Aerobic oxidation of some arylthiols catalyzed by a new manganese (III) schiff-base complex

Article abstract of DOI:10.1080/10426500902839830

Bis(2-hydroxybenzene)phthaldiimine (BHBPDI) as a new quadridentate Schiff base ligand and its [Mn(III)(BHBPDI)Cl] complex were synthesized and characterized by analytical and spectral data (MS, FT-IR, UV-Vis, 1H NMR). The aerobic oxidation of thiols catalyzed by Mn(III)-complex using molecular oxygen was studied at room temperature. The reaction proceeded to yield the corresponding symmetric disulfides in good to high yields. The reaction was retarded when the reaction mixture was saturated with nitrogen gas. This system provides an efficient and convenient method for the oxidation of thiols to symmetric disulfides.

Full text of DOI:10.1080/10426500902839830

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Phosphorus, Sulfur, and Silicon and the
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Aerobic Oxidation of Some Arylthiols
Catalyzed by a New Manganese (III)
Schiff-Base Complex
Morteza Montazerozohori a & Leila Zamani Fradombe a
a Department of Chemistry , Yasouj University , Yasouj, Iran
Published online: 24 Feb 2010.
To cite this article: Morteza Montazerozohori & Leila Zamani Fradombe (2010) Aerobic Oxidation
of Some Arylthiols Catalyzed by a New Manganese (III) Schiff-Base Complex, Phosphorus, Sulfur, and
Silicon and the Related Elements, 185:3, 509-515, DOI: 10.1080/10426500902839830
To link to this article: http://dx.doi.org/10.1080/10426500902839830
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Phosphorus, Sulfur, and Silicon, 185:509–515, 2010
Copyright ꢀC Taylor & Francis Group, LLC
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500902839830
AEROBIC OXIDATION OF SOME ARYLTHIOLS CATALYZED
BY A NEW MANGANESE (III) SCHIFF-BASE COMPLEX
Morteza Montazerozohori and Leila Zamani Fradombe
Department of Chemistry, Yasouj University, Yasouj, Iran
Bis(2-hydroxybenzene)phthaldiimine (BHBPDI) as a new quadridentate Schiff base ligand
and its [Mn(III)(BHBPDI)Cl] complex were synthesized and characterized by analytical and
spectral data (MS, FT-IR, UV-Vis, 1H NMR). The aerobic oxidation of thiols catalyzed
by Mn(III)-complex using molecular oxygen was studied at room temperature. The reaction
proceeded to yield the corresponding symmetric disulfides in good to high yields. The reaction
was retarded when the reaction mixture was saturated with nitrogen gas. This system provides
an efficient and convenient method for the oxidation of thiols to symmetric disulfides.
Keywords Air; disulfides; Mn(III); oxidation; Schiff-base complex; thiols
INTRODUCTION
The oxidation of thiols is the most exploited method for disulfide synthesis, mainly
because a large number of thiols are commercially available and/or are easily synthesized.
The oxidation of thiols to the corresponding disulfides is also of interest from a biologi-
cal point of view.1–2 Thiols are among functional groups that can be easily over-oxidized
and, therefore, extensive studies have been carried out to develop controlled conditions to
prevent this from occurring. Various reagents or catalytic systems such as permanganates,3
tungstated or molybdated sulfuric acid/NaNO2,4 transition metal catalysts such as Schiff
base and porphyrin complexes,5 sodium perborate,6 ferric chloride,7 sodium chlorite,8 nitric
oxide,9 hydrogen peroxide10 and halogens,11 2,6-dicarboxypyridinium chlorochromate,12
monochloro poly(styrenehydantoin) beads,13 N-phenyltriazolinedione,14 and sulfuryl chlo-
ride15 among others have been utilized for the oxidation of thiols. Enzymatic16 and elec-
trochemical17 methods are also known to perform this oxidative transformation. Most of
the reported methods suffer from disadvantages such as expensive, rare, or toxic reagents.
However, recent demands for environmentally friendly chemical reactions have encouraged
chemists to develop clean and inexpensive reactions for the oxidation of thiols.18–20 Thus
a simple, mild, clean, and efficient oxidative method to oxidation of thiols to disulfide is
still needed. In this direction, molecular oxygen is a candidate for a green oxidant due to
its strength and lack of toxic byproducts. There are only rare reports on aerobic oxidation
Received 23 August 2008; accepted 22 February 2009.
The authors gratefully acknowledge partial support of this work by the Yasouj University, Yasouj, Iran.
Address correspondence to Morteza Montazerozohori, Department of Chemistry, Yasouj University, Yasouj
75918-74831, Iran. E-mail: mmzohori@mail.yu.ac.ir; mmzohory@yahoo.com
509

