Factors affecting the reactivity and selectivity in the oxidation of sulfides with tetra-n-butylammonium peroxomonosulfate catalyzed by Mn(III) porphyrins: Significant nitrogen donor effects

Article abstract of DOI:10.1016/j.poly.2010.11.026

The oxidation of aryl sulfides by tetra-n-butylammonium peroxomonosulfate (n-Bu₄NHSO₅) was carried out in the presence of six different manganese (III) tetraarylporphyrins [Mn(Por)s] as biomimetic catalysts and a number of nitrogen donors as co-catalysts. There is no noticeable difference between the reactivity of sulfides, in the presence of electron-rich Mn(por)s, whereas, for electron-deficient catalysts, conversion rates are different. Nevertheless, the over-oxidation of sulfoxide is more sensitive to both the nature of substituents attached to the sulfur atom in substrates as well as porphyrin complex structure. The degree of catalytic activity of Mn(Por)s for the formation of sulfone product increases as the following order: Mn(TPFPP)OAc < Mn[T(4-NO₂P)P]OAc < Mn(TDCPP)OAc < Mn(TPP)OAc < Mn(TMP)OAc < Mn[T(4-OMeP)P]OAc. Our results show that in the presence of electron-rich Mn(Por)s, the strong π-donor N-H imidazoles possess co-catalytic activity greater than that of strong σ-donor amines and weak π-donor pyridines. When electron-deficient Mn(Por)s were employed as catalyst, pyridines demonstrated a higher co-catalytic activity than that of N-H imidazoles. The pronounced effect of protic solvents on the rate and selectivity of oxidation reactions, particularly in the presence of electron-deficient Mn(Por)s has been observed. The outcome of our investigations accompanied by UV-Vis and Raman spectral data confirms the involvement of different active oxidant such as a high valent Mn-oxo species as well as a six-coordinate [(L)(Por)Mn-OHSO₄] complex.

Full text of DOI:10.1016/j.poly.2010.11.026

Polyhedron 30 (2011) 592–598
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Factors affecting the reactivity and selectivity in the oxidation of sulfides
with tetra-n-butylammonium peroxomonosulfate catalyzed by Mn(III) porphyrins:
Significant nitrogen donor effects
⇑
⇑
Abdolreza Rezaeifard , Maasoumeh Jafarpour , Heidar Raissi, Ebrahim Ghiamati, Aida Tootoonchi
Department of Chemistry, Faculty of Science, University of Birjand, Birjand 97179-414, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidation of aryl sulfides by tetra-n-butylammonium peroxomonosulfate (n-Bu4NHSO5) was carried
out in the presence of six different manganese (III) tetraarylporphyrins [Mn(Por)s] as biomimetic
catalysts and a number of nitrogen donors as co-catalysts. There is no noticeable difference between
the reactivity of sulfides, in the presence of electron-rich Mn(por)s, whereas, for electron-deficient cata-
lysts, conversion rates are different. Nevertheless, the over-oxidation of sulfoxide is more sensitive to
both the nature of substituents attached to the sulfur atom in substrates as well as porphyrin complex
structure. The degree of catalytic activity of Mn(Por)s for the formation of sulfone product increases as
the following order: Mn(TPFPP)OAc < Mn[T(4-NO2P)P]OAc < Mn(TDCPP)OAc < Mn(TPP)OAc < Mn(TM-
P)OAc < Mn[T(4-OMeP)P]OAc. Our results show that in the presence of electron-rich Mn(Por)s, the strong
Received 9 October 2010
Accepted 23 November 2010
Available online 30 November 2010
Keywords:
Porphyrin
Peroxomonosulfate
Sulfide
Oxidation
p
-donor N–H imidazoles possess co-catalytic activity greater than that of strong
r-donor amines and
Nitrogen donors
Mn-oxo species
weak -donor pyridines. When electron-deficient Mn(Por)s were employed as catalyst, pyridines demon-
p
strated a higher co-catalytic activity than that of N–H imidazoles. The pronounced effect of protic
solvents on the rate and selectivity of oxidation reactions, particularly in the presence of electron-
deficient Mn(Por)s has been observed. The outcome of our investigations accompanied by UV–Vis and
Raman spectral data confirms the involvement of different active oxidant such as a high valent Mn-
oxo species as well as a six-coordinate [(L)(Por)Mn–OHSO4] complex.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
sulfides and sulfoxides are invaluable substrates in transition me-
tal catalyzed oxidation reactions, especially regarding the electro-
The selective oxidation of organic molecules is fundamentally
vital in life science and immensely useful in industry. Among the
myriad ways by which such reactions may be performed are those
catalyzed by naturally occurring metalloenzymes [1,2]. The enzy-
matic oxidation of sulfides to sulfoxides by heme peroxidases such
as horseradish peroxidase (HRP), chloroperoxidase (CPO), lactoper-
oxidase (LPO) and lignin peroxidase (LiP) is an important process
from both the synthetic and mechanistic points of view [3]. Organ-
ic sulfides and sulfoxides are interesting substrates that have many
similarities but possess some dissimilarity in their reactions with
different oxidants. It is worthy to note that organic sulfides behave
always as strong nucleophiles, whereas, sulfoxides act as biphilic
substrates if strong and rather unselective oxidants are used. It is
well known that organic sulfoxides are less powerful nucleophiles
compared to the corresponding organic sulfides and thus less reac-
tive towards electrophilic reagents [4,5]. Thus mechanistically,
philic character of high valent metal-oxo species [6]. Accordingly,
a number of studies have been focused on the understanding of
the factors that do control the activity and selectivity of oxygen
transfer to sulfides to acquire information about the active site
structure and to distinguish between the two possible reaction
mechanisms, namely direct oxygen atom transfer, ‘‘oxene process’’
(a in Scheme 1) or electron transfer (b in Scheme 1). Both are
involving in the initial formation of the high valent metal-oxo por-
phyrins [7–14].
