Kinetics and mechanism of ruthenium(III) catalyzed oxidation of L-proline by copper(III): A free radical intervention and decarboxylation

Article abstract of DOI:10.1134/S0036024406070053

The kinetics of ruthenium(III) catalyzed oxidation of L-proline by diperiodatocuprate(III) (DPC) in alkaline medium at constant ionic strength (0.10 mol dm^-3) has been studied spectrophotometrically using a rapid kinetic accessory. The reaction showed first order kinetics in [DPC] and [Ru^III] and apparently less than unit order dependence each in L-proline and alkali concentrations. A mechanism involving the formation of a complex between the L-proline and the hydroxylated species of ruthenium (III) has been proposed. The active species of oxidant and catalyst were [Cu(OH) ₂ (H₃IO₆)₂ (H₂IO ₆)₂]^4- and [Ru (H₂O) ₅OH]²⁺ respectively. The reaction constants involved in the mechanism were evaluated. The activation parameters were computed with respect to the slow step of the mechanism and discussed. Nauka/Interperiodica 2006.

Full text of DOI:10.1134/S0036024406070053

ISSN 0036-0244, Russian Journal of Physical Chemistry, 2006, Vol. 80, No. 7, pp. 1029–1033. © Pleiades Publishing, Inc., 2006.
CHEMICAL KINETICS
AND CATALYSIS
Kinetics and Mechanism of Ruthenium(III) Catalyzed Oxidation
of L-proline by Copper(III):
A Free Radical Intervention and Decarboxylation1
M. I. Hiremath and S. T. Nandibewoor
Postgraduate Department of Studies in Chemistry, Karnatak University, Dharwad-580 003, India
e-mail: stnandibewoor@yahoo.com
Received July 18, 2005
Abstract—The kinetics of ruthenium(III) catalyzed oxidation of L-proline by diperiodatocuprate(III) (DPC) in
alkaline medium at constant ionic strength (0.10 mol dm–3) has been studied spectrophotometrically using a
rapid kinetic accessory. The reaction showed first order kinetics in [DPC] and [RuIII] and apparently less than
unit order dependence each in L-proline and alkali concentrations. A mechanism involving the formation of a
complex between the L-proline and the hydroxylated species of ruthenium (III) has been proposed. The active
species of oxidant and catalyst were [Cu(OH)2 (H3IO6)2 (H2IO6)2]4– and [Ru (H2O)5OH]2+ respectively. The
reaction constants involved in the mechanism were evaluated. The activation parameters were computed with
respect to the slow step of the mechanism and discussed.
DOI: 10.1134/S0036024406070053
The periodate and tellurate complexes of copper in tion of acetone to various aldehydes with high ste-
reospecificity.
its trivalent state have been extensively used in the anal-
ysis of several organic compounds [1]. The kinetics of
self-decomposition of these complexes has been stud-
ied in some detail [2]. Diperiodatocuprate(III) (DPC) is
a versatile one-electron oxidant for various organic
compounds in alkaline medium. Its use as an analytical
reagent is now well recognized, and it is also used in
Ruthenium(III) acts as an efficient catalyst in many
redox reactions [11] involving different complexities
due to the formation of different intermediate com-
plexes, free radicals, and multiple oxidation states of
ruthenium [12]. The uncatalyzed reaction between
L-proline and DPC in alkaline medium has been stud-
evaluation of amino acids [3]. Movius [3] reported the ied previously [13]. A very small amount of ruthe-
nium(III) is sufficient to catalyze the reaction, and a
variety mechanisms are possible. Herein, we describe
the results of the title reaction in order to understand the
active species of the oxidant, reductant, and catalyst in
such a medium and arrive at a plausible mechanism.
reactivity of some alcohols with DPC. Copper(III) has
been shown to be an intermediate in the CuII catalyzed
oxidation of amino acids by peroxydisulphate [5]. The
use of diperiodatocuprate(III) as an oxidant in alkaline
medium is new and restricted to a few cases, due to the
fact of its limited solubility and stability in aqueous
medium [6]. Moreover, when the copper(III) periodate
complex is an oxidant, since multiple equilibria
between the different copper(III) species are involved,
it needs to be known which species is the active oxi-
dant.
EXPERIMENTAL
All chemicals used were of reagent grade. Stock
solution of L-proline (sd-fine chem) was prepared by
dissolving the appropriate amount of recrystallized
sample in double distilled water. The copper(III) perio-
date complex was prepared by the standard procedure
Amino acids have been oxidized by a variety of oxi-
dizing agents [7]. The oxidation of amino acids is of [14]. The purity of the complex was checked by its UV–
visible spectrum, which showed a broad absorption
band at 415 nm. The aqueous solution of copper(III)
was standardized by the back titration [15] method. CuII
solution was made by dissolving a known amount of
copper sulphate (BDH) in distilled water. Periodate
solution was prepared by weighing out the required
amount of sample in hot water and kept for 24 h. Its
concentration was ascertained iodometrically [16] at
neutral pH by phosphate buffer. The ruthenium(III)
solution was prepared by dissolving a known weight of
interest, as the oxidation products differ for different
oxidants [8, 9]. The study of amino acids is important
because of their biological significance and selectivity
relative to the oxidant to yield different products.
L-proline is a nonessential amino acid and an important
constituent of collagen. As per a recent report [10],
L-proline is considered to be the world’s smallest natu-
ral enzyme. It is used in catalyzing the aldol condensa-
1
The text was submitted by the authors in English.
1029

