Ru(III), Os(VIII), Pd(II) and Pt(IV) catalysed oxidation of glycyl-glycine by sodium N-chloro-p-toluenesulfonamide: Comparative mechanistic aspects and kinetic modelling

Article abstract of DOI:10.1002/poc.1379

The Ru(III)/Os(VIII)/Pd(II)/Pt(IV)-catalysed kinetics of oxidation of glycyl-glycine (Gly-Gly) by sodium N-chloro-p-toluenesulfonamide (chloramine-T; CAT) in NaOH medium has been investigated at 308 K. The stoichiometry and oxidation products in each case

Full text of DOI:10.1002/poc.1379

Research Article
Received: 21 October 2007,
Revised: 2 February 2008,
Accepted: 25 March 2008,
Published online in Wiley InterScience: 2 June 2008
(
www.interscience.wiley.com) DOI 10.1002/poc.1379
Ru(III), Os(VIII), Pd(II) and Pt(IV) catalysed
oxidation of glycyl–glycine by sodium
N-chloro-p-toluenesulfonamide: comparative
mechanistic aspects and kinetic modelling
a
a
R. V. Jagadeesh and Puttaswamy *
The Ru(III)/Os(VIII)/Pd(II)/Pt(IV)-catalysed kinetics of oxidation of glycyl–glycine (Gly-Gly) by sodium N-chloro-p-
toluenesulfonamide (chloramine-T; CAT) in NaOH medium has been investigated at 308 K. The stoichiometry and
oxidation products in each case were found to be the same but their kinetic patterns observed are different.
Under comparable experimental conditions, the oxidation-kinetics and mechanistic behaviour of Gly-Gly with CAT
in NaOH medium is different for each catalyst and obeys the underlying rate laws:
0
S x
Rate ¼ k [CAT] [Gly-Gly] [Ru(III)][OH ]
t
x
y
S z
Rate ¼ k [CAT]
Rate ¼ k [CAT]
t
t
[Gly-Gly] [Os(VIII)] [OH ]
x
S y
[Gly-Gly] [Pd(II)][OH ]
0
x
S y
Rate ¼ k [CAT] [Gly-Gly] [Pt(IV)] [OH ]
t
S
Here, Ts ¼ CH3C6H4SO2ꢀ and x, y, z < 1 in all the cases. The anion of CAT, CH
3 6 4 2
C H SO NCl , has been postulated as
the common reactive oxidising species in all the cases. Under comparable experimental conditions, the relative ability
of these catalysts towards oxidation of Gly-Gly by CATare in the order: Os(VIII) > Ru(III) > Pt(IV) > Pd(II). This trend may
be attributed to the different d-electronic configuration of the catalysts. Further, the rates of oxidation of all the four
catalysed reactions have been compared with uncatalysed reactions, under identical experimental conditions. It was
found that the catalysed reaction rates are 7- to 24-fold faster. Based on the observed experimental results, detailed
mechanistic interpretation and the related kinetic modelling have been worked out for each catalyst. Copyright ß
2008 John Wiley & Sons, Ltd.
Keywords: Ru(III)/Os(VIII)/Pd(II)/Pt(IV) catalysis; oxidation-kinetics; glycyl–glycine; chloramine-T; alkaline medium
study is expected to throw some light on the mechanism of
INTRODUCTION
metabolic conversion of Gly-Gly in the biological systems. Hence,
the present kinetic study gives an impetus, as the Gly-Gly is a
biologically important substrate. Also, no one has examined the
role of platinum group metal ions as catalysts in N-haloamine
oxidation of Gly-Gly.
The chemistry of N-haloamines is of interest due to their diverse
[
1]
chemical behaviour. These compounds resemble hypohalites in
their oxidative behaviour and they are more stable than
[
2,3]
hypohalites.
Consequently, these compounds react with a
wide range of functional groups and effect a variety of molecular
changes. The prominent member of this class of compounds
is sodium N-chloro-4-methylbenzenesulfonamide, commonly
The studies on the use of platinum group metal ions, either
alone or binary mixtures, as catalysts in many redox reactions,
[1]
[
12]
have been gaining interest.
Common platinum group metal
known as chloramine-T (p-CH
3
C
6
H
4
SO
2
NClNaꢁ3H
2
O and abbre-
ions employed in homogeneous catalysis involving redox
reactions are Ruthenium (III) chloride (Ru(III)), Osmium(VIII) oxide
(Os(VIII)), Palladium(II) chloride (Pd(II)), Platinum(IV) chloride
(Pt(IV)), Iridium(III) chloride (Ir(III)) and Rhodium(III) chloride
(Rh(III)). The mechanisms of these catalysis are quite complicated
due to the formation of different intermediate complexes, free
viated as CAT). Mechanistic aspects of many of its reactions have
[
1,4–6]
been well documented.
However, a very little information is
available in the literature on CAT reactions with dipeptides.
Dipeptides are useful biomaterials in many biological,
pharmaceutical, analytical and synthetic applications. Glycyl–
Glycine (Gly-Gly) is the first member of the dipeptide series and
a review of literature reveals that there are many reports on
[
13,14]
radicals and differing oxidising states of these catalysts.
[
7,8]
the kinetics of its hydrolysis.
manganese-(III),
It has also been oxidised by
[
9]
[10]
[11]
bromamine-T,
bromamine-B
in acid
*
Correspondence to: Puttaswamy, Department of Chemistry, Central College
Campus, Bangalore University, Bangalore-560 001, India.
E-mail: pswamy_chem@yahoo.com
medium. However, literature survey revealed that there are no
efforts being made from the kinetic and mechanistic viewpoints
on the oxidation of Gly-Gly by N-haloamines in alkaline medium.
Hence, there was a need for understanding the oxidation
mechanism of this dipeptide in alkaline medium, so that this
a
R. V. Jagadeesh, Puttaswamy
Department of Chemistry, Central College Campus, Bangalore University,
Bangalore-560 001, India
J. Phys. Org. Chem. 2008, 21 844–858
Copyright ß 2008 John Wiley & Sons, Ltd.

OXIDATION OF GLY-GLY BY SODIUM N-CHLORO-P-TOLUENESULFONAMIDE
Although many complexes of these metal ions with various
organic and inorganic substances have been reported, an
extensive literature survey reveals the absence of comprehensive
studies on the oxidation-kinetics and mechanism of dipeptides
by any of these catalysts. Consequently, such studies provide an
insight into the interaction of metal ions with the substrate in
biological systems.
This background instigated us to carry out the title reaction in
order to interpret the mechanism and to understand the catalytic
redox chemistry of Gly-Gly in the presence of platinum group
metal ions. Preliminary experimental results revealed that the
reactions of Gly-Gly with CAT in alkaline medium without a
catalyst were sluggish, but the reactions become facile in the
presence of a micro quantity of platinum group metal ions
like Ru(III), Os(VIII), Pd(II) and Pt(IV). The main objectives of the
present investigation are to: (i) elucidate plausible mechanisms,
surface to eliminate any photochemical effects. The oxidant and
requisite amount of substrate, NaOH, Ru(III)/Os(VIII)/Pd(II)/Pt(IV)
catalyst, NaClO (wherever mentioned) solutions and water (to
4
keep the total volume constant for all runs), were taken in the
tube and thermostatted at 308 K until thermal equilibrium was
attained. A measured amount of CAT solution, which was also
thermostatted at the same temperature, was rapidly added to the
mixture and was shaken intermittently for uniform concentration.
