Imidazole Catalyzed Oxidations with Organic Peroxides: A Kinetic Study

Article abstract of DOI:10.1246/bcsj.77.313

Imidazole and 1-methylimidazole catalyze the oxidation of indigo dyes with m-chloroperoxybenzoic acid (MCPBA) in organic and mixed aqueous solutions. The kinetics of these reactions were studied in CH₃OH/CH₃CN and CH₃OH/H₂O solvent systems. The reaction rates decreased with the solvent polarity. The catalytically active species is proposed as an MCPBA-Im adduct which is in equilibrium with MCPBA and the imidazole. The equilibrium constant (K ～ 70) for the formation of the MCPBA-Im adduct was determined in CH₃OH/H₂O (8:2). The catalyzed rates are at least three-orders of magnitude faster than the uncatalyzed rates. The electron-rich, 1-methylimidazole, increases the rate of oxidation of indigo blue via MCPBA more than imidazole itself. A mechanism similar to the Michaelis-Menten type was proposed for these reactions. The initial products from oxidation of indigo dyes by MCPBA are the epoxides, which undergo a slow ring-opening in the presence of water to form 1,2-diol products.

Full text of DOI:10.1246/bcsj.77.313

Ó 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 313–319 (2004)
313
Imidazole Catalyzed Oxidations with Organic Peroxides:
A Kinetic Study
ꢀ
Khaled Q. Shawakfeh and Ahmad M. Al-Ajlouni
Department of Applied Chemical Sciences, Jordan University of Science and Technology,
Irbid 22110, P.O. Box 3030, Jordan
Received April 7, 2003; E-mail: aajlouni@just.edu.jo
Imidazole and 1-methylimidazole catalyze the oxidation of indigo dyes with m-chloroperoxybenzoic acid (MCPBA)
in organic and mixed aqueous solutions. The kinetics of these reactions were studied in CH3OH/CH3CN and CH3OH/
H2O solvent systems. The reaction rates decreased with the solvent polarity. The catalytically active species is proposed as
an MCPBA–Im adduct which is in equilibrium with MCPBA and the imidazole. The equilibrium constant (K ꢁ 70) for the
formation of the MCPBA–Im adduct was determined in CH3OH/H2O (8:2). The catalyzed rates are at least three-orders
of magnitude faster than the uncatalyzed rates. The electron-rich, 1-methylimidazole, increases the rate of oxidation of
indigo blue via MCPBA more than imidazole itself. A mechanism similar to the Michaelis–Menten type was proposed
for these reactions. The initial products from oxidation of indigo dyes by MCPBA are the epoxides, which undergo a slow
ring-opening in the presence of water to form 1,2-diol products.
The most common and cheapest methods for activating hy-
O
OH
drogen peroxide and molecular oxygen are to transfer them into
organic or inorganic peroxides.1–5 Organic and inorganic per-
oxides are widely used in the oxidation of many organic and in-
organic substrates. Of these, the epoxidation of olefins by per-
oxy acids is a very common reaction and useful for the synthe-
sis of many organic compounds. Mimoun2 has discussed the
possible mechanisms for the oxygen transfer from a peroxy
acid to an olefin. The most acceptable mechanism is the ‘‘but-
terfly mechanism’’, which involves a nucleophilic attack of the
olefin at the terminal oxygen atom of the hydroperoxide group
(I in Chart 1).6
This mechanism is not supported by the fact that these per-
oxy acids dissociate to their conjugate bases, where this electro-
philic oxygen carries a negative charge. In addition, the rate of
the epoxidation reaction is greatly enhanced by the acidity of
the peroxy acid (i.e., CF3CO3H > CH3CO3H and m-
ClC6H5CO3H > C6H5CO3H). Furthermore, ab initio calcula-
tions have shown that none of the peroxidic oxygen atoms of
the peroxy acids are electrophilic.7 Therefore, a nucleophilic at-
tack on a peroxy acid is more probable to occur at the electro-
philic carbon of the CO3H group. An alternative mechanism
considers the intramolecular hydrogen-bonding, which leads
to the zwitterion species (II) and the dioxirane species (III)2
as shown in Chart 2. Theoretical studies have shown that the
most stable species is the dioxirane (III).8 However, a base ad-
duct, such as pyridine, may increase the stability of II.