R2S=O + (Por)MIII
a
[O]
Por-MIII
(Por)MIV=O + R2S
Metal-oxo(porphyrin)
R2S + (Por)MIV=O
b
R2S=O
⇑
Corresponding authors. Tel.: +98 0561 2502516; fax: +98 0561 2502515.
E-mail addresses: rrezaeifard@birjand.ac.ir, rrezaeifard@gmail.com (A. Rezaei-
fard), mjafarpour@birjand.ac.ir (M. Jafarpour).
Scheme 1. Oxygen atom transfer mechanism vs. electron transfer oxygen rebound.
0277-5387/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2010.11.026

A. Rezaeifard et al. / Polyhedron 30 (2011) 592–598
593
2.4. Competitive oxidation reactions
O
S
O
O
Mn-Por/L
Mn-Por/L
n-Bu4NHSO5
+
S
S
An equimolar amount of methyl phenyl sulfoxide and diphenyl
sulfoxide (0.2 mmol) were reacted with n-Bu4NHSO5 (0.2 mmol)
and Mn(TPP)OAc or Mn(TMP)OAc (0.001 mmol) in the presence
of imidazole and pyridine (0.08 mmol) in CH2Cl2 (2 ml). Reaction
mixture was stirred at 20 2 °C for 5 min, and products were ana-
lyzed by GC using mesitylene as internal standard.
Scheme 2. Oxidation of sulfides to sulfoxides and sulfones using n-Bu4NHSO5
catalyzed by Mn(Por)s.
Tetra-n-butylammonium peroxomonosulfate (n-Bu4NHSO5),
which is an organic salt of OxoneÒ (2KHSO5, KHSO4, and K2SO4)
has been introduced for using in anhydrous conditions without re-
course to a buffered two phase system [15]. It has been employed
in limited oxidation reactions under non-catalytic conditions in
different organic solvents [15–19]. Followed by our promising
results on the oxygenation of hydrocarbons with n-Bu4NHSO5
catalyzed by manganese (III) tetraphenylporphyrin [Mn(TPP)OAc],
[20] various organic substrates have been successfully subjected to
oxidation using this versatile oxygen source under catalytic condi-
tions [21–24]. In this work, different factors that may affect the
reactivity and selectivity of oxygenation of sulfide to sulfoxide
and sulfone catalyzed by Mn(Por)s have been probed (Scheme 2).
Some evidences for involving two active oxidants in this catalytic
system have also been postulated.
3. Results and discussion
3.1. Preliminary studies
Knowing that the oxidant has ability to trigger sulfide oxidation
[15], we performed the blank experiment using of 0.2 mmol thio-
anisole with 3 equiv. n-Bu4NHSO5 (0.2 g) in 1 ml CH2Cl2 at 25 °C
under air afforded 27% methyl phenyl sulfoxide after 5 min as
the sole product according to GC analysis. At lower temperature
(20 2 °C) with 1.7 equiv. of oxidant (0.34 mmol, 0.12 g) and larger
amounts of solvent (2 ml), the formation of sulfoxide was dimin-
ished to less than 2% within 5 min and 8% after 30 min. Now, these
optimum reaction requirements can be used reliably for obtaining
the results under catalytic condition.
Preliminary catalytic experiments were addressed to the oxida-
tion of thioanisole (0.2 mmol) with n-Bu4NHSO5 (0.12 g, 1.7 equiv.)
in the presence of Mn(TPP)OAc (0.001 mmol, 0.5 mol% with respect
to the substrate) and imidazole axial base (Im) (0.02 mmol) in
CH2Cl2 (2 ml) at 20 2 °C under air after 5 min. The various molar
ratios of n-Bu4NHSO5/sulfide in the catalytic oxidation of thioani-
sole were used in the presence of Mn(TPP)OAc/Im with molar ratio
of 1/20 (Fig. 1). As illustrated in Fig. 1, the conversion of thioanisole
oxidation improved from 29% to 94% as the molar ratio of n-
Bu4NHSO5/sulfide increases from 1 to 2.5, while, the selectivity of
sulfoxide drops remarkably from 100% to 40%. Thus, the sulfone
product is resulted from over-oxidation of obtained sulfoxide dur-
ing the reaction. In the present study the catalytic activity of differ-
ent electronically and structurally Mn(Por)s (Fig. 2) have been
evaluated in the oxidation of three aryl sulfides using n-Bu4NHSO5
in the presence of various nitrogen donors and solvents.
2. Experimental
2.1. General remarks
Mn(OAc)2Á4H2O, OxoneÒ, and tetra-n-butylammonium hydro-
gen sulfate were purchased from Merck or Fluka Chemical Compa-
nies. Nitrogenous bases were purchased from Merck or Fluka.
BzImH was recrystallized before use [25]. Pyridine and 2,6-Me2Py
were distilled on KOH and kept over molecular sieves. The free
base porphyrins: TPPH2 [26], T(4-OMeP)PH2 [26], T(4-NO2P)PH2
[27], TMPH2 [28], and TDCPP [28], and TPFPP [29] were prepared
and purified by methods reported previously. Mn(Por)OAc com-
plexes were obtained using the Mn(OAc)2Á4H2O according to the
procedure of Alder et al. [30]. Mn(TPFPP)OAc was synthesized in
a manner similar to that described by Kadish et al. [31]. The freshly
prepared n-Bu4NHSO5 [32,33] was a much stronger oxidant than
commercially available samples.
3.2. Co-catalytic effects of nitrogen donors
The fast catalytic sulfide oxidation which takes place only when
both catalyst and nitrogen base are present, may be taken as an
indication of that a nitrogen donor coordinates to manganese cen-
tre and makes it prone to oxidation by the peroxomonosulfate [21].
