Thermodynamic stability and reactivity of silylated bis(oxy)iminium ions

Article abstract of DOI:10.1021/jo070633x

(Chemical Equation Presented) Silylation of various nitronates with trialkylsilyl triflates was investigated by applying NMR techniques. In several cases, a flexible nitronate-bis(oxy)iminium ion (as an ion pair with triflate as counterion) equlibrium was found, and its thermodynamic parameters were determined. Elevation of temperature or dilution shifts this equilibrium toward the reactants. Activation parameters for the C,C-coupling reaction of silylated bis(oxy)iminium ions with a series of reference nucleophiles were determined. Estimated electrophilicity of bis(oxy)iminium ions allows one to count on C,C-coupling when partner nucleophilicity is N > 4.

Full text of DOI:10.1021/jo070633x

Thermodynamic Stability and Reactivity of Silylated
Bis(oxy)iminium Ions
Yulia A. Khomutova,† Vladimir O. Smirnov,† Herbert Mayr,‡ and Sema L. Ioffe*,†
N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospekt 47,
119991 Moscow, Russian Federation, and Department Chemie und Biochemie,
Ludwig-Maximilians-UniVersta¨t Mu¨nchen, Butenandtstrasse 5-13 (Haus F), 81377 Mu¨nchen, Germany
iof@ioc.ac.ru
ReceiVed April 13, 2007
Silylation of various nitronates with trialkylsilyl triflates was investigated by applying NMR techniques.
In several cases, a flexible nitronate-bis(oxy)iminium ion (as an ion pair with triflate as counterion)
equlibrium was found, and its thermodynamic parameters were determined. Elevation of temperature or
dilution shifts this equilibrium toward the reactants. Activation parameters for the C,C-coupling reaction
of silylated bis(oxy)iminium ions with a series of reference nucleophiles were determined. Estimated
electrophilicity of bis(oxy)iminium ions allows one to count on C,C-coupling when partner nucleophilicity
is N > 4.
Introduction
Results and Discussion
Structural Characterization of the Siloxyiminium Ions 4.
Procedure. Silylated bis(oxy)iminium triflates 4 were obtained
by addition of ca. 2 equiv of the trialkylsilyltriflates 5a (Si )
Nitroalkanes 1 are important substrates in organic synthesis.1
While most synthetic applications proceed via nitronate anions
2 generated by deprotonation of nitroalkanes 1 with various
bases, we have previously shown2 that it is possible to convert
nitroalkanes into siloxyiminium ions 4, which are able to react
with a variety of nucleophiles and thus can be considered as
“umpoled” nitronate anions (Scheme 1).3
t
SiMe2 Bu) or 5b (Si ) SiMe3) to CD2Cl2 solutions of nitronates
3 in a NMR tube at approximately -70 °C using CH2Br2 as
internal standard. In order to avoid protonation of 3 by traces
of humidity, all reactions were performed under an inert
atmosphere in the presence of 2,6-di-tert-butyl-4-methylpyridine
as a proton trap.
The characteristic signals (NMR) of cations 4 are listed in
Table 1. Corresponding signals of initial nitronates 3 are shown
for comparison.
The data show that silylation of 3 causes a downfield shift
of the R-C-atom’s signal (C-3) by about 30 ppm. A downfield
shift of 1.5-2.0 ppm is observed for the signal H-CR (3-H).
The signals of H-Câ (4-H) show a smaller downfield shift.
All cations 4 have a signal in their 29Si NMR spectra near 50
ppm. It distinctly differs from the signal of the initial silyl triflate
near 45 ppm.4 (For more details of the configuration of cations
4 as well as their conformational analysis, see the Supporting
Information.)
In order to define the scope and limitations of C,C-coupling
of the silyl bis(oxy)iminium ions 4 with nucleophiles, we have
investigated the intermediates 4 by NMR (1H, 13C, and 29Si).
Now we will report on the study of equilibria 3 / 4 as well as
on the kinetics of the reaction of cations 4 with electron-rich
π-systems.
† Russian Academy of Sciences.
‡ Ludwig-Maximilians-Universta¨t Mu¨nchen.
(1) Ono, N. The Nitro Group in Organic Synthesis; Wiley-VCH: New
York, 2001.
(2) Smirnov, V. O.; Ioffe, S. L.; Tishkov, A. A.; Khomutova, Yu. A.;
Nesterov, I. D.; Antipin, M. Yu.; Smit, W. A.; Tartakovsky, V. A. J. Org.
