Kinetics and mechanism of oxidation of aryl methyl sulfoxides with (salen)MnIII/H2O2 catalytic system

Article abstract of DOI:10.1016/j.molcata.2009.11.002

The kinetics of (salen)Mn^III complexes-catalysed oxidation of aryl methyl sulfoxides with hydrogen peroxide in 80% acetonitrile-20% water solvent mixture at 25 °C has been followed spectrophotometrically. The reaction is first-order in (salen)Mn^III, zero-order in hydrogen peroxide and fractional-order in sulfoxide. Also, it has been found that nitrogenous bases affect the oxidation, while free-radical inhibitor does not. However, an increase in the water content of the solvent mixture causes an increase in the rate of reaction. Stoichiometry between H₂O₂ and sulfoxide has been found to be 1:1 and the product analysis confirms the formation of sulfone and the regeneration of (salen)Mn^III complex. These observations have been well analyzed in favor of a Michaelis-Menten type mechanism, involving a manganese(III)-hydroperoxide complex as the reactive species. Using the derived rate law, the oxidant-substrate complex formation constant, K and the oxidant-substrate complex decomposition rate constant, k₂ have been evaluated. The proposed mechanism has been well supported by electronic-oxidant and electronic-substrate effect studies.

Full text of DOI:10.1016/j.molcata.2009.11.002

Journal of Molecular Catalysis A: Chemical 317 (2010) 104–110
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Kinetics and mechanism of oxidation of aryl methyl sulfoxides with
III
(
salen)Mn /H O catalytic system
2
2
∗
Arunachalam Chellamani , Paramasivan Sengu, Naina Mohamed Ismail Alhaji
Department of Chemistry, Manonmaniam Sundaranar University, Abishekapatti, Tirunelveli 627012, India
a r t i c l e i n f o
a b s t r a c t
The kinetics of (salen)MnIII complexes-catalysed oxidation of aryl methyl sulfoxides with hydrogen
Article history:
Received 12 September 2009
Accepted 1 November 2009
Available online 6 November 2009
◦
peroxide in 80% acetonitrile–20% water solvent mixture at 25 C has been followed spectrophotomet-
III
rically. The reaction is first-order in (salen)Mn , zero-order in hydrogen peroxide and fractional-order
in sulfoxide. Also, it has been found that nitrogenous bases affect the oxidation, while free-radical
inhibitor does not. However, an increase in the water content of the solvent mixture causes an increase
in the rate of reaction. Stoichiometry between H2O2 and sulfoxide has been found to be 1:1 and
Keywords:
III
(
Salen)Mn complexes
III
the product analysis confirms the formation of sulfone and the regeneration of (salen)Mn complex.
Hydrogen peroxide
Organic sulfoxides
Mechanism
These observations have been well analyzed in favor of a Michaelis–Menten type mechanism, involv-
ing a manganese(III)–hydroperoxide complex as the reactive species. Using the derived rate law, the
oxidant–substrate complex formation constant, K and the oxidant–substrate complex decomposition
rate constant, k2 have been evaluated. The proposed mechanism has been well supported by electronic-
oxidant and electronic-substrate effect studies.
©
2009 Elsevier B.V. All rights reserved.
1
. Introduction
aryl sulfides proceeds by a Michaelis–Menten type mechanism.
Organic sulfoxides may behave as electrophiles as well as nucle-
ophiles, depending on the nature of the oxidant [34–37]. Keeping
this biphilic nature of sulfoxides in mind, we extended our study to
sulfoxides also with a view to compare the behaviors of sulfides and
Transition metal complexes of porphyrin [1–5], Schiff-base
[
6–19] and pthalocyanin [20] ligands have widely been explored
in the past decades as catalysts in the reactions of oxygen atom
transfer from terminal oxidants to organic substrates. The activity
of these complexes varied with the nature of ligands, coordination
sites, metal ions and the terminal oxidants. During the past years,
we are investigating on the catalytic efficiency of (salen)M com-
plexes (salen = N,N -ethylenebis(salicylideneaminato) and M = Mn,
III
sulfoxides towards (salen)Mn /H O catalytic system. Hereunder,
2
2
we report the results of a detailed kinetic study on the oxidation of
III
several aryl methyl sulfoxides with H O2 catalysed by (salen)Mn
2
III
complexes, 1a–f.