Initially, we investigated the effect of co-catalyst concentration on
2.2. Instrumentation
Purity determinations of the products were accomplished by GC
on
a
Shimadzu GC-16A instrument using
a 25 m CBP1-S25
(0.32 mm ID, 0.5
l
m coating) capillary column. IR spectra were re-
corded on a Perkin–Elmer 780 instrument. UV–Vis spectra were re-
corded on a 160 Shimadzu spectrophotometer. NMR spectra were
recorded on a Brucker Avance DPX 500 MHz instrument. Mass
spectra were recorded on a Shimadzu GC–MS-QP5050A. Raman
spectra were recorded on a Nicolet 910 FT Raman. The excitation
of the Raman spectrum was achieved by the near-infrared line at
60
Sulfoxide%
Sulfone%
50
40
30
20
10
0
1.064 l
m (9398.5 cmÀ1) from a 3 W cw Nd:YAG laser. About
2000 scans at a resolution of 2 cmÀ1 were needed to ensure a high
signal-to-noise ratio.
2.3. General procedure for oxidation of sulfides
To a mixture of sulfide (0.2 mmol), Mn(por)OAc (0.001 mmol)
and nitrogen donor (0.08 mmol) in CH2Cl2 (2 ml) was added freshly
prepared n-Bu4NHSO5 (0.34 mmol, 0.12 g). Reaction mixture was
stirred at 20 2 °C for the appropriate reaction time, and the yields
of products were determined by GC using mesitylene as internal
standard.
0.25 0.5 0.75
1
1.25 1.5 1.7
2
2.5
Oxidant/Sulfide Molar Ratios
Fig. 1. Reaction profile for the oxidation of thioanisole (0.2 mmol) using various
molar ratios of n-Bu4NHSO5/sulfide in the presence of Mn(TPP)OAc/Im with molar
ratio of 1/20.

594
A. Rezaeifard et al. / Polyhedron 30 (2011) 592–598
the oxidation of thioanisole. In the absence of Im, a high conversion
rate for oxidation of thioanisole was observed (73%/5 min), while a
trace amount of sulfone product was appeared (<3%) indicating
that the presence of nitrogen donor is essential for over-oxidation
reaction. An increase in the Im/Mn(TPP)OAc molar ratio up to 80:1
improved the yield of sulfone product rather than the total conver-
sion (Fig. 3). However as demonstrated in Fig. 3, further increase in
this molar ratio led to decrease in both the molar ratio of sulfone to
sulfoxide and in the total conversion of respective sulfide, i.e. thio-
anisole, confirming the formation of catalytically inactive six coor-
dinate species of Mn(TPP)(Im)2 [34,35].
The results of thioanisole oxidation by using n-Bu4NHSO5 under
the catalytic influence of five different Mn(Por)s, in association
with three classes nitrogen donors as co-catalysts including the
strong pure
r
-donor amines (pKa = 10.75–11.12), the strong
p-do-
nor imidazoles (pKa = 5.53–7.86) and weak
p-donor pyridines
(pKa = 1.86–6.65), are given in Table 1.
The general order of the co-catalytic activities of the various
classes of the nitrogen donors for Mn(TPP)OAc, Mn(TMP)OAc was
amines < pyridines < imidazoles, whereas, an inverted order was
observed for electron-deficient Mn(TPFPP)OAc and Mn[T(4-NO2P)-
P]OAc (Table 1). The order of the co-catalytic activities of the imi-
dazoles in association with Mn(TPP)OAc, 1-MeIm < Im < BzIm and
in combination with Mn(TMP)OAc, 1-MeIm < Im = BzIm are rather
similar in terms of both sulfide conversion and sulfone selectivity.
However, significant decrease in sulfone selectivity in the case of
hindered Mn(TMP)OAc when compared to Mn(TPP)OAc especially
in combination with bulky BzIm indicates that the steric effects are
quite pronounced in the second step of oxidation. 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 the reactiv-
ity of the sulfur atom. The much lower co-catalytic activity of 1-
MeIm than those of the N–H imidazoles in the presence of both
the catalysts clearly demonstrates the significance of the occur-
rence of N–HÁ Á ÁB (B = nitrogen donor or anionic species) hydrogen
bonding in the case of the N–H imidazoles enhancing substantially
their donor abilities toward the Mn(Por)s catalysts [36–38].
The order of the co-catalytic activities of the imidazoles in the
presence of electron-deficient Mn(TPFPP)OAc and Mn[T(4-NO2P)-
P]OAc, Im < BzIm < 1-MeIm are the same in terms of both sulfide
conversion and sulfone yield. The higher co-catalytic activity of
1-MeIm than those of the N–H imidazoles in combination with
both the catalysts may be related to large electrophilicity at their
Mn centers generating rapidly the inactive six coordinated species
Fig. 2. Mn-porphyrins utilized in this study.
Sulfone
90
80
70
60
50
40
Sulfoxide
0
1
10
20
50
80 100 200
(PorMnL2) with strong p-donor N–H imidazoles. Decreasing in co-
Im/Mn(TPP)OAc
catalytic activity of Im in association with electron-deficient
Mn(Por)s at co-catalyst/catalyst molar ratio > 20 confirms this
claim (Figs. 4 and 5). For electron-rich catalysts, a co-catalyst/cat-
Fig. 3. The effect of Im/Mn(TPP)OAc molar ratios on the selectivity of sulfone in the
oxidation of thioanisole using n-Bu4NHSO5.
