Kinetics and mechanism of (salen)MnIII-catalysed oxidation of organic sulfides with sodium hypochlorite

Article abstract of DOI:10.1002/poc.620

The oxidation of organic sulfides with several substituted oxo(salen)manganese(V) complexes was investigated in 90% acetonitrile-10% water and the reaction is second-order overall, first-order each in sulfide and complex. Electron-releasing substituents i

Full text of DOI:10.1002/poc.620

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2003; 16: 589–597
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.620
Kinetics and mechanism of (salen)MnIII-catalysed oxidation
of organic sul®des with sodium hypochlorite
Arunachalam Chellamani* and Sivalingam Harikengaram
Department of Chemistry, Manonmaniam Sundaranar University, Tirunelveli 627 012, India
Received 23 July 2002; revised 13 January 2003; accepted 25 January 2003
ABSTRACT: The oxidation of organic sulfides with several substituted oxo(salen)manganese(V) complexes was
investigated in 90% acetonitrile–10% water and the reaction is second-order overall, first-order each in sulfide and
complex. Electron-releasing substituents in sulfides and electron-withdrawing substituents in oxo(salen)manga-
nese(V) complexes accelerate the rate of oxidation. The second-order rate constants for the oxidation of p-substituted
phenyl methyl sulfides follow a linear Hammett relationship with ꢀ = À1.85. However, correlation between log k2 and
2s is excellent with ꢀ = 0.48 for the oxidation of thioanisole by substituted oxomanganese(V) complexes. The rate of
oxidation of alkyl phenyl sulfides and dialkyl sulfides with oxo complexes was also examined and the reactions show
a moderate steric effect. Substituent, acid and solvent effect studies reveal the operation of an SN2 mechanism.
Copyright  2003 John Wiley & Sons, Ltd.
KEYWORDS: (salen)MnIII complexes; sodium hypochlorite; organic sulfides; catalysed oxidation; mechanism
INTRODUCTION
with the oxidation of compounds of heteroatoms are very
limited.18,19 The fact that cytochrome P-450 model
complexes can readily catalyse the oxygenations of
nitrogen and sulfur compounds makes the study of the
reactivity of oxometal complexes towards organosulfur
compounds20 interesting and useful in understanding the
mechanism of biologically important oxygen atom
transfer processes. Recently, NaOCl, a cheap and readily
available representative of the family of single-oxygen
donors, has been used effectively in the presence of
manganese–salen catalysts to epoxidize a variety of
olefins.21,22 Jacobsen and co-workers21a,b used chiral
salen-based manganese(III) catalysts for epoxidation
reactions with the terminal oxidants PhIO and NaOCl.
They reported that both the terminal oxidants PhIO
Transition metal-catalysed transfer of oxygen atoms to
organic substrates is of interest in the study of bio-
inorganic mechanisms and the development of efficient
catalysts in the laboratory and industrial organic synth-
esis.1 Currently, synthetic metallaporphyrins and metal-
lasalens are receiving considerable interest as models of
the cytochrome P-450 class of enzymes.2 With single
oxygen donors, e.g. dioxygen,3 H2O2,4 percarboxylic
acids,5 iodosylarenes,6 hypochlorite,7 sodium per-
chlorite8 and activated N-oxides,9 these compounds form
high-valent oxometal complexes, which, like monooxy-
genases, are capable of oxygenating organic substrates.
Several mechanisms have been proposed for oxygen
transfer by hypervalent oxometal species. Oxygen
transfer may proceed via electron transfer,10 radical
addition,11 p-radical cation,12 carbocation formation,13
metallaoxetane formation14 and combinations of these
mechanisms.15 Kochi and co-workers studied epoxida-
and NaOCl produce a common oxo intermediate,
‡
[(salen)Mnv
=
O] . Adam and co-workers22 studied the
(salen)MnIII-catalysed epoxidation of chiral allylic al-
cohols, oxidation of silyl enol ethers and ketene acetals
with PhIO–NaOCl in the presence of 4-phenylpyridine
N-oxide. They also established that in the absence of a
chlorine source (CH2Cl2, ClÀ), the reaction between
(salen)MnIII complexes and PhIO–NaOCl leads to
oxo(salen)manganese(V) complexes.22c Selective oxida-
tion of sulfides to sulfoxides has been of continuing
interest, for which numerous methods have been devel-
oped. Transition metal-catalysed sulfoxidations have
been reported for oxidants such as H2O2, PhIO and t-
BuOOH.23 Oxidation of organic sulfides with aqueous
sodium hypochlorite has been reported.24 Only very few
catalytic oxidation of organic sulfides with NaOCl have
tion of olefins with Crv O and Mnv
= =O complexes
and showed that oxochromium(V)16 has an electro-
philic character and oxomanganese(V)17 a radical-like
character.
Although a large number of reports on the reactivity of
oxometal complexes have appeared, most of them
describe alkene epoxidation studies, and those dealing
*Correspondence to: A. Chellamani, Department of Chemistry,
Manonmaniam Sundaranar University, Tirunelveli 627 012, India.