510
M. MONTAZEROZOHORI AND L. Z. FRADOMBE
of thiols to disulfides.21–23 In continuation of our current studies in the selective oxidation
of thiols,4,5 we wish to report a very efficient and simple method for catalytic oxidative
coupling of some aryl thiols into the corresponding disulfides using molecular oxygen
using a new Mn(III) Schiff-base complex [Mn(III)(BHBPDI)Cl] as catalyst (1) (Scheme 1)
including a new quadridentate ligand, bis(2-hydroxybenzene)-phthaldiimine (BHBPDI),
under mild conditions.
N
N
Mn
O
O
Cl
Scheme 1 Structure of [Mn (III)(BHBPDI)Cl] complex (1).
RESULTS AND DISCUSSION
The ligand was prepared based on common method, by condensation of a 1:2 molar
ratio of phthaldialdehyde and 2-aminophenol in ethanol under reflux. Elemental analysis,
MS, FT-IR, and 1H NMR confirmed the synthesis of ligand. The mass spectrum of the ligand
shows some main peaks at 318 (M++2), 314 (M+−2), 298 (M+ OH), 282 (M+−2OH),
223 (M HOC6H4) that confirm the molecular structure of the ligand. The IR spectral data
of BHBPDI using KBr pellets are in agreement with the structure. The broad absorption
band centered at around 3306 cm−1 is readily ascribed to the O H stretching. The ab-
sorption band at 1634 cm−1 is due to the C N stretching of the ligand that is appeared
at lower frequency than that for C O stretching (1686 cm−1) of phthaldialdehyde as
starting material. Also, elimination of two sharp absorption bands in 3375 and 3304 cm−1
and a very broad one centered in 2851 cm−1, as the characteristic bands of NH2 and OH
of 2-aminophenol as starting amine, are in good agreement with the molecular structure.
1
The solution NMR data of the BHBPDI also confirm the structure of the ligand. The H
NMR spectrum recorded in DMSO-d6 solution shows two broad singlets at 10.22 and 8.88
ppm for two OH groups of the ligand, probably because one of them has been involved
in the intramolecular hydrogen bonding [O H. . . N] in a stable conformer in the solution.
The 1H NMR spectrum also exhibits the expected peaks at 7.57 (m, 4H) for phthaldiimine
ring and 7.16 (m, 2H, CH N), 6.93 (m, 2H), 6.61 (m, 2H), and 6.35 (m, 4H) ppm for
two aminophenol aromatic rings of the ligand. The synthesis of [Mn(III)(BHBPDI)Cl] was
similar to the experimental procedure reported previously in the literature.24,26 Elemental
analysis, IR spectra, and conductivity resulted in satisfactory data. The absorption band
at 1653 cm−1, which can be assigned to the coordinated C N stretching, is a char-
acteristic band of the complex (1). The conductivity measurement in methanol indicates
that the Mn(III) complex is neutral (ꢀ = 20.57 µS/cm).25,26 During our investigations on
the development of organic transformations utilizing Schiff base complexes as catalysts
under aerobic or non-aerobic conditions, we found that 4-bromothiophenol was oxidized
under aerobic conditions effectively in the presence of [Mn(III)(BHBPDI)Cl] as catalyst (1).
Therefore, this compound was selected as a typical substrate for the optimization of the
conditions. At first, we investigated various solvents to run this conversion. As shown in
Figure 1, ethanol was found to be the most effective solvent for this oxidation.

MANGANESE (III) SCHIFF-BASE COMPLEX CATALYST
511
b c
100
80
60
a
40
20
0
C2H5OH CH2Cl2 CHCl3 CH3CN
n-
DMF
Hexane
Solvent
Figure 1 The solvent effect on the aerobic oxidative coupling of 4-bromothiophenol after 1 h using 10% mol of
catalyst (1). The labels of a, b, and c refer to the percent conversion at three temperatures of 0◦C, room temperature,
and 40◦C in ethanol after 60, 60, and 50 minutes, respectively.
Figure 1 also shows the performance of the reaction at 0◦C, room temperature, and
40◦C in ethanol. At 0◦C, the conversion was decreased (25% conversion), whereas the
selectivity was reduced significantly at elevated temperature more than room temperature,
and several unidentified byproducts were formed. Further, when the oxidative coupling of
4-bromothiophenol was done using different amounts of catalyst (1), the reaction was found
to be completed in shorter reaction time with increase in mole percent of catalyst (1) up to
10%. The oxidation reaction did not occur at the same condition in the absence of catalyst
(1) (Figure 2) and/or when using the ordinary Mn(III) salt such as Mn(III) acetate as the
catalyst, even after prolonged reaction time. To confirm the role of the molecular oxygen as
an oxidant in this reaction, two parallel reactions were designed so that in one reaction, the
air is bubbled and in the other, nitrogen gas is bubbled. Under nitrogen, oxidative coupling
of thiols did not occur. This observation refers to the key role of molecular oxygen in this
reaction (Scheme 2).
(1)
Mn(III)-Complex
RSSR
2 RSH
EtOH / Aerobic condition
Scheme 2 Oxidative coupling of thiols.
120
100
80
60
40
20
0
0
5
10
15
20
25
%mole of catalyst
Figure 2 The effect of mole% of Mn(III) complex (1) on the aerobic oxidative coupling of 4-bromothiophenol
after 1 h.