ꢀ
Cr, or Ru) in oxidising organic sulfides and sulfoxides with PhIO,
H O and NaOCl as terminal oxidants [21–31]. Among the var-
2
2
ious terminal oxidants, hydrogen peroxide has the distinction
of being designated as ‘green reagent’ because of its non-toxic
side-product, water and its cheap availability. In reactions involv-
ing hydrogen peroxide and alkyl hydroperoxides [5,6,32,33], the
metal complexes have been shown to be effective in activating
the peroxide group and minimizing the importance of free-radical
pathways, thus making the reactions proceed faster with less-
complicated mechanisms. Recently we have reported [29] that
III
the (salen)Mn -catalysed hydrogen peroxide oxidation of alkyl
∗ Corresponding author. Tel.: +91 462 2333887; fax: +91 462 2322973.
E-mail address: achellamani@yahoo.co.in (A. Chellamani).
1
381-1169/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2009.11.002

A. Chellamani et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 104–110
105
2
. Experimental
sulfones determined from HPLC analyses ranged between 70 and
9
0 depending on the complex and the sulfoxide employed.
2.1. Materials
3
. Results and discussion
All the aryl methyl sulfoxides were prepared by known methods
1
[
38]. The purity of sulfoxides was checked by H NMR ad HPLC anal-
3.1. Identification of the reactive species
III
yses. (Salen)Mn complexes 1a–f were synthesized by the reported
method [21]. The results of IR and UV–vis spectral studies of all the
complexes were found to be identical with the literature data [10].
When an aqueous solution of H O2 (0.04 mol dm−3) is added to
2
−
3
a clear brown solution of 1a (0.0004 mol dm ; ꢀmax ∼350 nm) at
◦
Acetonitrile (GR, E. Merck India) was first refluxed over P O5 for 5 h
25 C, the solution gets darkened (ꢀ
shifts to ∼420 nm) with a
2
max
and then distilled. EPR spectra were recorded with a JEOL JES-TE 100
brisk evolution of dioxygen as represented in Eq. (2):
X-band EPR spectrometer in CH CN.
3
H O → 2H O + O
(2)
2
2
2
2
This establishes the catalase-like activity of (salen)MnIII complexes
as reported for the other mono- and di-nuclear manganese, iron and
cobalt complexes in acetonitrile [40–43]. Nitrogenous bases such
as 1-methylimidazole, 2-methylimidazole and pyridine accelerate
the dismutation of H2O2 as indicated by the more vigorous evo-
lution of oxygen, thus increasing the catalase-like activity of 1a–f
by binding to the metal centre [5]. Addition of MPSO to the solu-
tion greatly diminishes oxygen evolution and the UV–vis spectral
analysis of the solution shows a progressive shifting of ꢀmax from
∼420 nm back to ∼350 nm. The catalyst-like nature of the complex
is evident from the fact that same changes are observed on addition
of fresh quantities of H2O2 and MPSO. However, the complex gets
decomposed to unknown products (the brown colour of the solu-
tion gets bleached) after few (10–12) additions. Thus it is clear that
mixing the solutions of 1a and H2O2 produces a reactive species
(ꢀmax ∼420 nm) which is responsible for the catalase-like activ-
ity as well as for the oxidation of sulfoxides. Another interesting
observation is that when MPSO is added to the mixture of 1a and
H2O2 in the presence of a nitrogenous base, 1-methylimidazole, 2-
methylimidazole or pyridine, oxygen evolution is not diminished,
indicating that oxidation of MPSO does not start. This shows that
the nitrogenous bases have strong binding affinity for the metal
centre of the reactive species than MPSO, and binding of sulfoxide
to the reactive species is an essential condition for the oxidation
reaction to proceed.