E-mail: achellamani@yahoo.co.in
Copyright  2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 589–597

590
A. CHELLAMANI AND S. HARIKENGARAM
been described.25 Siedlecka and Skarzewski25b studied
the oxidation of sulfides with sodium hypochlorite
catalysed by 2,2,6,6-tetramethylpiperidine-1-oxyl and
reported that only 33% of sulfoxide was obtained without
catalyst whereas 95% of sulfoxide was produced with
catalyst.
tuned by introducing electron-withdrawing and electron-
releasing substituents and bulky groups in the ligand.
EXPERIMENTAL
Materials
We have initiated a systematic study on the oxygena-
tion reactions of organosulfur compounds with oxometal
complexes by taking Cr, Mn and Ru as metal ions.
Recently we reported the mechanism of oxidation of
organic sulfides and sulfoxides with PhIO catalysed by
metal–salen complexes.26–28 We initially proposed single
electron transfer from organic sulfide to the oxometal ion
as the rate-controlling step in the oxygen atom transfer
reaction from several cationic oxo(salen)manganese(V)26
complexes to sulfide. However, in a subsequent study, by
comparing the reactivity of organic sulfides and sulf-
oxides towards the same oxidant, oxo(salen)mangane-
se(V), a common mechanism involving the electrophilic
attack of the oxygen of the oxidant at the sulfur centre of
the substrate was proposed.28 Similarly, the selective
oxidation of organic sulfides to sulfoxides with oxo(sa-
len)chromium(V) complexes proceeds through the elec-
trophilic attack of oxygen at the sulfur center of the
organic sulfide.27
Thioanisole, para-substituted thioanisoles and alkyl
phenyl sulfides were prepared by known methods26a,31
and were purified by distillation under reduced pressure
or recrystallization from suitable solvents. The physical
constants of these sulfides were found to be identical with
literature values.26a,31 Further, the sulfides showed no
impurity peaks in 1H NMR spectra, and the HPLC
analyses proved the presence of single entity in each
sulfide. The dialkyl sulfides purchased from Aldrich were
used as such. Sodium hypochlorite (s.d.fine) was
determined by an iodometric method. Acetonitrile (GR,
Merck) was first refluxed over P2O5 for 5 h and then
distilled.
The [(salen)MnIII]‡PF6À complexes 1a–f were synthe-
sized according to reported procedures.17,28 The results
of IR and UV–visible spectral studies of all the com-
plexes were found to be identical with literature data.17
The oxo(salen)manganese(V) complexes 2a–f were
obtained by mixing equimolar quantities of complex
and sodium hypochlorite. As oxomanganese(V) com-
plexes undergo autodecomposition, the solutions were
prepared freshly for each kinetic run.
In this paper, we report the kinetics and mechanism of
the oxidation of thioanisoles with oxo(salen)mangane-
se(V) complexes 2a–f generatedÀ in situ from the
corresponding [(salen)MnIII]‡PF6
NaOCl as represented in Eqn. (1).
complexes and
Kinetic measurements
The kinetic measurements were carried out in 90%
acetonitrile–10% water at 20 Æ 0.1°C under pseudo-first-
order conditions ([sulfide] >[oxo complex]) using a
Perkin-Elmer UV–visible spectrophotometer (Lambda
3B) fitted with thermostated cell compartments. Reaction
mixtures for kinetic runs were prepared by quickly
mixing the solutions of the oxo complex and sulfide in
varying volumes so that in each run the total volume was
5 ml. The progress of the reaction was monitored by
following the decay of oxo complex at 680 nm.
The rate constants were obtained from the slopes of
linear plots of log(At À A?) versus time, where At is the
absorbance at time t and A? is the experimentally
determined infinity point. The first-order self-decom-
position rate constants k1(dec) of oxo(salen)manganese(V)
complexes were determined from the first-order plots up
to 50–60% of reaction. The plots for the decay of the oxo
complex in the presence of sulfide were linear over 40%
of reaction, and the pseudo-first-order rate constants
k1(obs) were determined from the disappearance of the oxo
complex up to this extent. The values of k1 were obtained
from k1 = k1(obs) À k1(dec) and the second-order rate
constants were obtained from k2 = k1/[sulfide]. The
The active species in the present reaction is considered
to be the oxo(salen)manganese(V) complex, as proposed
by Jacobsen and co-workers21a,b and others21c,22 in the
(salen)MnIII-catalysed NaOCl oxidation of achiral deri-
vatives. We could not isolate this oxo(salen)mangane-
se(V) complex and the present spectral data are similar to
those in earlier reports.17,26,28 Even though there is no
report on the structural characterization of the oxo(sa-
len)manganese(V) complex, recent theoretical work29
suggested a triplet ground state for this species. Groves et
al.30a and others30b–e have characterized oxomangane-
se(V)–porphyrin complexes in recent years. We chose the
salen ligand because it is similar to porphyrin and the
electronic and steric nature of the metal complex can be
Copyright  2003 John Wiley & Sons, Ltd.
