Oxidation of secondary alcohols and ethers by dimethyldioxirane

Article abstract of DOI:10.1139/v99-011

The oxidation of several series of secondary alcohols 2-9, ethers 10- 17, and related derivatives 18 and 19, by dimethyldioxirane, 1, in acetone at 25°C produced the corresponding ketones in good to excellent yields for all but two cases. (The exceptions: oxidation of 1-methoxy-2-methyl-1- phenylpropane (48%) and 1-methoxy-2,2-dimethyl-1-phenylpropane (24%).) The oxidation of the secondary alcohols was found to yield k₂ values that were roughly 10-fold greater than those of the corresponding methyl ethers. The rate constant for oxidation of a silyl ether was slightly lower than that for the corresponding methyl ether while that for the ester derivative was roughly half the value. For oxidation of alcohols and methyl ethers, the k₂ values became smaller as the R'' series (Me, Et, nPr, iPr, and tBu) increased in steric bulk (p* = 1.7; r = 0.998 and p* = 3.2; r = 0.95, respectively). The Hammett study for the oxidation of the methyl ethers of α-methyl-p- benzyl alcohols (10, 20-25) yielded a ρ value of -0.74. The activation parameters for oxidation of the parent compound of the ether series (1- methoxy-1-phenylethane) were ΔH(+) = 14.8 ± 0.5 kcal/mol, Δ(+) = -21.9 eu, ΔG(+) = 21.3 kcal/mol, k₂ (25°C) = 1.6 x 10^-3 M^-1 s^-1. The mechanistic aspects of the oxidation are discussed in relation to two mechanistic extremes: (a) direct insertion of the oxygen atom into the CH bond and (b) direct abstraction of the H by dimethyldioxirane to yield a caged-radical pair, with subsequent coupling to hemi-ketal intermediates that fragment to yield acetone, alcohol or water, and ketone as the final products.

Full text of DOI:10.1139/v99-011

308
Oxidation of secondary alcohols and ethers by
dimethyldioxirane
Alfons L. Baumstark, Franci Kovac, and Pedro C. Vasquez
Abstract: The oxidation of several series of secondary alcohols 2–9, ethers 10–17, and related derivatives 18 and 19,
by dimethyldioxirane, 1, in acetone at 25ЊC produced the corresponding ketones in good to excellent yields for all but
two cases. (The exceptions: oxidation of 1-methoxy-2-methyl-1-phenylpropane (48%) and 1-methoxy-2,2-dimethyl-1-
phenylpropane (24%).) The oxidation of the secondary alcohols was found to yield k2 values that were roughly 10-fold
greater than those of the corresponding methyl ethers. The rate constant for oxidation of a silyl ether was slightly
lower than that for the corresponding methyl ether while that for the ester derivative was roughly half the value. For
′′
oxidation of alcohols and methyl ethers, the k2 values became smaller as the R series (Me, Et, nPr, iPr, and tBu)
increased in steric bulk (ρ* = 1.7; r = 0.998 and ρ* = 3.2; r = 0.95, respectively). The Hammett study for the
oxidation of the methyl ethers of α-methyl-p-benzyl alcohols (10, 20–25) yielded a ρ value of –0.74. The activation
parameters for oxidation of the parent compound of the ether series (1-methoxy-1-phenylethane) were ∆H‡ = 14.8 ±
0.5 kcal/mol, ∆S‡ = –21.9 eu, ∆G‡ = 21.3 kcal/mol, k2 (25ЊC) = 1.6 × 10–3 M–1 s–1. The mechanistic aspects of the
oxidation are discussed in relation to two mechanistic extremes: (a) direct insertion of the oxygen atom into the C —H
bond and (b) direct abstraction of the H by dimethyldioxirane to yield a caged-radical pair, with subsequent coupling
to hemi-ketal intermediates that fragment to yield acetone, alcohol or water, and ketone as the final products.
Key words: dimethyldioxirane, oxidation.
