Oxidation of thiols with molecular oxygen catalyzed by cobalt(II) phthalocyanines in ionic liquid

Article abstract of DOI:10.1039/b305888c

An efficient procedure for catalyst solubility, recycling and easy product isolation in oxidation of thiols to disulfides with molecular oxygen catalyzed by cobalt(II) phthalocyanines dissolved in ionic liquid at room temperature is reported.

Full text of DOI:10.1039/b305888c

Oxidation of thiols with molecular oxygen catalyzed by cobalt(II
)
phthalocyanines in ionic liquid†
S. M. S. Chauhan,* Anil Kumar and K. A. Srinivas
Department of Chemistry, University of Delhi, Delhi-110 007, India.
E-mail: smschauhan@chemistry.du.ac.in
Received (in Cambridge, UK) 27th May 2003, Accepted 30th July 2003
First published as an Advance Article on the web 8th August 2003
An efficient procedure for catalyst solubility, recycling and
easy product isolation in oxidation of thiols to disulfides with
molecular oxygen catalyzed by cobalt(II) phthalocyanines
dissolved in ionic liquid at room temperature is reported.
Phthalocyanines (Pcs) constitute a remarkably versatile and
robust class of compounds with diverse technological applica-
tions.1–4 Metallophthalocyanines have been used as efficient
biomimetic catalysts for oxidation, reduction and other reac-
tions of different organic compounds.5 On the other hand, alkyl
mercaptans and other sulfur containing impurities present in
petroleum are major environmental pollutants. They are also
corrosive and poisoners of different catalysts during the
processing and purification of petroleum. Further, the oxidation
of thiols to disulfides has also been involved in many synthetic,
biological6 and oil-sweetening processes.7,8 The oxidation of
thiols to disulfide has been achieved by molecular oxygen in
basic aqueous H2O2 in the absence and presence of catalyst and
homogeneous medium under oxygen atmosphere (Fig. 1). A
mixture of thiophenol and 1a in 2 was stirred for 45 min in the
presence of molecular oxygen to give diphenyl disulfide in 95%
yield. Similarly, the oxidation of other thiols with molecular
oxygen catalyzed by 1a or 1b in 2 was carried out and the results
are shown in Table 1.‡
with other oxidants under different reaction conditions.9,10
Efficient oxidation of thiols to disulfides catalyzed by transition
metal phthalocyanines in aqueous, water-in-oil and oil-in-water
microemulsions has been reported by us and others.11,12
However, the main drawbacks of these catalysts are the poor
solubility in common organic solvents and the formation of m-
oxo-dimers in the case of water-soluble tetrasulfonated phthalo-
cyanines which decrease the reactivity in aqueous media. Thus,
metallophthalocyanines have been immobilized on a variety of
solid supports for stability and easy catalyst recovery but these
supported catalysts also often suffer from drawbacks such as
lower activity than soluble catalysts due to either chemical or
diffusional restrictions.13,14
The 1,3-dialkylimidazolium tetrafluoroborates [bmim][BF4]
are an important class of ionic liquids and they are environmen-
tally benign solvents due to their unique chemical and physical
properties such as excellent chemical and thermal stability with
ease of reuse, miniscule vapour pressure, unique misibilities and
non-flammability.15,16 They behave macroscopically like polar
aprotic solvents but are non-coordinating. They have been
employed as solvents for many organic and enzymatic trans-
formations,17,18 in liquid–liquid separation19 and recycling of
organometallic catalysts.20,21 Further, imidazolium cation con-
taining ionic liquids have also been shown to enhance the rate of
reaction with facilitation of easy catalyst recovery.22 Herein, we
report the oxidation of selected thiols to disulfides with
molecular oxygen catalyzed by cobalt(II) phthalocyanines
dissolved in imidazolium ionic liquid under mild reaction
conditions.
Fig. 1
The disulfides were extracted with ethyl acetate from the
ionic liquid after completion of the reaction leaving behind the
catalyst solubilized in the ionic liquid. Evaporation of the ethyl
acetate under reduced pressure gave the disulfide, which was
further purified by column chromatography over a silica gel
column using hexane as eluent. The formation of products was
confirmed by the comparison of physical and spectroscopic data
as well as HPLC retention time with those of authentic samples.
UV-visible spectral studies of the oxidation of thiophenol with
molecular oxygen catalyzed by 1a in 2 show that the formation
of disulfide resulted from the dimerization of RS· radicals which
in turn formed from the reaction of Co(
)–O2 with RS2.†10 The
I
recovered blue ionic liquid containing CoPc was reused for
further catalytic reactions and found to have comparable
activity upto 5–6 recycling experiments with a slight decrease in
the yields of the products in each cycles. Similar results were
obtained for 1b but yields of disulfides were higher than that for
1a. Further, 1b showed better catalytic activity than 1a in the
oxidation of thiols with molecular oxygen at room tem-
perature.
The yield of diphenyl disulfide in the oxidation of thiophenol
with molecular oxygen catalyzed by 1a in organic solvents such
as THF and DMF were 58 and 82%, respectively, as compared
to 95% in ionic liquid under similar conditions. Further, longer
reaction times were required in these organic solvents (1–3 h)
than in ionic liquids for complete conversion of thiols to
disulfides and separation of the product from the reaction
mixture is tedious.
The required cobalt(II) phthalocyanine [CoPc] (1a) and
cobalt(II) tetranitrophthalocyanine [CoTNPc] (1b) were synthe-
sized by a template method using phthalic anhydride or
3-nitrophthalic anhydride, urea and CoCl2.†9,23 The catalysts
were purified by a reported procedure and their structures were
confirmed by IR and UV-visible spectroscopic data. Catalysts
1a–b were freely dissolved in [bmim][BF4] (2) by simple
mixing and the oxidation of different thiols was carried out in
In summary, efficient biomimetic oxidation of thiols to
disulfides with molecular oxygen catalyzed by cobalt(II)
† Electronic supplementary information (ESI) available: experimental. See
http://www.rsc.org/suppdata/cc/b3/b305888c/
2348
CHEM. COMMUN., 2003, 2348–2349
This journal is © The Royal Society of Chemistry 2003