Table 1
The effect of co-catalytic activity of different nitrogenous donors on the oxidation of thioanisole catalyzed by Mn(Por)s.a
c
Axial
ligand
pKa
Mn(TPP)OAc
Mn(TMP)OAc
Conversion %b
Mn[T(4-NO2P)P]OAc
Mn(TPFPP)OAc
Conversion %b
Conversion %b
Sulfone
Sulfone
Conversion %b
Sulfone
Sulfone
selectivity %b
selectivity %b
selectivity %b
selectivity %b
ImH
6.953
85
77
90
82
85
75
77
76
41
25
45
33
28
24
33
28
84
73
85
71
72
70
68
57
25
18
25
18
20
20
24
11
46
76
66
88
91
68
78
66
15
25
20
40
44
28
30
29
50
68
52
79
93
73
74
69
10
18
15
24
39
17
23
20
1-MeIm
BzImH
Py
2,6-Me2Py
4-CNPy
Piperidine
NEt3
6.95
5.532
5.25
6.65
1.86
11.123
10.75
ImH: imidazole; 1-MeIm: 1-methylimidazole; BzImH: benzimidazole; Py: pyridine; 2,6-Me2Py: 2,6-dimethylpyridine; 4-CNPy: 4-cyanopyridine; Et3N, triethylamine.
a
The reactions were run under air at 20 2 °C for 5 min. The molar ratio for sulfide:n-Bu4NHSO5:axial ligand:Mn(Por) is 200:340:80:1.
GC yields based on the mesitylene as internal standard. All the reactions were run at least in triplicate, and the data represent an average of these reactions.
See Refs. [44] and [45].
b
c

A. Rezaeifard et al. / Polyhedron 30 (2011) 592–598
595
alyst molar ratio up to 100 is required for observing a decrease in
co-catalytic efficiency (Figs. 4 and 5) [34,35].
100
80
97
100
Pyridines are generally much weaker co-catalysts than those of
imidazoles, in the presence of the Mn(Por) catalysts [39–42]. How-
ever, in this study the co-catalytic activities of pyridines and imida-
zoles were similar to each other except for electron-deficient
Mn(TPFPP)OAc and Mn[T(4-NO2P)P]OAc, which pyridines were
more efficient co-catalysts than those of imidazoles at co-cata-
lyst/catalyst ratio > 20 (Fig. 5).
89
87
73
Im
60
61
More interesting is the relatively high co-catalytic activity of
weak base 4-CNPy (pKa = 1.86) containing an electron-withdraw-
ing CN group in the presence of electron-deficient Mn(TPFPP)OAc
and Mn[T(4-NO2P)P]OAc compared to N–H imidazoles (Table 1).
The less electron-donating ability of pyridines retarding the forma-
tion of inactive PorMnL2 species may be a reason for this superior-
ity (Fig. 5).
Py
40
TPP
T(4-OMeP)P T(4-NO2P)P
Mn-Por
Fig. 6. The durability of Mn(Por)s catalysts during the oxidation of thioanisole
using n-Bu4NHSO5 in the presence of Im and Py (see footnote of Table 1 for the
molar ratios).
These suggestions were further supported by the evaluation of
stability of Mn(por)s during the oxidation reaction. Three para-
substituted Mn(por)s having different electronic groups and with
very similar steric environments at their Mn centers were used
in the oxidation of thioanisole (Fig. 6). In combination of Im a lower
stability of electron-rich Mn[T(4-OMeP)P]OAc was observed than
those of simple Mn(TPP)OAc and especially electron-deficient
Mn[T(4-NO2P)P]OAc reflecting the higher catalytic activity of
Mn[T(4-OMeP)P]OAc than two others alongside Im. The durability
of 100% with Mn[T(4-NO2P)P]OAc under this condition can be re-
lated to that of the formation of the inactive six coordinated spe-
cies with Im (PorMnIm2). On the other hand, in combination
with weak
p-donor Py, a lower stability was observed for
100
Mn[T(4-NO2P)P]OAc, indicating more effective catalytic activity
of this electron-deficient catalyst than those of electron-rich ones.
The unexpected high co-catalytic activity of hindered 2,6-
Me2Py in association with Mn(TPFPP)OAc versus all of the nitroge-
nous donors employed in this study can be resulted from some
attractive interactions between the C–H bonds of their methyl sub-
stituents and ortho-C–F groups of Mn(TPFPP)OAc [35,43].
TMP
80
TPP
The co-catalytic activities of the amines in the presence of the
different Mn(Por)s in this study are under the influence of their
basicities and steric hindrances. Less hindered piperidine with
more basicity (pKa = 11.12) showed the higher co-catalytic effi-
ciency than that of Et3N (pKa = 10.75) in terms of sulfone selectivity
especially, in combination with sterically more demanding
Mn(TMP)OAc (Table 1).
60
TPFPP
The comparison of the outcome of this study for the co-catalytic
activity of different nitrogen donors with those obtained in the oxi-
dation of olefins with n-Bu4NHSO5 [35] convinced us that the co-
catalytic activity of different nitrogen donors is more pronounced
in the Mn(por)s catalyzed oxidation of olefins with respect to oxi-
dation of sulfides with n-Bu4NHSO5. This is mainly due to en-
hanced reactivity of sulfides relative to olefins towards oxidation
with various oxidants [4,5]. However, the second step of oxidation
of sulfides depends more strongly on the concentration (Fig. 3) and
also the nature of the axial base (Table 1). The highest sulfone
selectivity obtained with molar ratio of 80:1 of Im to catalyst
may be an evidence for this suggestion (Fig 3). Moreover, the
40
1
5
10 20 50 100 200
Im/MnPor Molar Ratio
Fig. 4. Co-catalytic activity of Im in the presence of electron-rich and electron-
deficient Mn(Por)s as a function of different co-catalyst/catalyst molar ratios (see
footnote of Table 1 for the molar ratios).
100
Py
80
strong
p-nitrogen donors such as N–H imidazoles that facilitate
the formation of high-valent Mn-oxo species enhance the selectiv-
ity of sulfone product (vide infra) [35].
