Mechanism of the dimethyldioxirane oxidation of N,N-dimethylanilines

Article abstract of DOI:10.1039/a706772k

Relative rates of dimethyldioxirane oxidation of a number of para-substituted N,N-dimethylanilines in acetone at 5°C are compared with those of reactions with methyl iodide and other oxidants. The reactions with dimethyldioxirane followed the Hammett relationship with a ρ value of -1.0. Measurement of the second order rate constants for the dimethyldioxirane reactions in aqueous acetonitrile containing potassium nitrate at 21°C, showed better correlation with the Hammett relationship (ρ = 0.89) than with the Okamoto-Brown model (ρ⁺ = 0.56). The reaction rates are accelerated greatly in the presence of water such that the respective pseudo first order rate constants for the oxidation of N,N-dimethyl-4-nitroaniline in acetone and water are 6.3 × 10^-3 and 5.86 s^-1, respectively. All of the data are consistent with a concerted electrophilic mechanism and there is no evidence of free radical or electron transfer reactions.

Full text of DOI:10.1039/a706772k

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Mechanism of the dimethyldioxirane oxidation of N,N-dimethyl-
anilines
,
a
a
,b
P. Christopher Buxton,* Julie N. Ennis, Brian A. Marples,*
b
b
Victoria L. Waddington and Todd R. Boehlow (in part)
a
SmithKline Beecham Pharmaceuticals, New Frontiers Science Park (South), Third Avenue,
Harlow, Essex, UK CM19 5AW
b
Loughborough University, Loughborough, Leicestershire, UK LE11 3TU
Relative rates of dimethyldioxirane oxidation of a number of para-substituted N,N-dimethylanilines in
acetone at 5 ЊC are compared with those of reactions with methyl iodide and other oxidants. The reactions
with dimethyldioxirane followed the Hammett relationship with a ñ value of ؊1.0. Measurement of
the second order rate constants for the dimethyldioxirane reactions in aqueous acetonitrile containing
potassium nitrate at 21 ЊC, showed better correlation with the Hammett relationship (ñ ؍ 0.89) than with
؉
the Okamoto–Brown model (ñ ؍ 0.56). The reaction rates are accelerated greatly in the presence of
water such that the respective pseudo first order rate constants for the oxidation of N,N-dimethyl-4-
؊3
؊1
nitroaniline in acetone and water are 6.3 × 10 and 5.86 s , respectively. All of the data are consistent
with a concerted electrophilic mechanism and there is no evidence of free radical or electron transfer
reactions.
Introduction
neutral peroxides. Benzoyl peroxide dealkylates tertiary amines
13
and its rate determining step is thought to be electrophilic.
In recent years, dioxiranes 1 have been used to carry out a wide
variety of synthetically useful transformations. However, some
mechanistic details remain unclear and evidence has been pre-
sented suggesting that some dioxirane reactions may occur via
bis(oxyl) diradicals 2.
tert-Butyl hydroperoxide, also, dealkylates tertiary amines but is
thought to react via a homolytic process involving tert-butoxyl
1
14
radicals. tert-Butyl hydroperoxide in the presence of a
15
vanadium catalyst, which is reported to lead to N-oxidation
rather than dealkylation, was also used.
O
O
R
R
. O
R
R
.
O
Results and discussion
1
2
The competition reactions were carried out by mixing one
equivalent of the para-substituted N,N-dimethylaniline with
one equivalent of the unsubstituted N,N-dimethylaniline in an
acetone solution and adding one equivalent of dimethyl-
dioxirane–acetone solution. The reactions were carried out at
2
3
4
Epoxidation, sulfur oxidation and C᎐H insertion reac-
tions of dimethyldioxirane have been shown to be electrophilic
by means of linear free energy relationship (LFER) correlations
and a mechanistic study by Adam and Golsch concluded that
the dimethyldioxirane oxidation of nitrogen heteroarenes
follows an S 2 mechanism. Other studies, however, on the oxid-
ation of para-substituted benzaldehydes to the corresponding
acids and an LFER study on the oxidation of some α-methyl-
benzyl alcohols by Baumstark and co-workers have shown
evidence of radical character, as has recent work on alkene
epoxidation and alkane oxidation by Minisci and co-workers.
5
0
–5 ЊC and continued overnight to ensure total consumption of
the dimethyldioxirane, enabling 50% conversion to be assumed.
