnII-
AlIII-layered double hydroxide,17 cobalt chlorate,18 Fe (III)/NaI19 as cata-
lysts have been reported.
In continuation of our studies on oxidation using molecular oxygen as
20–23
a primaryoxidant,
we earlier reported that iron (III)-ethylenediamine-
tetraaceticacid (Fe(III)-EDTA) catalyzed oxidation of thiols to disulfides.24
Although this method worked well for the oxidation of different types of
thiols, there have been limitations for the oxidation of tertiaryand long
chain aliphatic thiols (C8 and above) and long reaction hours were required
due to their low ionization constants. Efforts to overcome this problem by
conducting the reaction at higher pH(>11) led to the precipitation of iron
25
oxide due to the low stabilityof Fe(III)-EDTA.
In search of a better
catalytic system we focused our attention towards metal phthalocyanines
which are used for catalytic oxidation of thiols present in petroleum frac-
tions under stronglyalkaline conditions. 26 We report herein for the first time
a simple and convenient synthetic method for the oxidation of thiols1 to
disulphides2 in near quantitative yields using molecular oxygen as an oxi-
dant and cobalt phthalocyanine tetrasulphonamide [CoPc(SO2NH2)4]27 or
tetrasodium salt of cobalt tetrasulphophthalocyanine [CoPc(SO3Na)4]28 as a
catalyst (Scheme 1).
Scheme 1.
A wide varietyof thiols consisting of aliphatic, aromatic, heterocyclic,
tertiaryand long chain aliphatic thiols was selectivelyoxidized to their
corresponding disulphides in near quantitative yields without any evidence
for the formation of corresponding sulphonic acid29 (Tables 1 and 2).

AEROBIC OXIDATION OF THIOLS
1153
Table 1. Cobalt Phthalocyaninetetrasulphonamide Catalyzed
Oxidation of Thiols (1) to Disulphides (2) with Molecular Oxygen
Reaction Time
(min)
Disulphide Yielda
(%)
EntryR
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
C6H5
40
25
30
35
22
23
25
140
28
30
35
35
40
230
50
96
95
87
97
84
93
86
84
88
83
88
85
92
80
95
96
97
98
4-H3C-C6H4
4-H3COC6H4
C6H5CH2
2-Pyridyl
n-C4H9
n-C5H11
tert-C5H11
n-C7H15
n-C8H17
n-C9H19
n-C10H21
n-C12H25
tert-C12H25
n-C14H29
n-C16H33
n-C18H37
n-C19H39
70
100
120
aIsolated yield.
Table 2. Tetrasodium Salt of Cobalt Tetrasulphophthalo-
cyanine Catalyzed Oxidation of Thiols (1) to Disulphides (2)
with Molecular Oxygen
Reaction Time
(min)
Disulphide Yielda
(%)
EntryR
1
2
3
4
5
6
7
4-H3C-C6H4
4-H3COC6H4
tert-C5H11
n-C9H19
n-C14H29
n-C18H37
n-C19H39
30
32
150
34
55
95
126
96
85
86
85
90
95
97
aIsolated yield.

