Kinetics and mechanism of meso-tetraphenylporphyriniron(III) chloride catalyzed oxidation of indole by peroxomonosulphate

Article abstract of DOI:10.14233/ajchem.2013.13953

Mechanistic study on meso-tetraphenylporphyriniron(III) chloride (TPP) catalyzed oxidation of indole by peroxomonosulphate in aqueous acetonitrile medium have been carried out. The reaction follows a fractional order (0.57) with respect to substrate and first order with respect to oxidant. The order with respect to catalyst was found to be one. Increase in percentage of acetonitrile decreased the rate of the reaction. The added H+ did not affect the reaction rate. The reaction fails to initiate polymerization and free radical mechanism is ruled out. Activation and thermodynamic parameters have been computed. Nucleophilic attack of the ethylenic bond on the persulphate oxygen in presence of mesotetraphenylporphyriniron( III) chloride is envisaged to explain the reactivity. A suitable kinetic scheme based on the observations was proposed. Significant catalytic activity is observed for the reaction system in the presence of meso-tetraphenylporphyriniron(III) chloride.

Full text of DOI:10.14233/ajchem.2013.13953

Asian Journal of Chemistry; Vol. 25, No. 8 (2013), 4311-4314
http://dx.doi.org/10.14233/ajchem.2013.13953
Kinetics and Mechanism of meso-Tetraphenylporphyriniron(III)
Chloride Catalyzed Oxidation of Indole by Peroxomonosulphate
*
D. KUNGUMATHILAGAM and K. KARUNAKARAN
Department of Chemistry, Sona College of Technology, Salem-636 005, India
Corresponding author: E-mail: thilagamchem@gmail.com; drkk@sonatech.ac.in
Received: 24 April 2012; Accepted: 9 February 2013)
*
(
AJC-12952
Mechanistic study on meso-tetraphenylporphyriniron(III) chloride (TPP) catalyzed oxidation of indole by peroxomonosulphate in aqueous
acetonitrile medium have been carried out. The reaction follows a fractional order (0.57) with respect to substrate and first order with respect
to oxidant. The order with respect to catalyst was found to be one. Increase in percentage of acetonitrile decreased the rate of the reaction. The
+
added H did not affect the reaction rate. The reaction fails to initiate polymerization and free radical mechanism is ruled out. Activation and
thermodynamic parameters have been computed. Nucleophilic attack of the ethylenic bond on the persulphate oxygen in presence of meso-
tetraphenylporphyriniron(III) chloride is envisaged to explain the reactivity. A suitable kinetic scheme based on the observations was
proposed. Significant catalytic activity is observed for the reaction system in the presence of meso-tetraphenylporphyriniron(III) chloride.
Key Words: Indole, Peroxomonosulphate, Meso-tetraphenylporphyriniron(III) chloride, Kinetics, Oxidation.
16
chemistry during the last decade . The metalloporphyrins can
catalyze the oxidation of a variety of organic substrates with
INTRODUCTION
Indole, an electron-rich heteroaromatic nitrogen compound,
is found in many naturally occurring compounds such as various
1
6,17
many different terminal oxidants . The catalytic properties
of the transition metal porphyrins are due to the fact that an
oxo-transition metal porphyrin intermediate is formed, which
can transfer the oxygen atom to a substrate or can accept an
1,2
plant alkaloids and fungal metabolites usually as metabolites
of tryptophan. Indole, on oxidation gives products which are
3
involved in significant biological processes . Indoles possess
16
4,5
6
significant biological activity . Lawson and Witkop as well
electron from the substrate . Many stable oxo-transition metal
complexes are known and have been characterized, but
several of these complexes are inert and non-reactive as
7
as Finch and Taylor have shown that N-bromosuccinimide
8
can be used to convert indoles to oxindoles. Rangappa et al.
18
oxygen transfer reagents. Groves and coworkers described
the use of meso-tetraphenylporphyriniron(III) chloride (TPP)
in combination with the lipophilic iodosylbenzene, first used
in vivo by Ullrich, for the epoxidation of olefins and the
hydroxylation of alkanes.
have examined the kinetics of oxidation of indole by chlora-
mines-T in the presence of Os(VIII) in alkaline medium.
Peroxomonosulphate can also used to convert indoles to
9
oxindoles .
–
5
Peroxomonosulphate (PMS) ion, HSO
, is an anionic
Literature survey reveals that no report was available on
the kinetics of oxidation of meso-tetraphenylporphyriniron(III)
chloride catalyzed oxidation of indole, hence we have carried
out this work with peroxomonosulphate.
oxidant which is usually represented as oxone,
KHSO ·KHSO ·K SO . Studies on the oxidation of organic
compounds and inorganic complexes by PMS have been
2
5
4
2
4
1
0
reported . The synthetic utility of potassium peroxomono-
1
sulphate as an oxidant has been studied in detail . It is a
1
EXPERIMENTAL
powerful oxidant and oxidizes alcohols, ketones, carboxylic
1
2,13
acids, alkenes, arenes, phenols, amines and sulfides . It
All the chemicals and solvents used were of analytical
grade (Merck, India). Indole, oxone and meso-tetraphenyl-
porphyriniron(III) chloride (TPP) (Sigma Aldrich) were used
as such.All the solutions used in the study were made by using
doubly distilled water. All the reagents were prepared freshly
and used in the reaction. All the reactions were carried out in
a thermostat and the temperature was controlled to ± 0.1 ꢀC.
14
hydroxylates alkanes and aromatic compounds . The peroxo
anion oxidation of indole has been carried out because its
distinct reactivity is appropriate for an independent and precise
15
kinetic study on the steps of the mechanism .
Oxidation of organic compounds catalyzed by synthetic
metalloporphyrin complexes has been one of the hot areas in