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(SALEN)MNIII-CATALYSED OXIDATION OF ORGANIC SULFIDES
591
precision of the rate constant values in all the kinetic runs
is given in terms of 95% confidence limit of Student’s t-
test. The thermodynamic parameters DH≠ and DS≠ were
evaluated using the Eyring equation by the method of
least squares.
The absorption spectrum of the final solution coincided
with that of the original (salen)MnIII complex. We have
tried to isolate the active (salen)MnV
=O species in the
following way. A solution of 2.6 Â 10À3 M 1a in
acetonitrile–water was treated with an equimolar quantity
of sodium hypochlorite at 20°C. The dark brown solution
of 2a was poured directly into a pool of diethyl ether
cooled to À40°C. The dark brown solid was filtered at
low temperature. The crude solid product was thermally
labile and could not be purified by recrystallization. Upon
dissolution, the crude solid was found to be impure
compared to an in situ generated solution of 2a, on the
basis of their reported spectroscopic characterization.
Therefore, the oxomanganese(V) complexes were gen-
erated in situ for the studies reported here.
Stoichiometry and product analysis
The stoichiometry of the reaction between the
(salen)MnV
=
O complex and sulfide was studied under
the
experimental conditions ([2a] = 0.0026 M;
[PhSMe] = 0.20 M). The reaction gave sulfoxide in ca
72% yield and MnIII complex in ca 95% yield with a
negligible amount of sulfone. Accordingly, the stoichio-
metry of the reaction can be represented by
‡
‡
O
MnVꢀsalen† ‡PhSMeÀ! MnIIIꢀsalen† ‡PhSOMe
Kinetics of oxygen atom transfer from oxoman-
ganese(V) to sul®des
ꢀ2†
The kinetics of oxygen atom transfer from oxomangane-
se(V) complexes to sulfides was studied spectrophoto-
metrically in 90% acetonitrile–10% water at 20°C by
monitoring the disappearance of oxo complex at 680 nm.
Under pseudo-first-order conditions, excellent linear
plots of log(At À A?) versus time were obtained; from
these plots, the pseudo-first-order rate constants k1(obs)
and hence k1 and k2 were determined (Table 1). The
excellent linearity of the log(At À A?) versus time plots
(r > 0.995) and at a constant initial concentration of
sulfide, the constancy of k1 values at different [2a]0
establish that the reaction is first-order in oxo(salen)-
manganese(V) complex (Table 1). The pseudo-first-order
The reaction mixture from an actual kinetic run was
subjected to vacuum evaporation and the residue was
then extracted with chloroform. The extract was dried
over anhydrous Na2SO4 and the solvent evaporated. The
product was dissolved in methylene chloride and gas
chromatographic analyses of the samples showed that
sulfoxide was the sole product. The yield of sulfoxide,
ranging from 70 to 85%, depended on the sulfide and
oxomanganese(V) complex employed.
RESULTS AND DISCUSSION
The (salen)MnIII complexes 1a–f are all readily soluble in
90% acetonitrile–10% water. The electronic spectra of
these clear brown solutions are characterized by absorp-
tion bands with lmax ꢁ 350 nm tailing to beyond 400 nm.
When a clear brown solution of (salen)MnIII in aceto-
nitrile–water is treated with equimolar quantities of
sodium hypochlorite, it immediately turns dark brown,
indicating the formation of oxomanganese(V) spe-
cies.21,22 The formation of oxo(salen)manganese(V)
species is invariably associated with the following two
changes: (i) the characteristic peak of (salen)MnIII at
lmax ꢁ 350 nm disappears and (ii) a new absorption band
at lmax ꢁ 530 nm appears (Fig. 1). The dark brown
solution, on standing, faded to the original light brown
within 2–3 h. When the same experiment was carried out
in the presence of thioanisole, the dark brown colour was
discharged to original light brown within 15–20 min and
phenyl methyl sulfoxide was isolated in 72% yield [Eqn.
(3)].
Figure 1. (a)Absorption spectrum of 1a (0.0026 M)in 90%
acetonitrile±10% water taken in a 1 cm cuvette; (b)
absorption spectrum of oxo(salen)manganese(V) complex
2a (0.0026 M)
(salen)MnIII
PhSMe
PhSOMe
ꢀ3†
----------------------À----!---
NaOCl
Copyright  2003 John Wiley & Sons, Ltd.