1154
RAO ET AL.
The protocol developed consists of simplybubbling molecular oxygen
into a solution of thiols in 1 N NaOH containing cobalt phthalocyanine-
tetrasulphonamide or tetrasodium salt of cobalt tetra-sulphophthalocyanine
catalyst (0.2 mol % of the thiol) under ambient conditions, until the com-
pletion of the reaction (indicated bychange of reaction mixture colour from
black to blue). Tertiarythiols were found to be less reactive and require
longer reaction time. In the case of aliphatic thiols the reactivitydecreases
with the increase in chain length and accordinglythe thiols with longer chain
length require more reaction time.
Results presented in Tables 1 and 2 show that both cobalt phthalo-
cyaninetetrasulphonamide and tetrasodium salt of cobalt tetrasulpho-
phthalocyanine exhibit comparable catalytic activities in the oxidation of
different types of thiols. It appears that catalytic activity relates to the cobalt
phthalocyanine, and the groups attached (SO2NH2, SO3Na) are essentially
required for solubilizing the cobalt phthalocyanine in aqueous medium.
To evaluate the catalytic effects of cobalt phthalocyaninetetrasulphon-
amide and tetrasodium salt of cobalt tetrasulphophthalocyanine, experi-
ments were carried out under similar conditions but without catalysts.
The results showed that there was no oxidation of 1-octanethiol to cor-
responding disulphide even after 24 h.
With a view to determine the effect of pH, we studied the cobalt
phthalocyaninetetrasulphonamide catalyzed oxidation of thiophenol at
different pH. These results are presented in Table 3 and clearlyshow that
this reaction was highlydependent upon pH of the system and required less
reaction times at higher pH. To determine the effect of catalyst concentra-
tion in these reactions, experiments were carried out with thiophenol using
different quantities of cobalt phthalocyaninetetrasulphonamide catalyst.
These results in Table 4 show that the catalyst concentration does not
have a significant effect on the reaction time.
Table 3. Effect of pH in Cobalt Pthalocyaninetetrasulphon-
amide Catalyzed Oxidation of Thiophenol to Diphenyl
Disulphide with Molecular Oxygen
Reaction Time
(min)
EntryR
pH
C6H5
C6H5
C6H5
C6H5
1
2
3
4
7.00
8.50
11.0
14.00
145
65
50
40

AEROBIC OXIDATION OF THIOLS
1155
Table 4. Effect of Catalyst Concentration in Cobalt Pthalo-
cyaninetetrasulphonamide Catalyzed Oxidation of Thiophenol
to Diphenyl Disulphide with Molecular Oxygen
Catalyst Concentration
(mol %)
Reaction Time
(min)
EntryR
1
2
3
4
C6H5
C6H5
C6H5
C6H5
0.2
0.5
2.0
5.0
40
35
27
17
In conclusion, simplicity of the system, excellent yields, wide applic-
abilityand reasonable reaction times make cobalt phthalocayninetetra-
sulphonamide/tetrasodium salt of cobalt tetrasulphophthalocyanine
catalyzed oxidation with molecular oxygen an attractive and environmen-
tallyacceptable synthetic tool for the oxidation of thiols to disulphides.
EXPERIMENTAL
Cobalt phthalocyaninetetrasulphonamide was prepared following the
27
literature procedure reported byAgrawal et al.
and its structure was
established byFAB mass spectral and elemental analsyis. Tetrasodium
salt of cobalt tetrasulphophthalocyanine dihydrate was prepared from
4-sulphophthalic acid essentiallyemploiyng the procedure reported by
Weber and Busch.28
A typical procedure for the oxidation of thiol to disulphide is as
follows: molecular oxygen was bubbled into the stirred solution of
1-octanethiol (7.32 g, 50 mmol) and cobalt phthalocyanine-tetrasulphon-
amide (0.088 g, 0.1 mmol) in 1 N NaOH (250 ml, water), contained in a
500 ml two-necked round-bottomed flask fitted with a gas inlet tube and a
condenser for 5 min. The completion of the reaction was indicated bythe
change of reaction mixture colour from black to blue. The reaction mixture
was stirred further for an additional 5 min, and then extracted with dichlor-
omethane (2 Â 100 ml), organicallyphase dried over anhydrous sodium sul-
phate. Evaporation of the solvent and distillation at the reduced pressure
(185–187ꢀC at 6 torr) yielded dioctyl disulphide (6.04 g, 83%) as oily liquid.
Similarly, disulphides from aliphatic thiols (Entries 6–14, Table 1) were
prepared. The disulphides from aromatic heterocyclic thiols (Entries 1–5,
Table 1) and aliphatic thiols (Entries 15–18, Table 1) were prepared by
conducting the reactions using 10 mmol of thiol similarly, and evaporation

1156
RAO ET AL.
of the dichloromethane followed by recrystallization yielded pure disul-
phides. The products were identified bycomparing their physical spectral
data and GC retention times with those of authentic samples.
ACKNOWLEDGMENTS
We are thankful to the Director IIP, for his kind permission for
publishing these results. Suman L. Jain is thankful to CSIR, New Delhi
for the award of a research fellowship.
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Received in the Netherlands May12, 2001