Oxidation of ethers with dimethyldioxirane

Article abstract of DOI:10.1007/s11172-006-0127-0

Oxidation of a series of tert-butyl ethers Bu^tOR (R = Me, Et, CH₂CH₂Cl, Prⁱ, Buⁱ), diethyl ether, diisopropyl ether, 1,2-dimethoxyethane, diisobutoxymethane, 1,4-dioxane, and tetrahydrofuran with dimethyldioxirane (DMDO) was studied. The reaction kinetics obeys the second-order equation w = k[DMDO][ether]. The rate constants in a range of 5-50°C and the activation parameters of the reaction were determined. The solvent effect on the oxidation rate was studied. The oxidation products are the corresponding alcohols and carbonyl compounds. The competition between the nonradical (oxygen insertion) and radical mechanisms of the reaction is discussed. The reactions of the parent dioxirane and DMDO with a series of methyl ethers MeOR′ (R′ = Me, Et, CH₂CH₂F, Prⁱ) were studied by the density functional theory (DFT). The (U)B3LYP-6-311G(d,p) method was employed to calculate the geometry and energies of the reactants and transition states. The data obtained indicate a possible increase in the probability of oxidation via the radical route and an increase in the activation barrier for the substrates containing electron-withdrawing substituents.

Full text of DOI:10.1007/s11172-006-0127-0

2384
Russian Chemical Bulletin, International Edition, Vol. 54, No. 10, pp. 2384—2393, October, 2005
Oxidation of ethers with dimethyldioxirane
S. A. Grabovskiy,aꢀ Q. K. Timerghazin,b and N. N. Kabal´novaa
aInstitute of Organic Chemistry, Ufa Research Center of the Russian Academy of Sciences,
71 prosp. Oktyabrya, 450054 Ufa, Russian Federation.
Fax: +7 (347 2) 35 6066. Eꢀmail: chemox@anrb.ru; stas_g@anrb.ru
bCentre for Research in Molecular Modeling (CERMM) and Department of Chemistry and Biochemistry,
Concordia University, 7141 Sherbrooke West, Montreal, Quebec, H4B 1R6 Canada
Oxidation of a series of tertꢀbutyl ethers ButOR (R = Me, Et, CH2CH2Cl, Pri, Bui), diethyl
ether, diisopropyl ether, 1,2ꢀdimethoxyethane, diisobutoxymethane, 1,4ꢀdioxane, and tetraꢀ
hydrofuran with dimethyldioxirane (DMDO) was studied. The reaction kinetics obeys the
secondꢀorder equation w = k[DMDO][ether]. The rate constants in a range of 5—50 °C and
the activation parameters of the reaction were determined. The solvent effect on the oxidation
rate was studied. The oxidation products are the corresponding alcohols and carbonyl comꢀ
pounds. The competition between the nonradical (oxygen insertion) and radical mechanisms
of the reaction is discussed. The reactions of the parent dioxirane and DMDO with a series of
methyl ethers MeOR´ (R´ = Me, Et, CH2CH2F, Pri) were studied by the density functional
theory (DFT). The (U)B3LYPꢀ6ꢀ311G(d,p) method was employed to calculate the geometry
and energies of the reactants and transition states. The data obtained indicate a possible
increase in the probability of oxidation via the radical route and an increase in the activation
barrier for the substrates containing electronꢀwithdrawing substituents.
Key words: dimethyldioxirane, ethers, oxidation, kinetics, free radicals, ab initio calculaꢀ
tions.
The chemistry of dioxiranes was under intense develꢀ
gives ρ = –0.74 0.03 (25 °C).10 However, the mechaꢀ
nism of oxidation of different ethers was not studied sysꢀ
tematically. We have previously11,12 shown that C—H
bonds of alkanes and alcohols are oxidized with DMDO
via two parallel channels, nonradical and homolytic. The
ratio of these processes was determined. In the present
work, we studied the products and kinetics of the reacꢀ
tions of DMDO with aliphatic ethers 1—12 and the influꢀ
ence of different factors on the process to elucidate its
mechanism.
opment for the last two decades.1,2 A distinctive feature of
these cyclic peroxides is a combination of high reactivity
and selectivity of oxidation reactions.3 The works conꢀ
cerning the oxidation of ethers with dioxiranes are few
and, as rule, synthetic. For instance, the transformation
of steroid ethers to the corresponding ketones was studꢀ
ied.4 The oxidation of Oꢀisopropylidene diol derivatives
to 2ꢀhydroxyketones and the oxidation of benzyl ethers
were studied,5,6 and a nonradical nature was postulated
for these reactions. The oxidation of 1,3ꢀdihydroisoꢀ
benzofuran with dimethyldioxirane (DMDO) to 2ꢀhydrꢀ
oxymethylbenzaldehyde was assumed7 to involve radiꢀ
cals; however, the total composition of the products was
not reported. The study8 of the reaction products of dialkyl
ethers with dioxiranes suggested the radical nature of this
reaction, which has been developed further.9,10 The
study10 of the products and kinetics of oxidation of ethers
with DMDO concluded the competition of two processes:
insertion of the oxygen atom at the C—H bond of the
substrate and hydrogen atom abstraction followed by fast
recombination of the radical pair that formed. The effect
of oxygen on the yield of the products and conversion of
the substrate were demonstrated for paraꢀsubstituted anaꢀ
logs of 1ꢀmethoxyꢀ1ꢀphenylethane. Analysis of the kiꢀ
netic data in the framework of the Hammett equation
Experimental
NMR spectra were recorded on a Bruker AMꢀ300 specꢀ
trometer (Me4Si as standard, solvents (D3C)CO, CDCl3, and
CCl4). Benzene or dimethylsulfone was used as reference for
1H NMR estimations of the concentration.
Chromatographic analysis was carried out on a Chromꢀ5
instrument (column 3.7 m × 3 mm, SEꢀ30 stationary phase) or
on a Hewlett Packard 5890E SERIES II gas chromatograph
(capillary column HPꢀ5 (Crosslinked 5% PH ME Silicon)
30 m × 0.32 mm × 0.25 µm). nꢀDecane was used as internal
standard.
GCꢀMS analysis was carried out on an HP 5890 Series II
chromatograph (capillary column HPꢀ5, l = 60 m) with an
HP5972 massꢀselective detector.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2309—2318, October, 2005.
1066ꢀ5285/05/5410ꢀ2384 © 2005 Springer Science+Business Media, Inc.