The rate of epoxidation of olefins by transition metal perox-
HO
R
O
OH
O
R
O
R
O
O
II
III
Chart 2.
ides has been enhanced by the addition of base adducts, such as
pyridine derivatives.9 The activation of H2O2 by Mn(III)–por-
phyrin catalysts towards olefin epoxidation has been accelerat-
ed in the presence of pyridine or imidazole axial ligands.10 It
has been demonstrated that the ꢀ-donor ability of imidazole,
which is greater than that of pyridine, leads to more stable high
valence metal complexes.11
Imidazole, an essential component of many important bio-
logical compounds (enzymes), seems to play an important role
in enhancing the activity of inorganic and organic perox-
ides.12–19 We have found that imidazole increases the reactivity
of metal peroxide species and peroxy acids toward oxidation re-
actions. In addition, imidazole is more resistant to oxidation by
peroxides than pyridine.20
Here, we are presenting the effect of imidazole and 1-meth-
ylimidazole on the oxidation of organic dyes by m-chloroper-
oxybenzoic acid (MCPBA). The reactions of MCPBA with in-
digo dyes in the presence and absence of imidazole were inves-
tigated in organic and mixed aqueous solvents. The effect of
each substrate concentration on the reaction rate was studied.
A mechanism that involves the formation of an imidazole–per-
oxide adduct in the initial step, followed by its reaction with the
organic reductant has been proposed.
O
H
Experimental
O
R
O
Materials.
Indigo (96%), tripotassium indigotrisulfonate
I
(85%), and indigo cumine (90%) dyes, 2-phenylpropene, imida-
zole, and 1-methylimidazole (Aldrich), were used without further
Chart 1.
Published on the web February 10, 2004; DOI 10.1246/bcsj.77.313

314 Bull. Chem. Soc. Jpn., 77, No. 2 (2004)
Imidazole Catalyzed Oxidation with Peroxide
purification. The percent purity for each dye was considered in the
rate constant calculations when necessary. Water was purified by a
Milipore deionization system. Methanol and CH3CN (HPLC
grade, Aldrich) were used without further purification. Stock solu-
tions of indigo dyes (0.1–0.2 mM) (1 M = 1 mol dmꢂ3) were pre-
pared in CH3OH. Solutions of the water-soluble dyes, potassium
indigotrisulfonate, and indigo cumine, (1–5 mM) were prepared
in deionized water. Stock solutions of m-chloroperoxybenzoic acid
(Aldrich, 70%), 0.1–0.2 M, were prepared in water and standard-
ized daily by iodometric titration. Stock solutions of imidazoles
(0.5–1.0 M) were prepared in CH3OH.
g (2 mmol) of MCPB and 0.036 g (0.5 mmol) of imidazole in 25
mL of CH3OH/H2O (8:2) solution. The deep blue solution was
stirred until the solution color changed to light blue or greenish-
yellow. The methanol was removed at 40 ꢃC by a rotary evapora-
tor, and the remaining aqueous solution was washed with ether
(5 ꢅ 10 mL) to remove imidazole, excess MCPBA, and benzoic
ꢃ
acid. Water was then removed by a rotary evaporator at ꢁ70 C.
The product was washed twice with CH3CN and ether (25 mL
each), and characterized by UV–visible, IR, and NMR spectrosco-
py. The major product was identified as the epoxide of the indigo
dye. The 1,2-diol was also observed as a minor product in 5–10%
yield. The imidazole oxide was also observed in low yield ꢁ5%.
The NMR data for the epoxide product 2 was in good agreement
with literature values.21 1H NMR (DMSO-d6, ꢁ/ppm): 9.1 (s,
1H), 8.29 (s, 2H), 7.76 (d, JH,H ¼ 7:4 Hz, 1H), 7.1 (d, JH,H ¼ 8:0
Hz, 1H), 4.0 (s, NH). 13C NMR (DMSO-d6, ꢁ/ppm): 193.2
(C=O), 142.5, 138.5, 132.4, 129.2, 123.2, 118.2 (Aromatic car-
bons), 81.4 (epoxide carbons).
Equilibrium and Kinetic Experiments. The determination of
the equilibrium constant for the formation of the MCPBA–Im ad-
duct was made by scanning the UV spectrum of solutions contain-
ing a constant MCPBA concentration (0.5 mM) and different Im
concentration, (0.5–200 mM). The reactions were carried out in
ꢃ
MeOH/H2O (8:2) at 25 C using a 1 cm quartz cuvett. To insure
that equilibrium was established, waiting periods of 5–10 min were
applied before scanning. Absorbance values were read at several
wavelengths in the region 240–300 nm. The data were obtained
with a Shimadzu UV-2401 spectrophotometer. All data analysis
was carried out with the KaleidaGraph computer program.