The progress of the reaction was monitored by iodometric
determination of unreacted CAT in 5 ml of aliquots of the reaction
mixture withdrawn at different time intervals. The course of
the reaction was studied for more than two half-lives. The
0
ꢀ1
pseudo-first-order rate constants (k s ) calculated from the
linear plots of log [CAT] versus time were reproducible within
ꢃ3–5%.
(
ii) deduce appropriate rate laws, (iii) ascertain the various
Stoichiometry
reactive species, (iv) assess the relative rates of catalysts, (v)
understand the catalytic redox chemistry of Gly-Gly in the
presence of platinum group metal ions, (vi) find the catalytic
efficiency of catalysts and (vii) compare the reactivity with that
under uncatalysed oxidation.
Different ratios of CAT to Gly-Gly in the presence of
ꢀ2
ꢀ3
ꢀ5
ꢀ3
1
.0 ꢄ 10 mol dm
NaOH and 1.0 ꢄ 10 mol dm
catalyst
were equilibrated at 308 K for 24 h. The unreacted oxidant in the
reaction mixture was determined iodometrically. The analysis
showed that one mole of Gly-Gly reacts with two moles of oxidant
for all the catalysed reactions, confirming to the following
stoichiometry:
EXPERIMENTAL
NH2CH2CONHCH2COOH þ 2TsNClNa þ 3H2O ! 2HCHO þ 2CO2
þ
ꢀ
Materials
þ 2NH3 þ 2TsNH2 þ 2Na þ 2Cl
Chloramine-T (Merck) was purified by the method of Morris
(1)
[
3]
et al. An aqueous solution of CAT was prepared, standardised
where Ts ¼ CH
Product analysis
The reaction mixtures (1 mol of Gly-Gly and 2 moles of CAT in the
3 6 4 2
C H SO .
iodometrically and stored in amber coloured, stoppered bottles
[
3]
to prevent photochemical deterioration. The concentration of
stock solutions was periodically determined. Chromatographi-
cally pure Gly-Gly (Merck) was used as received. Aqueous
solutions of desired strength were prepared whenever required.
Ruthenium trichloride (Merck), palladium chloride (Arora-
Matthey) and platinum chloride (SD Fine Chem Ltd.) solutions
were prepared in 20 mM HCl while osmium tetroxide (BDH) was
prepared in 20 mM NaOH. Allowance for the amount of acid/alkali
present in the catalyst solutions was made while preparing the
reaction mixtures for kinetic runs. The ionic strength of the
ꢀ2
ꢀ5
ꢀ3
presence of 1.0 ꢄ 10 and 1.0 ꢄ 10 mol dm catalyst) were
allowed to progress for 2–3 h under stirred condition at 35 8C.
After completion of the reaction (monitored by TLC), the reaction
products were extracted with ether. Formaldehyde and
p-toluenesulfonamide (PTS) are identified as the main oxidation
product of Gly-Gly and reduction product of CAT, respectively.
Evaporation of the ether layer was followed by column
chromatography on silica gel (60–200 mesh) using gradient
elution (from dichloromethane to chloroform). After initial
separation, the products were further purified by recrystalisation.
Formaldehyde, the oxidation product of Gly-Gly was detected
ꢀ
3
system was maintained constant at a high value (0.3 mol dm
using a concentrated solution of NaClO in order to swamp the
)
4
reaction wherever the effect is noticed. The permittivity of the
reaction mixture was altered by the addition of methanol in
varying proportions (v/v), and the values of the dielectric
constant of water–methanol mixtures as reported in the
[
16]
[17]
by spot tests
and estimated as its 2,4-DNP derivative.
The
amount of formaldehyde was calculated from the weight of
hydrazone formed in the presence of each catalyst and the yield
of the formaldehyde was found to be 75–80%. Further, the
melting point of the 2,4-DNP derivative of formaldehyde was
found to be 165 8C (Lit. m.p. 166 8C). It was also observed that
formaldehyde does not undergo further oxidation under the
prevailing kinetic conditions.
[
15]
literature were employed. Solvent isotope studies were made
in D O (99.4% purity) supplied by Bhabha Atomic Research
2
Center, Mumbai, India. Reagent grade chemicals and doubly
distilled water were used throughout. UV-3101 PC UV–Vis-NIR
Scanning Spectrophotometer was used for studying the
formation of various complexes. Regression analysis of the
2
The reduction product of CAT, PTS (TsNH ), was detected by
x
experimental data was carried out on an f -100W scientific
calculator to obtain the regression coefficient, r.
[
18]
TLC. It was further confirmed by its melting point of 139 8C (Lit
m.p. 137–140 8C). Further, PTS has been quantitatively esti-
[
17]
mated by its reaction with xanthydrol to yield the correspond-
ing N-xanthyl-p-toluenesulfonamide. In a typical experiment,
equal quantities of separated PTS and xanthydrol (0.20 g) were
dissolved in 10 ml of glacial acetic acid. The reaction mixture was
stirred for 3 min at laboratory temperature and allowed to stand
for 90 min. The derivative was filtered, recrystalised with dioxane/
Kinetic measurements
The reactions were carried out under pseudo-first-order
conditions ([substrate] ) at constant temperature
0
ꢂ [oxidant]
0
in glass-stoppered Pyrex boiling tubes coated black on outside
J. Phys. Org. Chem. 2008, 21 844–858
Copyright ß 2008 John Wiley & Sons, Ltd.
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R. V. JAGADEESH AND PUTTASWAMY
Table 1. Effect of varying reactant concentrations on the reaction rate at 308 K
0
4
ꢀ1
)
1
0 k (s
3
ꢀ3
2
10 [Gly-Gly] (mol dm
ꢀ3
1
0 [CAT] (mol dm
)
)
Ru(III)
Os(VIII)
Pd(II)
Pt(IV)
0
0
0.20
0.50
1.00
2.00
4.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.50
1.00
2.00
3.00
4.00
5.69
5.74
5.78
5.70
5.81
5.70
5.78
5.71
5.80
5.79
10.11
9.98
10.0
10.1
10.0
7.41
10.0
14.1
17.40
20.1
3.16
3.10
3.12
3.00
3.20
2.14
3.12
4.46
5.51
6.56
4.20
4.29
4.26
4.21
4.30
4.21
4.26
4.20
4.19
4.30
NaOH] ¼ 1.00 ꢄ 10ꢀ mol dmꢀ3; [catalyst] ¼ 1.00 ꢄ 10ꢀ5 mol dmꢀ3; I ¼ 0.30 mol dmꢀ3 (in case of Ru(III) catalysis).
2
[
water (3:1) and dried at room temperature. This was weighed and
the recovery of PTS in all the cases is found to be 80–85%. The
liberated CO and NH were identified by conventional lime water
indicated by the linearity of the plots of log [CAT] versus time
(r > 0.9948). Further, the linearity of these plots with constancy of
the slopes obtained at various [CAT]0 indicated a first-order
2
3
and Nessler’s reagent tests, respectively.
0
dependence of the reaction rate on [CAT] . The pseudo-
first-order rate constants (k ) obtained are given in Table 1.
0
Under the same experimental conditions, the rate of the reaction
increased in [Gly-Gly] (Table 1) for Os(VIII)- and Pd(II)-catalysed
0
0
RESULTS
reactions and plots of log k versus log [Gly-Gly] were found to be
linear (r > 0.9925) with fractional slopes of 0.47 and 0.54,
Kinetic orders
respectively, indicating
[ G0 ly-Gly] in both cases. Further, the intercepts of the plots of
k versus [Gly-Gly] in these cases were linear (r > 0.9889) which
confirmed a fractional-order dependence of rate on [Gly-Gly] in
both the reactions. But the orders in [Gly-Gly] were found to be
zero in case of Ru(III) and Pt(IV) catalysis (Table 1).
a fractional-order dependence on
The kinetics of oxidation of Gly-Gly by CAT was investigated at
several initial concentrations of the reactants in NaOH medium in
the presence of Ru(III), Os(VIII), Pd(II) and Pt(IV) catalysts at 308 K
under identical experimental conditions. Despite the fact that the
stoichiometry and oxidation products were same in all the
catalysed reactions, the oxidation kinetic behaviour was different.