4
312 Kungumathilagam et al.
Asian J. Chem.
Kinetic measurements: The kinetic studies were carried
TABLE-1
PSEUDO-FIRST ORDER RATE CONSTANTS FOR
THE TPP CATALYSED OXIDATION OF INDOLE BY
PEROXOMONOSULPHATE AT 301 K
out in an aqueous acetonitrile medium. The reactions were
performed by maintaining a large excess of [indole] over
[
PMS]. The reaction mixture was homogeneous throughout
2
3
7
4
[
Indole] × 10
(mol dm )
[PMS] × 10
-3
(mol dm )
[TPP] × 10
-3
(mol dm )
CH CN
3
% (V/V)
kobs × 10
-1
(s )
-3
the course of the reaction. The reaction's progress was moni-
tored for atleast two half-lives by iodometric estimation of
unchanged oxidant at regular time intervals. The rate constants
3.0
2.5
2.5
2.5
2.5
2.5
1.5
2.0
3.0
3.5
2.5
2.5
2.5
2.5
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
2.0
6.0
8.0
10.0
80
80
80
80
80
80
80
80
80
80
80
80
80
1.52
1.70
2.09
2.17
2.45
2.62
2.42
1.01
0.82
0.59
3.20
4.50
7.81
4.0
5.0
6.0
7.0
(
[
kobs) were evaluated from the slopes of linear plots of log
titre] versus time.
Stoichiometry: Solutions of indole containing an excess
5.0
5.0
5.0
5.0
of PMS were kept overnight at room temperature. Titrimetric
estimation of the concentration of PMS consumed and assum-
ing that all the indole taken had reacted, the stoichiometry of
indole:PMS was found to be 1:1.
5.0
5.0
5
5
.0
.0
Product analysis: The reaction mixture after 48 h, was
2 4
extracted with chloroform, dried over anhydrous Na SO and
then evaporated. The residual brown solid obtained was
subjected to column chromatography using silica gel and eluted
with benzene and then evaporated. The melting point of the
compound was found to be 116-118 ꢀC confirms oxindole
3
3
3
3
.85
.80
.75
.70
r = 0.987
(
literature value is 118-120 ꢀC) as the main product of the
reaction. The product was also confirmed by UV-visible
absorption spectra at λmax of 397 nm (Fig. 7) which is in agree-
ment with literature values.
Data analysis: Correlation studies were carried out using
Microcal origin (version 6) computer software. The goodness
of the fit was discussed using the correlation coefficient, r, in
the case of simple linear regression and R in the case of multiple
linear regressions.
3.65
3
.60
RESULTS AND DISCUSSION
1
.2
1.3
1.4
log[indole]
1.5
1.6
Factors influencing the rate of TPP catalyzed oxidation
of indole by peroxomonosulphate such as [indole], [PMS],
+
TPP], [H ] and dielectric constant have been studied. Rate
[
and activation parameters were evaluated.
Fig. 1. Plot of log kobs versus log [indole] for TPP catalyzed oxidation of
indole by peroxomonosulphate in acetonitrile medium
+
Effect of [indole]: At a constant [PMS], [TPP], [H ] and
fixed percentage of acetonitrile, kinetic runs were carried out
with various initial concentrations of indole, which yielded
rate constants whose values depended on [indole]. The pseudo-
first order rate constants (kobs) thus obtained were found to
increase with [indole] (Table-1) over a range of [indole] used
7
000
r = 0.983
6000
5
4
3
2
1
000
000
000
000
000
0
-2
-3
(
3.0-7.0 × 10 mol dm ). The plot (Fig. 1) of log kobs versus
log [indole] is linear with a slope of 0.57 showing that the
reaction is fractional order in [indole]. The plot (Fig. 2) of
1
/kobs versus 1/[indole] is linear with definite intercept on the
rate ordinate, which indicates the operation of Michalis-
Menton mechanism, where K is the equilibrium constant for
the formation of Michalis-Menton complex and k the rate
3
4
constant for the decomposition of Michalis-Menton complex.
Usually first step is a fast pre-equilibrium and the electron
19
0
5
10
15
20
25
30
35
transfer step is rate determining .
Effect of [PMS]: The order of the reaction with respect
to PMS was to be strictly unity as shown by the linearity of
log [PMS] versus time plots, over 80 % of the reaction. Further,
the reaction rate decreases with increase in the concentration
1/[indole]
Fig. 2. Plot of 1/kobs versus 1/[indole] for TPP catalyzed oxidation of indole
by peroxomonosulphate in acetonitrile medium
-3
-3
of oxidant, [PMS] (1.5-3.5 × 10 mol dm ) at fixed [indole],
with various initial concentrations of [TPP], which yielded
rate constants whose values depended on [TPP]. The pseudo-
first order rate constants (kobs) thus obtained were found to
increase with [TPP] (Table-1) over a range of [TPP] used (0.2-
+
[
TPP], [H ] and percentage of acetonitrile (Table-1).
Effect of [TPP]: At a constant [indole], [PMS], [H ] and
fixed percentage of acetonitrile, kinetic runs were carried out
+