Results and Discussion
Oxidation of Indigo Dyes by MCPBA—The Uncatalyzed
Reaction. All reactions were carried out under pseudo-first-
order conditions with MCPBA in large excess over the indigo
dye. Kinetic data were collected by following the loss of the
dye by recording the absorbance change at a wavelength be-
tween 580–620 nm. The reactions were first-order in the [indi-
go] and in the MCPBA concentrations in CH3CN/CH3OH and
CH3OH/H2O. The pseudo-first-order rate constants obtained
by fitting the absorbance–time curves to Eq. 1 varied linearly
with the MCPBA concentration, indicating that the reaction
is first-order in [MCPBA] and second-order overall. The slope
Kinetic Studies.
The kinetic studies were carried out in
CH3OH/H2O (8:2) solutions at 25 ꢃC. When the solvent effect
was studied, mixtures of CH3OH/CH3CN were used. The temper-
ature was kept constant at 25 ꢄ 0:5 ꢃC throughout the entire series
of experiments. The ionic strength of the solution was not main-
tained for these reactions. Air (oxygen) had no effect on the reac-
tions and was not excluded. In all kinetic studies, however, the acid
concentration (MCPBA) was held constant. When the effect of
[MCPBA] was studied, the Hþ concentration was kept constant
by adding m-chlorobenzioc acid to the reaction solution. The effect
of the pH was studied by adding HClO4 (stock 1.0 M). Quartz cuv-
ettes with optical paths of 0.8 (VT ¼ 2:0 mL) and 1.0 cm (VT ¼ 3:0
mL) were used. The kinetic data were obtained by following the
disappearance of the blue color of the indigo dye in the region
580–620 nm using either a Shimadzu UV-2401 or a Unicam spec-
trophotometer.
is the second-order rate constant, (k2 ¼ 1:1 ꢅ 10ꢂ4
M )
ꢂ1 sꢂ1
for the oxidation of tripotassium indigotrisulfonate by MCPBA
ꢃ
in MeOH/H2O (8:2) at 25 C.
The Products. The reaction products were isolated and
identified by IR and NMR spectroscopy. The final major prod-
uct is the epoxide of indigo blue. The 13C NMR spectra showed
two new peaks at 81.1 ppm for the epoxide carbons, where the
peaks at ꢁ130 ppm for the alkene carbons disappeared. In ad-
dition, the 1,2-diol products were identified as a minor product
from this reaction. The epoxide undergoes slow ring opening by
water in the solution to give 1,2-diol (Scheme 1). A small
amount (<5% based on MCPBA) of imidazole oxide was also
observed.
Reaction mixtures were prepared in the spectrophotometer cell
with the last reagent added being MCPBA, or the reductant to op-
timize the kinetic conditions as explained later. The pseudo-first-
order rate constants were evaluated by nonlinear least squares fit-
ting of the absorbance–time curves to a single exponential equa-
tion, Eq. 1.
Abst ¼ Abs1 þ ðAbso ꢂ Abs Þ expðꢂk tÞ
ð1Þ
1
Products. Indigo blue (0.52 g, 1.0 mmol) was mixed with 0.35
Effect of Imidazoles. The reaction rates for the oxidation of
-
-
SO3
O
SO3
O
H
N
H
N
-
-
O3S
O3S
MCPBA
kuncat
-
-
N
H
N
H
O
SO3
SO3
O
O
2
1
H2O
Indigotrisulfonate
-
SO3
O
HO
H
N
-
O3S
-
SO3
N
H
OH
O
3
Scheme 1. Oxidation of indigo blue with MCPBA.

K. Q. Shawakfeh et al.
Bull. Chem. Soc. Jpn., 77, No. 2 (2004)
315
1.2
1.0
0.8
0.6
0.4
0.2
0.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
[Im] = 0.0
[Im] = 0.01 M
[Im] = 0.1 M
[Im] = 0.2 M
0
50
100
150
200
250
300
350
240
260
280
300
320
Time/ sec
Wavelength/ nm
Fig. 1. The absorbance–time curves for the oxidation of in-
digo blue (0.05 mM) with MCPBA (2.0 mM) in the pres-
ence and absence of imidazole. The absorbance change
was recorded at 600 nm due to the loss of indigo dye
Fig. 2. UV spectra of equilibrated MCPBA/1-MeImidazole
mixtures in CH3OH/H2O (8:2) at 25 ꢃC. The total MCPBA
concentration is 0.5 mM. Reading downward at 282 nm,
the 1-MeImidazole concentrations are 0.0 ( ), 10 ( ),
20 ( ), 50 ( ), 100 ( ), 200 ( ) mM. Approximate iso-
sbestic point at 269 nm.
