Nitrogen donor-controlled chemoselectivity of reaction in oxidation of sulfides with tetra-n-butylammonium hydrogen monopersulfate catalyzed by a partially β-brominated meso-tetraphenylporphyrinatomanganese(III) acetate: A clue to the nature of active oxidant

Article abstract of DOI:10.1080/17415990903569551

Highly efficient and rapid oxidation of different sulfides to the corresponding sulfones with tetra-n- butylammonium hydrogen monopersulfate (TBAO) in the presence of catalytic amounts of Mn(TPPBr2)OAc at room temperature is reported. Contrary to other nitrogen donors, using 4-cyanopyridine as co-catalyst leads to an increase in the ratio of sulfoxide to sulfone in the products. Comparison of the chemoselectivity of reaction in the presence of different nitrogen donors as co-catalyst shows the involvement of a high-valent Mn-oxo species as well as the six-coordinate Mn(TPPBr₂)(HSO5)(B) (B = nitrogen donors) complex in sulfide oxidation reactions with TBAO.

Full text of DOI:10.1080/17415990903569551

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Nitrogen donor-controlled
chemoselectivity of reaction in
oxidation of sulfides with tetra-
n-butylammonium hydrogen
monopersulfate catalyzed by a
partially β-brominated meso-
tetraphenylporphyrinatomanganese(III)
acetate: a clue to the nature of active
oxidant
Saeed Rayati a , Saeed Zakavi b & Vahid Noroozi a
a Department of Chemistry , K.N. Toosi University of Technology ,
PO Box 16315-1618, Tehran, 15418, Iran
b Department of Chemistry , Institute for Advanced Studies in
Basic Sciences , 45195-1159 Gava Zang, Zanjan, Iran
Published online: 17 Mar 2010.
To cite this article: Saeed Rayati , Saeed Zakavi & Vahid Noroozi (2010) Nitrogen donor-controlled
chemoselectivity of reaction in oxidation of sulfides with tetra-n-butylammonium hydrogen
monopersulfate catalyzed by a partially β-brominated meso-tetraphenylporphyrinatomanganese(III)
acetate: a clue to the nature of active oxidant, Journal of Sulfur Chemistry, 31:2, 89-95, DOI:
10.1080/17415990903569551
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Journal of Sulfur Chemistry
Vol. 31, No. 2, April 2010, 89–95
Nitrogen donor-controlled chemoselectivity of reaction in
oxidation of sulfides with tetra-n-butylammonium hydrogen
monopersulfate catalyzed by a partially β-brominated
meso-tetraphenylporphyrinatomanganese(III) acetate:
a clue to the nature of active oxidant
Saeed Rayatia*, Saeed Zakavib and Vahid Noroozia
aDepartment of Chemistry, K.N. Toosi University of Technology, PO Box 16315-1618, Tehran 15418, Iran;
bDepartment of Chemistry, Institute for Advanced Studies in Basic Sciences, 45195-1159 Gava Zang,
Zanjan, Iran
(Received 30 September 2009; final version received 13 December 2009)
Highly efficient and rapid oxidation of different sulfides to the corresponding sulfones with tetra-n- butyl-
ammonium hydrogen monopersulfate (TBAO) in the presence of catalytic amounts of Mn(TPPBr2)OAc at
room temperature is reported. Contrary to other nitrogen donors, using 4-cyanopyridine as co-catalyst leads
to an increase in the ratio of sulfoxide to sulfone in the products. Comparison of the chemoselectivity of
reaction in the presence of different nitrogen donors as co-catalyst shows the involvement of a high-valent
Mn-oxo species as well as the six-coordinate Mn(TPPBr2)(HSO5)(B) (B = nitrogen donors) complex in
sulfide oxidation reactions with TBAO.
Keywords: electron-deficient Mn(III) porphyrin; oxidation; tetra-n-butylammonium hydrogen monoper-
sulfate; catalyst; sulfone
1. Introduction
The oxidation products of sulfur organic compounds, especially sulfones, are valuable synthetic
intermediates for the preparation of biologically and chemically important compounds (1, 2).
Also, they have been used as versatile intermediates in organic syntheses (3). In the Ramberg–
Backlund reaction (4) and Julia olefination (5, 6), sulfones are converted to alkenes. A novel
polyalkyl ether/polyaryl ether sulfone copolymer has been shown to be useful for producing a
medical material to be used to contact the blood (7).
Polysulfone is widely used in food processing, electrical applications, medical instrumentation
and trays for holding instruments during sterilization, chemical processing equipment, corrosion-
resistant piping, tower packing, pump parts, filter modules and membranes (8).
*Corresponding author. Email: rayati@kntu.ac.ir
ISSN 1741-5993 print/ISSN 1741-6000 online
© 2010 Taylor & Francis
DOI: 10.1080/17415990903569551
http://www.informaworld.com