512
M. MONTAZEROZOHORI AND L. Z. FRADOMBE
Table I Aerobic oxidation of thiols to the corresponding disulfides catalyzed by [Mn(III)(BHBPDI)Cl] (1) in
ethanol at room temperature
Entry
1
Thiol
Disulfidea
Time (h)
5
Yieldb(%)
90
Mp (◦C) [Lit.]
78–8027
SH
MeO
F
S
MeO
F
S
OMe
2
3
2.9
4
89
92
Oil27
S
SH
S
F
Cl
80–835
Cl
Cl
Cl
S
SH
S
Cl
Cl
4
5
3.58
4.5
94
90
40–435,26
58–6027
SH
S
CH3S
CH3S
S
SCH3
S
SH
S
N
N
N
N
N
N
N
N
N
N
N
N
6
7
N
O
3
1
85
94
200–20127
N
N
O
S
S
SH
SH
O
145–1475,27
S
S
8
9
1
91
93
90
72–735,27
90–9227
Cl
S
Cl
Br
SH
S
Cl
1
Br
S
SH
S
Br
177–1795
N
10
2.8
N
S
N
S
SH
S
S
S
11
12
1.75
1.66
95
90
55–565
SH
SH
S
S
N
N
N
133–1355,27
N
N
N
N
N
N
S
S
aIdentified by physical, spectral data, and comparison with those reported in the literature.
bRefers to isolated yields.
Finally, the oxidative coupling of a variety of thiols was examined using the op-
timized reaction conditions, and oxidation was observed in high yields during 1–5 h of
reactions (Scheme 1 and Table I). When an ethanolic solution of Mn(III) complex in aer-
ated conditions was treated with a typical thiol, the initial dark brown color changed to
a light brown solution. Based on the literature, the dark brown and light brown solutions
of catalyst (1) refer to Mn(III) and Mn(II) complexes, respectively.22 With regard to the
literature and our observations, the proposed reaction mechanism is as shown in Scheme 3.
The Mn(III) complex oxidizes the thiol molecules to thionyl radicals and is itself reduced
to Mn(II) complex. Thionyl radicals are coupled to give the related disulfides. Then the
Mn(III) complex is regenerated by treatment with molecular oxygen and entered to the next
cycle of oxidation.
As shown in Table I and with regard to mechanism of Scheme 3, the thiols including
electronegative substituents, especially in 2-positions (Table I, entries 1, 2, 3, and 4) are
oxidized slower than others due to the instability of thionyl radicals and steric effect,