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592
A. CHELLAMANI AND S. HARIKENGARAM
Table 1. Rate constants for the oxidation of MPS by 2a in 90% acetonitrile±10% water at 20°Ca
102[MPS]0 (M)
103[2a]0 (M)
104k1(obs) (sÀ1 b
)
104k1(dec) (sÀ1 c
)
104k1 (sÀ1 d
)
103k2 (MÀ1 sÀ1 e
)
10
10
10
10
10
10
5
15
20
40
50
100
1.00
1.60
2.00
2.60
3.00
3.60
2.60
2.60
2.60
2.60
2.60
2.60
10.0 Æ 0.2
9.97 Æ 0.17
9.58 Æ 0.21
9.96 Æ 0.19
10.2 Æ 0.2
10.3 Æ 0.1
7.75 Æ 0.15
11.8 Æ 0.2
14.1 Æ 0.3
22.0 Æ 0.6
26.8 Æ 0.6
51.3 Æ 1.4
5.44 Æ 0.09
5.72 Æ 0.16
5.24 Æ 0.09
5.52 Æ 0.03
5.79 Æ 0.16
5.99 Æ 0.05
5.52 Æ 0.03
5.52 Æ 0.03
5.52 Æ 0.03
5.52 Æ 0.03
5.52 Æ 0.03
5.52 Æ 0.03
4.56 Æ 0.11
4.25 Æ 0.01
4.34 Æ 0.12
4.44 Æ 0.16
4.41 Æ 0.04
4.31 Æ 0.05
2.23 Æ 0.12
6.28 Æ 0.17
8.58 Æ 0.27
16.5 Æ 0.6
21.3 Æ 0.6
45.8 Æ 1.4
4.56 Æ 0.11
4.25 Æ 0.01
4.34 Æ 0.12
4.44 Æ 0.16
4.41 Æ 0.04
4.31 Æ 0.05
4.46 Æ 0.24
4.19 Æ 0.11
4.29 Æ 0.14
4.15 Æ 0.15
4.26 Æ 0.12
4.58 Æ 0.14
a
As determined by a spectrophotometric technique following the disappearance of oxomanganese(V) at 680 nm; the error quoted in k values is the 95%
confidence limit of Student’s t-test.
Estimated from pseudo-first-order plots over 40% reaction.
b
c
Estimated from first-order plots over 50–60% reaction in the absence of sulfide.
d
Obtained as k1 = k1(obs) À k1(dec)
.
e
Individual k2 values estimated as k1/[sulfide].
rate constant, k1, for the oxidation of methyl phenyl
sulfide (MPS) by the oxomanganese(V) complex in-
creases with increase in substrate concentration. The plot
of k1 versus [MPS]0 is a straight line passing through the
origin (Fig. 2; r = 0.997). The double logarithmic plot of
k1 versus [MPS]0 is linear (r = 0.999) with a slope of
unity. Hence the reaction is overall second-order, first-
order in each reactant. Similar results were obtained for
the oxidation of substituted phenyl methyl sulfides with
oxomanganese(V) complexes 2a–f. Hence the rate law
can be depicted as
Àd‰2Š
0
ˆ k2‰2Š ‰sulfideŠ
ꢀ4†
0
0
dt
Addition of pyridine N-oxide (PyO), a donor ligand, to
the dark brown solution of 2a caused no change in the
absorption spectrum of oxomanganese(V). The effect of
donor ligand on the reaction rates was determined by
measuring k1 at various concentrations of added PyO.
The rate data in Table 2 indicate that PyO has no
appreciable effect on the reaction rate. The constant k2
values at different [PyO] indicate that PyO does not bind
with oxomanganese(V) species. If binding of PyO
occurred as in the case of oxochromium(V) complexes,16
then changes in the absorption spectra and reaction rates
would have been observed. This conclusion is consistent
with the observation of Powell et al.9 that the manganese
porphyrins lack affinity for a sixth axial ligand. Similar
results were observed in the (salen)MnIII-catalysed PhIO
oxidation of olefins17 and sulfides.26
To study the effect of acidity on the reaction rate, the
rates at different concentrations of trichloroacetic acid
were measured and are given in Table 3. The rate of
oxidation increases significantly with increase in con-
centration of acid. To understand the nature of the
transition state, the kinetics of oxidation of methyl phenyl
sulfide were measured at various solvent compositions
and the rate data are included in Table 3. The reaction
rate increases as the amount of water in the solvent is
increased.
Figure 2. Plots of k1 versus [sul®de] for the oxidation of (a) p-
nitrothioanisole with 2a; (b)thioanisole with 2b; (c)diethyl
sul®de with 2a; (d)thioanisole with 2e; (e)thioanisole with
2a; (f)thioanisole with 2c; (g) p-methoxythioanisole with 2a;
(h)thioanisole with 2d in 90% acetonitrile±10% water at
20°C; [2] = 0.0026 M
Substituent effect
The second-order rate constants for the oxidation of para-
substituted thioanisoles with 2a are given in Table 4.
Copyright  2003 John Wiley & Sons, Ltd.