Oxidation of ethers with dimethyldioxirane
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
2385
Acetone (analytical grade) was purified by distillation.
Dimethyldioxirane was synthesized according to a known proꢀ
cedure.13 Identification and analysis of DMDO were carried out
as described earlier.3 Dioxirane was extracted in CCl4 according
to a known procedure.14
Synthesis of tertꢀbutyl ethers. 2ꢀMethoxyꢀ2ꢀmethylproꢀ
pane (1), 2ꢀethoxyꢀ2ꢀmethylpropane (2), and 2ꢀisopropoxyꢀ2ꢀ
methylpropane (4) were synthesized by a known procedure.15
2ꢀ(2ꢀChloroethoxy)ꢀ2ꢀmethylpropane (3) and 1ꢀtertꢀbutoxyꢀ2ꢀ
methylpropane (5) were synthesized similarly in 20 and 25%
yields, respectively. 2ꢀ(2ꢀChloroethoxy)ꢀ2ꢀmethylpropane was
washed with water to a minimum content of alcohols, dried with
molecular sieves 4A, and distilled on a rectification column.
The other ethers were washed 20—40 times with an equal amount
of water to remove alcohols and then were refluxed for 12 h over
metallic sodium and distilled. The purity of the resulting ethers
was <99.9%.
flask. Substrate 1—12 (2.1 mmol) was added with stirring, the
flask was tightly closed with a ground stopper, and the solution
was left to stir (T = 25 °C). After a time period necessary for the
reaction to occur by 95% (according to the kinetic data), a
standard solution of nꢀdecane was added, and the resulting mixꢀ
ture was analyzed. The products were identified by GCꢀMS.
The obtained mass spectra agree well with those in the NIST
library (1999). Quantitative analysis was carried out on an
HP 5890E Series II chromatograph.
The products of 2ꢀethoxyꢀ2ꢀmethylpropane (2) oxidation
with dimethyldioxirane were also studied by 1H NMR in a
CCl4—CD2Cl2 solution. A solution of DMDO in 2 mL of CCl4
(0.1 mol L–1, 0.2 mmol) was placed in a flatꢀbottom flask
(10 mL) equipped with a magnetic stirrer and saturated with
oxygen filling the flask volume with oxygen. A solution (0.2 mL)
of 2ꢀethoxyꢀ2ꢀmethylpropane (1 mol L–1, 0.2 mmol) in CCl4
was added with stirring, the flask was tightly closed with a ground
stopper, and the solution was left to stir for 36 h (T = 25 °C).
The resulting solution (sample of 0.4 mL) was placed in a tube
to record NMR spectra, a solution (0.15 mL) of benzene
(0.11 mol L–1) in CD2Cl2 (standard for quantitative analysis)
was added, and the 1H NMR spectrum was recorded. The prodꢀ
ucts were identified by the characteristic signals of Me groups.
No signal of the aldehyde proton was observed.
2ꢀEthoxyꢀ2ꢀmethylpropane
(2).
B.p.
72.5
°C,
d25 0.742 g cm–3. H NMR (acetoneꢀd6, 300 MHz), δ: 1.09 (t,
3 H, CH2CH3, J = 7.15 Hz); 1.16 (s, 9 H, CMe3); 3.38 (q, 2 H,
OCH2, J = 7.15 Hz). 13C NMR (acetoneꢀd6, 75 MHz), δ: 16.4
(CH2CH3); 27.6 (CCH3); 56.7 (OCH2); 72.1 (OC).
1
2ꢀ(2ꢀChloroethoxy)ꢀ2ꢀmethylpropane (3). B.p. 134.5 °C,
d
25 0.946 g cm–3. 1H NMR (CDCl3, 300 MHz), δ: 1.18 (s, 9 H,
CMe3); 3.53—3.65 (m, 4 H, OCH2CH2Cl, J = 4.32 Hz).
13C NMR (acetoneꢀd6, 75 MHz), δ: 27.4 (Me); 44.5 (CH2Cl);
62.8 (OCH2); 73.3 (OC).
Solutions of the products were analyzed for a content of
peroxides by the iodometric method.
Quantum chemical calculations. Density functional theory
(DFT) calculations were performed using a combination of the
threeꢀparameter hybrid exchange Becke functional (B3)16,17 with
the Lee—Yang—Parr correlation functional (LYP),18 which is
designated hereinafter as B3LYP. The standard twoꢀ and threeꢀ
exponential 6ꢀ31G** and 6ꢀ311G** basis sets19 were used. All
geometric parameters for molecules, intermediates, and transiꢀ
tion states were optimized without symmetry restraints. The
nature of optimized structures was confirmed by analysis of the
eigenvalues of the Hessian matrix, being the matrix of the secꢀ
ond derivatives of the energy by the coordinates, which were
calculated analytically.20 The calculated energies were corrected
for the zeroꢀpoint vibration energies taken without a scaling
factor. All calculations were performed using the Gaussian 98
program package.21
2ꢀIsopropoxyꢀ2ꢀmethylpropane (4). B.p. 87.6 °C,
d
25 0.737 g cm–3. 1H NMR (CDCl3, 300 MHz), δ: 1.05 (d, 6 H,
CHMe2, J = 6.15 Hz); 1.14 (s, 9 H, CMe3); 3.74 (m, 1 H,
CHMe2, J = 6.15 Hz). 13C NMR (CDCl3, 75 MHz), δ: 25.0
(CHCH3); 28.3 (CCH3); 63.4 (OCH); 73.0 (OC).
1ꢀtertꢀButoxyꢀ2ꢀmethylpropane (5). B.p. 111.5 °C,
1
d20 0.755 g cm–3. H NMR (acetoneꢀd6, 300 MHz), δ: 0.88 (d,
6 H, CHMe2, J = 6.7 Hz); 1.15 (s, 9 H, CMe3); 1.61—1.76 (m,
1 H, CH2CHMe2); 3.09 (d, 2 H, OCH2, J = 6.4 Hz). 13C NMR
(acetoneꢀd6, 75 MHz), δ: 19.6 (CHCH3); 27.6 (CCH3); 29.6
(CH2CHMe2); 68.5 (OCH2); 72.1 (OC).
Dioxane (6), 1,2ꢀdimethoxyethane (7), diisobutoxymeꢀ
thane (8), ethoxyethane (9), 2ꢀisopropoxypropane (10), and
tetrahydrofuran (11) were dried above metallic sodium and disꢀ
tilled under argon. 1,3ꢀDioxolanꢀ2ꢀone (12) was distilled under
reduced pressure.