60
3.3. The electronic and steric effects of porphyrin ligand
The oxidation of methylphenyl, allylphenyl and diphenyl sul-
fides in the presence of Im were carried out to evaluate the cata-
lytic activity of six different electronically and structurally
Mn(Por)s (Fig. 2). The outcome which has been shown in Table 2,
indicates that there is no appreciable difference between the reac-
tivity of sulfides, in the presence of electron-rich catalysts,
whereas, for electron-deficient Mn(por)s, conversion rates are dif-
ferent and remarkably lower than those of electron-rich ones. Nev-
ertheless, the selectivity of sulfone products was more sensitive to
the nature of substituents attached to the sulfur atom as well as
40
Im
20
1
10 20 50 100 200
L/Mn[T(4-NO2P)P]OAc Molar Ratio
Fig. 5. Co-catalytic activities of Py and Im in the presence of Mn[T(4-NO2P)P]OAc as
a function of different co-catalyst/catalyst molar ratios(see footnote of Table 1 for
the molar ratios).

596
A. Rezaeifard et al. / Polyhedron 30 (2011) 592–598
Table 2
The effect of porphyrin structure on the catalytic activity of Mn(Por)s in the oxidation of aryl sulfides using n-Bu4NHSO5 in the presence of Im.a
S
S
S
Mn-Por
Conversion %b
Sulfone selectivity %b
Conversion %b
Sulfone selectivity %b
Conversion %b
Sulfone selectivity %b
Mn(TPP)OAc
Mn[T(4-OMeP)P]OAc
Mn(TMP)OAc
Mn(TDCPP)OAc
Mn[T(4-NO2P)P]OAc
Mn(TPFPP)OAc
85
84
85
81
46
51
41
42
26
23
15
10
83
90
94
74
60
55
65
81
76
34
37
23
82
89
90
73
51
64
35
50
46
22
18
15
a
The reactions were run under air at 20 2 °C for 5 min. The molar ratio for sulfide:n-Bu4NHSO5:Im:Mn(Por) is 200:340:80:1.
GC yields based on the mesitylene as internal standard. All the reactions were run at least in triplicate, and the data represent an average of these reactions.
b
stereo-electronic properties of the Mn(Por) catalysts. Accordingly,
the extent of catalytic activity of Mn(Por)s for over-oxidation
reaction (sulfone selectivity in Table 2) was found to follow the or-
der of: Mn(TPFPP)OAc < Mn[T(4-NO2P)P]OAc < Mn(TDCPP)OAc <
Mn(TPP)OAc < Mn(TMP)OAc < Mn[T(4-OMeP)P]OAc.
m)2]OAc species, arises by co-ordination of methanol to its Mn
center causing the reaction to proceed toward the formation of sul-
fone product [48,49].
3.5. Competitive reaction
This trend obeys in the oxidation of all sulfides except for
thioanisole, in which Mn(TPP)OAc is more efficient catalyst than
Mn(TMP)OAc. As mentioned in the previous section, the low
catalytic activity of electron-deficient Mn[T(4-NO2P)P]OAc and
Mn(TPFPP)OAc in combination with strong p-donor Im, arises from
the formation of catalytically inactive six coordinate species
Based on the above results we found out that the second step of
oxidation of sulfide, i.e. over-oxidation of sulfoxide is more delicate
to the nature of substrate, catalyst and co-catalyst than the first
step. Accordingly, the competitive oxidation of equimolar amount
of methyl phenyl sulfoxide and diphenyl sulfoxide with different
electronic and steric properties using n-Bu4NHSO5 was carried in
the presence of both Mn(TPP)OAc and bulky Mn(TMP)OAc, along-
side Im and Py as co-catalysts (Fig. 7). By using less hindered
Mn(TPP)OAc catalyst and Im co-catalyst, the oxidation of both of
the sulfoxides proceeded identically and the same conversions
were observed (65% and 62%), whereas, under the co-catalytic
influence of Py, the oxidation of less hindered methyl phenyl sulf-
oxide favored and the higher yield of the related sulfone was
achieved (77% versus 42%). When the hindered Mn(TMP)OAc was
employed, the steric factors became dominant in the presence of
both the co-catalysts. Nevertheless, more significant steric effect
was appeared in the presence of Py (75% versus 23%) rather than
Im (72% versus 68%) (Fig. 7). The similar conversion rates occurred
in the oxidation of both of the sulfoxides using Im as co-catalyst is
owing to the formation of a high valent Mn-oxo species in the oxi-
dation reaction (Fig. 7). On the other hand, different conversions
with Py, could be related to the involvement of bulky and weak
electrophilic six coordinate adduct [(Py)(TPP)Mn–OHSO4)] during
the reaction.
[35,43,46].
3.4. Solvent effects
The results of thioanisole oxidation by n-Bu4NHSO5 in different
solvents under the influence of simple Mn(TPP)OAc and electron-
deficient Mn(TPFPP)OAc in the presence of Im have been summa-
rized in Table 3. There is no regular dependency between dielectric
constants of the solvents used and the efficiency of the catalytic
reactions. We believe that the pronounced effect of protic CH3OH
on the catalytic activities of both the Mn(TPP)OAc and
Mn(TPFPP)OAc is due to hydrogen bonding between the methanol
and coordinated peroxomonosulfate [47–49]. This facilitates the
heterolytic cleavage of [PorMnO–HSO4] and the formation of a high
valent Mn-oxo species [50–56].