2
.2. Kinetic measurements
The kinetic runs of the oxidation of aryl methyl, alkyl phenyl
sulfides with hydrogen peroxide in the presence of 1a–f were
◦
conducted in 80% acetonitrile–20% water at 25 ± 0.1 C under
pseudo-first-order conditions using 10–100-fold excess of sulfide
over H O . Reaction mixtures for the kinetic runs were prepared
2
2
III
by mixing the standard solutions of (salen)Mn , sulfoxide and
H O in varying volumes so that in each run the total volume
did not exceed 5.0 ml. The reaction mixture was shaken well and
quickly transferred to the 1 cm quartz cuvette. The progress of the
reaction was monitored by following the changes in absorbance
values at 400 nm in a PerkinElmer UV–vis spectrophotometer
2
2
(
Lambda 3B) fitted with thermostated cell compartments. At higher
complex concentrations the reactions were followed at higher
wavelengths in order to keep the absorbance values in a reliable
range.
The pseudo-first-order rate constants (kobs) for the initial
5–20% of the reaction were estimated from the slopes of lin-
1
ear least square plots of ln(At − A∞) versus time, where At is the
absorbance at time t and A∞ is the experimentally determined
infinity point. The precision of kobs values is given in terms of 95%
confidence limit of ‘Student’s t’ test [39]. The oxidant–substrate
complex decomposition rate constant (k ) and the complex forma-
tion constant (K) were then obtained from the slope and intercept
of double reciprocal plot of kobs versus [sulfoxide]o.
2
Based on UV–vis and EPR spectral studies and analysis of liter-
ature, we have clearly established (for a detailed discussion, see
Ref. [29]) that this reactive species is a low-spin, EPR inactive
2
.3. Stoichiometry and product analysis
III
III
Mn –hydroperoxide complex, [(salen)Mn OOH], the formation of
which is represented as:
To a solution of 1a (0.0004 M) and methyl phenyl sulfoxide
(
MPSO) (0.01 M) in acetonitrile was added an aqueous solution of
+
+
(salen)MnIII
]
+ H O + H O → [(salen)Mn OOH] + H O
III
(3)
[
2
2
2
3
H O (0.0004 M) and the composition of acetonitrile and water in
2
2
(C )
1
the solvent mixture was maintained at 80:20 by adding the solvents
in the required proportions at 25 C under nitrogen atmosphere.
◦
The kinetics of oxidation of aryl methyl sulfoxides was carried out
under such [complex] to [H O ] ratios (between 1:30 and 1:10)
2
2
The reaction gave methyl phenyl sulfone in ∼85% yield with neg-
that the dismutation of H O2 was insignificant (as evidenced by
2
ligible amount of sulfone, which established a 1:1 stoichiometry
the iodometric estimation of residual hydrogen peroxide) and only
the oxidation of sulfoxide proceeded.
between MPSO and H O2 as represented by Eq. (1):
2
III
(
salen)Mn
H O + ArSOMe −→ ArSO Me + H O
(1)
2
2
2
2
3.2. Kinetics of oxidation of aryl methyl sulfoxides
Product analyses were done using a Shimadzu LC-8A modular HPLC
system (reverse phase column (ODS), UV-detector at 258 nm) using
The kinetics of oxidation of aryl methyl sulfoxides by H O in
2
2
III
7
0% methanol as the mobile phase. A mixture of 1a (0.0006 M),
presence of (salen)Mn complexes, 1a–f, in 80% acetonitrile–20%
◦
MPSO (0.08 M) and H O2 (0.006 M) in 80% acetonitrile–20% water
water at 25 C were monitored by following the absorbance
2
was allowed to stand overnight. The reaction mixture was extracted
with chloroform and dried over anhydrous Na SO . The solvent
changes at 400 nm. The pseudo-first-order rate constants (kobs)
obtained for the oxidation of MPSO catalysed by 1a–f are listed
in Tables 1 and 2. Comparison of the first two entries in Table 1
shows that the catalysed oxidation of MPSO is ∼40 times faster than
the uncatalysed reaction. Hence the contribution of uncatalysed
2
4
was removed under reduced pressure. GC and TLC analyses of the
residue showed the product to be methyl phenyl sulfone (∼85%).