MANGANESE (III) SCHIFF-BASE COMPLEX CATALYST
513
.
2 Mn(II) complex + 2 RS+ 2 H+
a)
2 Mn(III) complex + 2 RSH
2 RS.
RSSR
b)
c)
+
2 Mn(II) complex
+ 1/2 O2 + 2 H
2 Mn(III) complex + 2 H2O
Scheme 3 Proposed reaction mechanism for oxidative coupling of thiols.
respectively. The thiols with heterocyclic rings (Table I, entries 5, 6, 10, 11, and 12) are
oxidized at the next preference, which is related to thiol-thione equilibriums of them in
solution that make them difficult to be oxidized. The thiols of entries of 7, 8, and 9 are
oxidized in shorter times with respect to others because of facility of oxidation due to relative
stability of produced thionyl radicals. Tenfold scale oxidation of 4-bromothiophenol (10
mmol) was also carried out. The results showed that oxidation took place in good yield
(less than 5% yield was lost). Reusability of catalyst was investigated by the use of it in
three sequential reactions that resulted in no efficient loss of catalytic activity (10% yield
was lost after the third use of it). Catalyst stability has been investigated by FT-IR and
UV-visible spectroscopy. The spectral data confirmed that the structure of the complex is
stable during the reactions.
EXPERIMENTAL
Chemicals were obtained from Merck or Fluka and used without further purification.
The reactions were monitored by TLC (silica-gel 60 F254, n-hexane:ethylacetate, 15:5).
The electronic absorption spectra were recorded with a Jasco UV-570 spectrophotometer.
1H NMR spectra were obtained with a Brucker DPX-250 and/or 500 MHz spectrometer.
Mass spectra were recorded on an AMD 604 spectrometer, EI-mode at 70 eV, FT-mode
at 0.005 V and/or Shimadzu model GC-MS QP5050. IR spectra were recorded on a FT-
IR Jasco-680 spectrophotometer. Molar conductivity of the complexes was measured by
Metrohm 712 model. Elemental analyses (CNHS) of samples were performed using a
CHNS-932 elemental analyzer by the central instrumental laboratory of Tarbiat Moallem
University of Tehran.
Preparation of the Ligand, Bis(2-Hydroxybenzene)phthaldiimine
(BHBPDI)
To a ethanolic solution (25 mL) of 2-aminophenol (0.546 g, 5 mmol), phthaldial
dehyde (0.335 g, 2.5 mmol) in 25 mL ethanol was gradually added, and the reaction
mixture was refluxed. The progress of the reaction was monitored by TLC. After 8 h,
cooled water (50 mL) was added to the reaction mixture to form a deep yellow precipitate
that was filtered, washed with water and ethanol/water mixture (15 mL) twice, and finally
recrystallized from ethanol. The 1,2-bis(2-hydroxybenzene)phthaldiimine was obtained in
76% yield. Mp = 117–120 ◦C. Elemental anal. calcd. (%)C20H16N2O2: C 48.93, H 4.62;
N 7.13. Found: C 49.27, H 4.72, N 6.96. IR (KBr, cm−1): 3306 (bs, νOH), 3061 (w,
CH-Aromatic), 2976 (w, CH-imine), 1634 (vs, νC N- imine) 1493 (s),1586 (s), 1453 (s,
νC C, aromatic ring), 1331 (m), 1302 (m), 1274 (m), 1209 (m, νC N), 1146 (m), 1123
(w), 1033 (m, C O), 928 (w), 896 (m), 747 (s), 727 (s), 657 (m), 566 (w), 471 (m). UV
1
(CH2Cl2), λmax: 223 and 306 nm. H NMR (DMSO-d6, ppm): 10.22 (bs, 1H), 8.88 (bs,

514
M. MONTAZEROZOHORI AND L. Z. FRADOMBE
1H), 7.57 (m, 4H, phthaldiimine benzene ring), 7.16 (m, 2H, CH N), 6.93 (m, 2H), 6.61
(m, 2H) and 6.35 (m, 4H). Mass (m/z): 318 (M++2), 314(M+−2), 298 (M+ OH), 282
(M+ OH), 223(M HOC6H4), 209, 196, 186, 168, 150, 129, 122, 113, 97, 93, 79, 77, 64,
51.
Preparation of [Mn(BHBPDI)Cl] Complex as the Catalyst
Manganese insertion into BHBPDI was achieved by heating the free-base BHBPDI
(1.58 g, 5 mmol) and MnCl2.4H2O (0.989 g, 5 mmol), respectively, in a similar method to
that reported in literature,7,10,24,26 under reflux and aerobic conditions, in ethanol (50mL),
for 4–6 h. After cooling, the solids were collected by filtration and washed twice with a
small portion of ethanol (10 mL) and then recrystallized from a water/ethanol mixture to
give 68% yield of product. Elemental analysis calculated for [Mn(III)(BHBPDI)Cl]. H2O,
{C20H16ClMnN2O3}, (%): C, 56.82; H, 3.81; N, 6.63; Found: C 56.47, H 4.06, N 6.86. Mp
(◦C) = (>340◦C, dec.): IR (KBr, cm−1): 3276(m, ν(H2O)), 3063, 1654, (s, ν ( C N)),
1593, 1496, 1453, 1243, 1200, 1036, 851, 814, 745, 704, 584, 476. UV(CH2Cl2), λmax
223, 300 and 431 nm. Molar conductivity in methanol is ꢀ = 20.57 µS/cm.
:
Aerobic Oxidation of Thiols Catalyzed by [Mn(III)(BHBPDI)Cl]
To the Mn(III) complex (0.1 mmol, 10% molar ratio) in methanol (20 mL), the thiol
(1 mmol) was added, and then the solution was aerated. The reaction mixture was stirred at
room temperature until TLC indicated the reaction was completed. The resulting solution
was concentrated under reduced pressure to yield a residue, which was passed through a
short pad of silica gel using ethyl acetate and hexane (1:2) as eluent to provide analytically
pure product in good to high yields. All products are known, and their physical and spectral
data were compared with those reported in the literature. The results are summarized in
Table I.
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