Relative reaction rates were determined by monitoring the
consumption of starting material by HPLC. In each case, the
N-oxide was the major product and no evidence was found for
dealkylation [reaction (1)]. The reactions of dimethyldioxirane
N
6,7
8
9
O–
10
11
Also, studies by Crandell and Curci and their co-workers on
the dimethyldioxirane oxidation of phenols and by Adam and
Schonberger on the oxidation of hydroquinones propose
electron transfer processes.
NMe2
N+Me2
O
O
1
2
+
(1)
X
X
We report an investigation into the nature of the dimethyl-
dioxirane oxidation of a series of para-substituted N,N-
dimethylanilines in an attempt to further probe the mechanism
of amine oxidations and explore the possibility of substituent-
induced changes of mechanism. The relative rates of oxygen
transfer by dimethyldioxirane to the nitrogen of several N,N-
dimethylanilines are described and compared to the relative
rates obtained for methyl iodide methylation, benzoyl peroxide
oxidation and tert-butyl hydroperoxide oxidation. The sub-
strates are sterically identical around the nitrogen so the only
factor affecting the rate of reaction is the electron demand of
the para-substituent. Methyl iodide was used because, as in
with each N,N-dimethylaniline were carried out to confirm
this and the products obtained were compared with N-oxides
prepared using performic acid.
16
The competition reactions were repeated, replacing the
dimethyldioxirane with (i) methyl iodide, (ii) benzoyl peroxide,
(iii) tert-butyl hydroperoxide and (iv) tert-butyl hydroperoxide
and vanadyl acetylacetonate. As with dimethyldioxirane, these
reactions were run at 0–5 ЊC and continued to completion. The
tert-butyl hydroperoxide reaction was also carried out at 70 ЊC
as this is the more usual temperature at which to carry out
14
dealkylations of tertiary amines with this reagent.
5
Adam’s study, it is reported to react with tertiary amines via
The results of the competition reactions are shown in Table 1
and were calculated by determining the ratio of the reactants
remaining at the end of the reaction. Here the unsubstituted
a well established S 2 mechanism. Benzoyl peroxide and
N
tert-butyl hydroperoxide, like dimethyldioxirane, are both
J. Chem. Soc., Perkin Trans. 2, 1998
265

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Table 1 Summary of the relative data for the reaction of para-substituted N,N-dimethylanilines in acetone with the reagents shown
krel
Methyl
iodide
0–5 ЊC
Benzoyl
peroxide
0–5 ЊC
tert-Butyl
hydroperoxide
0–5 ЊC
tert-Butyl
hydroperoxide
70 ЊC
tert-Butyl
hydroperoxide
ϩ catalyst
Dimethyldioxirane
0–5 ЊC
X
MeO
H
CI
2.13 ± 0.19
1.00 ± 0.00
0.87 ± 0.01
0.18 ± 0.04
3.65 ± 1.14
1.00 ± 0.00
0.70 ± 0.02
0.65 ± 0.10
27.6 ± 12.9
1.00 ± 0.00
0.06 ± 0.02
0.03 ± 0.02
1.19 ± 0.08
1.00 ± 0.00
0.86 ± 0.01
0.91 ± 0.08
0.89 ± 0.01
1.00 ± 0.00
0.52 ± 0.12
0.52 ± 0.10
1.03 ± 0.10
1.00 ± 0.00
1.13 ± 0.03
0.43 ± 0.06
NO2
Table 2 Summary of the relative rate data and substituent constants
used in the LFER plot for the reaction of dimethyldioxirane with
para-substituted N,N-dimethylanilines in acetone at 0–5 ЊC
Table 3 Run 3: absolute second order rate constants for the reactions
of dimethyldioxirane with para-substituted N,N-dimethylanilines in
50:50 acetonitrile–0.1  KNO solution
3
3
Ϫ1 Ϫ1
Run 1
Run 2
X
k /dm mol
s
krel
log krel
x
X
σa
σϩ b
krel
log krel
krel
log krel
MeO
H
CI
1777
1109
611
1.60
1.00
0.55
0.19
0.20
0.00
Ϫ0.26
MeO
H
Cl
Ϫ0.27
0.00
0.23
Ϫ0.78
0.00
0.11
2.13
1.00
0.87
0.18
0.33
0.00
Ϫ0.06
2.91
1.00
0.49
0.23
0.46
0.00
Ϫ0.31
NO2
213
Ϫ0.72
NO2
0.78
0.79
Ϫ0.74
Ϫ0.64
RH
0.998
ρ
Ϫ0.89
c
R
ρ
0.987
0.964
H
d
Ϫ0.99
Ϫ1.01
R
ρ
0.952
Ϫ0.58
OB
ϩ
e
R
ρ
0.963
Ϫ0.67
0.979
Ϫ0.71
OB
ϩ f
a
b
c
Taken from ref. 18. Taken from ref. 19. R , Correlation coefficient
H
d
for Hammett plot. ROB, Correlation coefficient for Okamoto–Brown
plot. ρ, Hammett constant. ρ , Okamoto–Brown constant.