After the organic product was chloroform extracted the left behind
brown residue gave an absorption spectrum which resembled that
reaction between MPSO and H O2 can be neglected. Furthermore,
2
III
of (salen)Mn complex. The product analyses carried out with
if sulfoxide is added sometime after mixing H O2 and complex,
2
other complexes and other sulfoxides also showed the formation
of corresponding sulfone as the product. The percentage yields of
the rate constants are lower than those obtained when sulfoxide
is added immediately by only 5–10% depending on the length of

1
06
A. Chellamani et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 104–110
Table 1
Pseudo-first-order rate constants for the oxidation of MPSO with H2O2 catalysed by 1a at 25 ◦C in 80% acetonitrile–20% watera.
02[MPSO] (M)
10 [H2O2] (M)
3
10 [1a]o (M)
4
10 kobs (s
4
−1
)
1
b
b
1
1
5.0
5.0
10.0
10.0
6.0
6.0
6.0
6.0
6.0
2.0
3.0
4.0
5.0
7.0
–
0.09 ± 0.02
3.51 ± 0.07
0.96 ± 0.03
1.05 ± 0.03
1.06 ± 0.02
1.05 ± 0.03
0.95 ± 0.03
1.00 ± 0.02
0.99 ± 0.03
1.08 ± 0.01
1.05 ± 0.02
1.04 ± 0.02
10.0
2.0
3.0
4.0
5.0
6.0
2.0
2.0
2.0
2.0
2.0
8
8
8
8
8
8
8
8
8
8
.0
.0
.0
.0
.0
.0
.0
.0
.0
.0
a
The kobs values were determined by a spectrophotometric technique following the absorbance changes at 400 nm over 15–20% of reaction; the
error quoted in kobs values is 95% confidence limit of ‘Student’s t’ test.
b
Estimated iodometrically by following the unreacted H2O2.
Table 2
Pseudo-first-order rate constants for the oxidation of MPSO with H2O2 catalysed by 1b–f at 25 ◦C in 80% acetonitrile–20% watera.
4
−1
)
[MPSO]o (M)
Complex 10 kobs (s
1
a
1b
1c
1d
1e
1f
0
0
0
0
0
.04
.08
.15
.30
.40
0.77 ± 0.01
1.04 ± 0.02
1.26 ± 0.01
1.51 ± 0.92
1.63 ± 0.03
0.32
0.62 ± 0.01
0.84 ± 0.01
1.00 ± 0.02
1.25 ± 0.02
1.31 ± 0.03
0.32
1.14 ± 0.03
1.54 ± 0.05
1.94 ± 0.04
2.36 ± 0.05
2.55 ± 0.04
0.35
2.90 ± 0.07
3.89 ± 0.06
4.78 ± 0.09
5.75 ± 0.09
6.05 ± 0.12
0.32
0.57 ± 0.01
0.78 ± 0.01
1.01 ± 0.03
1.24 ± 0.04
1.30 ± 0.03
0.36
0.43 ± 0.01
0.58 ± 0.01
0.82 ± 0.01
1.01 ± 0.01
1.05 ± 0.02
0.40
b
Order in MPSO
a
General conditions: [complex]o = 0.0004 M; [H2O2]o = 0.004 M.
Slope values of double logarithmic plots of kobs versus [MPSO]o.
b
the interval period. Therefore, the self-decay of the complex in the
absence of sulfoxide seems not to be substantial.
strate concentrations. The fractional-order dependence on [MPSO]o
is further evidenced from the fractional slopes (Table 2) shown
by the double logarithmic plots between kobs and [sulfoxide]o
(r > 0.990). It can be understood from the direct plots of kobs ver-
sus [sulfoxide]o (Fig. 1) that saturation kinetics is observed with
other aryl methyl sulfoxides also, establishing that the reaction
follows Michaelis–Menten type kinetics involving an intermedi-
ate complex formation between the oxidant and the sulfoxide. The
influence of nitrogenous bases on the reaction (Section 3.1) also
upholds this finding.