À
Furthermore, the by-product of HSO4 and active oxidant of
[(L)(Por)Mn@O]+ could be stabilized by hydrogen bonding to the
methanol enhancing the reaction rate. Accordingly, the other sol-
vents, i.e. CHCl3, CH2Cl2 and CH3CN with weaker acidities and
hydrogen bonding capabilities than methanol produce Mn@O spe-
cies in low yield (Table 3). Moreover, the effective solvation of the
polar sulfoxide generated in the course of the reaction by metha-
nol, increases the conversion of the sulfide to sulfoxide, preventing
the over-oxidation of sulfoxide to sulfone as can be observed
clearly for Mn(TPP)OAc. Nevertheless, in the case of Mn(TPFPP)OAc
with strong electrophilicity, both the conversion rate and sulfone
selectivity enhanced under the influence of methanol. This is due
to lapse in the formation of catalytically inactive [Mn(TPFPP)(I-
3.6. Spectroscopic studies
UV–Vis spectra show that addition of n-Bu4NHSO5 to a solution
of Mn(TMP)OAc (Soret, kmax = 478 nm), in CH2Cl2, containing Im
(100 equiv. with respect to catalyst), in the absence of thioanisole,
yields an intense Soret band, kmax = 408 nm (Fig. 8), due to the for-
mation of a high valent Mn-oxo species at room temperature
[43,57]. Addition of thioanisole (10
l
l) immediately gave the origi-
Table 3
The effect of solvent on the catalytic oxidation of thioanisole using n-Bu4NHSO5 catalyzed by Mn(Por)s in the presence of Im.a
Solvent
n
Mn(TPP)OAc
Mn(TPFPP)OAc
Conversion %b
Conversion %b
Sulfone selectivity %b
Sulfone selectivity %b
CHCl3
4.8
9.1
33
37
83
85
100
74
41
41
32
42
87
50
99
47
10
10
20
13
CH2Cl2
CH3OH
CH3CN
a
The reactions were run under air at 20 2 °C for 5 min. The molar ratio for sulfide:n-Bu4NHSO5:Im:Mn(Por) is 200:340:80:1.
GC yields based on the mesitylene as internal standard. All the reactions were run at least in triplicate, and the data represent an average of these reactions.
b

A. Rezaeifard et al. / Polyhedron 30 (2011) 592–598
597
nent band was appeared at 701.19 cmÀ1 which may be attributed
to a single Mn–O stretching mode ( Mn–O), probably owing to the
formation of the six coordinate [(Py)(TMP)Mn–O–OHSO3] complex
[60,61]. The use of Im as axial ligand in this study displaced the
band to 757.15 cmÀ1 which is ascertained to a stronger Mn@O
90
70
50
30
10
PhMe sulfone
Diphenyl sulfone
t
stretching mode
(tMn@O) due to formation of the [(Im)(-
TMP)Mn@O] species [62–64]. Thus, the formation of an Mn-oxo
species requires both a strong
non-electron-deficient Mn(Por).
p-donor axial ligand, and also a
3.7. Proposed mechanism
Based on our studies, we suggested a mechanism for oxygena-
tion of sulfides to sulfoxides and sulfones using n-Bu4NHSO5 as
oxidant under catalytic influence of Mn(Por)s in the presence of
nitrogen donors (Scheme 3). Utilizing the electron-deficient
Mn(por)s and
r-donor amines or weak p-donor pyridines in
non-protic solvents, a simple interaction between peroxomonosul-
fate anion with Mn(por) catalyst leads to a six coordinate adduct
[(L)PorMn–O–OHSO3] which is capable of converting sulfides to
sulfoxides in a moderately fast step (Scheme 3, Step a). Neverthe-
less, the oxidation of less nucleophile sulfoxides generated in the
course of the reaction, to the corresponding sulfone products using
this weak electrophile species, proceeds slowly (Scheme 3, Step b).
Furthermore, employment of electron-rich porphyrin catalysts and
Fig. 7. The competitive reaction between methyl phenyl sulfoxide and diphenyl
sulfoxide using n-Bu4NHSO5 catalyzed by Mn(Por)s in the presence of Im and Py.
The molar ratio for sulfoxides:n-Bu4NHSO5:axial ligand.:Mn(Por) is 200:200:80:1.
nal Soret (kmax = 478 nm) indicating the new species formed after
addition of the oxidant is kinetically competent for performing
substrate oxidation. On the other hand, when electron-deficient
Mn[T(4-NO2P)P]OAc and Mn(TPFPP)OAc was employed as catalyst,
the spectrum displayed no change [58]. This suggests that, in the
case of Mn(TMP)OAc, the active oxidizing species is a Mn-oxo
compound, whereas for electron-deficient ones the active oxidant
is a six coordinate [(Im)(Por)Mn–O–OHSO3] complex [58]. The
strong
p-donor axial ligands such as N–H imidazoles, in protic sol-
vents, lead to rapid formation of both sulfoxides and the related
employment of weak p-donor Py as co-catalyst, under similar con-
ditions, yielded no Mn-oxo species, and in the spectrum of both
Mn(TMP)OAc and Mn[T(4-NO2P)P]OAc no changes were observed,
indicating that the active oxygen intermediate is [(Py)(TMP)Mn–
O–OHSO3] complex [35]. This was further supported by the study
of the Raman spectra of the Mn(por) complex which has been pro-
ven to be especially useful since the manganyl stretch is enhanced
in resonance with the porphyrin electronic transitions [59]. Fig. 9
shows FT Raman spectrum of Mn(TMP)OAc before (Fig. 9A) and
after the addition of n-Bu4NHSO5 in the presence of Py (Fig. 9B)
and Im (Fig. 9C) in CH2Cl2. In combination with Py a new promi-
Fig. 9. The FT Raman spectra represent the conversion of (A) Mn(TMP)OAc (10 lM)
to (B) [(Py)(TMP)Mn–O–HSO4] complex after the addition of n-Bu4NHSO5
(1.7 equiv.) in the presence of Py (100 equiv.); and to (C) [(Im)(TMP)Mn@O]+ after
the addition of n-Bu4NHSO5 (1.7 equiv.) in the presence of Im (100 equiv.).