Results and Discussion
Procedure of kinetic experiments. The reaction kinetics was
studied from the consumption of dioxirane. The change in the
DMDO concentration during the reaction was monitored specꢀ
Reaction products. The oxidation products of ethers
with dimethyldioxirane are presented in Table 1.
Acyclic ethers are transformed by DMDO into semiꢀ
acetals, which decompose rapidly to the corresponding
alcohols and carbonyl compounds (Scheme 1).
Cyclic ethers are oxidized to the corresponding hyꢀ
droxy derivatives (Scheme 2), which are further transꢀ
formed into lactones.
In the most cases, the yields of the products based on
the DMDO consumed are lower than 100%, which is due
to their subsequent oxidation with dioxirane. Under the
studied conditions, compound 12 is not oxidized by
DMDO.
trophotometrically at λ
= 335 nm (ε = 13.5 L mol–1 cm–1
)
max
on a Specord Mꢀ40 instrument. A solution of ether was placed
in a quartz cell, and the temperature of the cell was maintained
at a specific level in the cell chamber of the spectrophotometer
at 7—50 °C. A required amount of a solution of DMDO was
rapidly added, oxygen (1.5 min) or argon was passed through the
solution if necessary, and the change in absorbance was moniꢀ
tored. The initial concentrations of DMDO and ethers 1—12
were varied within (0.7—6.5)•10–2 and (4.0—60.0)•10–2
mol L–1, respectively. Acetone, CCl4, or Me2CO—CCl4 and
Me2CO—ButOH mixtures were used as solvents.
Analysis of products. A solution of DMDO in 3 mL of acꢀ
etone (0.07 mol L–1, 0.21 mmol) was placed in a flatꢀbottom
flask (10 mL) equipped with a magnetic stirrer. The solution was
saturated with oxygen filling with oxygen the gas volume of the

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Grabovskiy et al.
Table 1. Oxidation products of ethers with dimethyldioxirane
in acetone at 25 °C and the ratio of reactants
[ether] : [DMDO] = 10 : 1
Scheme 1
Ether
Products
Yielda
(%)
ButOMe (1)b
ButOEt (2)
2ꢀMethylpropanꢀ2ꢀol
2ꢀMethylpropanꢀ2ꢀol
Ethanal
2ꢀMethylpropanꢀ2ꢀol
2ꢀChloroethanal
Chloroacetic acid
2ꢀMethylpropanꢀ2ꢀol
Acetone
2ꢀMethylpropanꢀ2ꢀol
2ꢀMethylpropanal
1,4ꢀDioxanꢀ2ꢀol
1,4ꢀDioxanꢀ2ꢀone
Methanol
2ꢀMethoxyethanal
Methoxyacetic acid
2ꢀMethylpropanꢀ1ꢀol
2ꢀMethylpropanal
Ethanol
60
99
97
41
12
3
97
97
98
96
11
44
32
4
ButO(CH2)2Cl (3)b
ButOPri (4)c
Scheme 2
ButOBui (5)
1,4ꢀDioxane (6)
MeO(CH2)2OMe (7)b
3
DMDO decomposition is 7•10–7 s–1 12
The resulting kd
.
BuiOCH2OBui (8)
Et2O (9)
96
96
99
99
98
99
96
value is in good accord with the thermolysis constant of
dioxirane in acetone.12
The rate constant kapp is independent of the oxygen
content in the reaction mixture and DMDO concentraꢀ
tion. The kinetic equation of the reaction has the followꢀ
ing form:
Ethanal
Propanꢀ2ꢀol
Acetone
Tetrahydrofuranꢀ2ꢀol
PriOPri (10)c
Tetrahydrofuran (11)
(12)d
—
Not oxidized
d[DMDO]/dt = k•[DMDO]•[RH] + kd•[DMDO].
The temperature plot of the rate constant in an interꢀ
val of 5—55 °C was studied and the activation parameters
a Based on the consumed dioxirane.
b The reaction was carried out in oxygen.
c Solvent CCl4.
for the ethers were determined (Table 2). The ∆H≠
,
298
∆S≠298, and ∆G≠ values were calculated by the Eyring
d 50 °C, solvent CCl4—acetone (5 : 1).
298
equation.22 The resulting parameters indicate that the oxiꢀ
dation can proceed via a concerted mechanism with the
simultaneous cleavage of several bonds in the molecules
Reaction kinetics. The reaction kinetics was studied
under the condition that [DMDO]0 << [RH]0, where
[DMDO]0 and [RH]0 are the initial concentrations of
dimethyldioxirane and the substrate, respectively. For
ethers 2, 4—6, and 9—11 and acetal 8, the kinetic curves
are described by the first order equation with a high correꢀ
lation coefficient (0.98—0.999) (Fig. 1). The apparent
firstꢀorder rate constants were calculated from the anꢀ
amorphoses of the kinetic curves
[DMDO]•102/mol L–1
ln[DMDO]
t/s 300
200
100
1.2
1.0
0.8
0.6
0.4
0.2
–4.5
–5.0
–5.5
–6.0
–6.5
kapp = k•[RH]n,
2
where k is the apparent rate constant, and n is the reaction
order with respect to ether. The plot of kapp vs. ether
concentration is linear and takes the following form:
1
kapp = k•[RH] + kd,
0
100
200
300
t/s
which indicates two channels of DMDO consumption:
the reaction with ether (k) and thermal decomposition (kd)
(Fig. 2). It follows from the plot in Fig. 2 that kd =
(2 6)•10–6 s–1; at this temperature, the rate constant of
Fig. 1. Kinetic curve of DMDO consumption in the reaction
with 2ꢀisopropoxypropane (10) (1) and its semilogarithmic anꢀ
amorphosis (2) ([10]0 = 0.968 mol L–1, 37 °C).

Oxidation of ethers with dimethyldioxirane
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
2387
kapp•105/s–1
[DMDO]•102/mol L–1
2.5
8
∆[DMDO]
2.0
1.5
1.0
0.5
6
4
2
600
1200
1800
t/s
0
0.5
1.0
1.5
2.0
2.5 [4]0/mol L–1
Fig. 3. Kinetic curve of DMDO consumption in the presence of
2ꢀ(2ꢀchloroethoxy)ꢀ2ꢀmethylpropane (3) ([3]0 = 0.94 mol L–1
[O2]0 = 0.011 mol L–1, 15 °C).