Analysis of rate data in Table 1 points out that changes in the ini-
tial concentration of complex do not have any influence on reaction
rate. This, coupled with the excellent linearity (r > 0.990) observed
in ln(At − A∞) versus time plots ensures the order of the reaction
with respect to complex is one. Also, the constant k
values at
obs
different initial concentration of H O2 indicate that the reaction is
2
zero-order with respect to H O . The k values measured at differ-
2
2
obs
ent initial concentrations of MPSO for reactions catalysed by 1a–f
Table 2) do not increase linearly with increasing concentration
(
The reaction rate for the 1a-catalysed oxidation of MPSO with
H O is almost unaffected (Table 3) by the presence of N-phenyl-1-
of sulfoxide and saturation kinetics is observed at higher sub-
2
2
anthranilic acid, a free-radical inhibitor. The retardation of reaction
rate by this inhibitor in the metal–cyclam complex catalysed epox-
II
idation [44] and by ionol in the (salen)Co catalysed epoxidation
[
45] with t-butyl hydroperoxide have been taken in favor of free-
radical oxidising species. Consequently, it may be inferred that free
radicals are not involved in the rate-controlling step of the present
reaction. The rate of the reaction increases with increase in water
content in the solvent mixture (Table 3). A plot of log kobs versus 1/ε
is linear (r = 0.997) indicating a charge-separated transition state in
the rate-determining step.
Table 3
Effects of N-phenyl-1-anthranilic acid and solvent composition in the 1a-catalysed
◦
a
oxidation of MPSO by H2O2 at 25 C .
03[Inhibitor]o (M)
10 kobs (s
4
−1
)
% Waterb
10 kobs (s
4
−1
)
1
0
2
4
6
8
.0
.0
.0
.0
.0
1.26 ± 0.01
1.20 ± 0.02
1.18 ± 0.02
1.19 ± 0.03
1.17 ± 0.02
10
20
30
40
50
0.84 ± 0.01
1.26 ± 0.01
1.83 ± 0.01
2.72 ± 0.07
3.92 ± 0.05
Fig. 1. Plots of kobs versus [sulfoxide]o showing saturation kinetics for the oxida-
tion of p-XC6H4SOMe with H2O2 catalysed by 1a. The lines are labeled with the
substituent X.
a
General conditions: [1a]o = 0.0004 M; [H2O2]o = 0.004 M; [MPSO]o = 0.15 M.
Rest was acetonitrile.
b

A. Chellamani et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 104–110
107
Scheme 1.
The reactive species in the present study is already shown to
III
be [(salen)Mn OOH] (Section 3.1). Based on the observed exper-
imental features, we propose that the oxidation of sulfoxides
by the (salen)MnIII/H O catalytic system proceeds by a mecha-
2
2
nism (Scheme 1), similar to that we proposed for the oxidation
of alkyl aryl sulfides by the same system [29]. The mechanism
involves the reversible formation of a ternary complex (C ) in
2
which both peroxide and the sulfoxide are bound to the com-
plex. Then the monooxidation of sulfoxide occurs by a nucleophilic
attack of the sulfoxide to the peroxide followed by H O release,
2
within the manganese peroxide–sulfoxide complex. The fact that
III
the (salen)Mn /H O system oxidizes both alkyl aryl sulfides and
2
2
aryl methyl sulfoxides by a similar mechanism is also evident
from the excellent linearity of the plot of log k2 of aryl methyl
sulfoxides versus log k2 of aryl methyl sulfides (Fig. 2, r = 0.996,
slope = 1.11 ± 0.17).
The proposed mechanism (Scheme 1) leads to the rate expres-
sion:
1
kobs
1
1
1
Fig. 3. Lineweaver–Burk plots for the oxidation of MPSO with H O2 catalysed by
2
=
+ k
k2K [ArSOMe]o
(7)
1
a–f.