("600 ¼ 1:9 ꢅ 104
M
ꢂ1 cmꢂ1) in CH3OH/H2O (8:2) at
ꢃ
25 C.
indigo dyes with MCPBA were greatly enhanced by the addi-
tion of imidazole and 1-methylimidazole to the reaction mix-
ture (Fig. 1). The absorbance changes with time due to the loss
of the indigo dye were curved but not exponential. Generally,
they exhibit an approach toward linearity during the first one-
third of the reaction, but become nearly exponential during
the later stages. Both the initial rates (calculated from the first
2–4% of the absorbance–time curves) and the pseudo-first-or-
der rate constants (calculated by fitting the last 10–20% of
the absorbance–time curves to Eq. 1) increase linearly with
the imidazole concentration even at high concentration ([Im]
= 0.05 M). This indicates that the reaction rate is first-order
in imidazole concentration in this range.
Reaction of MCPBA with Imidazole: Determination of
the Equilibrium Constant. The absorbance–concentration
diagram of solutions of MCPBA and imidazole (Fig. 2) is not
like a titration curve. It does, however, continue to increase
as more imidazole is added, until finally a plateau is reached.
This reaction was assumed to form a 1:1 adduct, MCPBA–
Im, in equilibrium with MCPBA and Im (Eq. 2).
3.5
3.0
2.5
2.0
1.5
0.00
0.05
0.10
0.15
0.20
[Me-Imidazole]/ M
Fig. 3. A plot of the absorbance at 280 nm of MCPBA/1-
MeImidazole solutions as a function of [1-MeImidazole]
at [MCPBA]T = 0.5 mM in MeOH/H2O (8:2) at 25 C.
The solid line is the calculated absorbance using Eq. 6a
ꢃ
N
k1
with the values of K, "1, and "2 given in Table 1.
CO3H
+
N
CO3H
NR
RN
k -1
A
Cl
Cl
of the MCPBA–Im adduct in Eq. 2a, and using the mass bal-
ance [MCPBA]T = [MCPBA] + [MCPBA–Im], we obtain
Eq. 2b.
ð2Þ
The changes in absorbance (Abs) due to the loss of MCPBA
and the formation of the MCPBA–Im adduct can be expressed
by Eq. 2a (assuming 1.0 cm path length).
Abs
"1 þ "2K½Imꢆ
1 þ K½Imꢆ
¼
ð2bÞ
[MCPBA]T
The data were obtained with a constant concentration of
MCPBA and over a range of concentrations of imidazole be-
tween 0.5 and 200 mM. The data at several wavelengths
(270–295 nm) were fit to Eq. 2b (Fig. 3). The calculated equi-
librium constants and the values of "1 and "2 at each wave-
length are summarized in Table 1.
Abs ¼ "1½MCPBAꢆ þ "2½MCPBA{Imꢆ
ð2aÞ
where "1 and "2 are the molar extinction coefficients for
MCPBA and MCPBA–Im adducts, respectively. Both, "1 and
"2, at each wavelength were determined independently when
[Im] = 0 and in the presence of a large excess of Im, respective-
ly. Substituting the equilibrium constant (K) for the formation
Kinetic Studies. The reaction profiles have suggested a

316 Bull. Chem. Soc. Jpn., 77, No. 2 (2004)
Imidazole Catalyzed Oxidation with Peroxide
Table 1. Equilibrium Constants for the Formation of
MCBPA–Im Adducts in CH3OH/H2O (8:2) at 25 ꢃC
30
25
20
15
10
5
Adduct
ꢂ/nm K1/Mꢂ1 "1/Mꢂ1 cmꢂ1 "2/Mꢂ1 cmꢂ1
MCBPA–Im
280 68 ꢄ 15
2870 ꢄ 50
1880 ꢄ 32
2970 ꢄ 45
1940 ꢄ 73
1350 ꢄ 35
290 76 ꢄ 6
43 ꢄ 12
MCPBA–MeIm 280 85 ꢄ 15
290 83 ꢄ 11
1680 ꢄ 54
175 ꢄ 21
N
k1
CO3H
N
+
CO3H
NR
RN
k -1
A
Cl
Cl
0
k2
0
5
10
15
20
25
IB
[Imidazole]/ mM
-
SO3
O
Fig. 4. A plot of the observed-first-order rate constant with
[imidazole] for the reaction of MCPBA (2.0 mM) and indi-
go blue (0.05 mM) in MeOH/H2O (8:2) at 25 C.
H
N
-
O3S
CO2H
+
ꢃ
-
N
H
O
SO3
Cl
O
Scheme 2. A reaction scheme for the oxidation of indigo
blue with MCPBA as catalyzed by imidazole.