,
Fig. 2. Apparent rate constant of DMDO consumption (kapp
vs. concentration of 2ꢀisopropoxyꢀ2ꢀmethylpropane (4) (solꢀ
vent acetone, 15 °C).
)
concentration. A similar phenomenon has been observed
earlier11,12 for the oxidation of alkanes, alcohols, and keꢀ
tones. The high rate of DMDO consumption after the
end of the induction period was found to be caused by the
involvement of DMDO in a chain radical process, which
is induced by free radicals.9,11,12,23—25 Since the rate conꢀ
stant of the reaction R• + O2 is rather high ((1—5)•109
L mol–1 s–1),26 the sharp acceleration of the reaction can
be due to the complete consumption of oxygen. In the
system under study, oxygen is an efficient inhibitor of the
chain radical process. According to the inhibitor theory,
the rate of O2 consumption is proportional to the initiaꢀ
tion rate (except for the contribution of hidden radical
processes), and the amount of the consumed inhibitor
(oxygen) makes it possible to determine the efficiency of
initiation. In the previous studies,11,12 we determined the
efficiency of free radical generation in the reactions of
DMDO through the [O2]0/∆[DMDO] ratio. The data on
to form new bonds, as it should occur according to the
mechanism of oxygen atom insertion at the C—H bond of
an ether molecule. However, these data are insufficient to
prove the "insertion" mechanism.
Ethers 1, 3, and 7 are oxidized with an induction
period (Fig. 3). Saturation of the initial solutions with
oxygen leads to a prolonged induction period. On the
other hand, when the initial O2 concentration was deꢀ
creased by bubbling with an inert gas, the induction peꢀ
riod decreased too. Before the kinetic curve inflection,
the rate of DMDO consumption remains the same as that
in the runs where the oxygen dissolved in the solution was
present during the whole experiment. The variation of the
concentrations of ethers and DMDO at a constant initial
concentration of oxygen does not substantially change
∆[DMDO]. The apparent rate constants in the region
before acceleration increase proportionally to the ether
Table 2. Kinetic parameters for the oxidation of ethers with dimethyldioxirane in acetone
Ether
ka•104
/L mol–1 s–1 /kcal mol–1
Ea
logA
∆H≠
–∆S≠
∆G≠
298
298
298
/kcal mol–1
/cal K–1 mol–1
/kcal mol–1
2ꢀMethoxyꢀ2ꢀmethylpropaneb
2ꢀEthoxyꢀ2ꢀmethylpropane
1.1 0.1
19
17.8 1.2
16.6 0.9
—
9.1 0.8
9.5 0.9
—
17
16.0 0.9
—
1
18.8 1.2
17.3 1.7
—
22
21
—
2
2
2
2ꢀ(2ꢀChloroethoxy)ꢀ2ꢀmethylpropaneb 0.78 0.02c
2ꢀIsopropoxyꢀ2ꢀmethylpropane
1ꢀtertꢀButoxyꢀ2ꢀmethylpropane
Diisobutoxymethane
2ꢀIsopropoxypropane
Ethoxyethane
1,4ꢀDioxane
1,2ꢀDimethoxyethaneb
Tetrahydrofuran
0.66 0.05
13
6.2 0.3
30
17.8 0.7
1.56 0.14
3.0 0.1
37 2c
13.3 0.7
11.6 0.8
13.7 0.9
12.0 0.7
12.6 0.8
16.8 0.7
14.5 0.4
—
5.6 0.5
5.6 0.8
6.9 0.7
6.3 0.3
6.5 0.7
8.5 0.6
7.1 0.3
—
12.7 0.7
10.9 0.8
13.1 0.9
11.4 0.7
12.0 0.8
16.2 0.7
13.9 0.5
—
35
35
29
31.9 1.1
31
22
3
4
2
23
21
22
21
21
23
3
3
2
2
3
3
3
2
3
2
27.9 1.1
—
22.2 1.8
—
a 25 °C.
b The rate constants were obtained from the initial regions of the kinetic curves of DMDO consumption; the solution was saturated
with oxygen prior to experiment.
c At 15 °C.

2388
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
Grabovskiy et al.
Table 3. Ratio of the initial concentration of O2 ([O2]0) to the
amount of DMDO consumed in the reaction with ethers at
different temperatures
able by standard analytical methods. This explains a disꢀ
advantage of the reaction products for the calculation of
the material balance. The preliminary results showed a
considerable contribution of radical processes to the reꢀ
actions of aldehydes with DMDO.
Ether
T/°C [O2]0/∆[DMDO]*
The reactions of the most part of ethers under study
with DMDO involve no radical processes (Scheme 4,
route a). The exceptions are the ethers containing elecꢀ
tronꢀwithdrawing substituents for which route b occurs,
probably, in part (see Scheme 4). This assumption is based
on the fact that no radical formation was observed for the
oxidation of alcohols of similar structure with DMDO. In
addition, in the case of ethers, whose oxidation affords no
aldehydes (for instance, compounds 4, 6, and 10), no
effect of O2 was observed on the kinetics and composition
of the reaction products.
1,2ꢀDimethoxyethane
45.2
50.8
54.8
24.5
30.0
35.9
40.2
49.7
15.0
0.24 0.03
0.24 0.03
0.24 0.03
0.57 0.05
0.48 0.05
0.44 0.05
0.28 0.04
0.27 0.03
0.95 0.05
2ꢀMethoxyꢀ2ꢀmethylpropane
2ꢀ(2ꢀChloroethoxy)ꢀ2ꢀmethylꢀ
propane
* The amount of dimethyldioxirane consumed during the inꢀ
duction period (see Fig. 3).
Scheme 4
the ratio of the initial oxygen concentration to the amount
of consumed DMDO in the reactions with ethers are
given in Table 3.
However, since the products of the reactions of ethers
with DMDO undergo further oxidation in some cases, the
stoichiometric ratio DMDO : O2 can differ from unity.
The oxidation products of ether 1 showed no methanal or
products of its transformation as, e.g., for the oxidation of
methanol11 with DMDO. This indicates the further oxiꢀ
dation of methanal. The authors of the single study27 of
the oxidation of aldehydes with DMDO found that the
free radical processes contribute substantially to the reacꢀ
tion. In the case of ether 1, the source of free radicals is,
most likely, the reaction of DMDO with methanal.
