The Kinetics of 1,3-Dipolar Cycloaddition of Vinyl Monomers to 2,2,5,5-Tetramethyl-3-imidazoline-3-oxides

Article abstract of DOI:10.1002/cplu.202100266

In our previous work [Edeleva et al. Chem. Commun. 2019, 55, 190–193], we proposed a versatile approach to the activation of the homolysis of an aldonitrone group–containing alkoxyamine by 1,3-dipolar cycloaddition to a vinyl monomer. Both nitroxide- and alkoxyamine-containing aldonitrones were found to be capable of reacting with the activated alkenes. In the present study, the kinetics of these reactions with 11 different vinyl monomers were investigated using EPR and NMR spectroscopy, and apparent activation energies as well as pre-exponential factors were determined. The influence of monomer structure on the rate of the 1,3-dipolar cycloaddition is discussed. For the vinyl monomers typically used in nitroxide mediated polymerization (styrene, methyl methacrylate) the rate coefficient of cycloaddition to the nitroxide is around k(353 K) ～4 ? 10^?4 L mol^?1 s^?1, whereas for n-butyl acrylate and methyl vinyl ketone we observed the fastest cycloaddition reaction with k(353 K)=8 ? 10^?3 and 4 ? 10^?2 L mol^?1 s^?1 respectively.

Full text of DOI:10.1002/cplu.202100266

A Multidisciplinary Journal Centering on Chemistry
Accepted Article
Title: The Kinetics of 1,3-Dipolar Cycloaddition of Vinyl Monomers to
2,2,5,5-Tetramethyl-3-imidazoline-3-oxides
Authors: Sergey A. Cherkasov, Anastasiya D. Semikina, Polina M.
Kaletina, Yulia F. Polienko, Denis A. Morozov, Alexander
M. Maksimov, Igor A. Kirilyuk, Elena G. Bagryanskaya, and
Dmitriy Parkhomenko
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemPlusChem 10.1002/cplu.202100266
Link to VoR: https://doi.org/10.1002/cplu.202100266
01/2020

10.1002/cplu.202100266
ChemPlusChem
ARTICLE
The Kinetics of 1,3-Dipolar Cycloaddition of Vinyl Monomers to
2,2,5,5-Tetramethyl-3-imidazoline-3-oxides
Sergey A. Cherkasova,b, Anastasiya D. Semikinaa,b, Polina M. Kaletinaa,b, Yulia F. Polienkoa, Denis A.
Morozova, Alexander M. Maksimova, Igor A. Kirilyuka, Elena G. Bagryanskayaa, Dmitriy A.
Parkhomenkoa*
[a]
[b]
S.A. Cherkasov, A.D. Semikina, P.M. Kaletina, Dr. Yu.F. Polienko, Dr. D.A. Morozov, Dr. A.M. Maksimov, Dr. I.A. Kirilyuk, Prof. E.G. Bagryanskaya, Dr.
D.A. Parkhomenko
N.N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS
9 Lavrentiev Ave., Novosibirsk 630090, Russia
E-mail: parkhomenko@nioch.nsc.ru
S.A. Cherkasov, A.D. Semikina, P.M. Kaletina
Novosibirsk State University
1 Pirogova str., Novosibirsk 630090, Russia
Supporting information for this article is given via a link at the end of the document.
Abstract: In our previous work [Edeleva et al. Chem. Commun.
2019, 55, 190-193], we proposed a versatile approach to the
activation of the homolysis of an aldonitrone group–containing
alkoxyamine by 1,3-dipolar cycloaddition to a vinyl monomer. Both
nitroxide- and alkoxyamine-containing aldonitrones were found to be
capable of reacting with the activated alkenes. In the present study,
the kinetics of these reactions with 11 different vinyl monomers were
investigated using EPR and NMR spectroscopy, and apparent
activation energies as well as pre-exponential factors were
determined. The influence of monomer structure on the rate of the
1,3-dipolar cycloaddition is discussed. For the vinyl monomers
typically used in nitroxide mediated polymerization (styrene, methyl
methacrylate) the rate coefficient of cycloaddition to the nitroxide is
around k(353K) ~ 4∙10-4 L mol-1 s-1, whereas for n-butyl acrylate and
methyl vinyl ketone we observed the fastest cycloaddition reaction
with k(353K) = 8∙10-3 and 4∙10-2 L mol-1 s-1 respectively.