2
where K, equal to k /k , is known as oxidant–substrate complex
1
−1
3.3. Substituent- and steric-effect studies
formation constantand givesameasure of the binding affinity ofthe
substrate to the oxidant and k2 is the complex decomposition rate
constant. Eq. (7) suggests that a plot of 1/kobs versus 1/[sulfoxide]o
should be a straight line with definite intercept on the ordinate. The
excellent linearity observed in the Lineweaver–Burk plots (Fig. 3)
supports the given mechanism. The values of k2 and K obtained
from the slopes and intercepts of these plots are collected in Table 4.
To get more insights into the mechanism, substituent and steric
effect studies were carried out with substituted aryl methyl sulfox-
The effect of substituents present at the 4-position of phenyl
ring of aryl methyl sulfoxides on the reaction rate is implied from
the k2 values in Table 4. Electron-donating substituents accelerate
the rate, while electron-withdrawing substituents retard it. Also, it
is seen that though k2 is highly sensitive to the nature of the sub-
stituents, K remains almost constant, thereby establishing that the
decomposition of the ternary complex is the rate-determining step.
The Hammett correlation of log k2 with ꢁp values shows excellent
linearity yielding a ꢂ value of −0.76 ± 0.03 (Fig. 4, r = 0.997). The
negative ꢂ value indicates that the sulfur atom of the sulfoxide is
more positively charged in the transition state than it is in the reac-
tant, thus establishing the nucleophilic character of sulfoxide. This
ꢂ value can be compared with (i) the ꢂ value (−0.85) observed for
the oxidation of aryl methyl sulfides with the same catalytic sys-
tem [29] and (ii) the ꢂ values (−2.44 and −2.57) observed for the
III
ides and substituted (salen)Mn complexes.
Table 4
Values of k2 and K for the oxidation of p-XC6H4SOMe with H2O2 catalysed by 1a–f
◦
at 25 C in 80% acetonitrile–20% water.
4
a
−1
Ka (M−1
)
Complex
X
10 k2 (s
)
1
1
1
1
1
1
1
1
1
1
1
a
a
a
a
a
a
b
c
d
e
f
OMe
Me
H
3.27
2.36
1.76
1.25
1.25
0.47
1.43
2.79
6.63
1.46
1.21
21.2
17.3
19.1
17.2
16.6
11.6
18.8
16.9
19.0
15.6
13.3
Cl
Br
NO2
H
H
H
H
H
Fig. 2. Plot of log k2 of p-XC6H4SOMe versus log k2 of p-XC6H4SMe. The points are
referred to by the substituent X.
a
Calculated from the slope and intercept of Lineweaver–Burk plots.

1
08
A. Chellamani et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 104–110
Fig. 4. Hammett plot for the 1a-catalysed oxidation of p-XC6H4SOMe with H2O2.
The points are referred to by the substituent X.
Fig. 5. Hammett plot for the oxidation of MPSO with H2O2 catalysed by 1a–d.
3.4. Reactivity selectivity principle
oxidations of aryl methyl sulfoxides by oxo(salen)manganese(V)
In order to test the applicability of RSP (reactivity selectivity
III
III
ion in (salen)Mn /PhIO [24] and (salen)Mn /NaOCl [28] cat-
alytic systems. The first comparison reveals the less nucleophilic
nature of sulfoxides than sulfides and the second comparison
principle—inverse relationship between the reactivity of a reagent
and its selectivity among a set of similar substrates) in the present
study, the k2 values (Table 5) obtained from the Lineweaver–Burk
III
the less electrophilic character of (salen)Mn OOH species than
plots for the reactions of various para-substituted phenyl methyl
oxo(salen)manganese(V) ion. It is worthwhile to mention here that
all the three possibilities of ꢂ values for sulfoxide oxidations being
same as [46,47], lower than [9,48,49] and higher than [24,28] those
for sulfide oxidations are found in the literature and in all these
studies, a common mechanism has been proposed for the oxidation
of both the substrates.