All the reactions were carried out under pseudo-first-order
0
0
0
0
The rate of the reaction increased with increasing [NaOH]
(Table 2) in all the cases. The log–log plots of rate versus [NaOH]
(r > 0.9901) showed that the orders in [OH ] were less than unity
(0.37–0.73), indicating a fractional-order dependence of rate on
[OH ] in all the four catalysed reactions. The reaction rate
increases with the increase in [Ru(III)] and [Pd(II)] and log–log
ꢀ
conditions, wherein [Gly-Gly]
Under the pseudo-first-order conditions of [Gly-Gly]
with other reaction conditions remaining constant, the order in
CAT] was found to be unity in all the four catalysed reactions, as
0
ꢂ [CAT]
0
.
0
ꢂ [CAT]
0
,
ꢀ
[
0
Table 2. Effect of varying NaOH and catalyst concentrations on the reaction rate at 308 K
0
4
ꢀ1
)
1
0 k (s
2
ꢀ3
5
ꢀ3
1
0 [NaOH] (mol dm
)
10 [catalyst] (mol dm
)
Ru(III)
Os(VIII)
Pd(II)
Pt(IV)
0.20
0.50
1.00
2.00
4.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
0.20
0.50
1.00
2.00
4.00
2.41
4.00
5.76
8.41
12.2.
1.14
2.86
5.76
11.6
23.2
5.42
7.61
10.0
13.6
17.7
3.00
6.10
10.00
17.7
1.06
2.01
3.12
5.76
9.01
0.74
1.47
3.12
6.41
12.61
2.26
3.17
4.26
5.47
7.06
1.16
2.51
4.26
7.67
14.0
29.6
¼ 1.00 ꢄ 10ꢀ mol dm ; [Gly-Gly]
3
ꢀ3
¼ 1.00 ꢄ 10ꢀ2 mol dmꢀ3; I ¼ 0.30 mol dmꢀ3 (in case of Ru(III) catalysis).
[
CAT]
0
0
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 844–858

OXIDATION OF GLY-GLY BY SODIUM N-CHLORO-P-TOLUENESULFONAMIDE
0
1/2
Figure 1. Plot of log k versus I
plots of rate versus [catalyst] were found to be linear (r > 0.9905)
with a slope of unity, indicating a first-order dependence of rate
on [Ru(III)] and [Pd(II)]. Whereas in case of [Os(VIII)] and [Pt(IV)]
catalysis, such plots were linear (r > 0.9965) with slopes less than
unity (0.80–0.85), indicating fractional-order kinetics in [Os(VIII)]
and [Pt(IV)]. All these results are recorded in Table 2.
0
Addition of PTS to the reaction mixture retards the rate in Pt(IV)
Figure 2. Plots of log k versus 1/D
ꢀ
4
catalysis and the rate constants at 4.0, 8.0, 14.0 and 20.0 ꢄ 10
ꢀ3
ꢀ4 ꢀ1
[
PTS] mol dm
were 4.26, 3.80, 2.92 and 2.45 ꢄ 10
s ,
0
respectively. Further, a plot of log k versus log [PTS] was linear
in other catalysed reactions. Hence, only in case of Ru(III)-
catalysed reaction, the ionic strength of the medium was
maintained at a constant concentration of 0.30 mol dm of
NaClO for kinetic runs in order to swamp the reaction.
The effect of dielectric constant of the medium on the reaction
rate was studied by adding methanol (0–30% v/v) to the reaction
mixture, while keeping other experimental conditions constant.
The rates were found to increase with increase in MeOH content
(
r ¼ 0.9899) with a negative slope of 0.25, indicating a negative
ꢀ3
fractional-order dependence of the rate on [PTS]. It also indicates
that PTS is involved in a fast pre-equilibrium to the rate-limiting
step in the proposed scheme for the Pt(IV) catalysis. But the rate
was unaffected by the addition of PTS in other catalysed reactions
and it signifies that the PTS is not involved in any step prior to the
rate-limiting step in the schemes proposed.
4
An increase in ionic strength of a reaction system by the
in case of Ru(III)- and Os(VIII)-catalysed reactions and the plots of
0
addition of NaClO decreased the rate of the reaction and the rate
log k versus 1/D were linear (Fig. 2; r > 0.9979) with positive
slopes. But in the case of Pd(II) catalysis, the slope of such a plot
was negative (Fig. 2; r ¼ 0.9990). However, negligible influence of
variation of dielectric constant on the rate was observed in case
of Pt(IV) catalysis. All these results were recorded in Table 3.
4
ꢀ
3
constants at 0.1, 0.2, 0.3, 0.4 and 0.5 mol dm ionic strength were
ꢀ
4
ꢀ1
1
2.8, 8.11, 5.76, 4.21 and 3.11 ꢄ 10 s0 , respectively, in case of
1/2
Ru(III) catalysis. Further, a plot of log k versus I gave a straight
line (Fig. 1; r ¼ 0.9926) with a negative slope of 1.5. But variation of
ionic strength showed negligible effect on the rate of the reaction
ꢀ
ꢀ
The rate remained constant with the addition of Cl or Br ions in
Table 3. Effect of varying dielectric constant of the medium on the reaction rate at 308 K
0
4
ꢀ1
)
1
0 k (s
%
MeOH (v/v)
D
Ru(III)
Os(VIII)
Pd(II)
Pt(IV)
0
76.73
72.37
67.48
62.71
5.76
6.81
9.00
10.0
12.3
15.2
20.0
3.12
2.41
1.61
1.00
4.26
4.30
4.20
4.16
1
2
3
0
0
0
12.01
ꢀ
3
ꢀ3
¼ 1.00 ꢄ 10ꢀ2 moldmꢀ3; [NaOH] ¼ 1.00 ꢄ 10ꢀ2 moldmꢀ3; [catalyst]¼ 1.00 ꢄ 10ꢀ5 mol dmꢀ3
[
CAT]
0
¼ 1.00 ꢄ 10 mol dm ; [Gly-Gly]
0
;
ꢀ3
I ¼ 0.30 mol dm (in case of Ru(III) catalysis).
J. Phys. Org. Chem. 2008, 21 844–858
Copyright ß 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

R. V. JAGADEESH AND PUTTASWAMY
Table 4. Temperature dependence on the reaction rate and values of activation parameters for the oxidation of Gly-Gly by CAT in
the presence and absence of catalyst
0
4
ꢀ1
)
1
0 k (s
Temperature (K)
Ru(III)
Os(VIII)
Pd(II)
Pt(IV)
2
3
3
3
3
98
03
08
13
18
2.84
3.74
5.76
8.08
11.6
58.6
5.69
7.82
10.0
14.0
18.2
42.7
1.20
2.01
3.12
5.11
8.00
75.7
1.79 (0.11)
2.71 (0.22)
4.26 (0.41)
6.61 (0.81)
9.59 (1.41)
68.2 (108)
ꢀ
1
E (kJ mol
)
a
6¼
ꢀ1
ꢀ1
DH (kJ mol
DG (kJ mol
DS (J K mol
log A
)
)
56.0 ꢃ 0.02
95.0 ꢃ 0.09
ꢀ125 ꢃ 0.21
9.69 ꢃ 0.11
39.6 ꢃ 0.01
93.1 ꢃ 0.16
ꢀ173 ꢃ 0.21
4.17 ꢃ 0.01
73.1 ꢃ 0.02
95.8 ꢃ 0.2
ꢀ75.3 ꢃ 0.10
9.33 ꢃ 0.10
65.3 ꢃ 0.06 (106 ꢃ 0.10)
95.0 ꢃ 0.12 (101 ꢃ 0.21)
ꢀ96.2 ꢃ ꢀ0.21 (15.7 ꢃ 0.41)
8.20 ꢃ 0.01 (14 ꢃ 0.09)
6¼
6¼
ꢀ1
ꢀ1
)
CAT] ¼ 1.00ꢄ 10 moldm ; [Gly-Gly] ¼ 1.00 ꢄ 10 mol dmꢀ3; [NaOH] ¼ 1.00ꢄ 10ꢀ2 moldmꢀ3; [catalyst] ¼ 1.00 ꢄ 10ꢀ5 mol dm
ꢀ
3
ꢀ3
ꢀ2
ꢀ3
;
[
0
0
ꢀ
3
I ¼ 0.30mol dm (in case of Ru(III) catalysis).