Résumé : L’oxydation de plusieurs séries d’alcools secondaires (2–9), d’éthers (10–17) et de dérivés apparentés (18 et
19), par le diméthyldioxirane (1) dans l’acétone, à 25ЊC, conduit aux cétones correspondantes avec des rendements
allant de bons à excellents (à l’exception des deux cas suivants : 1-méthoxy-2-méthyl-1-phénylpropane (48%) et 1-
méthoxy-1-phénylpropane (24%)). On a observé que les valeurs de k2 des alcools secondaires sont environ 10 fois plus
élevées que celles des éthers méthyliques correspondants. La constante de vitesse d’oxydation d’un éther silylé est
légèrement inférieure à celle de l’éther méthylique correspondant alors que celle du dérivé ester est approximativement
égale à la moitié de la valeur. Pour l’oxydation des alcools et des éthers méthyliques, les valeurs de k2 deviennent de
′′
plus en plus faibles avec une augmentation de l’empêchement stérique (Me, Et, nPr, iPr, t-Bu) de la série R (ρ* = 1,7;
r = 0,998 et ρ* = 3,2; r = 0,95 respectivement). Une étude de Hammett pour l’oxydation des éthers méthyliques des
alcools benzyliques 10 et 20–25 conduit à une valeur de ρ de –0,74. Les paramètres d’activation pour l’oxydation du
produit parent de la série des éthers (1-méthoxy-1-phényléthane) sont: ∆H = 14,8 ± 0,5 kcal/mol, ∆S = 21,9 ue; ∆G =
21,3 kcal/mol et k2 (25ЊC) = 1,6 × 10–3 M–1 s–1. On discute des aspects mécanistiques de l’oxydation en relation avec
deux mécanismes extrêmes : (a) insertion directe de l’atome d’oxygène dans la liaison C—H et (b) enlèvement direct
de l’hydrogène par le diméthyldioxirane, conduisant à une paire de radicaux en cage, suivie d’un couplage subséquent
formant des intermédiaires hémicétals qui se fragmentent ensuite pour donner de l’acétone, de l’alcool ou de l’eau et la
cétone comme produits finals.
Mots clés : diméthyldioxirane, oxydation.
[Traduit par la Rédaction] Baumstark et al. 312
considerably less reactive than methyl(trifluoromethyl)diox-
irane (4), dimethyldioxirane is often the reagent of choice
because of its facile, inexpensive preparation. For example,
dimethyldioxirane has been shown to be useful (1) for olefin
epoxidation, heteroatom oxidation, and oxidation of C—H
bonds of saturated hydrocarbons, phenyl-substituted hydro-
Dimethyldioxirane, 1, has become an extremely versatile
reagent for the fast, mild oxidation of a great variety of or-
ganic substrates (for reviews, see ref. 1) since the initial re-
port of its isolation and characterization (2, 3). Although
Received September 2, 1998.
A memorial to Professor P.D. Bartlett. Your teaching continues to inspire!
A.L. Baumstark,1 F. Kovac, and P.C. Vasquez. Department of Chemistry, Laboratory for BCS (Biological and Chemical
Sciences), Georgia State University, Atlanta, GA 30303, U.S.A.
1Author to whom correspondence may be addressed. Telephone: (404) 651-1716. Fax: (404) 651-1416.
e-mail: chealb@panther.gsu.edu
Can. J. Chem. 77: 308–312 (1999)
© 1999 NRC Canada

Baumstark et al.
309
Table 1. Second-order rate constants and product (ketone) yields for the oxidation of secondary alcohols, ethers, and related
compounds by dimethyldioxirane in acetone at 25ЊC.
b
R′
R
Z
k2 (M–1 s–1) a
% Yield
Relative reactivity
′′
Alcohol
2 c
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Bz
Me
Et
nPr
iPr
tBu
Ph
Cyclopropyl
Me
H
H
H
H
H
H
H
H
(2.24±0.06) × 10–2
(1.48±0.03) × 10–2
(1.45±0.03) × 10–2
(1.02±0.02) × 10–2
(6.9±0.1) × 10–3
(8.9±0.1) × 10–3
(1.97±0.03) × 10–2
(7.1±0.1) × 10–3
98
92
92
90
90
96
92
85
1.00
0.66
0.65
0.46
0.31
0.40
0.88
0.32
3
4
5
6
7
8
9
Ethers
10
11
12
13
14
15
16
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Bz
Me
Et
nPr
iPr
tBu
Ph
Cyclopropyl
Me
Me
Me
Me
Me
Me
Me
Me
Me
(1.63±0.03) × 10–3
1.03±0.02 × 10–3
(7.6±0.1) × 10–4
(2.76±0.03) × 10–4
(2.23±0.03) × 10–4
(1.79±0.04) × 10–3
(1.52±0.03) × 10–3
(8.4±0.1) × 10–4
90
81
86
48
24
84
88
80
0.073
0.046
0.034
0.012
0.010
0.080
0.068
0.038
17
Miscellaneous
18
19
Ph
Ph
Me
Me
SiMe3
Ac
(1.48±0.03) × 10–3
(9.2±0.1) × 10–4
95
96
0.066
0.041
aObtained under pseudo-first-order conditions; errors are the standard deviations between duplicate experiments.