0.6
0.5
0.4
0.3
0.2
0.1
0.0
mechanism similar to the Michaelis–Menten mechanism. It in-
volves, first, a reversible (equilibrium) reaction between
MCPBA and imidazole to form a reactive intermediate. This
step is followed by the reaction of the intermediate with the in-
digo dye, as shown in Scheme 2.
The rate of oxidation of indigo blue (IB) according to
Scheme 2 is expressed by Eq. 3.
initial rate (vi) ¼ k2½Aꢆ½IBꢆ
The rate equation was derived by means of the steady-state ap-
proximation of [A]. With the mass balance [Im]T = [Im] + [A],
the rate law can be expressed as follow:
ð3Þ
0
0.01
0.02
0.03
0.04
0.05
k1k2[Im]T½MCPBAꢆ½IBꢆ
[Indigo Blue]/ mM
vi ¼
ð4Þ
k1 þ k2½IBꢆ þ k1½MCPBAꢆ
Equation 4 can be simplified to Eq. 4a:
Fig. 5. Variation of the initial rate with [indigo blue] for the
reaction of MCPBA (1.0 mM) and indigo blue in the pres-
ence of imidazole (20 mM) in MeOH/H2O (8:2) at 25 ꢃC.
The solid line represents the calculated initial rate using
Eq. 8 with the values of k1, kꢂ1, and k2 given in Table 2.
k2[Im]T½MCPBAꢆ½IBꢆ
vi ¼
ð4aÞ
ðK1Þꢂ1 þ k2=k1½IBꢆ½MCPBAꢆ
Using the known values of K1, which were obtained independ-
ently from the reaction of the imidazole and MCPBA in the ab-
sence of the reluctant, leaves Eq. 4a with two unknown rate
constants, k1 and k2.
Variation of [Im]T. The initial rates (vi) were calculated
from experiments in which [Im]T varied over the range of
2.0–30 mM. The indigo dye and MCPBA were kept constant
at 0.05 and 1.0 mM, respectively. The initial rates showed a lin-
ear dependence on [Im]T, as expected from Eq. 4 (Fig. 4). The
rate constants k1, kꢂ1, and k2, were determined by studying the
variation of the intial rate with IB and MCPBA concentration.
ꢀAbsi
vi ¼
ð5Þ
ꢀt b ꢀ"
ꢇ ꢇ
molar absortivity of the reactants and the products at 600 nm
where b is the optical path length and ꢀ" is the difference in the
(ꢀ"600 ¼ 1:9 ꢅ 104
M
ꢂ1 cmꢂ1). The dependence of vi on
[IB], Fig. 5, was linear at [IB] < 0.01 mM. The dependence de-
creases and reaches a plateau at [IB] > 0.05 mM. This behav-
iour is common for catalytic reactions in which the ratio
k1[MCPBA]/k2[IB] varies during the reaction due to the de-
crease in [IB] with time, as shown in Eq. 4a. The data fits very
well into Eq. 4a, leading to k1 ¼ ð0:022 ꢄ 0:001Þ Mꢂ1 sꢂ1 and
k2 ¼ ð6:8 ꢄ 0:4Þ Mꢂ1 sꢂ1. The rate constant for the deforma-
Variation of [IB].
The reaction of indigo blue with
MCPBA in the presence of imidazole was studied at constant
[MCPBA] (1.0 mM) and [Im] (20 mM). The concentration of
IB was varied over the range of 0.002–0.05 mM. The initial rate
at each [IB] was calculated from the initial rate of the absorb-
ance change from the loss of IB at 600 nm:
tion of the MCPBA–Im adduct, k ¼ ð3:1 ꢄ 0:3Þ ꢅ 10ꢂ4
ꢂ1
s
ꢂ1, was calculated from the value of k1 and K1 ¼ 72 Mꢂ1
.
Variation of [MCPBA]. Values of the initial rates (vi) for

K. Q. Shawakfeh et al.
Bull. Chem. Soc. Jpn., 77, No. 2 (2004)
317
the values of the rate constants k1 and k2 for 1-methylimidazole
catalyzed reactions. The value of k was calculated from the
ꢂ1
4.0
3.0
2.0
1.0
rate constant k1 and the equilibrium constant K1. Table 2 sum-
marizes the values of all rate constants for both imidazole and
1-methylimidazole catalytic reactions. The equilibrium con-
stant for the formation of the MCPBA–MeIm adduct was a lit-
tle higher than that for MCPBA–Im formation, as shown in
Table 1. The rate constant k2 for the oxidation of indigo blue
with the MCPBA–MeIm adduct is about 4 times higher than
that with the MCPBA–Im adduct. The oxygen-transfer step,
rather than the pre-equilibrium step, is responsible for the great-
er activity of 1-methylimidazole over imidazole.