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(SALEN)MNIII-CATALYSED OXIDATION OF ORGANIC SULFIDES
593
Table 2. Effect of pyridine N-oxide on the rate of oxidation of thioanisole by 2a in 90% acetonitrile±10% water at 20°Ca
102[MPS]0 (M)
103[2a]0 (M)
102[PyO] (M)
104k1(obs) (sÀ1 b
)
104k1(dec) (sÀ1 c
)
104k1 (sÀ1 d
)
103k2 (MÀ1 sÀ1 e
)
10
10
10
10
10
2.60
2.60
2.60
2.60
2.60
2.5
5.0
10.0
20.0
25.0
9.74 Æ 0.12
10.0 Æ 0.2
9.70 Æ 0.11
10.7 Æ 0.1
10.3 Æ 0.2
5.44 Æ 0.07
5.39 Æ 0.05
5.02 Æ 0.10
5.89 Æ 0.08
5.60 Æ 0.11
4.30 Æ 0.05
4.61 Æ 0.15
4.68 Æ 0.01
4.81 Æ 0.02
4.70 Æ 0.09
4.30 Æ 0.05
4.61 Æ 0.15
4.68 Æ 0.01
4.81 Æ 0.02
4.70 Æ 0.09
a
As determined by a spectrophotometric technique following the disappearance of oxomanganese(V) at 680 nm; the error quoted in k values is the 95%
confidence limit of Student’s t-test.
Estimated from pseudo-first-order plots over 40% reaction.
Estimated from first-order plots over 50–60% reaction in the absence of sulfide.
Obtained as k1 = k1(obs) À k1(dec)
b
c
d
.
e
Individual k2 values estimated as k1/[sulfide].
Table 3. Effect of adding acid and changing the solvent
composition on the rate of oxidation of methyl phenyl sul®de
by 2a at 20°Ca,b
Those sulfides containing electron-releasing groups in the
benzene ring accelerate the rate whereas those with
electron-withdrawing groups retard the rate. A plot of log
k2 versus sp shows an excellent correlation with the r
103[acid]
(M)
‡
value of À1.85 Æ 0.04 (Fig. 3; r = 0.998). When s /sÀ
104k1 (sÀ1 c CH3CN:H2O (%, v/v) 104k1 (sÀ1 d
) )
values are used a satisfactory correlation is obtained
‡
(ꢀ = À1.05, r = 0.970). Both s and sÀ values were
0.5
1.0
5.0
10.0
20.0
6.22 Æ 0.03
9.80 Æ 0.27
38.0 Æ 1.3
67.7 Æ 2.3
157 Æ 6
90:10
85:15
80:20
75:25
70:30
8.58 Æ 0.25
15.8 Æ 0.6
18.9 Æ 0.5
23.0 Æ 0.9
28.2 Æ 1.1
employed simultaneously in the plot of log k2 with
‡
s /sÀ. Thus a better correlation is observed with s values
than s /sÀ values. The negative reaction constant
‡
indicates an accumulation of positive charge at the sulfur
centre, while the magnitude of r value indicates the
extent of charge development on the sulfur atom in the
transition state of the rate-determining step.32 Further, the
plot of log k2 against the oxidation potential (Eox) of
sulfides33 was linear with a correlation coefficient of
r = 0.980 and a slope of À3.49.
a
General condition: [2a] = 0.0026 M.
In the evaluation of rate constants, the self-decomposition of 2a at
b
different [acid] and solvent composition is taken into account.
c
[MPS] = 0.10 M; solvent = 90% CH3CN–10% H2O.
[MPS] = 0.20 M.
d
The influence of the electronic effect of the oxidant on
Table 4. Second-order rate constants for the oxidation of p-
XC6H4SMe and R2S by 2a±d in 90% acetonitrile±10% water
at 20°Ca
No.
oxo(salen)MnV
X (Eox, V)b
103k2 (MÀ1 sÀ1
)
1
2
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
OCH3 (1.26)
16.5 Æ 0.7
10.2 Æ 0.4
4.29 Æ 0.14
3.61 Æ 0.17
2.03 Æ 0.06
1.66 Æ 0.09
0.76 Æ 0.08
0.55 Æ 0.08
0.18 Æ 0.04
2.91 Æ 0.12
9.20 Æ 0.21
26.8 Æ 1.1
CH3 (1.41)
3
H (1.53)
4
F (1.54)
5
Cl (1.55)
6
Br
7
COOHc
8
COCH3 (1.73)
9
NO2 (1.85)
10
11
12
H
H
H
R
13
14
15
16
17
2a
2a
2a
2a
2a
Et
3.19 Æ 0.11
2.94 Æ 0.14
2.59 Æ 0.14
2.34 Æ 0.10
1.99 Æ 0.08
n-Pr
i-Pr
n-Bu
t-Bu
a
General conditions: [2] = 0.0026 M; [sulfide] = 0.20 M, unless noted other-
Figure 3. Hammett plot for the oxidation of aryl methyl
sul®des by 2a. The points are referred to by the numbers in
Table 4
wise.
b
c
Oxidation potential values of sulfides taken from Ref. 33.
[Sulfide] = 0.10 M.
Copyright  2003 John Wiley & Sons, Ltd.
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594
A. CHELLAMANI AND S. HARIKENGARAM
Table 5. Second-order rate constants for the oxidation of C6H5SR' by 2a±d in 90% acetonitrile±10% water at 20°Ca
À1
Oxo(salen)manganese(V) complexes 103k2 (M sÀ1
)
No.