Chem. 2004, 69, 8485-8488.
(3) N,N-Bis(hydroxy)iminium ions are discussed as evident intermediates
of the Nef reaction (see ref 1 and: Kornblum, N.; Brown, R. A. J. Am.
Chem. Soc. 1965, 87, 1742-1747). For activity of these cations in C,C-
coupling reactions, see: Prakash, S.; Schleyer, P. v. R. Stable Carbocation
Chemistry; John Wiley & Sons: New York, 1997; pp 525-539 and
references therein. However, the procedures cited in these papers are very
harsh for use in directed organic synthesis.
Nitronates 3a-g showed only a single set of NMR signals,
3
and their configuration was derived from the JH,H coupling
(4) Similar NMR spectra were fixed recently for cations generated by
silylation of some nitrones (Chalaye-Mauger, H.; Denis, J.-N.; Averbuch-
Pouchot, M.-T.; Vallee, Y. Tetrahedron 2000, 56, 791-804).
10.1021/jo070633x CCC: $37.00 © 2007 American Chemical Society
Published on Web 10/31/2007
9134
J. Org. Chem. 2007, 72, 9134-9140

ReactiVity of Silylated Bis(oxy)iminium Ions
SCHEME 1. New Reactivity of Aliphatic Nitro Compounds
SCHEME 2
constants5 and in some cases from NOE experiments. The
barriers of ring inversion (IR) for six-membered cyclic nitronates
can be assumed to be around 20-25 kJ/mol.6 Because such
processes are rapid for the NMR time scale, we can observe
only averaged conformations of 3a-g. The values of vicinal
3JH,H for protons attached to C-4 and C-6 atoms allow to
determine orientation of the substituents on these atoms in
nitronates 3a-g7 (the dominant conformations of 3a-g
derived by this analysis are depicted in the Supporting Informa-
tion).
comparison to that of the initial nitronate 3 (without taking into
account effects of solvent).9
Silylation of Nitronates 3 as an Equilibrium Process.
Silylation of nitronates 3 with the silyl triflates 5 may either
produce free cation 4 and anion TfO- or the ion pairs 4‚TfO-
(Scheme 2).
Since free (4 + OTf-) and paired (4‚OTf-) iminium triflates
cannot be differentiated by their NMR spectra, the degree of
ion-pairing was derived from the concentration dependence of
the equilibria. The expression of equilibrium constant KFI for
formation of the free ions 4 and OTf- (eq 1) and the equilibrium
constant KIP for formation of the ion pairs 4‚OTf- (eq 2) can
be summarized by eq 3, which is transformed into eq 4
It should be noted that in general the cations 4a,b,d-g and
corresponding nitronates 3a,b,d-g adopt analogous conforma-
tions (see Table 1 and the Supporting Information). The cation
4c is an exception: one can observe for this cation two
conformers (4ca and 4cb, ratio ≈ 1.5:1) with slow interconver-
sion at -80 to -40 °C.8 We do not detect line broadening of
the NMR signals of these conformers at -40 °C. The low
stability of intermediate 4c does not allow to perform NMR
experiments at elevated temperatures in order to determine the
barrier for interconvertion of the conformers. The calculation
of the IR barrier for hypothetic model cation 4, derived from an
unsubstituted six-membered cyclic nitronate and containing SiH3
fragment,6 does not show an increase of IR barrier in the
KFI ) [4][OTf-]/([3][5]) ) [4]2/([3][5])
KIP ) [4‚OTf-]/([3][5])
(1)
(2)
K ) [4]n/([3][5])
(3)
(4)
logK ) n‚log[4] - log([3]‚[5])
where [4] is the concentration of actually observed cationic spe-
cies (4 or/and 4‚OTf -). Plot of log([3]‚[5]) versus log[4] should
give a straight line with slope n and an intercept log K.
For the determination of the equilibrium constants 3 / 4
comparable amounts of nitronate 3 and 4 should be observable.
In most cases this equilibrium is completely shifted to the right,
however, Figure 1 shows an example where the conversion of
3f to 4f is incomplete and thus renders the determination of the
equlibrium constant.
(5) See Karplus curves for cyclohexanes (Hesse, M.; Meier, H.; Zeeh,
B. Spektroskopische Methoden in der organischen Chemie; Thieme:
Stuttgart, 1995; p 108) and N-containing heterocycles (Lambert, J. B.;
Takeuchi, Y., Cyclic Organonitrogen Stereodynamics; VCH: New York,
1992; p 170).