View Article Online
Table 1 Oxidation of thiols to disulfides with molecular oxygen catalyzed by 1a and 1b immobilized in ionic liquid 2a
Yield of
disulfideb(%)
mp or bp/mmHg
(Lit. mp or bp)
Run
Substrate
Catalyst
Time/min
Turnoversc
1
2
3
4
5
6
7
8
9
10
11
11
12
C6H5SH
C6H5SH
1a
1b
1a
1a
1a
1a
1a
1b
1a
1a
1b
1a
1b
45
45
35
60
60
60
45
45
45
45
45
60
60
95 (92)d,e,f
99 (95)d
99 (97)d
95 (89)d
83
2.07 3 104
2.16 3 104
2.44 3 104
2.36 3 104
2.50 3 104
1.89 3 104
2.29 3 104
2.39 3 104
1.40 3 104
1.64 3 104
1.73 3 104
2.52 3 104
2.69 3 104
57–59 (59–60)24
4-MeC6H4SH
2-H2NC6H4SH
2-Mercaptopurine
2-Mercaptopyridine
C6H5CH2SH
C6H5CH2SH
HOCH2CH2SH
n-C4H9SH
51–53 (52–56)25
90–92 (91–93)26
241–244
56–57 (57–58)25
70–73 (71–72)25
86 (81)d
93 (90)d
97
91 (84)d
92 (91)d
97
87 (85)d
93
113/7 (115/7)24
223–226 (226)25
n-C4H9SH
n-C8H17SH
n-C8H17SH
185–187/6 (187/6)24
a All reactions were carried out using thiol (1.0 mmol), CoPc (1.0 3 1022 mmol) in [bmim][BF4] (2.0 mL). b Yields are based on starting material and are
calculated by HPLC analysis (m-bondapak C18 reverse phase column, methanol : water = 60 : 40 v/v, flow rate = 0.5 ml min21, monitored at 220 nm).
c Based on yield of disulfide. d Isolated yields. e 7% of diphenyl disulfide was also obtained in absence of catalyst 1a. f Diphenylsulfide was formed in 58
and 82% yield with 1.26 3 104 and 1.79 3 104 turnovers in THF and DMF, respectively.
3 J. D. Spikes, Photochem. Photobiol., 1986, 43, 691.
4 N. B. McKeown, Chem. Ind., 1999, 92.
5 B. Meunier and A. Sorokin, Acc. Chem. Res., 1997, 30, 470.
phthalocyanines (1a and 1b) immobilized in imidazolium ionic
liquid (2) has been achieved. The reactions are clean and work-
up of the reaction products is easy. The catalysts (1a and 1b)
6 A. Vizi-Orosz and E. Milaeva, Transition Met. Chem., 1992, 17, 16.
immobilized in the ionic liquid (2) are easily recyclable and can
7 P. C. Jocelyn, in Biochemistry of the Thiol Groups, Academic Press,
be reused in further reactions without much loss of catalytic
New York, 1992.
activity. The cobalt(II) tetranitrophthalocyanine (1b) proved to
8 S. Basu, S. Satapathy and A. K. Bhatnagar, Catal. Rev. Sci. Eng., 1993,
be a better catalyst than cobalt(II) phthalocyanine (1a). Hence,
Co(II) phthalocyanine immobilized in ionic liquid can be used
for petroleum purification and oxidative treatment of waste
water or industrial effluents.
35, 571.
9 T. V. Rao, K. N. Rao, S. L. Jain and B. Sain, Synth. Commun., 2002, 32,
1151.
10 A. Z. Lesnugin, A. V. Anisimov and A. V. Tarakanova, Pet. Chem.,
2000, 40, 423.
11 S. M. S. Chauhan, A. Gulati, A. Sahay and P. N. H. Nizar, J. Mol. Catal.
A: Chem., 1996, 105, 159.
Notes and references
12 S. A. Brisenkova, Pet. Chem., 1991, 31, 379.