Values in the parentheses refer to oxidation of Gly-Gly by CAT in the absence of catalyst. Experimental conditions are the same as in
footnote without catalyst.
ꢀ
2
ꢀ2
ꢀ3
[19]
Hardy and Johnston have reported the establishment of the
following equilibria in alkaline solution of CAT:
the form of NaCl or NaBr (1.0 ꢄ 10 –5.0 ꢄ 10 mol dm ). As a
dependence of the rate on hydroxyl ion concentration was noted,
solvent isotope studies were made using D
catalysed reactions. Studies of the reaction rate in D
for Ru(III)-, Os(VIII)-, Pd(II)- and Pt(IV)-catalysed reactions revealed
2
O for all the four
TsNCl þ H2O Ð TsNHCl þ OHꢀ
ꢀ
(2)
(3)
2
O medium
0
TsNHCl þ H2O Ð TsNH2 þ HOCl
ꢀ
4
ꢀ4
ꢀ4
that k (H
2
O)was equal to 5.70 ꢄ 10 , 10.0 ꢄ 10 , 3.12 ꢄ 10
0
ꢀ
4
ꢀ1
ꢀ4
ꢀ4
and 4.26 ꢄ 10
s
, and k (D
2
O) was 6.7.0 ꢄ 10 , 11.2 ꢄ 10
,
If HOCl were to be the primary oxidising species as indicated in
Eqn (3), a first-order retardation of rate by added PTS would be
expected. However, no such effect was noticed in the present
study. If TsNHCl is the reactive species, retardation of the rate by
ꢀ
4
ꢀ4 ꢀ1
3
.85 ꢄ 10 and 4.85 ꢄ 10
s , respectively. The solvent isotope
0
0
effect, k (H
2
O)/k (D
2
O) was found to be 0.88, 0.89, 0.81 and 0.88 for
the four cases.
ꢀ
The reaction was studied at different temperatures (298–
[OH ] is expected (Eqn (2)), which is also contrary to the
3
18 K), keeping other experimental conditions constant. From the
experimental observations. Hence, in the present investigation,
0
ꢀ
linear Arrhenius plots of log k versus 1/T (r > 0.9904), values of
activation parameters for the overall reaction were evaluated
the rate of the reaction is accelerated by OH ions clearly
ꢀ
indicates that the anion TsNCl is the most likely oxidising species
(
Table 4). The addition of the reaction mixtures to aqueous
involved in the oxidation of Gly-Gly by CAT in all the four
catalysed reactions.
acrylamide monomer solutions did not initiate polymerisation,
indicating the absence of in situ formation of free radical species
in the reaction sequence. The control experiments were also
performed under the same reaction conditions but without the
oxidant, CAT.
Mechanism and rate law of Ru(III) catalysis
Ru(III) chloride acts as an efficient catalyst in many redox
[
24–26]
reactions, particularly in an alkaline medium.
Under the
ꢀ
ꢀ
experimental conditions [OH ] ꢂ [Ru(III)] and the fact that [OH ]
DISCUSSION
increases the rate, ruthenium (III) is mostly present as the
2
þ
hydroxylated species [Ru(H O) OH] and its formation in the
2
5
Chloramine-T acts as an oxidising agent in both acidic and
alkaline solutions. The oxidation potential of CAT-PTS redox
couple varies
following equilibrium is of importance in the reaction:
[
19]
with the pH of the medium, having values of
RuðH2OÞ6ꢅ3þ þ OHꢀ Ð ½RuðH2OÞ OHꢅ þ H2O
2þ
½
(4)
5
1
.139 V at pH 0.65, 0.778 V at pH 7.0 and 0.614 V at pH 9.7.
[
2]
Chloramine-T behaves like a strong electrolyte in aqueous
solutions, and depending on the pH of the medium, it furnishes
different equilibria in solutions.
species in acidified CAT solutions
acid TsNHCl, the dichloramine-T TsNCl
HOCl and also perhaps H O Cl. In alkaline CAT solutions, TsNCl
does not exist
TsNHCl, TsNCl , HOCl and OCl .
2þ
Hence, the hydroxylated species [Ru(H O) OH] complex ion
2
5
[
2,19–21]
The possible oxidising
are the conjugate-free
of ruthenium(III) has been assumed to be the reactive catalysing
species. Similar equilibria have been reported between Ru(III)-
catalysed oxidation of several other substrates with various
[
2,19–21]
2
, the hypochlorous acid
þ
[13,27–29]
oxidants in alkaline medium.
2
2
[
22,23]
and hence the expected reactive species are
The existence of a complex between the catalyst and oxidant
was evidenced from the UV–Vis spectra of both Ru(III) and
ꢀ
ꢀ
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 844–858

OXIDATION OF GLY-GLY BY SODIUM N-CHLORO-P-TOLUENESULFONAMIDE
Ru(III)-CAT mixture, in which a shift of Ru(III) from 352.6 to 338 nm
was observed, indicating the formation of a complex (Fig. 3). Such
type of oxidant–catalyst complex formation has also been
reported in other studies.
Further, for a general equilibrium
plot of log (1/DA) versus log (1/[CAT]) was also linear (r ¼ 0.9801)
and from the slope and intercept of the plot of 1/DA versus 1/
[CAT], the value of the formation constant, K, of the complex was
[
30,31]
2
deduced; it was found to be 6.06 ꢄ 10 .
In view of the above facts, a detailed mechanism for
Ru(III)-catalysed oxidation of Gly-Gly by CAT in alkaline medium
is shown in Scheme 1.
K
(
5)
M þ nS Ð ðMSnÞ
between two metal species, M and MS
The total concentration of CAT is [CAT] , then
t
n
having different
[
32]
ꢀ
extinction coefficients, Ardon
has derived the following:
½CATꢅ ¼ ½TsNHClꢅ þ ½TsNCl ꢅ
(7)
t
1
1
1
By substituting for [TsNHCl] from equilibrium (i) of Scheme 1 in
ꢀ
¼
þ
(6)
n
DA ½Sꢅ f1=DE½MTotalꢅKg DE½MTotalꢅ
Eqn (7) and solving for [TsNCl ], one obtains
ꢀ
where K is the formation constant of the complex, [S] is the
concentration of CAT, DE is the difference in extinction coefficient
between two metal species, [M]Total is the total concentration of
metal species and DA is the absorbance difference of solution
containing no S and one that contains a certain concentration of
S represented by [S]. Equation (6) is valid provided that [S] is so
much greater than [M]Total that the amount of S tied up in the
complex is negligible or it is subtracted from the initial
concentration of S. According to Eqn (6), a plot of 1/DA versus
/[S] or 1/[S] should be linear with an intercept in case of 1:1 or
:2 type of complex formation between M and S. The ratio of
intercept to slope of this linear plot gives the value of K.