Solvent Effect. The reaction of MCPBA with indigo dye in
the presence of imidazole was carried out in three different sol-
vent mixtures, CH3OH/H2O (8:2), CH3OH/CH3CN (9:1), and
CH3OH/CH3CN (1:1). The initial rate and the observed first-
order rate constant were found to have the following order
(Table 3):
0.0
0
10
20
30
40
[MCPB]/ mM
Fig. 6. Variation of the initial rate with [MCPBA] for the re-
action of MCPBA and indigo blue (0.05 mM) in the pres-
ence of imidazole (20 mM) in MeOH/H2O (8:2) at 25 ꢃC.
The solid line represents the calculated initial rate using
CH3OH/H2O < CH3OH/CH3CN (9:1)
< CH3OH/CH3CN (1:1)
Eq. 8 with the values of k1 ¼ 0:025 Mꢂ1 sꢂ1, k
¼
ꢂ1
3:2 ꢅ 10ꢂ4
s
ꢂ1, and k2 ¼ 7:1 Mꢂ1 sꢂ1
.
The solvent polarity could be the reason for this trend. Howev-
er, CH3OH and CH3CN have similar polarity, but a different
ability to form hydrogen bonds with the oxidant and imidazole.
This order indicates that the reaction decreases with the solvent
ability to form a H-bond and with the solvent nucleophilicity.
We think that the solvent nucleophilicity is affecting the reac-
tion. The solvent (as a nucleophile) may compete with imida-
zole in binding to the carbonyl-carbocation (in the zwitterion
species (II)), which would be more stable in the presence of
a nucleophile, such as imidazole (Eq. 6). It is also possible that
protic solvents, such as water or methanol, act as a proton donor
to the imidazole and reduce its nucleophilicity.
the reaction of MCPBA with indigo blue in the presence of imi-
dazole were calculated over a [MCPBA] range of 0.5–100 mM.
Imidazole and indigo blue were held constant at 20 and 0.05
mM, respectively. The variation of the initial rates with
[MCPBA] is shown in Fig. 6. The data fits well to Eq. 4, and
gave k1 ¼ 0:025 ꢄ 0:005 Mꢂ1 sꢂ1 and k2 ¼ 7:1 ꢄ 0:3 Mꢂ1 sꢂ1
.
The value of k was calculated from k1 and the equilibrium
ꢂ1
ꢂ1
constant K1: k ¼ k1=K1ð3:2 ꢄ 0:5Þ ꢅ 10ꢂ4 sꢂ1. The rate con-
stant values obtained from variation of the indigo dye concen-
tration and that obtained from variation of [MCPBA] were in
excellent agreement.
-
O
Imidazole versus 1-Methylimidazole. Under the same
conditions, the oxidation of indigo dye by MCPBA was about
4–5 times faster in the presence of 1-methylimidazole than with
imidazole. Fitting the data obtained from variation of the initial
rates with the initial concentration of indigo dye to Eq. 4a led to
OH
HN
N
R
O
O
O
H
O
N
ð6Þ
OH
R
O
R
O
N
H
4
Table 2. Rate Constants for the Oxidation of Indigo Dye by MCBPA as Catalyzed by
ꢃ
Imidazoles in CH3OH/H2O (8:2) at 25 C
Catalyst
Imidazole
1-Methylimidazole
k1/Mꢂ1 sꢂ1
k
ꢂ1/sꢂ1
k2/Mꢂ1 sꢂ1
0:023 ꢄ 0:002
0:039 ꢄ 0:005
ð3:1 ꢄ 0:5Þ ꢅ 10ꢂ4
7:0 ꢄ 0:6
ð4:6 ꢄ 1:0Þ ꢅ 10ꢂ4
32:4 ꢄ 2:9
Table 3. Solvent and Acidity Effect Results on the Oxidation of Indigo Dye with MCPBA as
ꢃ
Catalyzed by Imidazole at 25 CaÞ
Solvent
HClO4 added
i. r./M sꢂ1
kobs/sꢂ1
CH3OH/H2O (8:2)
0
0
0
ð1:2 ꢄ 0:2Þ ꢅ 10ꢂ7
ð3:4 ꢄ 0:3Þ ꢅ 10ꢂ7
ð6:2 ꢄ 0:6Þ ꢅ 10ꢂ7
ð3:6 ꢄ 0:4Þ ꢅ 10ꢂ7
ð2:1 ꢄ 0:3Þ ꢅ 10ꢂ7
ð1:2 ꢄ 0:2Þ ꢅ 10ꢂ7
0:008 ꢄ 0:001
0:027 ꢄ 0:002
0:042 ꢄ 0:002
0:028 ꢄ 0:002
0:017 ꢄ 0:002
0:009 ꢄ 0:002
CH3OH/CH3CN (9:1)
CH3OH/CH3CN (1:1)
CH3OH/CH3CN (1:1)
CH3OH/CH3CN (1:1)
CH3OH/CH3CN (1:1)
0.01 M
0.02 M
0.05 M
a) [MCPBA] = 1.0 mM, [IB] = 0.05 mM, [Im] = 20 mM.