Fig. 8. The UV–Vis spectra of Mn(TMP)OAc (10 lM) in the presence of Im
(100 equiv.) before (kmax = 478 nm) and after (kmax = 408 nm) the addition of n-
Bu4NHSO5 (1.7 equiv.) in CH2Cl2.

598
A. Rezaeifard et al. / Polyhedron 30 (2011) 592–598
[3] H.B. Dunford, in: Heme Peroxidase, John Wiley, New York, 1999.
O
O
O
[4] S. Patai, Z. Rappoport, C.J.M. Stirling, The Chemistry of Sulfones and Sulfoxides,
John Wiley and Sons, New York, 1988. pp. 56–94 (Chapter 3).
[5] M.C. Carreno, Chem. Rev. 95 (1995) 1717.
S
[6] M. Newcomb, R.E.P. Chandrasena, Biochem. Biophys. Res. Commun. 338 (2005)
394.
[7] J.T. Groves, P. Viski, J. Org. Chem. 55 (1990) 3628.
O
HO
O
O
O
[8] W.Y. Lu, J.F. Bartoli, P. Battioni, D. Mansuy, New J. Chem. 16 (1992) 621.
[9] A. Pordea, M. Creus, J. Panek, C. Duboc, D. Mathis, M. Novic, T.R. Ward, J. Am.
Chem. Soc. 130 (2008) 8085.
III
S
Mn
R1
R2
HO
[10] R. Ricoux, M. Allard, R. Dubuc, C. Dupont, J.-P. Mahy, Org. Biomol. Chem. 7
(2009) 3208.
[11] A. Marques, M. Marin, M.-F. Ruasse, J. Org. Chem. 66 (2001) 7588.
[12] S. Grivea, F. Bedioui, C. Adamo, J. Phys. Chem. A 105 (2001) 11304.
[13] E. Baciocchi, M.F. Gerini, O. Lanzalunga, A. Lapi, M.G.L. Piparo, Org. Biomol.
Chem. 1 (2003) 422.
[14] A.B. Penenory, J.E. Argüello, M. Puiatti, Eur. J. Org. Chem. (2005) 114.
[15] B.M. Trost, R. Braslau, J. Org. Chem. 53 (1988) 532.
[16] D. Mohajer, N. Iranpoor, A. Rezaeifard, Tetrahedron Lett. 45 (2004) 631.
[17] Z. Lei, Y. Yang, X. Bai, Adv. Synth. Catal. 348 (2006) 877.
[18] A. Rezaeifard, M. Jafarpour, M.A. Naseri, R. Shariati, Dyes Pig. 76 (2008) 840.
[19] V. Kikelj, K. Julienne, J.-C. Meslin, D. Deniaud, Tetrahedron Lett. 50 (2009)
5802.
[20] D. Mohajer, A. Rezaeifard, Tetrahedron Lett. 43 (2002) 1881.
[21] N. Iranpoor, D. Mohajer, A.-R. Rezaeifard, Tetrahedron Lett. 45 (2004) 3811.
[22] A. Rezaeifard, M. Jafarpour, G. Kardan Moghaddam, F. Amini, Bioorg. Med.
Chem. 15 (2007) 3097.
L
S
S
O
O
R2
R1
Slow
III
a) Moderately fast
Mn
L
c) Very fast
L
Mn
b) Slow
O
O
O
S
O
d) Fast
S
S
R1
R2
R1
R1
R2
R2
L=HIms
L=Pys, Amines
Electron-rich Porphyrins
Protic Solvents
Electron-deficient Porphyrins
Non-protic Solvents
Scheme 3. Proposed mechanism for oxidation of sulfide to sulfoxide and sulfone
using n-Bu4NHSO5 catalyzed by Mn(Por)s.
[23] A. Rezaeifard, M. Jafarpour, S. Rayati, R. Shariati, Dyes Pig. 80 (2009) 80.
[24] A. Rezaeifard, M. Jafarpour, M.A. Nasseri, R. Haddad, Helv. Chim. Acta 93 (2010)
711.
[25] D.D. Perrin, W.L.F. Armarego, D.R. Perrin, Purification of Laboratory Chemicals,
2nd ed., Pergamon Press, Elmsford, NY, 1980.
[26] A.D. Adler, F.R. Longo, J.D. Finarelli, J. Goldmacher, J. Assour, L. Korsakoff, J. Org.
Chem. 32 (1967) 476.
sulfones (Scheme 3, Steps c and d) indicating the formation of a
strong elctrophilic Mn-oxo species.
[27] A. Bettelheim, B.A. White, S.A. Raybuck, R.W. Murray, Inorg. Chem. 26 (1987)
1009.
4. Conclusions
[28] P. Hoffmann, A. Robert, B. Meunier, Bull. Soc. Chim. Fr. 129 (1992) 85.
[29] J.S. Lindsey, I.C. Schreiman, H.C. Hsu, P.C. Kearney, A.M. Marguerettaz, J. Org.
Chem. 52 (1987) 827.
[30] A.D. Adler, F.R. Longo, F. Kampas, J. Kim, J. Inorg. Nucl. Chem. 32 (1970) 2443.
[31] K.M. Kadish, C. Araullo-McAdams, B.C. Han, M.M. Franzen, J. Am. Chem. Soc.
112 (1990) 8364.
[32] B.R. Travis, B.P. Ciaramitaro, B. Borhan, Eur. J. Org. Chem. (2002) 3429.
[33] S. Campestrini, B. Meunier, Inorg. Chem. 31 (1992) 1999.
[34] B. Meunier, M.-E. Carvalho, O. Bortolini, M. Momenteau, Inorg. Chem. 27
(1988) 161.
[35] D. Mohajer, L. Sadeghian, J. Mol. Catal. A: Chem. 272 (2007) 191.
[36] L.C. Yuan, T.C. Bruice, J. Am. Chem. Soc. 108 (1986) 1643.