(6) Birukov, A. A. Unpublished quantium calculations of IR for model
six-membered cyclic nitronate. See also the data for the IR barrier of
cyclohexene: Clayden, J.; Greeves, N.; Warren, S.; Wothers, P. Organic
Chemistry; Oxford University Press: Oxford, 2001; p 471.
(7) The half-chair conformation of six-membered nitronates in the solid
state was established according to X-ray data. (For example, see: Tishkov,
A. A.; Lesiv, A. V.; Khomutova, Yu. A.; Strelenko, Yu. A.; Nesterov, I.
D.; Antipin, M. Yu.; Ioffe, S. L.; Denmark, S. E. J. Org. Chem. 2003, 68,
9477-9480). The same conclusion on dominant conformation of
cyclic nitronates could be made according to the NMR spectra of their
solution.
One can see that the ratio [4f]/[3f] decreases with increasing
dilution. From the slope of 0.96 (Figure 2) one can derive
n ) 1 (eqs 3 and 4) indicating that the silylation of 3f leads
-
to the exclusive formation of the ion pair10 4f‚OTf while
the concentration of free ions 4f is negligible under these
conditions.
(8) NOESY technique was applied to confirm that observed species are
conformers of cation 4c and the epimerization at C(6) does not occur.
Additive evidence on absence of epimerization is that cis-isomer of 4c in
course of C,C-coupling with 6h give rise to product of other configuration
than trans-isomer 4c.2
(9) For possible explanation of this fenomena see below.
(10) Within this discussion, we can’t distinguish tight ion pair and loose
ion pair.
J. Org. Chem, Vol. 72, No. 24, 2007 9135

Khomutova et al.
TABLE 1. Characteristic NMR Data for Cations 4 and Initial Nitronates 3 (CD2Cl2, 300 MHz, 200 K)
a 4ca:4cb ) 1.5:1, An ) p-C6H4OMe. b 215 K. c 210 K. d Ar ) p-C6H4NO2.
The degree of ionization, i.e., the ratio [4f]/[3f] decreases as
the temperature is increased (Figure 1). The Gibbs plot in Figure
3 indicates that the reaction of 3f with silyl triflate 5a is
exothermic but has a highly negative reaction entropy, in
agreement with observations for other ionization processes.
Compensation for the enthalpic and entropic terms occurs at T
) 195 K, where K ) 1. Similar observations have been made
for the reactions of silyl triflate 5a with some other nitronates
3 (Table 2).
One could draw the conclusion from the thermodynamic
parameters of equilibrium nitronate / iminium ion that the
decrease of concentration of reagents and the increase of
9136 J. Org. Chem., Vol. 72, No. 24, 2007

ReactiVity of Silylated Bis(oxy)iminium Ions
FIGURE 3. Gibbs plot for the equilibrating system 3f + 5a / 4f‚OTf-
(see the Supporting Information, pp 52-53).
FIGURE 1. Proton NMR spectra of equilibrating system 3f + 5a /
4f‚OTf- at various temperatures (CD2Cl2, 300 MHz). Peaks of cation
4f marked as “a” (H-3) and “a′” (H-4), peaks of starting nitronate 3f
marked as “b” (H-3) and “b′” (H-4) (see the Supporting Information,
pp 52-53).
For this purpose, we measured the kinetics of the reactions
of a few cations 4 with some π-nucleophiles which have been
defined as reference compounds in ref 12.12
The rates of the reactions of cations 4a,b with nucleophiles
1
were monitored by H NMR spectroscopy. For this purpose,
the nitronates 3a,b were quantitatively converted into the
iminium triflates (4a,b)‚OTf- in CD2Cl2 by treatment with the
silyl triflates 5a,b as described above. These solutions were
thermostated in an NMR magnet, and 1.5-2.0 equiv of the neat
nucleophiles 6a-g were added. Figure 4 depicts the decrease
of the cation 4a and of the nucleophile 6a as well as the increase
of the product 7a as the reaction 4a + 6a proceeds.
Analogous spectral changes were observed when (4a,b)‚OTf-
was combined with other reference nucleophiles 6. Typically,
15 min to 4 h were required to achieve 85% conversion of
cationic intermediates 4. In several cases, it was demonstrated
that the rate of the formation of product 7 was identical to the
rate of the disappearance of the minor reactant 4 (Scheme 3).