318 Bull. Chem. Soc. Jpn., 77, No. 2 (2004)
Imidazole Catalyzed Oxidation with Peroxide
The exact structure of the active intermediate (4) is not known.
The structure proposed in Eq. 6 is only one possibility.
H
O
R/
O
R
H
O
O
N
k1
O
O
Solution Acidity Effect. The effect of pH on the rate of ox-
idation of indigo blue with MCPBA as catalyzed by imidazole
was observed in the presence of different HClO4 concentrations
in the range 0.1–0.001 M. The reaction rate decreased with an
increase in acidity (Table 3). This result suggests that the bind-
ing of imidazole to MCPBA is a necessary step in the activation
of MCPBA towards the oxidation of indigo blue dye. The acid
reduces both the basicity and the nucleophilicity of imidazoles.
Imidazole Catalysis. It is very clear that imidazoles en-
hance the activity of MCPBA towards the oxidation of indigo
dyes. In addition, imidazoles were not oxidized significantly
in these reactions. The imidazole oxide product was less than
10% (based on the [MCPBA]) even at the highest imidazole
concentration used in this study, [Im] = 0.2 M ([Im]/[IB] >
2000). Also, the rate of oxidation of indigo dye by MCPBA in-
creases linearly with the Im concentration over the range used,
[Im] = 1.0–200 mM (Fig. 4). Imidazole adducts, like all elec-
tron-rich bases (or nucleophiles), are known to increase the ac-
tivity of metal peroxides by enhancing the homolytic or heter-
O
N
+
N
k-1
R
R
N
R/
IB
k2
OH
H
O
HN
O
O
N
H
N
O
O
R
O
O
NH
N
H
O
N
R/
Scheme 3. A suggested mechanism for the oxidation of an
olefin by MCPBA/imidazole.
zole. It seems that the higher electron-donating ability of 1-
methylimidazole is responsible for this higher activity.
The authors thank Jordan University of Science and Tech-
nology and the Department of Applied Chemical Sciences for
financial and other support.
olytic cleavage of the peroxide O–O bond ‘‘Push Effect’’.13
A
study on the oxidation of azo dyes by a H2O2/MnIII(porphyrin)
catalytic system in the presence of imidazoles, pyridines, and
benzoic acid found that imidazoles have the highest activity.14
It has been suggested that imidazoles play a crucial role in
coordinating to MnIII, and help in transferring one oxygen atom
to the dye.14 Imidazoles are also reported to enhance the activ-
ity and enantiomeric selectivity of the oxidation of olefins by
H2O2 catalyzed by arenesulfonimido derivatives (Eq. 7).15
The kinetics and the mechanisms of these reactions were not in-
vestigated to understand the role of imidazoles in enhancing the
activity.
References
1
5231 (1975).
R. P. Hanzlik and G. O. Shearer, J. Am. Chem. Soc., 97,
2
a) H. Mimoun, Angew. Chem., Int. Ed. Engl., 21, 734
(1982). b) D. Swern, ‘‘Organic Peroxides,’’ ed by D. Swern, Wiley
Inter-science, New York (1971), Vol. 2, p. 355. c) R. Sheldon, Top.
Curr. Chem., 164, 21 (1993). d) W. A. Herrmann, Angew. Chem.,
Int. Ed. Engl., 27, 1297 (1988).
3
(2000).
4
H. Yao and D. E. Richardson, J. Am. Chem. Soc., 122, 3220
O
S
NR
Ar
N
NH
R1
R2
R3
R4
ð7Þ
R1
R2
R3
R4
O
a) A. M. Al-Ajlouni and J. H. Espenson, J. Am. Chem. Soc.,
H2O2
+
117, 9243 (1995). b) K. Vessell and J. H. Espenson, Inorg. Chem.,
33, 5491 (1994). c) S. Yamazaki, J. H. Espenson, and P. Huston,
Inorg. Chem., 32, 4683 (1993).