1030
HIREMATH, NANDIBEWOOR
1/[OH–] × 10–1, dm3 mol–1
Since the initial reaction was too fast to monitor by
the usual method, measurements were made on a Hita-
chi 150–20 spectrophotometer connected to a rapid
kinetic accessory (HI-TECH SFA-12). All kinetic mea-
surements were performed under pseudo-first-order
conditions with [L-proline] excess over [DPC] at a con-
stant ionic strength (0.10 mol dm–3). The reaction was
initiated by mixing previously thermostatted solutions
of DPC and L-proline, which also contained the neces-
sary quantities of RuIII, KOH, and KNO3. The first-
order rate constants (kc) were evaluated by plots of
log[Absorbance] versus time. The first-order plots in
almost all cases were linear up to 80% completion of
the reaction, and kc were reproducible within an 5%
error. Since periodate is present in excess in DPC, the
possibility of oxidation of L-proline by periodate in
alkaline medium at 25°C was tested. The progress of
the reaction was followed iodometrically. However, it
was found that there was no significant reaction under
the experimental conditions employed compared to the
DPC oxidation of L-proline.
0
10
5
6
0
2
4
5
4
3
2
1
6
0
1
2
3
4
1/[L-proline] × 10–3, dm3 mol–1
III
Verification of rate law (3) [Ru ]/k versus 1/[L-proline]
c
III
–
and [Ru /k versus 1/[OH ] (Condition as in table).
c
RuCl3 (sd-fine chem) in HCl (0.20 mol dm–3) and stan-
dardized by EDTA titration [17]. All other reagents
were of analytical grade and their solutions were pre-
pared by dissolving the requisite amounts of the sam-
ples in doubly distilled water. KOH and KNO3 (BDH,
AR) were employed to maintain the required alkalin-
ity and ionic strength, respectively, in reaction solu-
tions.
RESULTS AND DISCUSSION
The reaction mixtures containing an excess DPC
concentration over L-proline, RuIII, and 0.02 mol dm–3
KOH and adjusted ionic strength (0.10 mol dm–3) was
allowed to react for 2 h at 25 0.1°C. The remaining
DPC was then analyzed spectrophotometrically. The
results showed 2 : 1 stoichiometry, as is given in the
equation
RuIII
NH–CH2–CH2–CH2–CH–COOH + 2CuIII + 2OH–
H2N–CH2–CH2–CH2–CHO + 2CuII + H2O + CO2↑.(1)
The main reaction products were identified as aminobu- tion, the reaction mixture containing acrylonitrile was
tyraldehyde by a spot test [18]. The presence of alde- kept for 24 h in an inert atmosphere. On diluting the
hyde was also confirmed by the IR spectrum showing reaction mixture with methanol, a white precipitate was
bands at 3443 cm–1 for –NH stretching, 1733 cm–1 for obtained, indicating the presence of free radical inter-
aldehyde > CO stretching, and 2956 cm–1 for aldehyde vention in the reaction. Blank experiments of either
–CH stretching, respectively. However, the other prod- DPC or L-proline alone with acrylonitrile did not
uct in alkaline medium is copper (II) hydroxide, identi- induce polymerization under the same conditions as
fied by a spot test [19] and UV–visible spectra. The those induced with reaction mixtures. Initially added
DPC concentration was varied in the range of from 2 × acrylonitrile decreases the rate, indicating free radical
10–5 to 2 × 10–4 mol dm–3, and the linearity of plots of intervention, which is the case in earlier work [20].
log [DPC] versus time (r > 0.9985, S ≤ 0.027) indicated Value kc increased with decreasing dielectric constant
a reaction order of unity in [DPC]. This was also con- (ꢀT) of the medium. The plot of logkc versus 1/ꢀT was
firmed by variation of [DPC], which did not result in linear (r > 0.9985, S ≤ 0.027), with a positive slope. The
any change in the pseudo-first-order rate constants kc rate constants increased with increasing concentration
(table). The reaction orders in [L-proline] and [ruthe- of KNO3; the plot of logkc C versus I1/2 was linear with
nium(III)] were found to be less than unity and equal to a positive slope (r > 0.9978, S ≤ 0.018). The title reac-
unity, respectively (table). Initially added reaction tion obeys the following experimental law:
products, such as copper(II) sulphate and aminobu-
tyraldehyde, did not show any significant effect on the
–d[DPC]/dt
= k[DPC][L-Proline]0.46[OH]0.77[RuIII].
rate of the reaction. The added periodate has no effect
on the rate of reaction. To test the free radical interven-
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 7 2006