A similar situation is observed for the oxidation of ethers 3
and 7: acids were found in the reaction products, and the
overall yield of the oxidation products was lower than
those of the reactions of other ethers. These facts can be
explained assuming a side reaction (Scheme 3).
Stoichiometry of O2 consumption can be estimated
with allowance for the oxidation reaction via route а (see
Scheme 4) followed by the decomposition of semiacetal
to aldehyde and alcohol. The further oxidation of aldeꢀ
hyde results in the formation of radicals via route b (see
Scheme 5
Scheme 3
The H atom abstraction from an aldehyde molecule
produces carbonꢀcentered radicals, which react rapidly
with oxygen to result in a chain of transformations charꢀ
acteristic of radical oxidation of aldehydes in the presence
of oxygen.28—30 The oxidation products of aldehydes
formed from ethers 1, 3, and 7 are volatiles hardly detectꢀ

Oxidation of ethers with dimethyldioxirane
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
2389
Scheme 3) and their transformation via the chainꢀradical
oxidation scheme28,29,31 (Scheme 5).
hence, the following correlation should be taken into acꢀ
count:
At deeper steps of oxidation and a low concentration
of O2, chain termination can occur by the recombination
of acyl radicals.31 Let us estimate the contribution of chain
propagation (H atom abstraction from an aldehyde molꢀ
ecule by the R2C(O)OO• radical) (Scheme 6).
{3/[4•(1 + 2ϖAld)]}•([O2]0/∆[DMDO]),
where ϖAld is the yield of aldehyde based on the conꢀ
sumed dioxirane in the inhibited process. However, a
possibility of radical formation in the reaction of DMDO
with ether (see Scheme 4, route b), as that found for the
oxidation of alcohols with DMDO,11 cannot be excluded
completely. This is related to the fact that the approach
proposed earlier12 does not allow one to determine which
initiation reactions are involved in each particular case.
Taking into account the results of studying the oxidation
products, we can propose that the initiation reaction is
the oxidation of aldehyde with dioxirane.
Scheme 6
Applying the quasiꢀstationary approximation, for the
In the case of ethers 2, 5, and 9—11 and acetal 8, the
rate of ether oxidation, under the reaction conditions,
exceeds the reaction rate of DMDO with the oxidation
products (aldehydes or alcohols). When the oxidation rates
of the product and ether are comparable (compounds 6
and 3) or the reaction rate with the product is much
higher (compounds 1 and 7), the deeper oxidation prodꢀ
ucts are observed (see Table 1).
To check the above assumption about the reaction
mechanism, we studied the interaction of ether 2 with
DMDO at an equivalent ratio of reactants in more detail.
In this case, the kinetic curve of DMDO consumption
has a break, whose time of appearance depends on the
oxygen concentration in the system, unlike the kinetic
curves observed in a ≤10ꢀfold excess of the substrate.
Analysis of the composition of the reaction mixture after
the complete consumption of dioxirane in an oxygenꢀ
saturated solution showed equal amounts of acetic acid,
2ꢀmethylpropanꢀ2ꢀol, and the starting ether, which also
differs from the composition of oxidation products of this
ether in excess substrate. Thus, the rate of the main reacꢀ
tion (ether + DMDO) decreases with a decrease in the
ether concentration, and the side reaction, aldehyde oxiꢀ
dation, begins to contribute substantially to the overall
process. In this case, aldehyde is completely consumed to
the acid.
concentrations of acyl and acyl peroxide radicals (k1 =
108—109 L mol–1 s–1 31
we obtain
)
d[R2C(O)OO•]/dt = kox[DMDO][Ald] –
– kt[R2C(O)OO•]2 = 0,
where [Ald] is the aldehyde concentration under the exꢀ
perimental conditions (~10–2—10–3 mol L–1), and kox is
the rate constant of the reaction of DMDO with aldehyde
(according to our preliminary data, it equals ~10–2—10–3
L mol–1 s–1). Since kt for acyl peroxide radicals with
sterically unhindered aliphatic substituents is29
(3.4—10.4)•107 L mol–1 s–1, the stationary concentraꢀ
tion of the R2C(O)OO• radicals can be calculated. Under
the experimental conditions, this concentration is about
~10–7 mol L–1 and, therefore, the ratio of the recombinaꢀ
tion rate of the acyl peroxide radicals and the rate of their
reaction with aldehyde is
1/2
wt/wabst = (kt1/2•kox •[DMDO]1/2)/(kabst[Ald]1/2) ≈ 10—5
(kabst = (1.1—3.9)•102 L mol–1 s–1).
Therefore, H atom abstraction from the aldehyde molꢀ
ecule by the R2C(O)OO• radical can be neglected in this
case. This conclusion agrees with the results of analysis of
the products: no formation of peracids was found. The
reaction of the oxyl radical with aldehyde can also be
neglected because of the low concentration of the latter
and the rate of destruction32 of this radical, which prevails
over the rate of H atom abstraction from the aldehyde
molecule by 103—102 times.
Thus, six O2 molecules are required to oxidize two
aldehyde molecules via route b (see Scheme 3), and three
O2 molecules are formed due to the recombination
of peroxide radicals (see Scheme 5). Therefore, the
3[O2]0/2∆[DMDO] ratio characterizes the escape of radiꢀ
cals into the bulk (∆[DMDO] is the amount of dioxirane
consumed during the induction period). However, aldeꢀ
hyde oxidation is preceded by the reaction with ether and,
Therefore, the low rate of ether oxidation can be a
reason for the appearance of side reactions, such as
the further oxidation of the products that formed, due to
which free radicals and the products of deeper oxidation
appear in the system under study. In some cases, the
process can be directed to the formation of only the priꢀ
mary oxidation products by the variation of the initial
ether concentration. Increasing this concentration, one
can achieve such a rate of the primary reaction that it will
be predominant.
Solvent effect on the oxidation rate. The oxidation of
the C—H bond with dioxiranes was proposed33—36 to proꢀ
ceed via a highly ordered transition state (TS).

2390
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
Grabovskiy et al.