13 A. Skorobogaty and T. D. Smith, J. Mol. Catal., 1982, 16, 131.
14 M. Sanchez, N. Chap, J-B. Cazaux and B. Meunier, Eur. J. Inorg.
Chem., 2001, 1775.
15 R. A. Sheldon, Chem. Commun., 2001, 2399.
16 J. Dupont, R. F. de Souza and P. A. Z. Suarez, Chem. Rev., 2002, 102,
3667.
17 C. M. Gordon, Appl. Catal. A, 2001, 222, 101.
18 U. Kragl, M. Eckstein and N. Kaftzik, Curr. Opin. Biotechnol., 2002,
13, 565.
19 J. G. Huddleston, H. D. Willauer, R. P. Swatloski, A. E. Visser and R.
D. Rogers, Chem. Commun., 1998, 1765.
20 K. A. Srinivas, Anil Kumar and S. M. S. Chauhan, Chem. Commun.,
2002, 2456.
21 H. Waffenschimidt and P. Wasserscheid, J. Mol. Catal. A: Chem., 2000,
164, 61.
22 D. W. Kim, C. E. Song and D. Y. Chi, J. Am. Chem. Soc., 2002, 124,
10278.
‡
The required ionic liquid [bmim][BF4] was prepared according to the
reported procedure20 by the alkylation of 1-methylimidazole with 1-bromo-
butane followed by substitution of bromide anion with tetrafluoroborate in
acetone.27 1H NMR (CDCl3, 300 MHz, d ppm): 8.98 (s, 1H, C–2H), 7.49 (s,
2H, C–4H & C–5H), 4.21 (t, J = 7.11 Hz, 2H, NCH2CH2CH2CH3), 4.01 (s,
3H, NCH3), 1.85 (p, J = 7.20 Hz, 2H, –NCH2CH2CH2CH3), 1.35 (sexet, J
=
7.20 Hz, 2H, NCH2CH2CH2CH3), 0.91 (t, J = 7.12 Hz, 3H,
NCH2CH2CH2CH3). 13C NMR (CDCl3, 75 MHz, d ppm): 13.27, 19.23,
31.87, 36.13, 49.57, 122.49, 123.79 and 136.12.
Typical procedure for oxidation of thiols with molecular oxygen
catalyzed by CoPc or CoTNPc immobilized in ionic liquid [bmim][BF4]:
The solid CoPc or CoTNPc (0.01 mmol) was immobilized in [bmim][BF4]
(2 mL) by stirring and thiol (1.0 mmol) was added to the above mixture in
a round bottomed flask. Oxygen gas was bubbled through the reaction
mixture while stirring for 45–60 min at room temp. Progress of the reaction
was monitored by HPLC. The ionic liquid layer was extracted with ethyl
acetate (3 3 10 mL). The combined extracts were concentrated under
reduced pressure and the residue was purified by column chromatography.
The [bmim][BF4] containing CoPc or CoTNPc was recovered and reused
for the next oxidation.
23 J. H. Weber and D. H. Busch, Inorg. Chem., 1965, 4, 469.
24 B. Karimi, H. Hazarkhani and D. Zareyee, Synthesis, 2002, 2513.
25 A. R. Hajipour and S. E. Mallakpour, J. Chem. Res. (S), 2000, 32.
26 S. Uemura, S. Tanaka and M. Okano, Bull. Chem. Soc. Jpn., 1977, 50,
220.
1 C. C. Leznoff and A. B. P. Lever, Phthalocyanines: Properties and
Applications, VCH, Weinheim, 1989–1996, vol. 1–4.
2 J. R. Darwent, P. Douglas, A. Harriman, G. Porter and M. C. Richoux,
Coord. Chem. Rev., 1982, 44, 83.
27 P. A. Z. Suarez, J. E. L. Dullius, S. Einloft, R. F. De Souza and J.
Dupont, Polyhedron, 1996, 15, 1217.
CHEM. COMMUN., 2003, 2348–2349
2349