The concentrations of cations 4 could always be precisely
derived from their 3-H or 4-H resonances relative to internal
standard. The evaluation of the kinetics was generally based
on the consumption of 4, employing the second-order rate law
in integrated form (eq 6). This equation could be transformed
into eq 7, in which [4]t is the only time-dependent variable
parameter.
FIGURE 2. Plot of log([3f]‚[5a]) vs log[4f] for the equilibrating system
3f + 5a / 4f‚OTf- at 192 K (see the Supporting Information, pp
52-53).
temperature move substantially the above-mentioned equilibrium
toward the initial nitronates.
Taking into account typical concentrations of NMR samples
and achievable temperature range, the NMR method can be
considered to be the most effective for direct registration of
these cationic intermediates.
[6]t [6]0
e([6] -[4] )kt
(6)
(7)
t
t
)
[4]t [4]0
ktd ) ln([4]t + d) - ln([4]t) - ln([4]0 + d) + ln([4]0)
The existence of cations 4 as ionic pairs could explain the
unexpectedly high barrier for IR processes for these intermediates
(cf. 4ca / 4cb), because the dissociation of ionic pairs can be
crucial for realization of ring inversion.
Kinetic Measurements. In order to define the synthetic
potential of the silylated bisoxyiminium ions 4 in C,C-coupling
reactions, we tried to determine their electrophilicity parameter
E according to eq 5 where s and N are nucleophile-specific
parameters.11
where [4]0 and [4]t refer to initial and current concentrations of
4, respectively, and d ) [6]t - [4]t (or [6]0 - [4]0).
The linear correlation between ln[([4a]t + d)/[4a]t] and t
(Figure 5) shows that second-order kinetics are followed as
described by eq 7. (Analogous plots for other C,C-coupling
reactions are shown in the Supporting Information.)
(11) Mayr, H.; Kempf, B.; Ofial, A. R. Acc. Chem. Res. 2003, 36, 66-
77.
(12) Mayr, H.; Bug, T.; Gotta, M. F.; Hering, N.; Irrgang, B.; Janker,
B.; Kempf, B.; Loos, R.; Ofial, A. R.; Remennikov, G.; Schimmel, H. J.
Am. Chem. Soc. 2001, 123, 9500-9512.
log(k20°C) ) s(N + E)
(5)
J. Org. Chem, Vol. 72, No. 24, 2007 9137

Khomutova et al.
FIGURE 4. Time-dependent proton NMR spectra during the C,C-coupling reaction 4a + 6a f 7a (CD2Cl2, 200 K, 300 MHz) (see the Supporting
Information, pp 17-20).
TABLE 2. Thermodynamic Parameters for the Equilibriums 3 + 5a / 4‚OTf-
equilibruim
n (averaged)
range of T, K
∆G0293, kJ/mol
∆H0, kJ/mol
∆S0, J/mol‚K
3f / 4f
3h / 4h
3i / 4i
3j / 4j
0.98 ( 0.05
1.04 ( 0.08
1.04 ( 0.06
0.84 ( 0.05
183 ÷ 207
190 ÷ 220
180 ÷ 210
210 ÷ 240
18.0 ( 1.1
1.7 ( 3.3
12.2 ( 2.3
9.3 ( 2.3
-35.5 ( 0.4
-29.0 ( 1.3
-26.4 ( 0.9
-59.5 ( 1.0
-182.7 ( 2.4
-104.8 ( 6.7
-131.7 ( 4.8
-234.9 ( 4.3
SCHEME 3
According to proton NMR spectra (Figure 4), the reaction
Analogously, other reactions 4 and 6 proceed with high
4a and 6a proceeds without the formation of any side products.
selectivity and efficiency.
9138 J. Org. Chem., Vol. 72, No. 24, 2007

ReactiVity of Silylated Bis(oxy)iminium Ions
FIGURE 7. Two possible approaches of C-nucleophile Nu to dominant
conformer of iminium ion 4.
SCHEME 4
FIGURE 5. Plot of ln{([4a]t + d)/[4a]t} vs dt (eq 7) for 4a + 6a
C,C-coupling reaction according to second-order kinetics (see the
Supporting Information, pp 17-20).
(oxy)iminium ions, which allows us to directly compare the rate
constants obtained for reactions with 4a and 4a′.