90 S. Rayati et al.
A group of compounds related to the sulfonamides have been used as antibacterial drugs. These
drugs inhibit the growth of bacteria rather than killing the organisms (8, 9).
The three most important methods for the preparation of simple sulfones are oxidation of sul-
fides, alkylation of sulfinate salts and reactions of sulfonic acid derivatives. The oxidation of
sulfides is an extremely useful and broadly applicable method for preparing sulfones. Goheen and
Bennett (10) have reported the use of concentrated nitric acid for the preparation of simple dialkyl
sulfones from the corresponding sulfides. Peracetic acid, generated in situ from hydrogen perox-
ide (i.e. 30% H2O2 in glacial acetic acid), is a very popular choice for this oxidation (11). Other
forms of H2O2 useful for this reaction include the urea complex (12) and the bis(trimethylsilyl)
derivative TMSOOTMS (13). Other oxidizing agents for this reaction are KMnO4 (14), MnO2
(15), NaClO4 (16), m-chloroperbenzoic acid (MCPBA) (17), sodium metaperiodate (18), sodium
perborate (19), etc. Oxone, as a safe and commercially available oxidant, has become widely
accepted for the oxidation of sulfides to sulfones. Commercially available Oxone is a mixture of
K2SO4, KHSO4 and potassium hydrogen monopersulfate, KHSO5. The tetrabutylammonium salt
of Oxone has been also used for oxidation of sulfides [tetra-n-butylammonium hydrogen monop-
ersulfate (TBAO)] (20, 21). We have recently reported the Mn(TPPBr2)OAc-catalyzed oxidation
of hydrocarbons with TBAO (22).
In the present work, highly efficient and chemoselective oxidation of sulfides to sulfones in the
presence of the β-di-brominated meso-tetraphenylporphyrinatomanganese(III) acetate is reported.
2. Results and discussion
Oxidation of various sulfides with TBAO in the presence of catalytic amounts of Mn(TPPBr2)OAc
and ImH at room temperature gives the corresponding sulfones as the sole product (Table 1). There
Table 1. Oxidation of sulfides with TBAO catalyzed by Mn(TPPBr2)OAC in the presence of ImH in CH2Cl2 at room
temperature.a
Yield of
Yield of
Entry
Substrate
Conversion %
sulfoxide (%)
sulfone (%)
Time (min)
S
1
2
3
100
100
100
–
–
100
100
82
2
2
2
S
S
18
S
4
5
97
90
7
90
70
2
2
S
20
Note: aThe molar ratios for oxidant:sulfide:ImH:catalyst are 200:100:10:1.

Journal of Sulfur Chemistry 91
is no substantial difference between the reactivity of sulfides carrying substituents with different
electronic and steric properties. It may be rationalized in terms of the high reactivity of organic
sulfides toward oxidation with different oxidants as well as the large size of the sulfur atom as the
reaction center that reduces the steric effects of the substituents. However, the yield of sulfone is
more sensible to the nature of substituents attached to the sulfur atom (compare 1 and 5). It seems
that the first step of oxidation is accompanied with an increase in the steric hindrance around the
reaction center and a decrease in reactivity of the sulfur atom.
2.1. Co-catalytic effect of nitrogenous bases
Four types of nitrogenous donors (Table 2) have been used in this study, including (i) σ and
π-donor imidazole (entry 1), (ii) strong pure σ-donor amine (entry 2), (iii) weak π-donor pyridines
(entries 3-5) and (iv) σ- and π-donor aminopyridine (entry 6). It is found that the strong π-donor
imidazole is the best co-catalyst. Aminopyridines with π-donor ability are generally better than
pyridines and amines (23).
Comparison of total conversions and the distribution of products in the presence of σ and/or
π-donor axial bases clearly show that the use of weaker nitrogen donors essentially decreases
the yield of sulfone relative to sulfoxide in the products. The second step of oxidation of sulfides
seems to be more affected by the donor ability of axial bases. It seems that the strong σ- and
π-nitrogen donors that facilitate the formation of high-valent Mn-oxo species direct the reaction
toward the formation of sulfone as the major product (vide infra).
2.2. Effect of co-catalyst/catalyst molar ratio
Oxidation of methyl phenyl sulfide in the presence of various ratios of imidazole to catalyst mainly
changes the ratio of sulfone to sulfoxide in the products rather than the total conversion (Table 3).
This observation may also be explained on the basis of the high reactivity of organic sulfides
toward oxidation reactions, which decreases the importance of electron donation from the axial
base to the metal center on the catalytic activity of the metalloporphyrin. However, the second
step of oxidation depends more strongly on the nature (Table 2) and concentration (Table 3) of
the axial base so that the highest yield of sulfone relative to sulfoxide has been obtained with 10:1
ratio of ImH to catalyst. The addition of ImH beyond this ratio leads to a decrease in the ratio of
sulfone to sulfoxide and the total conversion. This observation may be due to the formation of a
six-coordinate inactive complex, i.e. Mn(TPPBr2)(ImH)2 (24).
Table 2. Oxidation of methyl phenyl sulfide with TBAO catalyzed by Mn(TPPBr2)OAc in the presence
of different nitrogen donors in CH2Cl2 at room temperature.a
Entry
Axial ligand
Conversion (%)
Yield of sulfoxide (%)
Yield of sulfone (%)
1
2
3
4
5
6
7
8
9
ImH
2-EtImH
Et2NH
Py
2-MePy
4-CNPy
2-NH2Py
2,6-Cl2Py
None
100
99
92
89
97
86
97
80
90
0
27
21
18
35
46
21
38
34
100
72
71
71
62
40
76
42
56
Note: aThe molar ratios for oxidant:sulfide:ImH:catalyst are 200:100:10:1.