situ transformation.[25-26] Recently, we presented
a
new
technique for the activation of alkoxyamine homolysis by 1,3-
dipolar cycloaddition of a monomer to a cyclic aldonitrone-
derived alkoxyamine of the 3-imidazoline-3-oxide series
(Scheme 1).[26]
Scheme 1. The 1,3-dipolar cycloaddition reaction of monomers with 2,2,5,5-
tetramethyl-3-imidazoline-3-oxides.[27]
The reaction of the 1,3-dipolar cycloaddition of vinyl monomers
to 3-imidazoline-3-oxides was the subject of several studies
more than 30 years ago.[27-28] This reaction was shown to afford
predominantly 2-substituted tetrahydroimidazo[1,5-b]isoxazoles
(Scheme 1). Nonetheless, these papers provide neither the data
on the rates of 1,3-dipolar cycloaddition nor a quantitative
comparison of reactivity of different dipolarophiles. Furthermore,
cost-effective implementation of NMP is based on model-guided
process design, which requires a complete set of temperature-
dependent rate constants of alkoxyamine homolysis as well as
1,3-dipolar cycloaddition to nitroxides and alkoxyamines.[29-30]
In the present study, we performed kinetic measurements using
a cost-effective approach, EPR spectroscopy. The kinetic data
were processed, and the rate constants for the cycloaddition
were determined for 11 monomers (see Chart 1). The use of the
diverse monomers should elucidate the opportunities for the
application of the 1,3-dipolar cycloaddition method to NMP.
Aside from 10 commercially available monomers of different
Introduction
Nitroxide-mediated polymerization (NMP) is a versatile method[1-
11]
enabling the synthesis of narrowly dispersed polymers and
copolymers with ordered architecture and composition. Since
the pioneering studies of Solomon,[12-13] numerous researchers
have contributed to further development of NMP with the aim of
technological improvement for widespread use.[9] The rate of
alkoxyamine homolysis is the key factor that determines NMP
success (e.g., the time needed for effective monomer
conversion and polymer dispersity).
A
higher rate of
alkoxyamine homolysis allows for polymerization at lower
temperatures and the achievement of better control over the
molar mass of the polymer. On the other hand, the synthesis of
thermally unstable alkoxyamines poses problems with their
storage and transportation, e.g., gradual decomposition needs to
be prevented. A possible solution involves the development of
alkoxyamines with a tunable decomposition rate. Thus, the
current trends in NMP include designing approaches to
changing the alkoxyamine homolysis rate using external
factors.[14-17]
classes, we synthesized
a
perfluorinated monomer: 4-
perfluorotolyl vinyl sulfide (Chart 1). It should be noted that 4-
perfluorinated monomers and the corresponding (co-)polymers
receive much attention nowadays due to their enhanced
properties,
e.g.,
hydrophobicity
and
ionizing-radiation
resistance.[31-32]
During the polymerization, both nitroxide 1 and alkoxyamine 2
can react with vinyl monomers to form cycloadducts. Therefore,
in addition, kinetics of the reaction of the corresponding
Several methods are presented in the literature and involve
protonation,[18-20] complexation with metals,[21-24] or chemical in
1
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ARTICLE
alkoxyamine, 2, with each of four highly active dipolarophiles
(Scheme 1) were measured and compared with those of
nitroxide 1.
nitroxides, N1 and N2, with identical g-factors (which determine
the position of the central line) and identical linewidths but
slightly different hyperfine coupling constants a1 and a2 (Δa = a2
− a1 << a1, a2).
In general, lines in EPR spectra have a Voigt profile, which is a
convolution of Gaussian and Lorentzian profiles. For the sake of
simplicity, we performed subsequent analyses assuming a
Gaussian profile for the EPR spectrum lines. We defined the
position of the central line in the EPR spectra as x = 0; therefore,
the position of the high field line represents coupling constant aN.