[37] P. Obrien, D.A. Sweigart, Inorg. Chem. 24 (1985) 1405.
[38] F.A. Walker, M.W. Lo, M.T. Ree, J. Am. Chem. Soc. 98 (1976) 5552.
[39] D. Dolphin, T.G. Traylor, L.X. Xie, Acc. Chem. Res. 30 (1997) 251.
[40] Y.M. Goh, W. Nam, Inorg. Chem. 38 (1999) 914.
[41] W. Nam, S.Y. Oh, Y.J. Sun, J. Kim, S.K. Woo, W. Shin, J. Org. Chem. 68 (2003)
7903.
[42] W.J. Song, M.S. Seo, S.D. George, T. Ohta, R. Song, M.J. Kang, J. Am. Chem. Soc.
129 (2007) 1268.
[43] D. Mohajer, G. Karimipour, M. Bagherzadeh, New J. Chem. 28 (2004) 740.
[44] R.L. David, Handbook of Chemistry and Physics, 80th ed., vol. 8, CRC Press, New
York, 1999–2000. p. 8, 46–56.
[45] K. Schofield, Hetero-aromatic Nitrogen Compounds, Pyrroles and Pyridines,
Plenum Press, New York, 1967. p. 146.
[46] F.G.M. Gunter, P. Turner, J. Mol. Catal. A: Chem. 66 (1991) 121;
F.G.M. Gunter, P. Turner, J. Mol. Catal. A: Chem. (1988) 161.
[47] X.H. Mu, F.A. Schultz, Inorg. Chem. 31 (1992) 3351.
[48] M.W. Nee, J. Lindsay Smith, Dalton Trans. (1999) 3373.
[49] D. Mohajer, M. Abbasi, Eur. J. Inorg. Chem. (2008) 3218.
[50] N.A. Stephenson, A.T. Bell, Inorg. Chem. 45 (2006) 2758.
[51] N.A. Stephenson, A.T. Bell, Inorg. Chem. 45 (2006) 5591.
[52] N.A. Stephenson, A.T. Bell, J. Mol. Catal. A: Chem. 275 (2007) 54.
[53] N.A. Stephenson, A.T. Bell, J. Mol. Catal. A: Chem. 272 (2007) 108.
[54] T.G. Traylor, R. Popovitz-Biro, J. Am. Chem. Soc. 110 (1988) 239.
[55] T.G. Traylor, F. Xu, J. Am. Chem. Soc. 112 (1990) 178.
[56] K. Yamaguchi, Y. Watanabe, I. Morishima, J. Am. Chem. Soc. 115 (1993) 4058.
[57] D. Mohajer, R. Tayebee, H. Goudarziafshar, J. Chem. Res. S (1999) 168.
[58] D. Mohajer, Z. Solati, Tetrahedron Lett. 47 (2006) 7007.
[59] K. Nakamoto, Coord. Chem. Rev. 226 (2002) 153.
Our results demonstrate that, in the oxidation of aryl sulfides
using n-Bu4NHSO5/Mn(Por) system, there is no substantial differ-
ence between the reactivity of different sulfides in the presence of
electron-rich Mn(Por) catalysts, however, for electron-deficient cat-
alysts conversions were different. Nevertheless, the selectivity of
sulfone products was more sensitive to the nature of substituents
attached to the sulfur atom as well as stereo-electronic properties
of the Mn(Por) catalysts and nitrogen-donors. The low activity of
electron-deficient Mn[T(4-NO2P)P]OAc and Mn(TPFPP)OAc cata-
lysts especially for over-oxidation reactions reflect the diminishing
effect of electron-withdrawing substitute upon the catalytic
activity of Mn-catalyst. Our findings show that the orders of the
co-catalytic effects of the nitrogen donors are critically depend on
the steric hindrances and electronic structures of both the nitrogen
donors and Mn(Por) catalysts. No correlation was found between
the co-catalytic effects of the nitrogen donors and their pKa (BH+)
values. Moreover, in the presence of Mn(TPP)OAc and Mn(TMP)OAc
catalysts, the strong
p-donating N–H imidazoles demonstrate high-
er co-catalytic activity with respect to the weak
p
-donor pyridines.
Nevertheless, an inverted order was observed for electron-deficient
complexes. The order of the co-catalytic activities of nitrogen do-
nors could also be greatly influenced by their relative capabilities
for the formation of various LMnPor and L2MnPor species. An en-
hanced catalytic activity of electron-deficient Mn(Por)s in protic
solvents has been observed in this study. The experimental results
and spectroscopic evidences suggest the involvement of a high va-
lent Mn-oxo species as well as a six-coordinate [(L)(Por)Mn–
OHSO4] complex.
Acknowledgment
[60] T.H. Parsell, R.K. Behan, M.T. Green, M.P. Hendrich, A.S. Borovik, J. Am. Chem.
Soc. 128 (2006) 8728.
[61] C.E. MacBeth, R. Gupta, K.R. Mitchell-Koch, V.G. Young, G.H. Lushington, W.H.
Thompson, M.P. Hendrich, A.S. Borovik, J. Am. Chem. Soc. 126 (2004) 2556.
[62] R.S. Czernuszewicz, Y.O. Su, M.K. Stern, K.A. Macor, D. Kim, J.T. Groves, T.G.
Spiro, J. Am. Chem. Soc. 110 (1988) 4158.
Support of this work by Research Council of University of Birj-
and is acknowledged.
References
[63] J.T. Groves, M.K. Stern, J. Am. Chem. Soc. 110 (1988) 8628.
[64] T. Kitagawa, Y. Mizutani, Coord. Chem. Rev. 135/136 (1994) 685.
[1] H. Arakawa et al., Chem. Rev. 101 (2001) 953.
[2] T. Punniyamurthy, S. Velusamy, J. Iqbal, Chem. Rev. 105 (2005) 2329.