R' (Es)b
2a
2b
2c
2d
1
2
3
4
5
Me (0.00)
Et (À0.07)
Prn (À0.36)
Pri (À0.47)
But (À1.54)
ꢁc
4.29 Æ 0.14
4.00 Æ 0.24
3.19 Æ 0.16
2.94 Æ 0.15
1.39 Æ 0.11
0.31 Æ 0.01
0.998
2.91 Æ 0.12
2.50 Æ 0.16
1.96 Æ 0.12
1.63 Æ 0.08
0.63 Æ 0.08
0.41 Æ 0.02
0.999
9.20 Æ 0.25
8.85 Æ 0.23
6.55 Æ 0.22
5.73 Æ 0.12
2.56 Æ 0.12
0.36 Æ 0.02
0.997
26.8 Æ 1.1
25.1 Æ 0.9
22.3 Æ 0.9
18.0 Æ 0.5
9.55 Æ 0.35
0.29 Æ 0.03
0.991
r
a
General conditions: [2] = 0.0026 M; [sulfide] = 0.20 M.
Taft’s steric parameter taken from Ref. 34.
Obtained by correlating log(k2/k2Me) with Es.
b
c
the rate of oxidation of thioanisole was studied with
different 5,5'-substituted oxomanganese(V) complexes
2a–d. The second-order rate constants are given in Table
4. Electron-releasing substituents at the 5-positions of the
salen ligand decrease the rate and electron-withdrawing
substituents enhance the rate of oxidation. The plot of log
k2 versus 2sp is linear with a slope of 0.48 Æ 0.04 (Fig. 4;
r = 0.994). The positive r value indicates the build-up of
negative charge on the metal centre in the transition state
of the rate-determining step.
that the reactivity of alkyl phenyl sulfides decreases in the
order PhSMe > PhSEt > PhSPrn > PhSPri > PhSBut,
indicating that the steric effect exerted by the increasing
bulkiness of the alkyl group is predominant over the ‡I
(inductive) effect. Further, when log(k2/k2Me) is plotted
against Taft’s steric substituent constant Es,34 an
excellent correlation is obtained (r > 0.990). These facts
indicate that the reaction is sensitive to steric crowding at
the reaction centre, sulfur. Similar conclusions have been
arrived at in the oxidation of alkyl phenyl sulfides by
peroxoanions,35 phenyliodoso diacetate,36 Cr(VI),31
bis(2,2'-bipyridyl)copper(II)
permanganate37
and
Steric effect
(salen)MnIII-catalysed PhIO.26
To understand the reactivity of alkyl sulfides towards
2a and the role of steric effects in this reaction, the
kinetics of oxidation of several dialkyl sulfides with 2a
were studied and the relevant data are included in Table
4. The observed kinetic data indicate that the dialkyl
sulfides are oxidized more slowly than thioanisole and
the reaction is sensitive to steric effects. Further, the
effect of substituents at the 7-positions of the salen ligand
of oxomanganese(V) complexes on the reaction rate was
studied using 2a, 2e and 2f. The rate data provided in
Table 6 show that the presence of a methyl or phenyl
The oxidation of alkyl phenyl sulfides C6H5SR' (R' = Me,
Et, Prn, Pri and But) with oxo(salen)manganese(V)
complexes 2a–d was studied with a view to under-
standing the effect of the bulkiness of alkyl group on the
reaction rate. The rate constants listed in Table 5 show
Table 6. Second-order rate constants for the oxidation of
p-XC6H4SMe by 2a, 2e and 2f in 90% acetonitrile±10%
water at 20°Ca
À1
Oxo(salen)manganese(V) complexes 103k2 (M sÀ1
)
X
2a
2e
2f
OCH3
CH3
H
16.5 Æ 0.7
10.2 Æ 0.4
4.29 Æ 0.14
3.61 Æ 0.17
2.03 Æ 0.06
1.66 Æ 0.09
15.0 Æ 0.4
14.7 Æ 0.4
7.42 Æ 0.27
3.87 Æ 0.08
2.82 Æ 0.08
1.47 Æ 0.12
1.35 Æ 0.05
0.64 Æ 0.14
0.36 Æ 0.03
0.11 Æ 0.02
7.00 Æ 0.12
3.31 Æ 0.07
2.56 Æ 0.06
1.39 Æ 0.05
1.24 Æ 0.04
0.60 Æ 0.01
0.32 Æ 0.05
0.09 Æ 0.01
F
Cl
Br
COOHb 0.76 Æ 0.08
COCH3 0.55 Æ 0.08
NO2
0.18 Æ 0.04
a
Figure 4. Hammett plot for the oxidation of thioanisole by
substituted oxo(salen)manganese(V) complexes. The points
are referred to by the numbers in Table 4
General conditions: [2] = 0.0026 M; [sulfide] = 0.20 M, unless otherwise
noted.
[Sulfide] = 0.10 M.
b
Copyright  2003 John Wiley & Sons, Ltd.