The Gaussian profile is given by
(x−a)2
2 2
e−
1
f =
2
where a is the mean, and σ is dispersion and in case of EPR is
equal to lwpp/2, where lwpp is peak-to-peak line width.
Because in an EPR experiment, a derivative of absorption is
obtained, the shape of the EPR line is described as
(x−a)2
2 2
f ' = −
e−
(1)
x − a
2 3
The cumulative spectrum of two species N1 and N2 with molar
fractions c and (1 − c) has the following line shape:
2
(x−a1)2
2 2
)
e−
− (1− c)
e−(x−a2
(2)
x − a1
2 3
x − a2
2 3
2 2
f ' = cfa' + (1− c) fa' = −c

1
2
Assuming
, eq. (2) can be rewritten as eq. (3),
a = a2 − a1  
which gives the EPR line shape in case of a slight difference in
the hyperfine coupling constants.
(x−a1)2
2 2
(x−a1)2
2 2
e−
− (1− c)
e−
=
x − a1
2 3
x − a2
2 3
f '  −c

Chart 1. Structures of the monomers, nitroxide, and alkoxyamine that were
subjected to the 1,3-dipolar cycloaddition reaction.
(x−a1)2
2 2
(x−a1)2
−
−
x −ca1 −(1−c)a
x − a2 + ca
2 3
2 2
= −
= −
2 e
= −
e
=
2 3
(x−a1)2
e−
x − aapp
2 3
2 2
Results and Discussion
(3)
(4)
1,3-Dipolar cycloaddition of monomers to nitroxide 1
EPR spectroscopy is commonly used to investigate the kinetics
and mechanisms of radical reactions. To monitor the evolution of
a reagent and/or product concentration, which describes the
kinetics of the reaction, EPR spectra of the reagents and
products must be sufficiently different to be distinguished from
each other.
where
aapp (t) = a2 − c(t)a
Equation (4) shows that the evolution of the apparent hyperfine
coupling constant represents the kinetics of the reaction if ΔaN
<< σ. To validate eq. (4), we assume a unimolecular reaction of
nitroxide N1 transformation into N2 with rate coefficient k1:
푘
1
푁1 → 푁2
(5)
The 1,3-dipolar cycloaddition of vinyl monomers to 1 does not
meet the criterion of the sufficient difference in EPR spectra
because the structures of the reagent and product are quite
similar. The difference in the EPR spectra of 1 and of the
cycloadduct is due to the nitrogen hyperfine coupling constant
(aN), and ΔaN  0.03 mT for the reagent and product, while the
width of the EPR line is fivefold greater and is ~0.15 mT.[26] Thus,
EPR spectra of the initial and formed nitroxides overlap, and
only three EPR lines are observed with an apparent coupling
constant (aN,app) that changes during the reaction and depends
on the ratio between the concentrations of the initial and formed
nitroxide.
If the initial concentration of N1 is C0, then the evolution of the
nitroxides’ concentrations can be expressed as
[
]
(
) [
]
(
)
푁1 = 퐶0 exp −푘1푡 , 푁2 = 퐶0(1 − exp −푘1푡 )
(6)
Integral intensity of the EPR lines is proportional to
concentrations of the radicals. Accordingly, the cumulative
spectrum can be calculated as the sum of individual spectra
weighted by the concentrations:
( )
[
]
[
]
푆 푡 = 푁1 × 푆1 + 푁2 × 푆2
(7)
For the validation of the above-mentioned assumption, we
calculated cumulative EPR spectra S(t) (eq. (7)) by means of the
EasySpin toolbox[33] for the N1 transformation reaction with the
following parameters: g1 = g2 = 2, aN1 = 1.4 mT, aN2 = 1.43 mT,
and Gaussian line broadening with different lwpp values (0.05,
0.1, 0.15, and 0.3 mT). Rate coefficient 푘1 was set to 3 ×
10−3푠−1, which is in the typical range of reported rate coefficients,
and the initial concentration of N1 was set to 1 M for simplicity.