e
f
ϩ
amine in each series has been given an arbitrary value of 1 and
the other rates in the series were calculated relative to this.
The results obtained indicate a similar qualitative trend for
dimethyldioxirane oxidation as for the electrophilic methyl
iodide and benzoyl peroxide reactions. The reactivity decreased
17
in the order MeO > H > Cl у NO2. The tert-butyl hydro-
peroxide reactions are less susceptible to changes in substit-
uent, the difference in rates being less marked, as would be
expected for a non-electrophilic reaction. These trends suggest
that the dimethyldioxirane oxidation of N,N-dimethylanilines is
electrophilic. All the reactions were run to completion, there-
fore, the relative rates obtained reflect a minimum difference in
reactivity.
Fig. 1 LFER plots of log (k /k ) versus σ for the reaction of dimethyl-
X
H
dioxirane with N,N-dimethylanilines
aniline were determined by controlled potential amperometry
on the polarographic reduction wave of dimethyldioxirane.
This avoids the large background interferences inherent with
spectroscopic methods for the detection of dimethyldioxirane.
The data obtained are summarised in Table 3.
18
The Hammett relationship was applied to the dimethyl-
dioxirane results giving an approximate ρ value of Ϫ0.99 (run 1,
Table 2). Similar treatment of the data with the Okamoto–
19
ϩ
Brown relationship gave ρ = Ϫ0.67. The highest negative ρ
value reported for a dimethyldioxirane reaction is Ϫ2.76 by
Murray and Gu for the C᎐H insertion reaction into para-
The reactions of dimethyldioxirane and para-substituted
N,N-dimethylanilines follow the Hammett relationship with a
ρ value of Ϫ0.89 which is similar to that obtained from the
relative rate data (Fig. 1). The agreement is good considering
that the two experimental approaches were conducted at differ-
ent temperatures and in different solvents. The electrochemical
measurements were carried out in aqueous acetonitrile at 21 ЊC
whereas the relative rate data were generated in acetone at
0–5 ЊC. The excellent correlation observed with the Hammett
plot of the absolute rate data suggests that this relationship is
4
substituted cumenes. Lower values (Ϫ0.77 and Ϫ0.76) have
3
been reported by Murray et al. for the electrophilic dimethyl-
dioxirane oxidation of para-substituted aryl methyl sulfides and
sulfoxides, respectively.
The dimethyldioxirane competition reactions were repeated
in a second run (Table 2) using a slightly different approach. In
this case the relative rates were calculated by comparing the
initial concentrations of anilines with the final concentrations
making no assumptions about the percentage conversion. The
20
more appropriate than that from the Okamoto–Brown plot
and lends further support to the hypothesis that the dimethyl-
dioxirane oxidation of substituted dimethylanilines is a con-
certed electrophilic process.
18
Hammett relationship was applied to data from both runs
Table 2) yielding a mean ρ value of Ϫ1.00 which suggests that
(
the conclusions drawn by Murray on the electrophilic nature of
the cumene and sulfide oxidations can be equally applied to the
dimethylaniline oxidations.