Ruthenium (III) in NaOH medium containing CAT showed an
absorption peak at 338 nm (lmax for the complex). The complex
formation studies were made at this lmax of 338 nm. In a set
of experiments, the solutions were prepared by taking
K1½CATꢅ ½OH ꢅ
ꢀ
t
½
TsNCL ꢅ ¼
(8)
ꢀ
½
From the slow and rate-determining step (rds) of Scheme 1,
H2OꢅþK1½OH ꢅ
ꢀ
Rate ¼ ꢀd½CATꢅ ¼ k2½TsNCl ꢅ½RuðIIIÞꢅ
(9)
t
ꢀ
By substituting for [TsNCl ] from Eqn (8) into Eqn (9), the
following rate law is obtained:
2
ꢀ
1
1
ꢀd½CATꢅt K K ½CATꢅ ½RuðIIIÞꢅ½OH ꢅ
1
2
t
Rate ¼
¼
(10)
ꢀ
dt
½H2Oꢅ þ K1½OH ꢅ
Rate law (10) is in good agreement with the experimental
results.
0
Since rate ¼ k [CAT] , Eqn (10) can be transformed into
t
ꢀ
K1k2½RuðIIIÞꢅ½OH ꢅ
ꢀ4
ꢀ3
ꢀ3
ꢀ3
k0 ¼
different amounts of CAT (2.0 ꢄ 10 –4.0 ꢄ 10 mol dm ) at
(11)
(12)
ꢀ
ꢀ5
½H2Oꢅ þ K1½OH ꢅ
constant amounts of RuCl (1.0 ꢄ 10 mol dm ) and NaOH
3
ꢀ
2
ꢀ3
(
1.0 ꢄ 10 mol dm ) at 308 K. The absorbance of these
1
½H2Oꢅ
1
¼
þ
ꢀ
solutions was measured at 338 nm. The absorbance of the
solution in the absence of CAT was also measured at
the same wavelength. The difference of these absorbances (with
and without CAT) gives the differential absorbance, DA. A plot of
0
k
K1k2½RuðIIIÞꢅ½OH ꢅ k2½RuðIIIÞꢅ
Based on Eqn (12), a plot of 1/k versus 1 /[OH ] at constant
0
ꢀ
[CAT]
linear (r ¼ 0.9822). From the values of slope and intercept of such
a plot, the equilibrium constant (K ) and decomposition
0 0
, [Gly-Gly] , [Ru(III)] and temperature was found to be
1
/DA versus 1/[CAT] was linear (r ¼ 0.9944) with an intercept
suggesting the formation of a 1:1 complex between CAT and
Ru(III) catalyst. Similar type of behaviour for the formation of
1
4
constant (k
11.1 dm mol
2
) were calculated and found to be 7.90 ꢄ 10 and
[
18,32,33]
3
ꢀ1 ꢀ1
s , respectively.
complex has been reported in earlier works.
Further, the
Figure 3. UV–Vis spectra of (A) CAT, (B) Ru(III) and (C) CAT þ Ru(III) in NaOH medium
J. Phys. Org. Chem. 2008, 21 844–858
Copyright ß 2008 John Wiley & Sons, Ltd.
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R. V. JAGADEESH AND PUTTASWAMY
Scheme 1. A detailed mechanistic scheme for Ru(III)-catalysed oxidation of Gly-Gly by CAT
The proposed mechanism and the derived rate law are
substantiated by the following experimental facts:
For a reaction involving a fast pre-equilibrium H or OH ion
strength on the rate for a reaction involving two ions is given by
the relationship:
þ
ꢀ
þ
ꢀ
0
1=2
transfer, the rate increases in D O since D O and OD are two to
log k ¼ log k þ 1:02Z Z I
(13)
2
3
0
A B
[
34–36]
three times stronger acids and stronger bases,
respectively,
þ
ꢀ
than H
3
O
and OH ions. In the present study, the observed
O) < 1 is due to the greater
basicity of OD compared to OH and is conforming to the fact.
A B 0
Here, Z and Z are the valency of the ions A and B, and k and k
0
0
O)/k (D
ꢀ
solvent isotope effect of k (H
2
2
are the rate constants in the presence and absence of the added
0
ꢀ
1/2
electrolyte, respectively. A plot of log k against I should be
However, the magnitude of increase of rate in D
2
O is small
linear with a slope of 1.02 Z
A B A B
Z . If Z and Z have similar signs, the
0
0
(
k (H
2
O)/k (D
2
O) ¼ 0.88) compared to the expected value of 2 to 3
quantity Z Z is positive and the rate increases with the ionic
A
B
times greater, which can be attributed to the fractional-order
strength, having a positive slope, while if the ions have dissimilar
charges, the quantity Z is negative and the rate would
ꢀ
dependence of rate on [OH ].
A B
Z
The ionic strength (I) effect on the reaction rates has been
described according to the theory of Bronsted and Bjerrum,
which postulates the reaction through the formation of an
activated complex. According to this theory, the effect of ionic
decrease with the increase in ionic strength, having a negative
slope. In the present case, a primary salt effect is observed as the
rate decreases with increase in ionic strength of medium,
[
37]
[
37]
supporting the involvement of ions of opposite sign in the
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J. Phys. Org. Chem. 2008, 21 844–858

OXIDATION OF GLY-GLY BY SODIUM N-CHLORO-P-TOLUENESULFONAMIDE
0
rate-limiting step (Scheme 1). The Debye–Huckel plot of log k
than without Ru(III). Further, the catalyst Ru(III) suitably modifies
the reaction path by lowering the energy of activation (Table 4).
1
/2
against I gave a straight line with a slope of ꢀ1.5. In the present
system, two positive ions and one negative ion are involved in the
rate-limiting step (Scheme 1) and the expected slope of ꢀ2 has
not been found. This may be due to the fact that the ionic
strength employed in the present investigation is beyond the
formal Debye–Huckel limiting law. Alternatively, there could be
formation of ion pairs in concentrated solutions, as suggested by
[
45]
It has been pointed out by Moelwyn-Hughes
that, even in
presence of the catalyst, the uncatalysed reactions also proceed
simultaneously, so that
x
k1 ¼ k0 þ KC½catalystꢅ
(15)
[
37]
Here, k
obtained in the presence of Ru(III) catalyst and k
uncatalysed reaction, K is the catalytic constant and x is the order
1
is the observed pseudo-first-order rate constant
Bjerrum.
0
is that for the
In the present case, addition of methanol to the reaction
mixture increased the reaction rate. The effect of changing
solvent composition on the rate of reaction has been described in
C
of the reaction with respect to [Ru(III)]. In the present
investigation, ‘x’ value was found to be unity. Then the value
0
[
38–44]
detail in various monographs
and a plot of log k versus 1/D
of K
C
is calculated using the Eqn (16):
was linear, with a positive slope. The dependence of the rate
constant on the dielectric constant of the medium is given by the
following:
k0
KC ¼ k1 ꢀ
(16)
x
½
RuðIIIÞꢅ
ꢀ
ꢁ
NZAZBe2
0
0
The values of K have been evaluated at different temperatures
C
ln k ¼ ln k ꢀ
(14)
0
DRTr
6
(298–318 K) and K was found to vary with temperature. Further, a
C
plot of log K
C
versus 1/T was linear (r ¼ 0.9992) and the values of
0
In this equation, k is the rate constant in a medium of infinite
0
energy of activation and other activation parameters for the
Ru(III) catalyst were computed and are summarised in Table 5.