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(SALEN)MNIII-CATALYSED OXIDATION OF ORGANIC SULFIDES
595
group at the 7-positions slightly reduces the rate. Thus the
steric effect observed in the present study is small. A
similar observation has been reported in the oxidation of
alkyl aryl sulfides with PhIO catalysed by (salen)MnIII
and (salen)RuIII complexes.26,38
(ꢀ = À1.19), oxo(phosphine)ruthenium(IV) complexes19
(ꢀ = À1.56), Ce(IV)41 (ꢀ = À3.3) and Fe(III)–polypyridyl
complexes55 (ꢀ = À3.2), a SET mechanism has been
proposed. Hence the low or high magnitude of the r value
cannot be taken as evidence for the operation of a SET or
SN2 mechanism in a particular reaction. According to
Miller et al.,40 a decision on reaction mechanism simply
based on the magnitude of the r value cannot be reliable.
Reactions54–57 that involve rate-limiting SET from sulfur
to yield radical cation intermediates are known to give
Mechanism
There are two possible mechanisms for the oxidation of
organic sulfur compounds by metal ions and other
oxidants. One is the nucleophilic attack of organic sulfide
on the oxidant, known as the SN2 mechanism. The second
mechanism involves a single electron transfer (SET)
from sulfide to oxidant. Oxidants such as peroxybenzo-
ate,39 hydroperoxidase,40 Cr(VI),31 Ce(IV)41 and oxo-
ruthenium(IV)19 oxidize sulfides by a SET mechanism.
The oxidation of sulfides by peroxoanions,35 phenylio-
doso diacetate,36 bis(2,2'-bipyridyl)copper(II) permanga-
‡
better Hammett correlations when s substituent con-
stants are used. In the present study, as log k2 is better
‡
correlated with s rather than s /sÀ, a single electron
transfer is not likely the rate-limiting step of the reaction.
If the transition state resembles a radical cation, as
predicted by the Hammond postulate58 for a SET mech-
anism, a better correlation should have been observed
‡
with s values. Hence the observed better correlation of
log k2 with s than s /sÀ may be taken as a clue for the
‡
nate,37
molybdenum
peroxypolyoxoanions,42
operation of an SN2 mechanism in the present reaction.46
Furthermore, the substituted oxo(salen)manganese(V)
complexes 2a–d display an excellent Hammett correla-
tion of log k2 with 2sp in favour of electrophilic attack of
oxidant on the sulfide sulfur.16 The excellent correlation
of the rates of alkyl phenyl sulfides with Es values may
favour nucleophilic attack of sulfide on the oxidant.59
The observed significant increase in the rate of oxidation
with increase in the concentration of trichloroacetic acid
demonstrates the electrophilic nature of the oxidant. The
addition of acid leads to the protonation of the oxidant28
and the protonated species is more electrophilic, thereby
favouring the reaction. The rate enhancement with
increase in the polarity of the medium indicates the
formation of a charge-separated transition state which is
in favour of the SN2 mechanism for the oxo(salen)man-
ganese(V) oxidation of organic sulfides. Similar results
have been observed in the oxidation of aryl methyl
sulfides with a similar oxidant, oxo(salen)chromium(V)
complexes,27 and in the PhIO oxidation of aryl methyl
sulfides and sulfoxides catalysed by (salen)MnIII com-
plexes.28 Further, from the linear log k2 versus Eox plot
we can obtain useful information on the mechanism of
the reaction by comparing the results observed in the
present study with the recent observations made by Goto
et al.60 on sulfoxidation catalysed by high-valent
sulfamyloxaridines,43a pyridinium hydrobromide perbro-
mide,43b pyridinium halochromates,44 permanganate45,46
oxo(salen)manganese(V)28
and
oxo(salen)chro-
mium(V)27 follow an SN2 mechanism. Further, the
mechanism of oxidation may be a continuum between
these two extremes, SET and SN2.47a Pross established
that the SN2–SET continuum has general significance.47b
However, for the oxidation of sulfides by PhIO catalysed
by metallaporphyrin,18 a clear distinction between the
SET and SN2 mechanisms has not been made, hence both
mechanisms have been proposed for this oxidation
reaction.
In the present investigation, since we identified oxo-
manganese(V) species as the crucial intermediate for the
catalytic oxidation, let us now consider how the oxygen
atom is actually transferred from the oxo species to the
sulfide in the rate-limiting step. The observed first-order
dependence with respect to sulfide and the absence of
kinetic saturation (even at the high concentration of
sulfide) indicate that prior coordination of sulfide to
oxomanganese(V) is unimportant. Substituent effect
studies can give an insight into the mechanism of the
‡
[(salen)MnV
=
O] oxidation of organic sulfides. In the
present study a r value of À1.85 was obtained. In the
oxidation of sulfides by hydrogen peroxide48 (ꢀ = À1.13),
periodate (ꢀ = À1.40), permanganate45a (ꢀ = À1.52),
peroxydisulfate35a (ꢀ = À0.56), lead tetracetate50
(ꢀ = À2.1), pyridinium halochromates44 (ꢀ = À2.1),
Lewis acid-catalysed permanganate46 (ꢀ = À1.11), per-
ruthenate51 (ꢀ = À0.66), (salen)MnIII-catalysed PhIO28
(ꢀ = À1.86), and oxo(salen)chromium(V) complexes27
(ꢀ = À2.7), an SN2 mechanism has been postulated. On
the other hand, in the oxidation of thioanisoles by tert-
butyl p-chloroperoxybenzoate39 (ꢀ = À1.68), Cr(VI)31
(ꢀ = À2.07), singlet oxygen52 (ꢀ = À1.63), peroxymono-
sulfate53 (ꢀ = À1.0), isoalloxazine hydroperoxide40
(ꢀ = À1.68),
carboxylato-bound
chromium(V)54
Scheme 1.