KINETICS AND MECHANISM OF RUTHENIUM(III) CATALYZED OXIDATION
1031
Effect of variation of [DPC], [L-proline], [OH–], and [RuIII] on RuIII catalysed oxidation of L-proline by alkaline DPC at
25°C, I = 0.10 mol dm–3
kc × 102, s–1
[DPC–] × 104 [L-proline] × 103
[OH–]
[RuIII] × 106 kt × 102, s–1 ku × 102, s–1
experiment
(3)
0.2
0.4
0.8
1.0
2.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.6
0.6
0.6
0.6
0.6
0.3
0.6
0.8
1.0
3.0
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.01
0.02
0.04
0.08
0.10
0.02
0.02
0.02
0.02
0.02
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
4.0
2.0
4.0
8.0
10.0
20.0
1.31
1.33
1.33
1.34
1.32
0.91
1.34
1.59
1.98
2.91
0.78
1.34
2.34
3.47
4.08
0.74
1.34
2.44
3.03
6.25
0.24
0.22
0.25
0.25
0.23
0.20
0.25
0.35
0.62
0.81
0.16
0.25
0.29
0.36
0.39
0.25
0.25
0.25
0.25
0.25
1.07
1.11
1.08
1.09
1.09
0.71
1.09
1.24
1.36
2.10
0.62
1.09
2.05
3.11
3.69
0.49
1.09
2.19
2.78
6.00
1.09
1.09
1.09
1.09
1.09
0.70
1.09
1.26
1.40
1.96
0.59
1.09
1.90
3.02
3.42
0.54
1.09
2.18
2.73
5.46
These experimentally determined orders and other kinetic parameters can be accommodated in the following
scheme, which involves free radical intervention:
K1
[Cu(OH)2(H3IO6) ]3– + OH–
[Cu(OH)2(H3IO6) (H2IO6) ]4– + H2O,
2
2
2
CH2–CH2
CH2 CH–COO–
K2
[Ru(H2O) OH]2+
Complex(C) + H2O,
5
N
H
Complex(C) + [Cu(OH)2(H3IO6) (H2IO6) ]4–
2
2
CH2–CH2
.
k
+ Cu(OH)2 + H2IO36– + H3IO62– + [Ru(H2O) OH]2+ + CO2↑,
5
slow
CH2 CH
N
H
CH–CH
+ [Cu(OH)2(H3IO6) (H2IO6) ]4–
.
2
2
CH CH
N
H
fast
+ OH–
H2N–H2C–CH2–CH2–CHO + Cu(OH)2 + H2IO36– + H3IO62–.
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 7 2006