The E parameters derived for cations 4a,a′ from reactions
with different nucleophiles vary by 2.5 units showing that
eq 5 does not precisely describe these reactions. However with
E ≈ -4 to -6 one can conclude that, at lowered temperatures,
reactions with nucleophiles which have N > 4 could be re-
alized.
One could underline also that the selectivity of cation 4a
toward π-systems with triorganostannyl leaving group is rather
close to that of benzhydrilium ions, while in the reactions of
cation 4a with silyl-capped π-systems almost no selectivity
toward nucleophiles was found.
The analysis of previous results2 and stereochemical outcome
of C,C-coupling products 7 show that the distal approach
of the nucleophile (relative to C(6) of cation 4) is strongly
preferable (see Figure 7). It is remarkable that an ana-
logous approach of the dipolarophile was strongly prefer-
able in [3 + 2]-cycloaddition reactions with six-membered cyclic
nitronates.13 Probably, this is connected with the peculiar
structure of the cations 4. Namely, C(6) atom in cations (as
well as in the initial nitronates) is more withdrawn from O-N-
C(3)-C(4) plane than C(5) on the opposite side.6,7,14 Cor-
respondingly, the proximal (to C(6)) approach is hampered in
comparison to the alternative.
FIGURE 6. Eyring plot for the reaction of 4a with 6a (see the
Supporting Information, pp 17-20).
Because of the decomposition of the cations 4 at elevated
temperature, the kinetics could only be determined in the range
from -90 to -40 °C. From Eyring plots of ln(k/T) vs 1/T (see
Figure 6 and Supporting Information), one can derive the
activation parameters listed in Table 3. It is found that ∆Sq has
a large negative value as previously reported for other reactions
of cationic electrophiles with neutral π-systems.12
We observed the only example of stereoselectivity violation
in C,C-coupling reactions when single conformer of cation 4
was detected. Despite the presence of only one dominant
conformation of cation 4a′ (see Table 1, entry 2), its C,C-
coupling with CH2dCMe(OTBS) (under the kinetic measure-
ment conditions) gave product contaminated with a slight
amount of the alternative diastereomer (for -70 °C dr ≈1:9,
for -40 °C dr ≈ 1:4; see the Supporting Information for details).
Discussion
While the reaction of 4a with 6g and 4a′ with 6d has
been studied at variable temperature (Table 3), the reaction of
4a with 6d was only investigated at -50 °C. The first two
entries of Table 4 show that the variation of the silyl group
in the enol ether derived from acetophenone does not affect
the kinetics. Therefore, one can conclude that the desilyl-
ation of cationic intermediate A is not the rate-determining step
in the reaction cascade 4 + 6 (Scheme 4).
(13) Denmark, S. E.; Seierstad, M.; Herbert, B. J. Org. Chem. 1999, 64,
884-901.
(14) Nesterov, I. D.; Lesiv, A. V.; Ioffe, S. L.; Antipin, M. Yu. MendeleeV
Commun. 2004, 281-282.
Entries 2 and 3 of Table 4 indicate that nature of the silyl
group also does not affect the electrophilicity of the silyl bis-
J. Org. Chem, Vol. 72, No. 24, 2007 9139

Khomutova et al.
TABLE 3. Rate Constants and Activation Parameters for the Reactions of 4 with 6
ref N and (s)
k2 (20 °C),
cat. 4
nucleophile
product
parameters12
E
L‚mol-1‚s-1
∆Gq20°C, kJ/mol
∆Hq, kJ/mol
∆Sq, J/mol‚K
4a
6a
6e
6f
6g
6a
6b
6c
6d
6h
7a
7a
7c
7d
7e
7f
7b
7g
7h
5.46 (0.89)
3.09 (0.90)
5.13 (0.90)a
6.22 (0.96)
5.46 (0.89)
4.41 (0.96)
5.41 (0.91)
6.22 (0.96)
10.32 (0.79)
-3.69
-3.37
-4.21
-6.08
-3.70
-4.54
-5.45
-5.99
-10.21
37.7
0.55
6.55
1.37
36.6
0.74
0.92
1.65
1.23
62.9 ( 3.7
73.2 ( 4.0
67.1 ( 4.4
70.9 ( 1.7
63.0 ( 3.6
72.4 ( 2.3
71.9 ( 2.3
70.5 ( 1.7
71.2 ( 1.1
42.9 ( 1.5
49.5 ( 1.8
45.2 ( 1.9
51.1 ( 0.7
41.7 ( 1.5
39.2 ( 1.0
43.9 ( 1.0
51.7 ( 0.7
31.4 ( 0.5
-68.1 ( 7.4
-80.7 ( 7.8
-74.9 ( 8.6
-67.6 ( 3.3
-72.4 ( 7.2
-113.6 ( 4.4
-95.6 ( 4.3
-64.1 ( 3.1
-135.9 ( 2.3
4a′
4b
a s value for 6f was estimated the same as for 6a and 6e in ref 12.