The effect of changes in the electronic and steric nature of salen
ligand on the rate of oxidation was also investigated using 1a–f
for the oxidation of MPSO. The k2 values listed in Table 4 point
out that electron-withdrawing substituents at the 5-positions of
salen ligand enhance the rate, while electron-donating substituents
III
sulfoxides with H O2 catalysed by each of the (salen)Mn com-
2
plexes 1a–d were subjected to Hammett correlations. The last row
of Table 5 contains ꢂ values for substituent variation in the phenyl
methyl sulfoxide for each complex, and the last column shows ꢂ
values for substituent variation in the complex for each sulfoxide.
The ꢂ values along the row are almost the same and so are the
ꢂ values along the column, indicating that selectivity of sulfoxide
or complex remains the same with increase in reactivity along the
complex or sulfoxide series. This observation leads to the conclu-
sion that RSP is not applicable in the present study. As a further
verification, we subjected the k values in Table 5 to a mathematical
2
retard the rate. The Hammett correlation of log k with 2ꢂp shows a
2
treatment formulated by Exner [50] using Eqs. (8) and (9):
good correlation with a ꢂ value of 0.33 ± 0.03 (Fig. 5, r = 0.992). The
positive ꢂ value indicates a build-up of negative charge on metal
centre in the transition state of the rate-determining step. The kobs
and k2 values measured at different initial concentrations of MPSO
for the complexes 1a, 1e and 1f (Tables 2 and 4) show that the
log kFi = a + b log k + εi
(8)
(9)
Si
ꢀ
ꢀ
ilog kFi
−
N
ilog kSi
ꢃ =
ꢀ
presence of methyl or phenyl group at 7 and 7 positions of salen
where k and kSi are the rate constants for the reactions of fast and
Fi
ligand significantly reduces the rate of the reaction. Also, the lower
values of K for 1e and 1f than 1a imply that the association between
the complex and sulfoxide is sensitive to steric environment of the
complexes.
slow reagents (complexes) respectively with each sulfoxide, ε is
i
the error of the log kFi versus log kSi correlation, and ꢃ is the mean
difference. The values of b and ꢃ were calculated for all of the six
possible combinations of one fast and one slow reagent (among the
Table 5
Values of k2 for the oxidation of p-XC6H4SOMe with H2O2 catalysed by 1a–d at 25 ◦C in 80% acetonitrile–20% watera.
Complex 10 k2b (s
4
−1
)
ꢂc (r)
X
1
b
1a
3.27
2.36
1.76
1.25
1.25
0.47
1c
5.20
3.51
2.79
2.00
1.97
0.73
1d
OMe
Me
H
Cl
Br
2.46
1.95
1.43
1.02
1.01
0.37
12.4
8.40
6.63
4.73
4.68
1.74
0.343 ± 0.021 (0.996)
0.311 ± 0.030 (0.991)
0.326 ± 0.029 (0.992)
0.326 ± 0.029 (0.992)
0.329 ± 0.026 (0.994)
0.330 ± 0.024 (0.994)
NO2
d
ꢂ
(r)
−0.768 ± 0.015 (0.999)
−0.763 ± 0.030 (0.997)
−0.774 ± 0.041 (0.994)
−0.767 ± 0.042 (0.994)
a
b
c
General conditions: [complex]o = 0.0004 M; [H2O2]o = 0.004 M.
Calculated from the intercept of Lineweaver–Burk plots.
The values obtained by correlating log k2 with 2ꢁp for the complexes 1a–d with a given sulfoxide.
The values obtained by correlating log k2 with ꢁp for the various sulfoxides with a given complex.
d

A. Chellamani et al. / Journal of Molecular Catalysis A: Chemical 317 (2010) 104–110
109
Table 6
Results of correlations between log kFi and log kSi according to Eq. (8).