The proposed mechanism is supported by the observed
moderate values of energy of activation and other thermodyn-
amic parameters. The fairly high positive values of the free energy
of activation and of the enthalpy of activation suggest that the
transition state is highly solvated, while fairly high negative
entropy of activation (Table 4) indicates the formation of a rigid
associated transition state. Addition of the reduction product of
CAT, PTS, did not alter the rate, indicating its non-involvement in
the pre-equilibrium with the oxidant. Similarly, addition of halide
ions has no effect on the rate indicating that there is no role for
halide ions in the reaction. All these observations support the
proposed mechanism.
dielectric constant, Z
A B
e and Z e are the total charges on the ions A
and B, r is the radius of the activated complex, R, T and N have0
6¼
their usual meanings. This equation predicts a linear plot of log k
versus 1/D with a negative slope if the charges on the ions are of
the same sign and a positive slope if they are of opposite sign. The
positive dielectric effect observed in the present study (Table 3)
[
41]
clearly supports
the involvement of dissimilar charges in the
rate-limiting step (Scheme 1).
It was felt reasonable to compare the reactivity of CAT towards
Gly-Gly in the absence of Ru(III) catalyst, under identical
experimental conditions, in order to evaluate the catalytic
efficiency of Ru(III). The reactions were carried out at different
temperatures (298–318 K) and from the plots of log k versus 1/T
0
(r > 0.9914), activation parameters are evaluated for the
uncatalysed reactions also (Table 4). However, the Ru(III)-
catalysed reactions were found to be about 14 times faster than
uncatalysed reactions. The activation parameters evaluated for
the catalysed and uncatalysed reactions explain the catalytic
effect on the reaction. The catalyst Ru(III) forms a complex (X) with
the oxidant, which increases the oxidising property of the oxidant
Mechanism and rate law of Os (VIII) catalysis
Osmium tetroxide is known to be an efficient catalyst in the
oxidation of several organic compounds by various oxidants in
[
29,46–48]
aqueous alkaline medium.
It has been shown that
osmium is stable in its þ8 oxidation state and exists in the
C C
Table 5. Values of catalytic constant (K ) at different temperatures and activation parameters calculated using K values
K
C
Temperature (K)
Ru(III)
Os(VIII)
Pd(II)
Pt(IV)
2
3
3
3
3
98
03
08
13
18
27.3
35.2
53.5
72.7
102
5.57
7.59
9.58
13.2
16.8
0.61
1.00
1.52
2.41
3.70
2.98
4.42
6.84
10.4
14.5
ꢀ
1
E
a
(kJ mol
)
51.0
39.2
68.2
59.8
6¼
ꢀ1
ꢀ1
DH (kJ mol
DG (kJ mol
DS (J K mol
log A
)
)
48.4 ꢃ 0.01
65.5 ꢃ 0.16
ꢀ54.6 ꢃ 0.13
10.3 ꢃ 0.10
36.7 ꢃ 0.06
69.7 ꢃ 0.14
ꢀ106 ꢃ 0.14
7.66 ꢃ 0.01
65.5 ꢃ 0.01
74.5 ꢃ 0.12
ꢀ28.9 ꢃ 0.10
11.7 ꢃ 0.11
57 ꢃ 0.1
70.3 ꢃ 0.24
ꢀ43.0 ꢃ ꢀ0.61
10.6 ꢃ 0.10
6¼
6¼
ꢀ1
ꢀ1
)
ꢀ3
ꢀ3
¼ 1.00 ꢄ 10ꢀ2 moldmꢀ3; [NaOH] ¼ 1.00 ꢄ 10ꢀ2 moldmꢀ3; [catalyst]¼ 1.00 ꢄ 10ꢀ5 mol dm
ꢀ3
[CAT]
0
¼ 1.00 ꢄ 10 mol dm ; [Gly-Gly]
0
;
ꢀ
3
I ¼ 0.30 mol dm (in case of Ru(III) catalysis).
J. Phys. Org. Chem. 2008, 21 844–858
Copyright ß 2008 John Wiley & Sons, Ltd.
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R. V. JAGADEESH AND PUTTASWAMY
Figure 4. UV–Vis Spectra of (A) CAT, (B) Gly-Gly and (C) CAT þGly-Gly, (D) Pd(II) and (E) Pd(II) þ CAT þGly-Gly in NaOH medium
[
49–55]
following equilibria:
III
By substituting for [X ] from Eqn (20) into Eqn (21), the
following rate law was obtained:
OsO4 þ OH þ H2O Ð ½OsO4ðOHÞðH2Oꢅꢀ
OsO4ðOHÞðH2OÞꢅ þ OHꢀ Ð ½OsO4ðOHÞ ꢅ þ H2O
ꢀ
(17)
ꢀ
K5K6K7k8½CATꢅ ½Gly ꢀ Glyꢅ½OsðVIIIÞꢅ½OH ꢅ
ꢀ
2ꢀ
t
½
(18)
Rate ¼
2
ꢀ
ꢀ
½
H2Oꢅ þ K5½OH ꢅ þ K5K6½Gly ꢀ Glyꢅ½OH ꢅf1 þ K7½OsðVIIIÞꢅg
ꢀ
2ꢀ
The complexes [OsO
4
(OH)(H
2
O)] and [OsO
4
(OH)
2
]
, which
, with octahedral geometries
(22)
2
ꢀ
can be reduced to [OsO
are less likely to form higher coordination species with the
oxidant/substrate. It is more realistic to postulate OsO , which has
as the active catalyst species than can
2
(OH)
4
]
0
Since rate ¼ k [CAT] , Eqn (22) can be transformed into
t
ꢀ
4
K5K6K7K8½Gly ꢀ Glyꢅ½OsðVIIIÞꢅ½OH ꢅ
k0 ¼
[
50]
tetrahedral geometry,
ꢀ
ꢀ
½
H2Oꢅ þ K5½OH ꢅ þ K5K6½Gly ꢀ Glyꢅ½OH ꢅf1 þ K7½OsðVIIIÞꢅg
effectively form a complex with the oxidant/substrate species.
Spectroscopic evidence for the complex formation between
CATand Gly-Gly was obtained from UV–Vis spectra of Gly-Gly, CAT
and a mixture of both (Fig. 4). Absorption maxima in alkaline
medium appear at 236 nm for Gly-Gly, 223 nm for CATand 227 nm
for a mixture of both. A bathochromic shift of 4 nm from 223 to
27 nm of CAT suggests that complexation occurs between CAT
and Gly-Gly.
Furthermore, the first-order dependence of rate on [CAT]
(
23)
Scheme 2 and the rate law (23) are supported by the following
facts:
0
0
Solvent isotope studies show that k (H O)/k (H O) < 1. This is
generally correlated with the fact that OD ion is a stronger base
than OH and in base catalysed reactions, enhancement of rate
in D O medium is expected
present study. In the present investigation, a plot of log k versus
1/D is linear with a positive slope supporting the proposed
Scheme 2, where a positive ion and a dipole
2
2
ꢀ
ꢀ
2
[
34–36]
2
and the same is noticed in the
0
0
and
, [OH ] and
ꢀ
fractional-order dependence on each of [Gly-Gly]
Os(VIII)] indicate that the intermediate complex formed from the
0
[
43,56]
[
are involved in
oxidant and substrate, which interacts with the catalyst. The
possible oxidising species here would be TsNCl . In the light of
these considerations, the Gly-Gly-CAT oxidation mechanism is
formulated as given in Scheme 2 to explain all the observed
experimental results.
the rate-limiting step. The negligible influence of variation of the
ionic strength, addition of PTS and halide ions on the rate of the
reaction, and also the evaluated activation parameters are in
good agreement with the mechanism proposed and the rate law
derived.