In our work, the electron-rich imidazoles may play a similar
role to the one observed in metal peroxides. Both the metal cen-
ter and the MCPBA carbonyl are electopositive and can be at-
tacked by the imidazole bases. Therefore, imidazoles may en-
hance the reactivity of MCPBA by stabilizing the intermediate
(4) (Eq. 6), and therefore weaken the O–O bond of the MCPBA
peroxide. A similar effect has been suggested for the catalytic
activation of metal peroxides by imidazoles towards oxidation
reactions.13
Based on the kinetic results, and the effect of solvent polarity
(or nucleophilicity) and the solution pH, the schematic mecha-
nism as described in Scheme 3 is proposed for the oxidation of
an olefin (such as indigo dyes) by a MCPBA/imidazole system.
5
3969 (1996).
A. M. Al-Ajlouni and J. H. Espenson, J. Org. Chem., 61,
6 R. D. Bach, M. N. Glukhotsev, and C. Gonzalez, J. Am.
Chem. Soc., 120, 9902 (1998).
7
(1977).
8
L. M. Hjelmeland and G. Loew, Tetraherdon, 33, 1029
a) W. R. Wadt and W. A. Goddard, J. Am. Chem. Soc., 97,
3015 (1975). b) D. Cremer, J. Am. Chem. Soc., 101, 7199 (1979).
a) W. Wang and J. H. Espenson, J. Am. Chem. Soc., 120,
9
11335 (1998). b) W. A. Herrmann, H. Ding, R. M. Kratzer, F. E.
Kuhn, J. J. Hailder, and R. W. Fischer, J. Organomet. Chem.,
549, 319 (1997). c) J. Rudolph, K. L. Reddy, J. P. Chiang, and
K. B. Sharpless, J. Am. Chem. Soc., 119, 6189 (1997).
10 S. Banfi, A. Maiocchi, F. Montanari, and S. Quici, Gazz.
Chim. Ital., 120, 123 (1990).
Conclusion
The ability of an imidazole to increase the reactivity of
MCPBA to oxidize indigo dyes depends mainly on the elec-
tronic nature of the imidazole. Nevertheless, 1-methylimida-
zole and imidazole were found to have similar equilibrium con-
stants for the formation of MCBPA–Im adducts (Table 1) 1-
methylimidazole is about 4–5 times more active than imida-
11 S. Belanger and A. L. Beauchamp, Inorg. Chem., 35, 7836
(1996).
12 W. Nam, H. J. Han, S. Oh, Y. J. Lee, M. Choi, S. Han, C.
Kim, S. K. Woo, and W. Shin, J. Am. Chem. Soc., 122, 8677
(2000).
13 K. Yamaguchi, Y. Watanabe, and I. Morishima, J. Am.

K. Q. Shawakfeh et al.
Bull. Chem. Soc. Jpn., 77, No. 2 (2004)
319
Chem. Soc., 115, 4058 (1993).
14 J. Tokuda, R. Ohura, and T. Iwasaki, Text. Res. J., 69, 956
(1999).
15 a) R. Kluge, H. Hocke, and M. Schulz, Tetrahedron: Asym-
metry, 8, 2513 (1997). b) R. Kluge, M. Schulz, and S. Liebsch,
Tetrahedron, 52, 2957 (1996).
16 S. L. Newmyer, J. Sun, T. M. Loehr, and P. R. Ortiz de
Montellano, Biochemistry, 35, 12788 (1996).
17 T. Uno, A. Takeda, and S. Shimabayashi, Inorg. Chem., 14,
1599 (1995).
18 V. L. Pecoraro, M. J. Bladwin, and A. Gelasco, Chem. Rev.,
94, 807 (1994).
19 a) A. Berkessel, M. Frauenkron, T. Schwenkreis, and A.
Steinmetz, J. Mol. Catal. A: Chem., 117, 339 (1997). b) M. M.
Miller and D. C. Sherrington, J. Catal., 152, 368 (1995). c) F.
Montanari, S. Banfi, and S. Quici, Pure Appl. Chem., 61, 1631
(1989).
20 Unpublished results on the oxidation of pyridine and imida-
1
zoles with CH3ReO3/H2O2 systems. H-NMR experiments have
shown that pyridine oxidizes faster than imidazole by CH3ReO3/
H2O2 in CD3CN.
21 R. Hoessel, S. Leclerc, J. Endicott, M. Nobel, A. Lawrie, P.
Tunnah, M. Leost, E. Damiens, D. Marie, D. Marko, E.
Niederberger, W. Tang, G. Eisenbrand, and L. Meijer, Nat. Cell
Biol., 1, 60 (1999).