Results
Complexes (F & S)
a and 1b
1
1c and 1b
1d and 1b
1c and 1a
1d and 1a
1d and 1c
r
0.998
0.997
0.997
0.999
0.999
0.999
b
ꢃ
0.99 ± 0.04
1.00 ± 0.03
1.00 ± 0.04
1.01 ± 0.01
1.01 ± 0.01
1.00 ± 0.00
0.093
0.291
0.668
0.197
0.574
0.377
four complexes) with a series of similar substrates (six sulfoxides)
plex and will thus exhibit less positive charge on S atom and a
weaker influence of the substituents, leading to a low ꢂ value. The
less sensitivity of sulfoxides in the first and second systems can be
attributed to the less nucleophilic character of sulfoxides compared
to the corresponding sulfides in accordance with the observation
in the earlier studies [51] that in the electrophilic oxidation by
peroxides the reactivity of sulfoxides is considerably less affected
by substituent effects than that of sulfides. The less sensitivity of
sulfoxides in the fifth system is in contrast with the Hammond
postulate for a common SN2 mechanism and has been attributed
(
Fig. 6). The results summarized in Table 6 show an indifferent RSP
(
RSP(?)) in all the cases as the value of b is nearly one. As expected
for a RSP(?), the lines in Fig. 6 are parallel to one another and pro-
duce no magic point. It may be mentioned here that a strong RSP has
been observed in our earlier studies of oxidation of aryl methyl sul-
fides and sulfoxides with (salen)MnIII/PhIO and (salen)Mn /NaOCl
catalytic systems [22,24,26,28].
III
3.5. Comparison of reactivity of various catalytic systems
V
towards organic sulfides and sulfoxides
to the poor electrophilic nature of oxo(salen)Cr –sulfoxide adduct
V
compared to the oxo(salen)Cr complex.
Comparison of the studies of oxidations of aryl methyl sul-
III
fides and the corresponding sulfoxides by (salen)Mn /H O
2
2
4. Conclusion
III
III
[
29,present study], (salen)Fe /PhIO [9,48], (salen)Mn /PhIO
III
III
[
21,24], (salen)Mn /NaOCl [26,28] and (salen)Cr /PhIO [25,49]
Aryl methyl sulfoxides are efficiently oxidized to the corre-
catalytic systems discloses some interesting results. In all the five
systems, both the substrates act as nucleophiles and the sulfides
III
sponding sulfones by the (salen)Mn /H O catalytic system. Based
2
2
on kinetic results and substituent effect studies with respect to
both sulfoxide and complex a Michaelis–Menten type mecha-
III
are more reactive. The same reactive species, (salen)Mn OOH
V
in the first system and oxo(salen)M in the next three systems,
III
nism involving (salen)Mn OOH complex as reactive species has
oxidizes both sulfides and sulfoxides with a similar mechanism,
Michaelis–Menten type in the first two systems and SN2 type in
the third and the fourth systems. In the fifth system, the mecha-
nism is similar (SN2 type), but the reactive species are different,
been proposed. The results in the present study have been com-
pared with those obtained in our earlier studies of oxidation
III
of organic sulfides and sulfoxides with the (salen)Mn /PhIO,
III
III
(
salen)Mn /NaOCl, (salen)Fe /PhIO and (salen)CrIII/PhIO catalytic
V
V
oxo(salen)Cr for sulfide oxidation and oxo(salen)Cr –sulfoxide
adduct for sulfoxide oxidation. The reaction constant (ꢂ) values
observed for substituent variation in the para-position of phenyl
ring of sulfides and sulfoxides show that sulfides are more sensi-
tive in the first, second and fifth systems and sulfoxides are more
sensitive in the third and fourth systems. The more sensitivity of
sulfoxides in the third and fourth systems is in consistent with the
Hammond postulate for a common SN2 mechanism for both the
substrates. That is, as the sulfides are electron-rich compared to the
corresponding sulfoxides, they have an earlier transition state for
interaction with the electrophilic oxo(salen)manganese(V) com-
systems. Both Hammett correlations and Exner’s method of math-
ematical verification point out an indifferent RSP in the present
study.
Acknowledgement
A.C. thanks Manonmaniam Sundaranar University for the finan-
cial assistance from UGC-UAG.
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