TABLE 4. Dependence of k2 Value at -50 °C on the Nature of the
Silyl Group in Cation 4 and Nucleophile 6
After registering the control spectra, the cold sample was taken
out from the magnet, and 2 equiv (13-30 µL) of silylation agent
(5a or 5b) was quickly injected by means of a microsyringe. The
sample was shaken twice and immediately put into the magnet.
Then, spectra (1H, 13C, 29Si) of the resulted mixture were registered.
The overheating of sample during these manipulations is negligible.
It is evident from the spectral picture, that, in most cases, the starting
nitronate 3 was fully converted into the siloxyiminium ion 4.
However, for nitronates 3f,h-j, there is an temperature-dependent
equilibrium 3 + 5 / 4.
General Procedure for Kinetic Measurements. The starting
siloxyiminium ions 4a-b were generated as described above. After
the registration of cation spectra, the sample was taken out of the
magnet, the precise quantity in 0.5-1.0 excess of neat nucleophile
6 (10-20 µL) was injected with a microsyringe, the tube was
shaken twice and immediately returned into the magnet. Solid
nucleophiles (6e,f) were placed into the NMR tube after the
nitronate and dissolved in CD2Cl2 simultaneously during preparation
of the NMR sample; the reaction began after addition of silyl triflate
5a.
Concentrations of cations 4 and nucleophile 6 were measured
by the integration of corresponding signals in the proton spectra of
that sample and comparison with the integrate intensity of the signal
of standard (CH2Br2). Precise concentration of standard was
determined before addition of silyl triflate 5 by comparison of its
signal intensity with signals intensities of nitronates, which were
precisely weighted in the course of sample preparation. Kinetic
measurements of the C,C-coupling reaction were proceeded until
85% conversion of cation 4 into product 7 was achieved (from 15
min to 4 h), and 12-24 spectra were thus registered. The
temperature was calibrated before and after of each experiment with
standard sample of neat methanol. The data taken after the second
minute of reaction were used for linear correlations. Amounts of
reactants for generation of cations 4 and kinetic measurements are
specified in the Supporting Information.
Conclusion
The interaction of iminium ions derived from cyclic six-
membered nitronates toward neutral C-centered π-nucleophiles
was investigated. NMR was considered to be method of choice
for such studies. It was found, that the reactivity of these cations
cannot be precisely described by Mayr’s equation. In several
cases, ion pairing of this type of iminium ion (with a triflate as
a counterion) was detected by the concentration dependence of
the equilibria with corresponding starting nitronate.
Experimental Section
Acknowledgment. This work was performed with the
financial support of the Russian Basic Research Foundation
(Grant Nos. 03-03-04001 and 05-03-08175) and Deutsche
Forschungsgemeinschaft MA 673/19.
General Procedure for the Generation of Siloxyiminium Ions
4. A standard sample of neat methanol was used for precise control
of the probe temperature. The temperature was measured in the
beginning and at the end of each experiment. Starting nitronate 3
(3.0-20.0 mg, 0.03-0.15 mmol) and 2,6-di(tert-butyl)-4-meth-
ylpyridine (0.5 equiv, 3.0-15.0 mg) were placed into a 5 mm NMR
tube which was evacuated and filled with dry argon. Under inert
atmosphere, 0.50 mL of CD2Cl2 was added, the tube was stoppered
with a rubber septum, and 2.0 µL of internal standard (CH2Br2)
was injected by means of a microsyringe. Then the sample was
put into the precooled magnet (T ) 200 K). Control spectra (1H,
13C) of initial nitronate 3 were recorded at the temperature of
experiment.
Supporting Information Available: Full experimental proce-
dures for generation of cations 4, their NMR data, data on
equilibrium studies, kinetic data (concentrations and rate constants
of the individual kinetic runs), and preparation procedures and
characterization for new compounds 7. This material is available
free of charge via the Internet at http://pubs.acs.org.
JO070633X
9140 J. Org. Chem., Vol. 72, No. 24, 2007