Vol. 25, No. 8 (2013)
Kinetics and Mechanism of meso-Tetraphenylporphyriniron(III) Chloride Catalyzed Oxidation of Indole 4313
-
.0 × 10 mol dm ). A linear plot was obtained between
7
-3
1
TABLE-3
EFFECT OF DIELECTRIC CONSTANT
ON THE REACTION RATE AT 301 K
a
logkobs and log [TPP] (Fig. 3) with a slope of 1.07, indicating
that the order of the reaction with respect to catalyst was one.
4
-1
CH CN:H O
3
D
kobs × 10 (s )
0.31
2
9
8
7
6
5
0:10
0:20
0:30
0:40
0:50
41.47
45.48
49.71
53.83
57.95
2.09
4.0
3.8
3.6
3.4
3.2
3.0
2.76
5.42
r = 0.986
8.95
-
3
-3
-2
-3
[
×
PMS] = 2.5× 10 mol dm , [indole] = 5.0 × 10 mol dm , [TPP] = 4.0
-7
-3 a
10 mol dm . Values are calculated from the values of pure solvent.
TABLE-4
EFFECT OF TEMPERATURE ON THE REACTION RATE
Temp. (K)
4
kobs × 10 (s )
301
306
312
318
323
-1
1.86
2.09
3.34
4.66
5.73
-
3
-3
-2
-3
PMS] = 2.5 × 10 mol dm , [indole] = 5.0 × 10 mol dm , [TPP] =
[
4
-7
.0 × 10 mol dm . Acetonitrile:water = 40:10.
-3
6.9 7.0 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8
-2.50
log[catalyst]
r = 0.988
Fig. 3. Plot of log kobs versus log [catalyst] showing the effect of catalyst
concentration reaction rate
-2.55
+
Effect of [H ]: The reaction rates measured with various
-2.60
+
-3
[
(
H ] (0.08-0.40 mol dm ) and were found to be the same
Table-2). Such kinetic behaviour indicates the nonexistence
-
2.65
2.70
of any protonation equilibrium with respect to both PMS and
indole in the experiment.
-
TABLE-2
EFFECT OF [H ]CONCENTRATION ON THE REACTION RATE
+
0
.0031
0.0032
/T
0.0033
+
H ] × 10 (mol dm )
2
-3
4
-1
[
kobs × 10 (s )
3.40
1
8.0
1
2
3
4
6.0
4.0
2.0
0.0
3.06
Fig. 4. Plot of ln kobs/T versus 1/T showing the effect of temperature on
reaction rate
3.31
3.46
-
1
#
-1
-1
#
-1
.84 kJ mol , ∆S = -194.10 J K mol , ∆G = 66.65 kJ mol .
3.93
7
The large negative value of entropy of activation (∆S ) obtained
is attributed to the severe restriction of solvent molecules
-3
-3
-2
-3
[
4
PMS] = 2.5 × 10 mol dm , [indole] = 5.0 × 10 mol dm , [TPP] =
-3
.0 × 10 mol dm , acetonitrile:water = 40:10, temperature = 301 K.
#
-7
22
around the transition state .
Mechanism: Peroxomonosulphate exists as HSO
acidic solution. Although many peroxy anions are effective
Effect of dielectric constant: In order to determine the
23
–
5
in
effect of dielectric constant (polarity) of the medium on rate,
the oxidation of indole by peroxomonosulphate was studied
in aqueous acetonitrile mixtures of various compositions
–
24
5
nucleophiles. HSO is weak nucleophile . Inspite of the fact
that free radicals can arise from the facile homolysis of the
(
Table-3). The data clearly reveals that the rate increases with