bNMR yield (±4%) from reactions with at least a 3 to 1 ratio of dioxirane to substrate.
cReference 5f.
carbons, aldehydes, and alcohols (5). In addition, there have
been several reports on the use of 1 for the removal of
benzyl protecting groups of alcohols (6) as well as the oxi-
dation of other ethers, acetals, and orthoesters (7). Similar
conversions have been carried out with methyl(trifluoro-
methyl)dioxirane (8). An earlier study (5e) of the reaction of
1 with benzaldehydes to produce benzoic acids indicated
that the formal CH insertion process involved radical inter-
mediates. Recently, a mechanistic study (5f) on the reaction
of substituted α-methylbenzyl alcohols with dimethyldioxi-
rane yielded results that could be interpreted as consistent
with a direct reaction to yield caged-radical pairs. CH inser-
tion studies on more difficult-to-oxidize positions by dio-
xiranes have been interpreted (8) to be consistent with a
concerted insertion process. We report here a study of the
oxidation of a series of secondary alcohols, the correspond-
ing ethers, and related compounds by dimethyldioxirane in
acetone.
responding methyl ethers, 10–17, and related compounds (1-
trimethylsilyloxy-1-phenylethane, 18; 1-acetoxy-1-phenylethane,
19; 1-(p-anisyl)-1-methoxyethane, 20; 1-(p-tolyl)-1-methoxy-
ethane, 21; 1(p-fluorophenyl)-1-methoxyethane, 22; 1-(p-
chlorophenyl)-1-methoxyethane, 23; 1-(p-bromophenyl)-1-
methoxyethane, 24; 1-(p-trifluoromethylphenyl)-1-methoxy-
ethane, 25) produced the corresponding ketones in good to
excellent yields in most cases (reaction [1]). Generally, un-
der similar conditions the yields of ketones were higher from
the oxidation of alcohols 2–9 than from that of the methyl
ethers 10–17.
In most cases, at least a threefold excess of 1 was neces-
ssary to achieve efficient conversion. For all but the slowest
oxidation reactions, use of an inert (N2) atmosphere resulted
in product yields within experimental error of those obtained
when open to the atmosphere. Low yields of ketones were
obtained in two cases, the dimethyldioxirane oxidation of 1-
methoxy-2-methyl-1-phenylpropane, 13, and 1-methoxy-2,2-
dimethyl-1-phenylpropane, 14 (48% and 24%, respectively).
The products (ketones) were isolated and identified by com-
parison of NMR and GC–MS data with those of authentic
samples. The product yields are listed in Table 1.
The reaction of excess dimethyldioxirane, 1, with second-
ary alcohols (1-phenylethan-1-ol, 2; 1-phenylpropan-1-ol,
3; 1-phenylbutan-1-ol, 4; 1-phenyl-2-methylpropan-1-ol,
5; 1-phenyl-2,2-dimethylpropan-1-ol, 6; benzhydrol, 7; α-
cyclopropylbenzyl alcohol, 8; 1-phenyl-2-propanol, 9), cor-
Kinetic studies of the oxidation of 2–19 were carried out
in acetone at 25ЊC by UV techniques. Excellent linear corre-
lations were obtained under pseudo-first-order conditions
with either the dioxirane or the substrate in at least a 10-fold
OZ
O
O
O
O
k2
C
R'
H
[1]
acetone
R"
R"
R'
1
2 - 25
excess
© 1999 NRC Canada

310
Can. J. Chem. Vol. 77, 1999
Table 2. Effect of temperature and added solvents on the second-order rate constants for oxidation of 1-
methoxy-1-phenylethane (10) by dimethyldioxirane.