In the case of reaction systems possibly involving radiꢀ
cal (biradical) intermediates, the problem appears for
choosing an adequate method for quantum chemical calꢀ
culations. The standard calculation schemes based on the
use of restricted wave functions for closed shells (restricted
Hartree—Fock or Kohn—Sham methods) are inapproꢀ
priate in such situations. It has previously39 been shown
that the wave functions calculated40—43 for the transition
state of the reactions of dioxiranes with alkanes, aldeꢀ
hydes, and alcohols have singletꢀtriplet instability, i.e.,
there is another unrestricted solution of the Schrödinger
(or Kohn—Sham) equation with nonequivalent α and
β orbital manifolds, which is lower in energy. Calculation
methods that take into account the multiconfigurational
character of the wave function are necessary for the adꢀ
equate description of systems of biradical nature. Howꢀ
ever, similar computations are computationally intensive
and timeꢀconsuming. The DFT calculations of (bi)radicals
using the hybrid B3LYP functional in the unrestricted
variant (UB3LYP) with broken α—βꢀsymmetry were
shown44,45 to give results, whose quality is comparable
with the stateꢀofꢀtheꢀart multiꢀreference coupled cluster
calculations (MRꢀAQCC). In this work, we used the
UB3LYP method to calculate the reactions of dioxiranes
with ethers. Generally, in calculations of singlet biradicals
in the framework of the unrestricted hybrid DFT methꢀ
ods, such as UB3LYP, fairly strong spin contamination is
inevitable: the <S2> value can be considerably higher than
zero, which indicates a significant radical character of the
molecular wave function. It is known46 that the thermoꢀ
chemical and geometric parameters calculated by the hyꢀ
brid DFT methods are less prone to the influence of spin
contamination than those obtained by the HF, MP2, CC,
and other methods.
S—H is solvent molecule
Solvents capable of forming a hydrogen bond stabilize
the negative charge on the oxygen atom, thus facilitating
substrate oxidation.36
Analysis of the data obtained (Table 4) shows a weak
increase in the rate constant of oxidation of ethers 2, 5,
and 10 with DMDO in the presence of ButOH compared
to solvents for which the effect of the hydrogen bond is
less pronounced (acetone, CCl4).
Structure of substrate and its reactivity. Based on the
partial rate constants, we can arrange the substrates in the
following series of reactivity: 6 < 3 ≈ 4 ≈ 1 ≈ 7 < 8 < 5 < 9 <
< 2 < 11 < 10. The low reactivity of ether 4 compared to
other ethers containing the tertiary C—H bond in the
αꢀposition to the oxygen atom is caused, probably, by
steric hindrance of the oxidized bond and a high sensitivꢀ
ity of DMDO to steric factors.1,2 An increase in the numꢀ
ber of oxygen atoms near the reaction center of an ether
molecule decreases the rate of substrate oxidation in the
order 6 ≈ 7 ≈ 1,3ꢀdioxocycloalkanes37,38 < ethers. Ethers
are oxidized by an order of magnitude more slowly than
alcohols,10,11 which is related, most likely, to a lower σ
value for the substituents at the reaction center and to an
increase in steric hindrance in the case of ethers due to
the replacement of the OH group by OR.
Quantum chemical calculations. We performed quanꢀ
tum chemical calculations for several model systems. The
geometry of the reactants, products, and transition states
and the activation energies for the oxidation of methoxyꢀ
methane, methoxyethane, 2ꢀmethoxypropane, and
1ꢀfluoroꢀ2ꢀmethoxyethane with the parent dioxirane
(DO), viz., CH2O2 (in the case of methoxymethane with
dimethyldioxirane as well), were calculated.
The detailed UB3LYP/6ꢀ311G(d,p) study of the poꢀ
tential surface energy of a dioxirane—methoxymethane
system revealed the first order saddle point (Fig. 4), which
was attributed to the transition state of the reaction. The
geometry of this saddle point (TSꢀ1, see Fig. 4) is similar
to the structure of the transition state described earlier for
Table 4. Effect of the solvent nature on the rate constant of
O(2)
Hβ,E
oxidation of ethers with dimethyldioxirane (k/L mol–1 s–1
)
O(1)
Ether
Solvent
T/°C
k•104
H
2ꢀEthoxyꢀ2ꢀmeꢀ
thylpropane
CCl4
Acetone
6.7
6.7
6.7
4.8
4.8
4.8
4.8
4.8
4.8
1.9 0.1
3.0 0.2
6.6 0.2
1.5 0.1
3.0 0.1
3.6 0.1
5.7 0.1
6.8 0.1
7.8 0.1
C
CD
O
Acetone—ButOH (1 : 1)
CCl4
1ꢀtertꢀButoxyꢀ
ꢀ2ꢀmethylꢀ
propane
2ꢀIsopropoxyꢀ
propane
Acetone
Acetone—ButOH (1 : 1)
CCl4
Acetone
Acetone—ButOH (1 : 1)
TSꢀ1
Fig. 4. Transition state TSꢀ1 for the oxidation of methoxyꢀ
methane with dioxirane calculated by the UB3LYP/6ꢀ311G**
method.

Oxidation of ethers with dimethyldioxirane
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
2391
Table 5. Calculated activation barriers* (kcal mol–1) to the oxiꢀ
dation of ethers with DO and DMDO
Hβ,E
O(2)
O(1)
Transition
state
Basis
set
B3LYP
UB3LYP
DO DMDO
DO DMDO
O
H
CD
C
MeOMe
TSꢀ1
6ꢀ31G**
6ꢀ311G**
20.7 25.1
18.3 22.0
—
—
14.5 17.1
MeOEt
TSꢀ2
TSꢀ3
6ꢀ31G**
6ꢀ311G**
6ꢀ31G**
6ꢀ311G**
17.6 20.7
15.7 18.1
16.7 20.7
14.7 18.1
—
13.5
—
—
—
—
—
TSꢀ2
12.7
MeOCH2CH2F
TSꢀ4
TSꢀ5
TSꢀ6
6ꢀ31G**
6ꢀ311G**
6ꢀ31G**
6ꢀ311G**
6ꢀ31G**
6ꢀ311G**
21.3 26.3
—
14.3
—
13.5
—
—
—
—
—
—
—
19.7
18.8 22.7
16.7
19.7 24.1
—
C
—
17.5
MeOPri
—
13.7
O
H
CD
TSꢀ7
6ꢀ31G**
6ꢀ311G**
15.0 17.1
13.5
—
12.2
—
—
—
O(1)
* Relative to the reactants, including ∆ZPVE.