Although the relative rate measurements enable a competi-
tive assessment to be made between substrates they provide no
direct measure of the rate of reaction itself. In a third approach,
therefore, absolute second order rate constants for reactions of
dimethyldioxirane each with a known excess of the dimethyl-
The dimethyldioxirane oxidations show obvious similarities
to reactions of dimethylanilines with methyl iodide (Menschut-
21
kin reaction) which has a reported ρ value of Ϫ3.30 at 35 ЊC
in 90% aqueous acetone. Whereas, in the latter reactions the
transition state is thought to have developed almost a full posi-
tive charge, the reactions with dimethyldioxirane clearly have
much less charge development (Fig. 2) and may be a result, in
2
66
J. Chem. Soc., Perkin Trans. 2, 1998

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2
6,27
Me
δ+
Me
could be important in some dimethyldioxirane oxidations
and very recently ab initio model studies on primary amine
oxidations with dimethyldioxirane has confirmed the import-
δ−
δδ+
δδ− δ+
.
..... ..........
......... .........
Ar N
Ar N Me
I
O
O
H
δ−
O
H
28
Me
ance of solvent and hydrogen bonding effects.
Me
The rate constants observed in water are comparable to those
obtained for the oxidation of N,N-dimethyl-4-nitroaniline
using (trifluoromethyl)methyldioxirane in a relatively unreac-
tive solvent such as acetonitrile. It is speculated that the pres-
ence of water promotes faster reaction times and yields without
recourse to the use of fluorinated dioxiranes which are experi-
mentally more difficult to handle.
Fig. 2 Transition states for the reactions of dimethylanilines with MeI
and dimethyldioxirane
Table 4 Pseudo first order rate constants for the reaction between
dimethyldioxirane and N,N-dimethyl-4-nitroaniline in various solvents
at 25 ЊC
Ϫ1
Solvent
k/s
ε
All the data obtained are consistent with conclusions that the
dimethyldioxirane oxidation of dimethylanilines is electrophilic
in nature and does not involve the bis(oxyl) diradical 2 or elec-
tron transfer. In this respect the mechanism agrees with the
conclusions drawn by Curci and co-workers for the epoxidation
Water
Water–acetone
Acetone
5.86
78.5
—
20.7
10.4
37.5
37.5
Ϫ1
Ϫ3
Ϫ4
Ϫ4
1.55 × 10
6.30 × 10
4.10 × 10
4.50 × 10
6.88
8.00 × 10
5.05
6.52
5.19
6.34
Dichloroethane
Acetonitrilea
29
and oxygen insertion into alkane C᎐H bonds.
Acetonitrile
Ϫ5
Ethyl acetate
6.02
pH 1, 0.1  Buffer soln
pH 10, 0.1  Buffer soln
pH 5, 0.1  Buffer soln
pH 5, 0.01  Buffer soln
—
—
—
—
Experimental
Materials
Acetone was distilled from potassium permanganate prior to
a
use. Oxone (2KHSO ؒKHSO ؒK SO ) was supplied by Aldrich
(
Trifluoromethyl)methyldioxirane.
5 4 2 4
and used without further treatment. N,N-Dimethylaniline and
N,N-dimethyl-4-nitroaniline were supplied by Lancaster and
used as received. Dimethyldioxirane–acetone solutions were
part, of steric crowding in the transition state. Hydrogen bond-
ing is also thought to be important (see below).
30
synthesised according to literature procedure. Methyl iodide,
The rapidity of the reactions in aqueous acetonitrile is sur-
prising in view of our earlier observation that N,N-dimethyl-4-
nitroaniline was effectively unchanged after many hours of
reaction with dimethyldioxirane in acetone. Interestingly oxid-
ation of N,N-dimethyl-4-nitroaniline at 0 ЊC with dimethyl-
dioxirane (10 equiv.) in 50% aqueous acetone on a preparative
scale showed significant conversion to the N-oxide after only
one hour. The H NMR spectrum of the N-oxide in the reaction
mixture showed the expected peaks at δ 8.37 (2H , d, J 9), 8.21
(
benzoyl peroxide, tert-butyl hydroperoxide and vanadyl acetyl-
acetonate were supplied by Lancaster and used as received.
N,N-Dimethyl-4-methoxyaniline
and
N,N-dimethyl-4-
chloroaniline were prepared according to literature procedures
using sodium cyanoborohydride and para-formaldehyde in
31
1
glacial acetic acid. The purity was checked using H NMR
32
spectroscopy and melting point data. The N-oxides of the
1
N,N-dimethylanilines were prepared using performic acid also
3
16
according to literature procedures.
2
2H , d, J 9) and 3.7 (NMe , s). The reaction mixture also con-
2
tained some starting material and a minor impurity that we
were unable to isolate.