k2 (M–1 s–1)
Temperature ±0.3ЊC
Solvent
(1.37±0.03) × 10–3
(1.63±0.03) × 10–3
(2.59±0.03) × 10–3
(4.65±0.05) × 10–3
(5.74±0.06) × 10–3
(1.05±0.01) × 10–2
(2.44±0.03) × 10–3
(1.73±0.03) × 10–3
(1.82±0.02) × 10–3
20.0ЊC
25.0ЊC
30.0ЊC
35.0ЊC
40.0ЊC
45.0ЊC
25.0ЊC
25.0ЊC
25.0ЊC
Acetone
Acetone
Acetone
Acetone
Acetone
Acetone
H2O–acetone (1.0:5.5 by vol)
CH2Cl2–acetone (1.0:1.0 by vol)
CHCl3–acetone (1.0:1.0 by vol)
Table 3. Product yields (ketone) and second-order rate constants
for the oxidation of 1-methoxy-1-arylethanes (secondary p-
substituted benzyl methyl ethers) by dimethyldioxirane in
acetone at 25ЊC.
chosen for futher characterization. Unlike epoxidation reac-
tions (5e) with 1, the ether oxidations did not show large
changes in rate upon addition of water to the solvent. This
result is similar to that found (5f) for the dimethyldioxirane
oxidation of α-methylbenzyl alcohols. Rate accelerations
that correlated with the hydrogen bond donor capacity of the
medium have been noted (5c) for the reaction of 1 with cis-
1,2-dimethylcyclohexane. Additional solvent effect studies
including some intramolecular hydrogen-bonding effects
have been described (5g). The rate constants for the oxida-
tion of 10 were determined at 5Њ intervals from 20° to 45ЊC
(see Table 2). The activation parameters determined by the
Arrhenius method were found to be ∆H‡ = 14.8 ±
0.5 kcal/mol; ∆S‡ = –21.9 e.u.; ∆G‡ = 21.3 kcal/mol; k2
(25ЊC) = 1.63 × 10–3 M–1 s–1. In addition, the oxidation of p-
substituted analogs of 10 showed that electron-donating groups
increased the rate of oxidation while electron-withdrawing
groups decreased it. A Hammett plot of the second-order
rate constants at 25ЊC for oxidation of 10 and 20–25 by 1
showed an excellent LFER against σp constants with a ρ
value of –0.74 ± 0.03. These data are shown in Table 3.
Compound
p-X
% Yield a
k2 (M–1 s–1) b
20
21
10
22
23
24
25
MeO
Me
H
F
Cl
98 (98)
84 (94)
83 (90)
76 (89)
78 (82)
74 (80)
61 (62)
(3.10±0.05) × 10–3
(2.22±0.05) × 10–3
(1.63± 0.03) × 10–3
(1.31±0.03) × 10–3
(1.21±0.03) × 10–3
(1.11±0.03) × 10–3
(7.2±0.1) × 10–4
Br
CF3
a±4% under N2 (O2); 3 to 1 ratio of dioxirane to substrate.
bPseudo-first-order conditions with 10-fold excess of dioxirane.
excess. Either approach yielded k2 values within experimen-
tal error (±5%) as expected for this second-order reaction.
For convenience, most experiments were carried out with a
10-fold excess of dioxirane. Reproducibility between dupli-
cate runs was excellent (generally 1–2%). In general, the k2
values for oxidation of the alcohol series were at least 10-
fold greater than those for the corresponding methyl ether
series. The k2 values for 13 and 14, the only compounds for
which the product yields were poor, were the lowest in the
entire set of data. Formal replacement of the methyl ether in
10 by either the trimethylsilyl ether (18) or the acetate (19)
yielded k2 values that were slightly lower with excellent
product yields. Formal replacement of the phenyl group in
compounds 2 and 10 by a benzyl group (9 and 17) produced
k2 values that were three- and twofold slower, respectively.