92 S. Rayati et al.
Table 3. Effect of various ImH/catalyst molar ratios on oxidation of methyl phenyl sulfide with
TBAO in CH2Cl2 at room temperature.a
Entry
Xb
Conversion %
Yield of sulfoxide (%)
Yield of sulfone (%)
1
2
3
4
0
5
10
20
90
95
100
91
34
32
0
56
63
100
76
15
Notes: aThe molar ratios for oxidant:sulfide:ImH:catalyst are 200:100:X:1. bImH/catalyst.
2.3. Effect of the ratio of oxidant to sulfide
The reaction was performed using different molar ratios of TBAO to methyl phenyl sulfide. The
2.0:1 ratio has been found to be the optimized one for the selective oxidation of the sulfide to
methyl phenyl sulfone in the presence of a catalyst (Table 4).
2.4. Proposed mechanism
UV–VIS spectra of the reaction solution after the addition of TBAO (up to 2.5 h) show no
observable change (Figure 1). The formation of a six-coordinate Mn(III) complex under reaction
condition resulted in no detectable change of the Soret band (23).
Table 4. Effect of various TBAO/sulfide molar ratios on oxidation of methyl phenyl sulfide in
presence of ImH in CH2Cl2 at room temperature.a
Xb
Conversion (%)
Yield of sulfoxide (%)
Yield of sulfone (%)
1.0
1.5
2.0
67
80
100
40
34
0
27
46
100
Notes: aThe molar ratios for oxidant:sulfide:ImH:catalyst are X:100:10:1. bTBAO/sulfide.
1.6
A
1.4
1.2
B
1
0.8
0.6
0.4
0.2
0
300
400
500
600
700
800
Figure 1. UV–VIS spectra of a solution of Mn(TPPBr2)OAc and methyl phenyl sulfide in CH2Cl2 in the presence of
imidazole (a) 2 min after the addition of TBAO and (b) 2.5 h after the addition of TBAO.