Instrumentation
NMR spectra were recorded using a Bruker AC 250 spec-
trometer. Amperometric measurements were recorded using a
BAS CV50W voltametric analyser with a hanging mercury
drop electrode and using the timebase mode for the
kinetic measurements. Spectrophotometric measurements were
recorded on a Beckmann 640i spectrophotometer using the
kinetics accessory. HPLC was performed using: Perkin-Elmer
series 3 pump, Perkin-Elmer LC55B spectrophotometric
detector and Spectraphysics integrator. A Hewlett Packard
The possibility of a strong solvent effect, as has been
22–24
observed
in other dioxirane oxidations, was investigated
kinetically by monitoring the loss of the UV absorbance of
N,N-dimethyl-4-nitroaniline when reacted with an excess of
dimethyldioxirane. The pseudo first order rate constants were
calculated in a number of solvents and the results are summar-
ised in Table 4.
With the exception of acetonitrile the pseudo first order rate
constant decreases with the relative permittivity of the medium.
This is a similar trend to that observed in the Menschutkin
reaction in which, as here, factors other than relative permit-
1
090 diode array detector was also used. The chromatographic
conditions used for the first run were as follows: column,
µBondapak C18 10 µm 3.9 × 300 mm; eluent, 6:4 methanol–
2
1
tivity are involved. Significantly there is a large increase in the
reaction rate in water and the increased reaction rate remains
relatively constant irrespective of the pH or ionic strength. The
magnitude of the rate increase in water is almost certainly not
due to the effects of relative permittivity alone and probably
arises from the strong hydrogen bonding nature of the solvent.
Such solvents would be expected to stabilise both the transition
state (Fig. 2) and the N-oxide reaction products which readily
water (NO and Cl), 1:1 methanol–water (MeO); flow rate, 1 ml
2
Ϫ1
min ; detection, UV@240 nm; injection volume, 20 µl.
The second set of results were obtained using the HPLC
method given below: column, Spherisorb S5 C1 15 cm × 4.6
mm; eluent, 0.05  phosphate buffer, 0.1% triethylamine pH 3.5
Ϫ1
(
NO and Cl), pH 4.0 (MeO); flow rate, 1 ml min ; detection,
2
UV@210 nm; injection volume, 20 µl.
25
form stable hydrates. Direct evidence of the latter was
1
observed particularly in the H NMR spectrum of N,N-
Relative rate studies of N,N-dimethylanilines
dimethyl-4-nitroaniline N-oxide. D O solutions of the N-oxide
An equimolar stock solution of N,N-dimethylaniline and para-
substituted N,N-dimethylaniline in acetone was prepared. An
aliquot of this solution was removed, placed in a flask covered
with aluminium foil to protect it from light, and cooled to
0–5 ЊC in an ice bath. One equivalent of dimethyldioxirane
solution was then added to the flask and the reaction stirred
magnetically overnight. Starch–iodide paper showed no
remaining oxidant. A 50 µl sample was removed and analysed
by HPLC. Two determinations were made for each reaction and
each reaction was carried out in duplicate.
2
3
2
showed resonances at δ 8.19 (2H , d, J 9), 7.89 (2H , d, J 9) and
.5 (NMe , s). Evaporation of the D O and suspension of the
3
2
2
1
hydrated residue in CDCl3 containing sieves afforded a
H
NMR spectrum identical to that of the N-oxide originally
isolated (see earlier). The role of hydrogen bonded solvents in
accelerating oxidation rates has previously been recognised by
Murray and Gu in studies of the dimethyldioxirane epoxidation
23
24
of ethyl (E)-cinnamate and oxidation of C᎐H bonds. We
and others have suggested intramolecular hydrogen bonding
J. Chem. Soc., Perkin Trans. 2, 1998
267

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The above procedure was repeated using aliquots of the same
stock solution and the following reagents: methyl iodide,
benzoyl peroxide and tert-butyl hydroperoxide. The tert-butyl
hydroperoxide reactions were also carried out at 70 ЊC and in
the presence of vanadyl acetylacetonate (0.025 equiv.). All the
reactions were run to completion. The relative rates were calcu-
lated by determining the ratio of reactants remaining at the end
of the reaction. This was achieved by first constructing a cali-
bration graph of actual ratio of unsubstituted aniline to substi-
tuted aniline versus observed peak area ratio for a series of
standard solutions containing varying known ratios of aniline
to substituted aniline. Using this graph, observed peak area
ratios of aniline to substituted aniline obtained from the chro-
matogram were converted to actual ratios. The reactions were
run to completion, consuming all the dioxirane, therefore, 50%
conversion could be assumed and the relative rates calculated
by taking the reciprocal of the ratio of aniline to substituted
aniline remaining at the end of the reaction. A summary of the
relative rate data obtained is given in Table 1.
were negligible) using standard linear regression statistics to
calculate the slopes and hence rate constants.