Oxidation of both the alcohol and methyl ether series
showed the same general trend: as the alkyl group in the α-
The oxidations of secondary alcohols and hydrocarbons
by 1 and methyl(trifluoro-methyl)dioxirane have been inter-
preted (1, 5a–d, 8) in terms of direct reaction without in-
volvement of a chain process. Recently, Ingold and co-
workers (9) have shown, based on radical rearrangement ar-
guments, that the oxidation of 2-cyclopropylpropane by 1 is
not a free-radical chain process. Historically, the AIBN cata-
lyzed O2 oxidation of benzyl methyl ether was found (10a)
to be faster than that of benzyl alcohol. The direct reaction
of 1 with secondary methyl ethers also shows different char-
acteristics from those of free-radical chain processes. The re-
sults clearly show that the oxidation of secondary alcohols
by 1 is at least 10-fold faster than that of the corresponding
methyl ethers and related derivatives. The Hammett ρ value
of –0.74 for oxidation of methyl ethers 10 and 20–25 by 1 is
essentially equivalent to that of –0.7, as was obtained (5f :
note, the ρ value reported was off by 2.303; correct value is
–0.7) for the oxidation of alcohol 2 derivatives. Free-radical
reactions on benzyl ethers, hydrogen atom abstraction by
alkyl peroxy radicals on benzyl phenyl ether (5, 9a), or by
benzoyloxy radicals on dibenzyl ether (5, 10b), were found
to show ρ values of –0 and –0.65, respectively. The direct
reaction of p-nitroperbenzoic acid with substituted benzyl
′′
position (R , Table 1) becomes bulkier, the k2 value becomes
smaller (Me > Et > nPr > iPr > tBu). An excellent correla-
tion (r = 0.998) was obtained for a plot of log k2 vs. σ* with
a Taft ρ* value of 1.7 for compounds 2–6. The correspond-
ing methyl ether series, 10–14, showed a marginal correla-
′′
tion (r = 0.95) with a ρ* of 3.2. The k2 value for R =
cyclopropyl in both series is only slightly lower than for R =
′′
methyl, and higher than for R = ethyl. A minor discrepancy
′′
between the two series is for R = phenyl: k2 for 7 falls be-
′′ ′′
tween 5 (R = iPr) and 6 (R = tBu); and k2 for 15 is slightly
′′
faster than that for 10 (R = Me). The kinetic data (second-
order rate constants k2) are summarized in Table 1.
The oxidation reaction of 1-methoxy-1-phenylethane (10)
and p-substituted analogs 20–25 by dimethyldioxirane was
© 1999 NRC Canada

Baumstark et al.
311
Scheme 1.
OZ
C
OZ
C
HO
O
OH
fast
R'
O
R'
R"
R"
a
b
O
C
cage
OZ
R'
O O
O
k2
H
ZOH
R"
R'
‡
R"
OZ
H
OZ
O
δ
δ
OH
R'
C
O
R"
R'
O
R
methyl ethers was found (11a) to show a ρ+ value of –0.9.
The differences in activation parameters for oxidation of the
secondary methyl ethers by 1 vs. those of the secondary al-
cohols (5f) are roughly an increase of 3 kcal/mol in ∆H‡ and
a decrease in ∆S‡ (less negative). A ∆S‡ of approximately –
15 eu has been estimated (8) for the oxidation of cyclo-
hexanol by methyl(trifluoromethyl)dioxirane. The relative
formed regardless of pathway. The two mechanisms are not
necessarily mutually exclusive. The trajectory of approach
of the dioxirane to the C—H bond remains to be elucidated.
In conclusion, the oxidation of secondary alcohols, methyl
ethers, and related derivatives with excess dimethyldioxirane
generally produces high yields of ketones. Reaction times
are convenient if a 10-fold excess of dioxirane is employed.
The differences in reactivity with each series and between
different series of compounds show the method to be of syn-
thetic utility.