A model for EPR investigation of the reaction kinetics
To develop a method for determining the rate constant by
means of the evolution of the apparent coupling constant (aN,app),
we performed the following model validation via a line shape
analysis of a cumulative EPR spectrum. We selected two
2
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10.1002/cplu.202100266
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1.430
1.425
1.420
1.415
1.410
1.405
aN,app
lwpp = 0.05 mT
lwpp = 0.1 mT
lwpp = 0.3 mT
1.43-0.03*exp(-0.003*t)
1.400
0
500
1000
1500
350
351
352
353
354
355
Time, s
Magnetic field, mT
Figure 1. (Left) The calculated dependence of aN,app on time during the transformation of N1 (aN1 = 1.40 mT) to N2 (aN2 = 1.43) with rate coefficient
푘1 = 3 × 10−3푠−1 for various lwpp values; black: lwpp = 0.05 mT, red: lwpp = 0.1 mT, and magenta: lwpp = 0.3 mT. (Right) The calculated EPR spectrum of
nitroxide N1; arrows show the determination of aN,app
.
We determined apparent coupling constant aN,app over time as a
distance between low-field and center-field lines. In the course
of the reaction, aN,app changed from 1.40 to 1.43 mT owing to the
transformation of N1 to N2.
The calculation results for various lwpp values are presented in
Figure 1. The solid curve in Figure 1 was obtained using eq. (4).
If the difference in hyperfine coupling constants of N1 and N2
(ΔaN) is nearly equal to lwpp (black symbols, Figure 1), then the
evolution of aN,app only poorly reproduces the reaction kinetics.
By contrast, if lwpp > 3∆aN, then aN,app as a function of time
describes the reaction kinetics adequately.
obtained k effectively reproduces the model value starting from a
peak-to-peak line width of 0.1 mT.
Table 1. The output of the model for the determination of the apparent rate
constant for chemical reaction (5) depending on peak-to-peak line width (used
as input) for different line-broadening types
kobs, [10-3 s-1]
lwpp, [mT]
Gaussian
3.52
Lorentzian
3.62
Voight
3.62
3.16
3.07
3.02
Overmodulation
0.05
0.1
3.6
3.13
3.18
3.07
2.98
2.99
For the data-processing procedure, we fitted the function
obtained at aN,app(t) for different lwpp values using the following
exponential equation:
0.15
0.3
3.06
3.08
aN,app (t) = aN2 −aNe−kt
3.01
3.03
(8)
Table 1 presents the rate constant kobs determined in the models
with various lwpp values. As readers can see in Table 1, the
accuracy of the rate constant determination increases with the
increasing lwpp value. For lwpp > 0.15 mT, which is typical for
nitroxides, the obtained k effectively reproduces the model value
of 3  10−3 s−1 with 1–2% error.
Although the Gaussian profile allows us to present the above-
mentioned approach in the simplest way, broad EPR lines of the
Gaussian shape can rarely be seen in experiments. We
performed the same numerical analysis – as presented above
for the Gaussian profile – on other broadening types commonly
seen in experiments: Lorentzian, Voight, and overmodulation of
a narrow line (see Supporting Information [SI] for details). As
indicated in Table 1, regardless of the broadening type used, the
Experimental determination of a temperature-dependent
kinetic rate constant
The typical kinetic plots obtained after the processing of the
experimental data by the above-mentioned procedure are
presented in Figure 2, and all the results are summarized in
Table 2. The kinetic data obtained by EPR were processed
according to eq. (8). Activation energies and pre-exponential
factors were obtained via the Arrhenius equation for the rate
coefficient:
Ea
k(t) = Ae−
RT
(9)
3
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1.460
1.455
1.450
1.445
1.440
1.435
1.430
1.425
1.420
1.415
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
-6.0
-6.5
-7.0
-7.5
-8.0
-8.5
-9.0
-9.5
methyl vinyl ketone
n-butylacrylate
4-perluorotolyl
vinyl sulfide
NIPAM
acrylonitrile
353K
358K
363K
368K
373K
378K
4-vinylpiridine
methymethacrylate
n-butylvinyl ether
vinylacetate
1-vinylimidazole
styrene
3.0
0
3000
6000
9000
12000 15000 18000
2.6
2.7
2.8
2.9
3.1
x10-3
1/T, K-1
Time, s
Figure 2. Kinetics of the evolution of the apparent hyperfine constant (symbols) and the fit to eq. (8) (solid curves) at different temperatures (see the figure for the
values) for the reaction of 1,3-dipolar cycloaddition of 1-vinyl imidazole to nitroxide (left) and Arrhenius plots of this reaction for all the studied monomers (right).