ꢀ
The total effective concentration of CAT is
The reactivity of CAT towards Gly-Gly in the absence of Os(VIII)
catalyst was compared with the Os(VIII)-catalysed reaction, under
identical set of experimental conditions. Rate constants revealed
that the Os(VIII)-catalysed reactions are 24-fold faster than
ꢀ
II
III
½
CATꢅ ¼ ½TsNHClꢅ þ ½TsNCl ꢅ þ ½X ꢅ þ ½X ꢅ
(19)
t
ꢀ
II
By substituting for [TsNHCl], [TsNCl ] and [X ] from steps (i), (ii)
III
uncatalysed reactions (Table 4). The values of K
at different temperatures and from a plot of log K
C
were determined
versus 1/T
and (iii) of Scheme 2 in Eqn (19) and solving for [X ], we get
C
ꢀ
K5K6K7½CATꢅ ½Gly ꢀ Glyꢅ½OsðVIIIÞꢅ½OH ꢅ
(r ¼ 0.9948), values of activation parameters for Os(VIII) catalyst
III
t
½
X ꢅ ¼
ꢀ
ꢀ
were computed (Table 5).
½
H2Oꢅ þ K5½OH ꢅ þ K5K6½Gly ꢀ Glyꢅ½OH ꢅf1 þ K7½OsðVIIIÞꢅg
(
20)
Mechanism and rate law of Pd(II) catalysis
From the slow and rds of Scheme 2,
Palladium (II) chloride catalysis has been observed during various
redox reactions and the reactions in the presence of Pd(II)
have also shown a complex kinetics. Generally, the palladium
ꢀ
d½CATꢅt
III
Rate ¼
¼ k8½X ꢅ
(21)
dt
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 844–858

OXIDATION OF GLY-GLY BY SODIUM N-CHLORO-P-TOLUENESULFONAMIDE
Scheme 2. A detailed mechanistic scheme for Os(VIII)-catalysed oxidation of Gly-Gly by CAT
complexes are somewhat less stable, from both kinetic and
thermodynamic sense, than their platinum analogous. It is known
medium for Pd(II) catalysis. Absorption maxima were appeared
at 223 nm for CAT, 236 nm for Gly-Gly and 227 nm for mixture
of both (Fig. 4). The formation of other complex between
CAT þGly-Gly mixture and Pd(II) was also informed from UV–Vis
spectral studies. Absorption maxima in alkaline medium
appeared at 356 nm for Pd(II), 227 nm for CAT þGly-Gly and
340 nm for Pd(II) and CAT þGly-Gly mixture confirmative the
existence of complex between Pd(II) and CAT þGly-Gly mixture.
The complex formation studies have been made for Pd(II) and
CAT þGly-Gly mixture at 340 nm in alkaline medium. A plot of 1/
DA versus 1/[CAT] (r ¼ 0.9910) with an intercept suggests the
formation of 1:1 complex between Pd(II) and CAT þGly-Gly
mixture. Further, the 1:1 complex formation was also evidenced
[
13,57–59]
to exist as different complexes in alkaline solutions
and the possible Pd(II) complex species are [Pd(OH)Cl
2
ꢀ
,
3
]
2ꢀ
2ꢀ
2ꢀ
[
[
Pd(OH) Cl ] and [Pd(OH)4] . The species [Pd(OH) Cl] and
2
2
ꢀ
3
2
Pd(OH)
4
]
are not commonly found as they are insoluble.
ꢀ
Further, the rate increases with the increase in [OH ] and there
was no effect of [Cl ] on the rate of reaction which clearly rules
ꢀ
2ꢀ
2ꢀ
out [Pd(OH)Cl
3
]
as the reactive species. Hence, [Pd(OH)
2
Cl ]
2
complex ion has been assumed to be the reactive species in the
present study.
UV–Vis spectral studies revealed that there is transient
existence of complex between CAT and Gly-Gly in alkaline
J. Phys. Org. Chem. 2008, 21 844–858
Copyright ß 2008 John Wiley & Sons, Ltd.
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R. V. JAGADEESH AND PUTTASWAMY
Scheme 3. A detailed mechanistic scheme for Pd(II)-catalysed oxidation of Gly-Gly by CAT
from the linearity of the plot log 1/DA versus log 1/[CAT]
From the slow and rds of Scheme 3,
(
r ¼ 9899). From the slope and intercept of the plot 1/DA versus 1/
[
CAT], the value of formation constant, K was found to be
ꢀd½CATꢅt
V
Rate ¼
¼ k12½X ꢅ½PdðIIÞꢅ
(26)
2
3
.5 ꢄ 10 .
dt
Furthermore, the kinetic data led the authors to assure that
V
ꢀ
By substituting for [X ] from Eqn (25) into Eqn (26), the
following rate law is derived:
TsNCl is the reactive species of CAT. On the basis of the
experimental findings, Scheme 3 is proposed for CAT oxidation of
Gly-Gly in the presence of Pd(II) in alkaline medium.
The total effective concentration of CAT is
ꢀ
K10K11K12½CATꢅ ½Gly ꢀ Glyꢅ½PdðIIÞꢅ½OH ꢅ
t
Rate ¼
(27)
ꢀ
ꢀ
½
H2Oꢅ þ K10½OH ꢅ þ K10K11½Gly ꢀ Glyꢅ½OH ꢅ
ꢀ
V
½
CATꢅ ¼ ½TsNHClꢅ þ ½TsNCl ꢅ þ ½X ꢅ
(24)
The rate law (27) satisfies all the experimental observations and
hence the proposed mechanism and the derived rate law are
t
ꢀ
By substituting for [TsNHCl] and [TsNCl ] from steps (i) and (ii)
valid.
Since rate ¼ k [CAT]
V
0
of Scheme 3 in Eqn (24) and solving for [X ], one gets
t
, Eqn (27) can be transformed into
ꢀ
ꢀ
K10K11K12½CATꢅ ½Gly ꢀ Glyꢅ½OH ꢅ
K10K11K12½Gly ꢀ Glyꢅ½PdðIIÞꢅ½OH ꢅ
t
k0 ¼
Rate ¼
(25)
(28)
ꢀ
ꢀ
ꢀ
ꢀ
½
H2Oꢅ þ K10½OH ꢅ þ K10K11½Gly ꢀ Glyꢅ½OH ꢅ
½H2Oꢅ þ K10½OH ꢅ þ K10K11½Gly ꢀ Glyꢅ½OH ꢅ
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J. Phys. Org. Chem. 2008, 21 844–858

OXIDATION OF GLY-GLY BY SODIUM N-CHLORO-P-TOLUENESULFONAMIDE
Figure 5. UV–Vis spectra of (A) CAT, (B) Pt(IV) and (C) Pt(IV) þ CAT in NaOH medium
of several substrates using various oxidants in alkaline med-
ium.
Rate of reaction decreases on decreasing dielectric constant of
the medium. This observed solvent effect leads to the reported
[
60–62]
[
43]
The formation of a complex between Pt(IV) and oxidant was
evidenced from UV–Vis spectra of both Pt(IV), and Pt(IV)-CAT in
which a shift of Pt(IV) from 360 to 347 nm was observed,
indicating the formation of a complex (Fig. 5). According to
Eqn (6), a plot of 1/DA versus 1/[CAT] (r ¼ 0.9912) with an intercept
suggests the formation of 1:1 complex between Pt(IV) and CAT.