′′
reactivity as a function of R within the alcohol and methyl
ether series shows similar trends. Plots of log k2 vs. σ* con-
stants give reasonable correlations with the greater sensitiv-
ity found in the ether series. The sign of the ρ* is opposite
that of the Hammett ρ value and opposite that found (11b)
for ρ* for the reaction of p-nitroperbenzoic acid with substi-
tuted alkenes. The ρ* of reaction [1] presumably is strongly
influenced by steric effects. The data showed a poor correla-
tion with the Taft steric parameter (ES). The lower reactivity
of methyl ether series coupled with the larger range of rela-
tive reactivity could be viewed as a steric effect. Alterna-
tively, it could be argued that the alcohols undergo faster
oxidation by 1 due to hydrogen bonding with 1 and (or) the
solvent. Our deuterium isotope effect study (5f) for the oxi-
dation of 2 with dimethyldioxirane has shown a kOH/kOD
value of 1.09. This value could represent a large secondary
effect or perhaps an extremely small primary effect, indica-
tive of a hydrogen bond between the alcohol OH and the
dioxirane in the transition state.
The reaction of dimethyldioxirane with secondary alco-
hols, ethers, and related derivatives can be viewed in terms
of two mechanistic extremes: (a) a direct insertion process
and (b) a caged-radical process (Scheme 1). The direct inser-
tion process has been postulated (1, 8) to involve an
“oxenoid” atom insertion into the side of the C—H bond via
a multicentered transition state. Our previous study (5f) of
the oxidation of α-methylbenzyl alcohols by 1 was inter-
preted in terms of a caged-radical process in which direct re-
action of 1 and the substrate yields a singlet pair of radicals
that are not solvent separated. For the caged-radical process
to be valid, combination of the radicals must be faster than
both rearrangement of the carbon-based radical and escape
from the cage. In addition, for the oxidation of methyl ethers
(Z = Me), the caged-radical process must yield a hemiketal,
which would be expected to rapidly fragment to the final
products. The concerted process has been favored (8) as the
simplest explanation due, in part, to the lack of detection of
(hemiketal) intermediates for secondary alcohol and hydro-
carbon oxidation. At present, the data do not clearly distin-
guish between these two possibilities. Note that for
secondary ether oxidation, hemiketal intermediates must be
A. General
Infrared spectra were recorded on a Perkin–Elmer system
2000 FT-IR spectrophotometer. All
1H NMR and 13C NMR
spectra were obtained in chloroform-d1 at 30ЊC on a Varian
300 MHz spectrometer. The GS–MS data were obtained on a
Shimadzu GC-17A gas chromatograph coupled to a Shimadzu
QP-5000 mass spectrometer.
All solvents were either spectral or HPLC grade (Aldrich).
Dimethyldioxirane (1) was prepared and isolated by a modi-
fied version (12) of the general method developed by Murray
and Teyaraman (3a). The redistilled acetone solution (≤0.1
M) of 1 was stored at –20ЊC with little or no decomposition
after several weeks (over MgSO4). Initial dioxirane concen-
tration was determined by NMR techniques (integration of
thioanisole and oxidation product signals) for reaction of the
stock solution with a known quantity of thioanisole. The
value was within experimental error (±5%) of that deter-
mined by monitoring the change in absorbance at 330 nm
before and after reaction with excess 2,3-dimethyl-2-butene
(added neat via syringe) and dividing by the ε value of 12.9.
All the final products (ketones) and alcohols 2, 3, 6, 7, and 9
were commercially available (Aldrich) and were used with-
out further purification. Alcohols 4, 5, and 8 were prepared
from the corresponding ketones (Aldrich) by conventional
NaBH4 reduction and were purified by vacuum distillation.
Methyl ethers 10–17 and 20–25 were prepared in -65%
yields by a two-step procedure: first, the parent ketones of
alcohols 2–9 were converted to their corresponding dimethyl
ketals (13) by treatment with methyl orthoformate in metha-
nol (HCl catalyst); after purification by vacuum distillation
the dimethyl ketals were converted to the corresponding
methyl ethers (14a) by treatment with AlCl3 in ethyl ether,
followed by LiAlH4; the methyl ethers were purified by vac-
uum distillation. The boiling points of the methyl ethers
were in good agreement with literature values (14). Silyl
© 1999 NRC Canada

312
Can. J. Chem. Vol. 77, 1999
ether 18 and acetate 19 were prepared from alcohol 2 by
standard silylation (Me3SiCl–Et3N) and acylation (Ac2O)
procedures, and were purified by vacuum distillation. All
compounds were characterized by physical and spectral
(1H NMR, IR, and GC–MS) data.