O(2)
Hβ,E
TSꢀ3
the TS of the reaction of dioxirane with a C—H bond,40
which was calculated by the B3LYP method for closed
shells (RB3LYP) and assigned to the TS of the concerted
insertion of an oxygen atom. However, in this case,
<S2> = 0.436, i.e., TSꢀ1 has a pronounced biradical charꢀ
acter. The transition state found by the RB3LYP method
has singletꢀtriplet instability and, as shown earlier39 for
isobutane oxidation, does not describe the experimentally
observed regularities. On going from methoxymethane to
2ꢀmethoxypropane, differences between the geometric
parameters of the TS and the energies calculated by the
RB3LYP and UB3LYP methods decrease. This indicates
a decrease in the probability that the reaction proceeds
via the radical mechanism. The conformation of ether
Fig. 5. Transition states TSꢀ2 and TSꢀ3 for the oxidation
of methoxyethane with dioxirane calculated by the
UB3LYP/6ꢀ311G** method.
exerts no substantial effect on the energy barrier to its
oxidation (Table 5, Fig. 5).
The effect of electronegative substituents was studied
for 1ꢀfluoroꢀ2ꢀmethoxyethane. The introduction of a fluoꢀ
rine atom into an ether molecule (Tables 5 and 6, Fig. 6)
destabilizes the carbocationic center in the substrate molꢀ
ecule in the transition state and increases the activation
barrier. The parameters calculated by the B3LYP and
UB3LYP methods differ significantly. Electronegative
Hβ,E
O(2)
O(2)
O(1)
O(2)
CD
O
Hβ,E
O
H
CD
O(1)
C
CD
F
O(1)
Hβ,E
H
H
C
O
F
C
F
TSꢀ4
TSꢀ5
TSꢀ6
Fig. 6. Transition states TSꢀ4, TSꢀ5, and TSꢀ6 for the oxidation of 1ꢀfluoroꢀ2ꢀmethoxyethane with dioxirane calculated by the
UB3LYP/6ꢀ311G** method.

2392
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
Grabovskiy et al.
Table 6. Geometric parameters for the transitions states of oxidation of methyl ethers with dioxirane (distances (r) and angles (ω))
Transition Calculation
state method
r/Å
ω/deg
C—H H...O(1) O(1)—O(2) СD—O(1) СD—O(2) Hβ,E—O(1) C—H—O(1) H...O(1)—C—O(2)
MeOMe
TSꢀ1а B3LYP/6ꢀ311G**
1.371
1.129
1.332
1.086
1.982
1.921
2.016
1.456
1.410
1.494
MeOEt
1.446
1.411
1.444
1.410
MeOPri
1.437
1.321
1.362
1.323
2.533
2.735
2.452
149.5
166.8
137.8
168.2
176.6
163.9
UB3LYP/6ꢀ311G** 1.219
TSꢀ1b B3LYP/6ꢀ31G**
TSꢀ2a B3LYP/6ꢀ311G**
UB3LYP/6ꢀ311G** 1.214
TSꢀ3a B3LYP/6ꢀ311G**
1.451
1.317
1.175
1.347
1.168
1.347
1.944
1.894
1.948
1.888
1.326
1.359
1.327
1.359
2.576
2.661
2.566
2.720
157.9
167.2
161.7
171.1
174.0
177.2
174.3
177.3
1.321
UB3LYP/6ꢀ311G** 1.212
TSꢀ7a B3LYP/6ꢀ311G**
1.285
1.213
1.356
1.912
1.871
1.332
1.357
2.552
2.615
165.0
170.8
176.2
177.5
UB3LYP/6ꢀ311G** 1.210
1.411
MeOCH2CH2F
1.466
TSꢀ4a B3LYP/6ꢀ311G**
UB3LYP/6ꢀ311G** 1.239
TSꢀ5a B3LYP/6ꢀ311G**
UB3LYP/6ꢀ311G** 1.219
TSꢀ6a B3LYP/6ꢀ311G**
1.430
1.093
1.294
1.141
1.334
1.117
1.329
2.009
1.944
1.973
1.908
1.988
1.908
1.319
1.364
1.322
1.362
1.322
1.362
2.467
2.772
2.707
2.842
2.570
2.919
149.2
175.5
154.3
169.3
153.7
174.5
162.9
174.5
176.4
180.0
170.6
178.8
1.416
1.462
1.414
1.456
1.353
1.386
UB3LYP/6ꢀ311G** 1.220
1.411
substituents should increase the probability of the reacꢀ
tion to proceed via the radical route.
This work was financially supported by the Russian
Science Support Foundation and the Division of Chemꢀ
istry and Materials Science of the Russian Academy of
Sciences (Program of Fundamental Research "Theoretiꢀ
cal and Experimental Study of the Chemical Bond Naꢀ
ture an Mechanisms of the Most Important Chemical
Reactions and Processes").
The data obtained on the oxidation products, kinetics
of the process, and solvent effect on the process rate can
be rationalized in the framework of a mechanism that
assumes the nonradical or hidden radical oxidation of the
αꢀC—H bond of an ether molecule (Scheme 7).
References
Scheme 7
1. W. Adam, L. P. Hadjiarapoglou, R. Curci, and R. Mello,
in Organic Peroxides, Ed. W. Ando, J. Wiley and Sons,
Chichester, 1992, 195.
2. S. A. Grabovskii and N. N. Kabal´nova, in Sovremennyi
organicheskii sintez [Modern Organic Synthesis], Khimiya,
Moscow, 2003, 252 (in Russian).
3. W. Adam and L. Hadjiarapoglou, Top. Curr. Chem., 1993,
164, 45.
4. F. R. van Heerden, J. T. Dixon, and C. W. Holzapfel, Tetraꢀ
hedron Lett., 1992, 33, 7399.
5. R. Curci, L. Daccolti, A. Dinoi, C. Fusco, and A. Rosa,
Tetrahedron Lett., 1996, 37, 115.
6. B. A. Marples, J. P. Muxworthy, and K. H. Baggaley, Synlett,
1992, 646.
7. P. Bovicelli, A. Sanetti, R. Bernini, and P. Lupattelli, Tetraꢀ
hedron, 1997, 53, 9755.
8. M. Ferrer, F. Sanchezbaeza, J. Casas, and A. Messeguer,
Tetrahedron Lett., 1994, 35, 2981.
9. F. Minisci, L. H. Zhao, F. Fontana, and A. Bravo, Tetraꢀ
hedron Lett., 1995, 36, 1895.
10. A. L. Baumstark, F. Kovac, and P. C. Vasquez, Can. J. Chem.,
1999, 77, 308.
However, it cannot be excluded that some ethers conꢀ
taining electronꢀwithdrawing substituents (for instance,
compound 3) can exhibit a competition between the
nonradical and homolytic mechanisms, which assumes
hydrogen atom abstraction from an ether molecule by
dioxirane.