25
oxygen-oxygen bond , an ionic mechanism is favoured in
certain reactions involving oxidations by peroxides. In the
present investigation no observed polymerization in the
presence of acrylonitrile rules out a free radical process. Hence
ionic mechanism was favoured in this study. Generally, the
enhancement of the electrophilic activity of peroxide will
minimize the importance of undesirable free radical pathways,
decrease in the percentage of acetonitrile, i.e., with increasing
dielectric constant or polarity of the medium and leads to the
influence that there is a charge development in the transition
state involving a more polar activated complex than the
2
0,21
reactants , a neutral molecule [indole] and a mono negative
–
ion (HSO ) suggesting a polar (ionic) mechanism.
5
Test for free radical intermediates: No turbidity was
observed when a freshly distilled acrylonitrile monomer was
added to the deaerated reaction mixture indicating the absence
of free radical intermediates.
26
resulting in a mixture of products . The Michaelis-Menton
dependence of the oxidation rate on [indole] confirms the
formation of complex in a rapid pre-equilibrium.
The first step is the formation of a complex between oxone
and TPP which was confirmed from the UV-visible spectra
Rate and activation parameters: The effect of tempe-
rature was studied in the range of 301-323 K and the results
were shown in Table-4. The Arrhenius plot of ln kobs/T versus
(
Fig. 5). This complex further decomposed and give Por-Fe(V)
27
=
=
O is in agreement with the literature study . This Por-Fe(V)
O may further react with the indole to form a complex (Fig. 6)
1
/T was found to be linear (Fig. 4). The value of energy of
-1
-1
#
a
activation (E ) was found to be 10.36 kJ mol K and ∆H =

4
314 Kungumathilagam et al.
Asian J. Chem.
O
O
K
III
PorFe
III
PorFe Cl
1
S
O
+
H
O
O
S
O
O
O
HCl
O
O
2
4
K
SO
2
O
V
PorFe
O
K
3
V
PorFe
+
N
H
fast
N
H
INDOLE
k
Wavelength (nm)
slow
4
Fig. 5. UV spectrum showing the complex formation between TPP and
PMS at 237 nm
+
III
O
+
PorFe
N
H
OXINDOLE
Scheme-I: Probable mechanism for the meso-tetraphenyl porphyriniron(III)
chloride catalysed oxidation of indole by peroxomonosulphate
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Fig. 6. UV spectrum showing formation of intermediate complex at 694
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(
9
Fig. 7). This type of product was already reported . The
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Wavelength (nm)
Fig. 7. UV spectrum showing the formation of product at 397 nm after
the decomposition of intermediate complex at 694 nm