0004), and to the Georgia State University Research Fund
for financial support of this work.
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Murray and Gu. Daquan. J. Chem. Soc. Perkin Trans. 2, 451
(1994); (d) D. Kuck, A. Schuster, C. Fusco, M. Fiorentino, and
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dron Lett. 35, 8751 (1994); (g) R.W. Murray and H. Gu. J.
Phys. Org. Chem. 2, 751 (1996).
B. Kinetic studies
Kinetic experiments were performed using a Shimadzu
UV-3101PC UV-VIS-NIR scanning spectrometer. The cell
temperature (25ЊC unless specified) was maintained via a
constant temperature circulating bath (± 0.3ЊC) and was checked
before and after each kinetics run using a YSI model 425C
telethermometer with a number 423 probe. The following is
a general, representative procedure for the kinetics studies:
dimethyldioxirane solutions (1.000 mL, 10 equiv.) of known
concentration (≤0.100 M) in dried acetone were placed in a
1 cm UV cell at the desired temperature. After temperature
equilibration, 1.0 equiv. of the substrate in 0.100 mL acetone
was added via syringe and the solution was rapidly mixed by
bubbling air via disposable pipet. The reaction was moni-
tored by measuring the change in absorbance at 380 nm vs.
time (pseudo-first-order conditions). The ε value at 380 nm
for 1 is 4.51 ± 0.03. The value of the relative absorbance
was determined by subtraction of the final value from the in-
stantaneous value as a function of time. The pseudo-first-
order rate constants (kobs) were calculated by plotting the ln
(relative absorbance) versus time and were linear for at least
two half-lives. The second-order rate constants (k2) were
calculated by dividing the pseudo-first-order rate constant
(kobs) by the initial concentration of dimethyldioxirane.
Reproducibility between runs was generally better than 5%
of the k2 value. Correlation coefficients were excellent for
all experiments. Several experiments were repeated under
pseudo-first-order conditions with the substrate in 10-fold
excess. The rate constants were found to be essentially iden-
tical (within experimental error) to those determined with 1
in excess, confirming the validity of the approach.
6. (a) R. Csuk and P. Dörr. Tetrahedron, 50, 9983 (1994); (b) B.A.
Marples, J.P. Muxworthy, and K.H. Baggaley. Synlett, 646
(1992).
7. (a) R. Curci, L. D’Accolti, M. Fiorentino, C. Fusco, W. Adam,
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4225 (1992); (b) A. Messegues, M. Fener, F. Sanchez-Baeza,
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activation and homogenous catalytic oxidation. Edited by L.I.
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Chem. 164, 45 (1993); (c) R. Curci, A. Dinoi, and M.F. Rubino.
Pure Appl. Chem. 67, 811 (1995).
9. R. Vanni, S.J. Garden, J.T. Banks, and K.U. Ingold. Tetrahe-
dron Lett. 7999 (1995).
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50, 4609 (1985).
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(1950).
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Barton. J. Org. Chem. 45, 2566 (1980).
C. Product studies
The products from the reaction of compounds 2–25 with 3
equiv. of isolated dimethyl-dioxirane at room temperature
(-1.5 mmol of substrate) were analyzed after 6–24 h by
GC–MS. The GC–MS data showed the ketones to be formed
in high yield in all cases except for oxidation of 13 and 14.
The GC–MS data were in excellent agreement with those of
authentic ketone samples. After removal of the solvent (ace-
tone), the residue was taken up in CDCl3 and then was ana-
1
1
lyzed by H NMR spectroscopy. The H NMR data were in
agreement with those of authentic (commercial) ketone sam-
ples. For most cases (within experimental error), little or no
reduction in final ketone yields was observed for reactions
carried out under inert (N2) atmosphere. The percentage
yields (±4%) were determined (with anisole as added inter-
1
nal standard) from the H NMR spectral data.
Acknowledgment is made to the National Science Foun-
dation (CHE-9017230), to the U.S. Army ERDEC SEAS
subcontract via Clark Atlanta University (DAAA 15–94-K–
© 1999 NRC Canada