Oxidation of ethers with dimethyldioxirane
Russ.Chem.Bull., Int.Ed., Vol. 54, No. 10, October, 2005
2393
11. S. A. Grabovskii, E. S. Suvorkina, N. N. Kabal´nova, S. L.
Khursan, and V. V. Shereshovets, Izv. Akad. Nauk, Ser.
Khim., 2000, 1338 [Russ. Chem. Bull., Int. Ed., 2000,
49, 1332].
12. S. L. Khursan, S. A. Grabovskii, N. N. Kabal´nova, E. G.
Galkin, and V. V. Shereshovets, Izv. Akad. Nauk, Ser. Khim.,
2000, 1344 [Russ. Chem. Bull., Int. Ed., 2000, 49, 1338].
13. R. W. Murray and R. Jeyaraman, J. Org. Chem., 1985,
50, 2847.
14. M. Ferrer, M. Gibert, F. Sanchezbaeza, and A. Messeguer,
Tetrahedron Lett., 1996, 37, 3585.
15. J. F. Norris and G. W. Rigby, J. Am. Chem. Soc., 1932,
54, 2088.
28. K. U. Ingold, Acc. Chem. Res., 1969, 2, 1.
29. G. E. Zaikov, J. A. Howard, and K. U. Ingold, Can. J. Chem.,
1969, 47, 3017.
30. N. A. Clinton, R. A. Kenley, and T. G. Traylor, J. Am.
Chem. Soc., 1975, 97, 3746.
31. E. T. Denisov, N. I. Mitskevich, and V. E. Agabekov,
Mekhanizm zhidkofaznogo okisleniya kislorodsoderzhashchikh
soedinenii [Mechanism of LiquidꢀPhase Oxidation of Oxygenꢀ
Containing Compounds], Nauka i Tekhnika, Minsk, 1975,
336 pp. (in Russian).
32. E. T. Denisov, Konstanty skorosti gomoliticheskikh
zhidkofaznykh reaktsii [Rate Constants of Homolytic Liquidꢀ
Phase Reactions], Nauka, Moscow, 1971, 712 pp. (in
Russian).
16. A. D. Becke, Phys. Rev. A, 1988, 37, 785.
17. A. D. Becke, J. Chem. Phys., 1993, 98, 5648.
18. C. Lee, W. Yang, and R. G. Parr, Phys. Rev., 1988, B41, 785.
19. W. J. Hehre, L. Radom, P. v. R. Schleyer, and J. A. Pople,
Ab Initio Molecular Orbital Theory, J. Wiley and Sons, New
York, 1985, 548 pp.
33. W. Adam and A. K. Smerz, J. Org. Chem., 1996, 61, 3506.
34. W. Adam, R. Curci, L. Daccolti, A. Dinoi, C. Fusco,
F. Gasparrini, R. Kluge, R. Paredes, M. Schulz, A. K. Smerz,
L. A. Veloza, S. Weinkotz, and R. Winde, Chem. Eur. J.,
1997, 3, 105.
20. H. B. Schlegel, Adv. Chem. Phys., 1987, 67, 249.
21. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski, J. A.
Montgomery, R. E. Stratmann, J. C. Burant, S. Dapprich,
J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain,
O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi,
B. Mennucci, C. Pomelli, C. Adamo, S. Clifford,
J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui,
K. Morokuma, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz,
A. G. Baboul, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin, D. J.
Fox, T. Keith, M. A. AlꢀLaham, C. Y. Peng, A. Nanayakkara,
C. Gonzalez, M. Challacombe, P. M. W. Gill, B. Johnson,
W. Chen, M. W. Wong, J. L. Andres, C. Gonzalez,
M. HeadꢀGordon, E. S. Replogle, and J. A. Pople,
Gaussian 98, Revision A.7, Gaussian, Inc., Pittsburgh PA
(USA), 1998.
35. R. Curci, A. Dinoi, C. Fusco, and M. A. Lillo, Tetrahedron
Lett., 1996, 37, 249.
36. R. W. Murray and H. Gu, J. Phys. Org. Chem., 1996, 9, 751.
37. Z. F. Akbalina, G. M. Abushakhmina, N. N. Kabal´nova,
S. S. Zlotskii, and V. V. Shereshovets, Zh. Obshch. Khim.,
2002, 72, 1493 [Russ. J. Gen. Chem., 2002, 72, 1406 (Engl.
Transl.)].
38. A. L. Baumstark, F. Kovac, and P. C. Vasquez, Heterocycl.
Commun., 2002, 8, 9.
39. M. Freccero, R. Gandolfi, M. SarziꢀAmadè, and A. Rastelli,
J. Org. Chem., 2003, 68, 811.
40. G. V. Shustov and A. Rauk, J. Org. Chem., 1998, 63, 5413.
41. M. N. Glukhovtsev, C. Canepa, and R. D. Bach, J. Am.
Chem. Soc., 1998, 120, 10528.
42. G. V. Shustov and A. Rauk, Can. J. Chem., 1999, 77, 74.
43. X. H. Du and K. N. Houk, J. Org. Chem., 1998, 63, 6480.
44. D. Cremer, E. Kraka, and P. G. Szalay, Chem. Phys. Lett.,
1998, 292, 97.
22. H. Eyring, J. Chem. Phys., 1935, 3, 107.
23. F. Minisci, L. H. Zhao, F. Fontana, and A. Bravo, Tetraꢀ
hedron Lett., 1995, 36, 1697.
24. A. Bravo, F. Fontana, G. Fronza, F. Minisci, and L. H.
Zhao, J. Org. Chem., 1998, 63, 254.
45. R. Crehuet, J. M. Anglada, D. Cremer, and J. M. Bofill,
J. Phys. Chem. A, 2002, 106, 3917.
46. W. Koch and M. C. Holthausen, A Chemist´s Guide to Denꢀ
sity Functional Theory, 2nd ed., J. Wiley and Sons, New
York, 2001, 313 pp.
25. M. Singh and R. W. Murray, J. Org. Chem., 1992, 57, 4263.
26. P. Neta, R. E. Huie, and A. B. Ross, J. Phys. Chem. Ref.
Data, 1990, 19, 413.
27. A. L. Baumstark, M. Beeson, and P. C. Vasquez, Tetraꢀ
hedron Lett., 1989, 30, 5567.
Received March 18, 2005;
in revised form June 25, 2005