1032
HIREMATH, NANDIBEWOOR
The probable structure of complex (C) is
complex is proved kinetically by the non-zero intercept
of the plot of 1/kc versus 1/[proline] (figure) (r > 0.9935,
S ≤ 0.043). The observed modest enthalpy of activation
and relatively low value of the entropy of activation
indicate that oxidation presumably occurs by an inner
sphere mechanism. This conclusion is supported by
earlier work [22]. The DPC oxidation of L-proline in
aqueous alkaline medium proceeds at a measurable rate
in the absence of a RuIII catalyst. Hence, in the presence
of the catalyst, the reaction is understood to occur in
parallel paths with contributions from the uncatalyzed
and catalyzed paths. Thus, the total rate constants (kT)
are equal to the sum of the rate constants of the cata-
lyzed (kc) and uncatalyzed (ku) reactions.
+
O
OH
CH–CH
⎛
⎜
⎜
⎜
⎜
⎝
⎞
OH2
⎟
CH CH–C–O–Ru–OH2
⎟
.
OH2⎟
N
H
OH2
⎟
⎠
Spectral evidence for complex formation between
catalyst and substrate was obtained from the UV–Vis
spectra of the ruthenium(III) species and a mixture of
ruthenium(III) and L-proline. A bathochromic shift,
λ
max, of about 6 nm from 224 to 230 nm is observed,
together with hyperchromicity at λmax 230 nm. Analo-
gous effects upon complex formation between ruthe-
nium(III) and the substrate have been observed in other
The scheme leads to the rate law given in the equa-
investigations [21]. Furthermore, the formation of the tion:
kK1K2[L-proline][OH–][DPC][RuIII]
---------------------------------------------------------------------------------------------------------------------------------------
Wcat
=
,
(2)
1 + K1[OH–] + K2[L-proline] + K1K2[L-proline][OH–]
where Wcat = Wtotal – Wuncat, Wi is the rate of the reaction the low concentrations of DPC and RuIII used, they
(i). The terms (1 + K1[DPC]) and (1 + K2[RuIII]) also approximate to unity. Hence, Eq. (2) becomes the equa-
should be in the denominator of Eq. (1). But in view of tion
kK1K2[L-proline][OH–][RuIII]
Wcat
[DPC]
---------------------------------------------------------------------------------------------------------------------------------------
--------------- = kcat = ktotal – kuncat
=
.
(3)
1 + K1[OH–] + K2[L-proline] + K1K2[L-proline][OH–]
The above equation can be rearranged to the following (r > 0.997, S ≤ 0.050) at different temperatures. The
values of k × 104 dm3 mol–1 s–1 were 3.60 0.20, 4.10
form, which is used for verification of the rate law:
0.22, 4.74 0.28, and 5.54 0.30 at 20, 25, 30, and 35°C,
[RuIII]
1
respectively. From these data the values of Ea, ∆H#, and
-----------------------------------------------------------
--------------- =
kK1K2[L-proline][OH–]
kc
∆S# are obtained as 21.4 1.0, 18.9 1.0 kJ mol–1 and
−92 4 J K–1 mol–1, respectively; Ea, ∆H#, and ∆S# are
(4)
1
1
k
1
30.6 1.5, 29.5 1.0 kJ mol–1, and −181 10 J K–1 mol–1
for the uncatalyzed reaction [11]. The difference in the
activation parameters for the catalyzed and uncatalyzed
reactions explains the catalytic effect on the reaction.
The catalyst RuIII forms the complex with L-proline,
which shows a greater reducing property than L-proline
itself. Hence, the catalyst, RuIII, lowers the energy of
activation, i.e., it provides an alternative pathway with
lower activation parameters for the reaction.
------------------------------------
--
+
+ ------------------------ + .
kK1[OH–]
kK2[L-proline]
According to Eq. (4), the plots of [RuIII]/kobs versus
1/[L-proline] (r > 0.997, S ≤ 0.050) and [RuIII]/kc versus
1/[OH–] are linear (r > 0.997, S ≤ 0.050) which is veri-
fied in the figure. The slopes and intercepts of plots lead
to the values of K1, K2, and k, which were found to be
8.77 0.50 dm3 mol–1, 1.30 0.05 × 103 dm3 mol–1,
and 4.10 0.2 × 104 dm3 mol–1 s–1, respectively. Using
these values, the rate constants under different experi-
mental conditions were calculated by Eq. (3) and com-
pared with experimental data (table). Experimental and
calculated values agreed reasonably well, supporting
the assumptions of the scheme. The value of K1 is in
good agreement with earlier work [23]. The rate con-
stants k, of the slow step of scheme were obtained from
the intercept of the plot of [RuIII]/kobs versus 1/[L-Pro-
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