Further, the plot of log 1/DA versus log 1/[CAT] was also linear
conclusion,
that the decrease in dielectric constant should
decrease the reaction rate for the reaction involving a negative
ion and dipolar molecule as shown in Scheme 3, which
substantiates the proposed mechanism. The increase of reaction
rate in D O medium supports the proposed mechanism since it is
2
ꢀ
ꢀ[34–36]
well known that OD is a stronger base than OH
ion and
hence exerts a stronger acceleration effect on the reaction. The
proposed mechanism is also supported by the activation
parameters and also negligible effect of PTS, halide ions and
ionic strength on the rate of the reaction. Further, energy of
activation of Pd(II)-catalysed and -uncatalysed reactions were
computed and it was found that Pd(II)-catalysed reactions were
nearly eightfold faster than uncatalysed reactions (Table 4). The
(
[
r ¼ 9816). From the slope and intercept of the plot 1/DA versus 1/
CAT], the value of formation constant, K, of the complex was
2
found to be 4.29 ꢄ 10 .
ꢀ
Based on the experimental results, it is likely that TsNCl itself
acts as the reactive oxidant species in the present case also.
Considering the above facts and all the observed experimental
data, the reaction Scheme 4 can be suggested for Pt(IV)-catalysed
oxidation of Gly-Gly by CAT in alkaline medium.
C
values of K and activation parameters for Pd(II) catalyst were
determined (Table 5) by the plot of log K versus 1/T (r ¼ 0.9981).
C
The total effective concentration of CAT is
ꢀ
VII
VIII
Mechanism and rate law of Pt(IV) catalysis
½
CATꢅ ¼ ½TsNHClꢅ þ ½TsNCl ꢅ þ ½X ꢅ þ ½X
ꢅ
(32)
t
Pt(IV) catalysis during variety of redox-reactions is well reported
ꢀ
VII
By substituting for [TsNHCl], [TsNCl ] and [X ] from steps (i), (ii)
and (iii) of Scheme 4 in Eqn (32) and solving for [X ], we get
[
13,33,60,61]
in the literature.
Chloroplatinic acid, H (PtCl ), is the
VIII
2
6
starting material in platinum(IV) chemistry. In aqueous solution,
[
13,33,62]
ꢀ
chloroplatinic acid ionises
as follows:
K14K15K16½CATꢅ ½PtðIVÞꢅ½OH ꢅ½H2Oꢅ
VIII
t
ꢀ
½
X
ꢅ ¼
ꢀ
½
TsNH2ꢅf½H2Oꢅ þ K14½OH ꢅ þ K14K15½PtðIVÞꢅ½OH ꢅg þ 1
H2½PtCl6ꢅ Ð ½PtCl6ꢅ ꢀ þ 2H
2
þ
(29)
(
33)
2
ꢀ
In an alkaline medium (pH > 8), [PtCl ] changes to [PtCl5
6
2
ꢀ
[33,60,62]
From the slow and rds (iv) of Scheme 4,
(
OH)] in a fast step
and follows as
PtCl6ꢅ ꢀ þ OHꢀ Ð ½PtCl5ðOHÞꢅ2ꢀ þ Clꢀ
2
ꢀd½CATꢅt
(30)
VIII
(34)
½
Rate ¼
¼ k17½X ꢅ
dt
ꢀ
2ꢀ
Further ligand (Cl ) replacements from [PtCl
5
(OH)] is also
VIII
By substituting for [X ] from Eqn (33) into Eqn (34), the
following rate law is governed:
[
33,60,62]
reported:
½
PtCl5ðOHÞꢅ ꢀ þ OHꢀ Ð ½PtCl4ðOHÞ2ꢅ2ꢀ þ Clꢀ
2
(31)
ꢀ
K14K15K16k17½CATꢅ ½PtðIVÞꢅ½OH ꢅ½H2Oꢅ
t
Rate ¼
ꢀ
ꢀ
However, the dihydroxy platinum (IV) species is quite
½TsNH ꢅf½H Oꢅ þ K ½OH ꢅ þ K K ½PtðIVÞꢅ½OH ꢅg þ 1
2
2
14
14 15
[
33,63]
unstable
in aqueous solutions and therefore consequently
(
35)
2
ꢀ
under the present experimental conditions [PtCl (OH)] may act
5
as the reactive species of Pt(IV) in alkaline medium. Similar
equilibrium have been reported in the Pt(IV)-catalysed oxidation
The rate law (35) is in complete agreement with the
experimental observations.
J. Phys. Org. Chem. 2008, 21 844–858
Copyright ß 2008 John Wiley & Sons, Ltd.
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R. V. JAGADEESH AND PUTTASWAMY
Scheme 4. A detailed mechanistic scheme for Pt(IV)-catalysed oxidation of Gly-Gly by CAT
0
Since rate ¼ k [CAT] , Eqn (35) can be transformed into Eqn (36):
(Table 5). Pt(IV)-catalysed reactions showed 10-fold faster than
t
0
the uncatalysed reactions (Table 4). The values of k obtained
0
ꢀ
ꢀ4 ꢀ1
s ) and by the plot of k versus
K14K15K16k17½CATꢅ ½PtðIVÞꢅ½OH ꢅ½H2Oꢅ
from the experiment (0.41 ꢄ 10
k0 ¼
t
ꢀ
ꢀ
ꢀ4 ꢀ1
s ) were consistent with each other,
indicating that both Pt(IV)-catalysed and -uncatalysed reactions
take place simultaneously.
½
TsNH2ꢅf½H2Oꢅ þ K14½OH ꢅ þ K14K15½PtðIVÞꢅ½OH ꢅg þ 1
Pt(IV) (0.39 ꢄ 10
(
36)
The observed dielectric and isotope effects are in accordance
We thought, it is worthwhile to compare the rate of oxidation
of Gly-Gly in the presence of these four catalysts with that of
uncatalysed reaction (in the absence of catalyst), under identical
experimental conditions. It was found that the catalysed
reactions are 7- to 24-fold faster than the uncatalysed reaction.
For the catalysed reactions, it is seen from the Table 4 that the
activation energy is highest for the slowest reaction and vice
[
43]
[34]
with the theories of Amis
and Collins and Bowman,
respectively. The negligible influence of ionic strength of the
medium and of added halide ions on the rate of the reaction, and
also the observed activation parameters further corroborate with
the suggested mechanism. Catalytic constants and activation
parameters with reference to Pt(IV) catalyst have been computed
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OXIDATION OF GLY-GLY BY SODIUM N-CHLORO-P-TOLUENESULFONAMIDE
versa. From the inspection of rate constants and the values of
energy of activation (Table 4), the relative reactivity of these
catalysts during the oxidation of Gly-Gly by CAT in alkaline
medium is in the order: Os(VIII) > Ru(III) > Pt(IV) > Pd(II). This may
be attributed to the d-electronic configuration of the metal ions.
acts as efficient catalysts in the oxidation of Gly-Gly brought
about by CAT in alkaline medium.
0
Acknowledgements
Osmium having d electronic configuration has greater catalytic
efficiency to oxidise the substrate compared to the other metal
ions used in the present study. Thus, the catalytic efficiency
decreases as the number of electrons in the d-orbital increases.
We gratefully acknowledge the UGC-DRS programme of our
Department for the encouragement and highly thankful to
Professor B. S. Sheshadri and Professor N. M. Nanje Gowda for
their helpful discussions.
8
Pd(II) having d electronic configuration is expected to have least
catalytic efficiency among the catalysts used. It is likely that
during the course of the reaction the metal ion momentarily
undergo reduction when the oxidant/oxidant-substrate complex
is attached to the metal ions and after this the metal ion gets back
to its original valence state as shown in Schemes 1–4. Ru(III) and
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