Journal of Sulfur Chemistry 93
MnIII
OAc
ImH
MnIII
ImH
ImH
HSO5
HSO5
OSO3
OSO3
ImH
HO
HO
R2SO2
R2SO
R2SO
R2SO
R2S
R2SO2
R2SO
MnIII
MnIII
ImH
MnIII
OAc
R2S
ImH
OSO3H
OSO3H
ImH
O
O
ImH
MnIII
MnIII
ImH
OAc
I
II
O
Mn
ImH
III
Figure 2. Proposed mechanism.
However, due to the high reactivity of sulfides toward oxidation with different oxidants, this
observation cannot exclude the involvement of a high-valent Mn-oxo species.
Good σ- and π-nitrogen donors facilitate the formation of high-valent Mn-oxo species in
reaction conditions (23, 25). The observed unusual behavior of 4-cyanopyridine may be evidence
for the existence of an equilibrium between a six-coordinate Mn(III) species (Figure 2, I) and
a high-valent Mn-oxo (Figure 2, III) species (Figure 2) that shifts toward the six-coordinate
moiety in the presence of 4-cyanopyridine. The comparison of 4-amninopyridine, pyridine and 4-
cyanopyridine (entries 4, 6 and 7, Table 2) clearly reveals the importance of 4-cyano group on the
chemoselectivity of the reaction. It is observed that 4-cyanopyridine, as a weaker σ- and π-donor
(or possibly a π-acceptor) relative to pyridine and 4-aminopyridine, shifts the reaction toward
the formation of sulfoxide. In accordance with this point, using 2,6-dichloropyridine (Table 2,
entry 8) mainly affects the yield of sulfone but has little effect on sulfoxide. It is suggested that
the lower ratios of III to II decrease the yield of sulfone relative to sulfoxide due to the lower
reactivity of II with respect to III (26). It may be observed from Table 2 (entries 6 and 9) that the
presence of 4-cyanopyridine has little effect on total conversion.
3. Conclusion
In summary, the distribution of oxidation products of sulfides with TBAO in the presence of the
partially brominated Mn(TPPBr2)OAc was found to be controlled by the electronic properties
of nitrogenous donors; 4-cyanopyridine shifts the reaction toward the formation of sulfoxide as
the major product. The distribution of products seems to provide a clue to the nature of active
oxidants in the metalloporphyrin-catalyzed oxidation of sulfides with different oxygen donors.

94 S. Rayati et al.
4. Experimental
1H NMR spectra were obtained in CDCl3 solutions with a BRUKER FT-NMR 250 (250 MHz)
spectrometer. The residual CHCl3 in conventional 99.8 at% CDCl3 gives a signal at δ = 7.26 ppm,
which was used for the calibration of the chemical shift scale. The electronic absorption spectra
were recorded on a double-beam spectrophotometer (Shimadzu, UV-240) in CH2Cl2. The reaction
products were analyzed by gas chromatography using a HP Agilent 6890 gas chromatograph
equipped with a HP-5 capillary column (phenyl methyl siloxane 30 m × 320 μm × 0.25 μm) and
a flame-ionization detector.
Chemicals were purchased from Merck or Fluka chemical companies. The free base meso-
tetraphenylporphyrin was prepared and purified as reported previously (27). n-Bu4NHSO5 was
prepared according to the literature (28).
4.1. Preparation of H2TPPBr2
β-Di-brominated meso-tetraphenylporphyrin (H2TPPBr2) was prepared from H2TPP and freshly
recrystalized N-bromosuccinimide (NBS) according to the method reported by Bhyrappa and
coworkers (29) with some modification. H2TPP (300 mg, 0.49 mmol) was dissolved in CHCl3
(80 ml). To this solution, freshly recrystallized NBS (180 mg, 0.98 mmol) was added. The reaction
mixture was stirred for 48 h in darkness and then CHCl3 was evaporated to dryness. The residue
was washed with methanol (2 × 20 ml) to remove any soluble succinimide impurities. UV–VIS
(CH2Cl2): λmax (nm): 423, 519, 594, 650. 1H NMR (CDCl3, 250 MHz): δ (ppm) 8.64–8.78 (m,
6H, β-pyrrole), 8.07–8.20 (m, 8, o-phenyl), 7.72–7.75 (m, 12, m- and p-phenyl).
4.2. Preparation of Mn(TPPBr2)OAc
Mn(TPPBr2)OAc was prepared and purified according to the previously reported method (30).
H2TPPBr2 (110 mg, 0.13 mmol) was dissolved in a minimum amount of methanol and then 30 ml
chloroform was added. Mn(OAc)2 · 4H2O (250 mg, 1.04 mmol) was added to the reaction mixture
and refluxed for a period of 4.5 h. UV–VIS (CH2Cl2): λmax (nm): 375, 475, 583 and 619.
4.3. General oxidation procedure
Stock solutions of the catalyst (0.003 M) and nitrogenous bases (0.5 M) were prepared in CH2Cl2.
In a 10 ml round-bottom flask, the reagents were added in the following order: sulfide (0.3 mmol),
catalyst (0.003 mmol, 1.0 ml), nitrogenous bases (0.03 mmol, 60 μl). TBAO (0.6 mmol, 0.247 g)
was then added to the reaction solution at 25 ◦C. The reaction solutions were analyzed immediately
by GC after stirring for 2 min.
Acknowledgements
We gratefully acknowledge practical support of this study by the Science Research Council of K.N. Toosi University of
Technology, Iran.
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