Note added in proof. Dimethyldioxirane oxidation of
N,N-dimethylaniline to the N-oxide has very recently been
33
reported.
Acknowledgements
We thank Loughborough University and SmithKline Beecham
for financial support.
References
1
2
(a) W. Adam and L. Hadjiarapoglou, Top. Curr. Chem., 1993, 164,
5; (b) R. Curci, in Advances in Oxygenated Processes, ed. A. L.
Baumstark, JAI Press, Greenwich, 1990, pp. 1–59.
R. W. Murray and D. L. Shang, J. Chem. Soc., Perkin Trans. 2, 1990,
349.
4
3 R. W. Murray, R. Jeyaraman and M. K. Pillay, J. Org. Chem., 1987,
5
2, 746.
4
5
R. W. Murray and H. Gu, J. Org. Chem., 1995, 60, 5673.
W. Adam and D. Golsch, Angew. Chem., Int. Ed. Eng., 1993, 32,
The dimethyldioxirane competition reactions were repeated
and analysed using quantitative HPLC. Again two determin-
ations were made for each reaction and each reaction was
done in duplicate. Standards of each N,N-dimethylaniline were
prepared and used to quantify the amounts of each present at
the end of the reaction. The relative rates were calculated using
7
37.
6
A. L. Baumstark, M. Beeson and P. C. Vasquez, Tetrahedron Lett.,
1989, 30, 5567.
7 F. Kovac and A. L. Baumstark, Tetrahedron Lett., 1994, 35, 8751.
8
A. Bravo, F. Fontana, G. Fronza, F. Minisci and A. Serri,
Tetrahedron Lett., 1995, 38, 6945.
9 F. Minisci, L. Zhao, F. Fontana and A. Bravo, Tetrahedron Lett.,
4
eqn. (2), where ∆C = change in concentration of reactants.
1
995, 36, 1697.
1
1
0 J. K. Crandall, M. Zucco, R. S. Kirsch and D. M. Coppert,
Tetrahedron Lett., 1991, 32, 5441.
1 A. Altamura, C. Fusco, L. D’Accolti, R. Mello, T. Prencipe and
R. Curci, Tetrahedron Lett., 1991, 32, 5445.
kp-substituted aniline
kaniline
∆Cp-substituted aniline
krel
=
=
(2)
∆Caniline
1
1
2 W. Adam and A. Schonberger, Tetrahedron Lett., 1992, 33, 53.
3 B. C. Challis and A. R. Butler, in The Chemistry of the Amino Group,
ed. S. Patai, Interscience, London, 1968, pp. 320–347, and references
therein.
Oxidation of the N,N-dimethylanilines with dimethyldioxirane
Solutions of each of the N,N-dimethylanilines in acetone
were oxidised using 2 equiv. of dimethyldioxirane–acetone
solution. Concentration of the product solution in vacuo gave
14 H. E. de la Mare, J. Org. Chem., 1960, 25, 2114; L. A. Harris and
1
J. S. Olcott, J. Am. Oil Chem. Soc., 1966, 43, 11.
the N-oxides as the only products by H NMR and HPLC.
1
1
5 L. Kuhnen, Chem. Ber., 1966, 99, 3384.
6 A. H. Kuthier, K. Y. Al-Mallah, S. Y. Hanna and N. A. I. Abdulla,
J. Org. Chem., 1987, 1710.
Constant potential amperometric rate studies
The absolute rates of reaction were determined by monitoring
the loss of current due to the polarographic reduction of
dimethyldioxirane on addition of a solution of the anilines. A
solution of dimethyldioxirane (0.13 m, 10 ml) prepared by
adding an appropriate volume of dimethyldioxirane in acetone
to a mixture of potassium nitrate solution (0.1 ) and
acetonitrile (50% v/v), was placed in the electrochemical cell.