Copyright  2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 589–597

596
A. CHELLAMANI AND S. HARIKENGARAM
Table 7. Second-order rate constants and activation parameters for the oxidation of p-XC6H4SMe by 2a in 90% acetonitrile±
10% water at four temperaturesa
À1
103k2 (M sÀ1
)
X
293 K
298 K
303 K
313 K
DH≠ (kJ molÀ1
)
ÀDS≠ (J KÀ1 molÀ1
)
OMe
Me
17.3 Æ 0.6
10.1 Æ 0.4
4.45 Æ 0.16
3.65 Æ 0.11
2.03 Æ 0.10
1.64 Æ 0.09
0.76 Æ 0.08
0.56 Æ 0.00
0.18 Æ 0.08
22.9 Æ 1.1
15.2 Æ 0.7
6.12 Æ 0.22
5.02 Æ 0.22
2.64 Æ 0.26
2.47 Æ 0.18
1.06 Æ 0.09
0.73 Æ 0.11
0.32 Æ 0.04
33.4 Æ 1.1
19.9 Æ 1.0
8.66 Æ 0.31
6.46 Æ 0.51
4.06 Æ 0.21
3.66 Æ 0.31
1.56 Æ 0.31
1.16 Æ 0.21
0.46 Æ 0.10
65.0 Æ 1.2
42.1 Æ 0.7
17.8 Æ 0.9
13.9 Æ 0.3
8.70 Æ 0.50
6.70 Æ 0.20
3.20 Æ 0.10
2.20 Æ 0.00
1.10 Æ 0.10
48.7
51.1
50.7
48.3
54.2
51.0
52.7
51.0
65.2
125
108
117
127
112
124
125
133
93.6
H
F
Cl
Br
COOH
COCH3
NO2
a
General conditions: [2a] = 0.0026 M; [sulfide] = 0.10 M.
intermediates of heme enzymes. Goto et al.60 observed a
slope of À10.5 when the reaction proceeds through an
electron transfer mechanism and À2.2 in the case of the
reaction proceeding via direct oxygen transfer. Hence the
observed slope of À3.5 in the present study is in favour of
a mechanism proceeding through direct oxygen transfer.
Based on similar arguments Sivasubramanian et al.61
proposed a direct oxygen transfer mechanism for the
oxo(salen)iron oxygenation of organic sulfides.
Based on the arguments presented above, the SN2
mechanism shown in Scheme 1 is proposed for the
oxidation of organic sulfides with sodium hypochlorite
catalysed by (salen)MnIII complexes. The proposed
mechanism involves the incipient formation of an
intermediate 1 in the rate-limiting electrophilic attack
of the oxidant on the sulfide sulfur. Then the intermediate
1 decomposes to give (salen)MnIII and sulfoxide as the
product. The proposed SN2 mechanism is supported by
the acceleration of rate by electron-attracting groups on
the 5,5'-positions of salen(2c and 2d) and by electron-
donating groups in aryl methyl sulfides.
The oxo(salen)manganese(V) oxidation of para-sub-
stituted phenyl methyl sulfides was carried out at four
different temperatures, and the thermodynamic par-
ameters evaluated using the Eyring equation are collected
along with k2 values in Table 7. The DH≠ (48–
65 kJ molÀ1) and DS≠ (À125 to À93 J KÀ1 molÀ1) values
are in favour of two electron transfer rather than single
electron transfer in the rate-limiting step of the
reaction.27,62,63 Although the correlation between DH≠
and DS≠ is poor (r = 0.812), a plot of log k2 at 20°C versus
log k2 at 40°C is linear (r = 0.997, slope = 0.92 Æ 0.03,
s = 0.048) indicating that all the sulfides are oxidized by a
similar mechanism.
lysed by (salen)MnIII complexes by varying the electro-
nic nature of the substrate and oxidant. Rate studies with
alkyl phenyl and dialkyl sulfides show the operation of a
moderate steric effect.
Acknowledgements
We thank Professor S. Rajagopal for helpful discussions.
S.H. thanks the UGC, New Delhi, and Manonmaniam
Sundaranar University, Tirunelveli, for the award of a
Fellowship under FIP. He also thanks the Principal and
management of P.M.T. College, Melaneelithanallur, for
granting permission to accept the Fellowship.
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