The solution was stirred at 600 rpm while the test aniline solu-
tion (50–250 µl, 0.06  in acetonitrile) was added to the cell.
The loss in current was monitored with time under the follow-
ing instrumental settings: potential = ϩ80 mV, sampling
time = 100 s, interval time = 50 ms, sensitivity = 1 µA, reference
1
7 The differences in the results with methyl iodide from those recorded
in the literature (ref. 21) may be a function of the different reaction
conditions or the lack of sensitivity of the method of analysis.
18 L. P. Hammett, J. Am. Chem. Soc., 1937, 59, 76.
1
9 C. H. Brown and Y. Okamato, J. Am. Chem. Soc., 1958, 80, 4979. It
is possible that a conjugative effect could be significant in an electron
transfer mechanism.
2
0 Note the lower correlation coefficient also for the Okamoto–Brown
plot for cumene oxidation (ref. 4).
21 K. B. Wiberg, Physical Organic Chemistry, Wiley, New York, 1964,
pp. 379, 405.
2
2 W. Adam, G. Asensio, R. Curci, M. E. González-Núñez and
R. Mello, J. Org. Chem., 1992, 57, 953.
ϩ
23 R. W. Murray and D. Gu, J. Chem. Soc., Perkin Trans. 2, 1993, 2203.
electrode Ag /Ag, temperature 21 ЊC. The pseudo first order
2
2
4 R. W. Murray and D. Gu, J. Chem. Soc., Perkin Trans. 2, 1994, 461.
5 J. P. Lorand, J. L. Anderson, Jr., B. P. Shafer and D. L. Verral II,
J. Org. Chem., 1993, 58, 1560.
rate constants were determined by plotting the ln I versus time
(
final current = zero). Second order rate constants were deter-
mined by plotting the pseudo first order rate constants versus
molar concentration of aniline. Standard linear regression
statistics were used to calculate the slope of these lines.
26 B. A. Marples, J. P. Muxworthy and K. H. Baggaley, Tetrahedron
Lett., 1991, 32, 533.
2
2
7 W. Adam and A. K. Smerz, Tetrahedron, 1995, 51, 13 039.
8 K. Miaskiewicz, N. A. Teich and D. A. Smith, J. Org. Chem., 1997,
6
2, 6493.
Spectrophotometric rate studies
2
9 W. Adam, R. Curci, L. D’Accolti, A. Dinnoi, C. Fusco,
F. Gasparini, R. Kluge, R. Paredes, M. Schulz, A. K. Smerz, L. A.
Veloza, S. Weinkotz and R. Winde, Chem. Eur. J., 1997, 3, 105.
0 W. Adam, J. Bialas and L. Hadjiarapoglou, Chem. Ber., 1991, 124,
The spectophotometric rates were determined by monitoring
the loss of UV absorbance of N,N-dimethyl-4-nitroaniline on
addition of dimethyldioxirane. A solution of N,N-dimethyl-4-
nitroaniline (0.03 m, 2.5 ml) in the appropriate solvent was
placed in a 1 cm cuvette and the solution stirred magnetically.
Dimethyldioxirane solution in acetone (0.07 , 60 µl) was
aspirated into the cell and the loss of absorbance as a function
of time was recorded with the following instrumental settings:
λ = 400–425 nm depending on λmax in each solvent, interval
time = 0.1 s, read average time = 0.05 s, run time = 30–3000 s,
temperature thermostatted at 25 ЊC. The pseudo first order rate
constants were determined by plotting ln absorbance versus
time (the absorbances of the N-oxides at these wavelengths
3
2
377.
3
3
1 G. W. Gribble and C. F. Nutaitis, Synthesis, 1987, 709.
2 (a) N,N-Dimethyl-4-methoxyaniline: D. G. Thomas, J. H. Billman
and C. E. Davis, J. Am. Chem. Soc., 1946, 68, 895; (b) N,N-dimethyl-
4-chloroaniline, D. P. Evans and R. Williams, J. Chem. Soc., 1939,
1199.
3
3 M. Ferrer, F. Sanchez-Baez and A. Messegner, Tetrahedron, 1997, 53,
1
5 877.
Paper 7/06772K
Received 18th September 1997
Accepted 10th November 1997
2
68
J. Chem. Soc., Perkin Trans. 2, 1998