The obtained activation energies of the 1,3-cycloaddition
reaction of nitroxide are in the ~47–70 kJ/mol range, in good
Table 2. Activation energies and pre-exponential factors for the 1,3-dipolar
cycloaddition reaction of different monomers to 1 and 2 as well as the k value
at 353 and 328 K.
agreement with the data presented in the literature for cyclic
aldonitrones.[34-35] The rate of 1,3-dipolar cycloaddition is often
related to electron-acceptor properties of a substituent in a
vinylic group of olefins.[36-39] We selected the temperature of
1
2
353K, which is typical for an NMP process, and compared the
rate constants of the cycloaddition to nitroxide. As one can see
in Table 2, the fastest monomers in the context of the 1,3-dipolar
cycloaddition to nitroxide are those that have a strong electron
acceptor in the vinyl moiety, e.g., a carbonyl or nitrile group. This
is true for methyl vinyl ketone, n-butyl acrylate, N-
isopropylacrylamide (NIPAM), and acrylonitrile. The much lower
rate constant of methyl methacrylate in comparison with n-butyl
acrylate is due to the methyl substituent instead of hydrogen at
the α position toward the acrylate group; this substituent causes
greater steric hindrance of the reactive center. The ether group
is much more electron-donating (according to the resonance
effect) than electron-withdrawing on an inductive one; therefore,
n-butyl vinyl ether and vinyl acetate yielded lower rates of the
1,3-dipolar cycloaddition reaction. Besides, aromatic groups are
characterized by an electron-donating resonance effect, which
was reflected in a lower reaction rate in the cases of styrene, 4-
vinylpyridine, 1-vinylimidazole, and 4-perfluorotolyl vinyl sulfide.
It should be noted that for a fluorine-substituted monomer, the
highest rate of 1,3-dipolar cycloaddition was observed in
comparison with other aromatic π-conjugated monomers. This
fact further confirms the assumption that electron acceptor
properties of a substituent in a vinylic group of a monomer
determine its rate of 1,3-dipolar cycloaddition to aldonitrone-
containing compounds.
Monomer
[a]
[b]
[a]
Ea,
A,
k353K
,
k328K
,
k328K,
[kJ/mol] [105 M-1s-1] [10-4 M-1s-1] [10-4 M-1s-1] [10-4 M-1s-1]
methyl vinyl ketone
n-butyl acrylate
NIPAM
47
55
63
53
2.8
12
400
80
110
19
96
19
47
30
5.4
5.5
5.3
7.3
acrylonitrile
1.6
23
4-perfluorotolyl vinyl
sulfide
59
3.3
6.6
1.5
-
4-vinylpyridine
methyl methacrylate
styrene
63
59
68
65
69
62
12
3.1
32
5.9
5.4
2.9
2.5
1.6
1.1
1.2
1.1
-
-
-
-
-
-
0.47
0.49
0.27
0.22
n-butyl vinyl ether
1-vinylimidazole
vinyl acetate
12
29
1.7
[a] Measured experimentally; [b] calculated using Arrhenius parameters.
The reaction of 2 with n-butyl acrylate, acrylonitrile, or NIPAM
was conducted with an excess of the monomer, so that the
consumption of 2 obeyed pseudo-first-order kinetics (Figure 3b).
Due to the high cycloaddition rate observed for methyl vinyl
ketone, the reaction of the latter with 2 was carried out at a 1:1
monomer-to-alkoxyamine ratio. In this case, reagent
consumption conformed to second-order kinetics (Figure 3c).
The obtained rate constants (Table 2) are close to those for the
analogous reaction with the corresponding nitroxide, 1.
1,3-Dipolar cycloaddition to alkoxyamine 2
1H NMR measurement of the 1,3-dipolar cycloaddition rate
constant toward alkoxyamine
2
was performed for the
monomers with the highest rate of cycloaddition to nitroxide,
namely methyl vinyl ketone, n-butyl acrylate, acrylonitrile, and
NIPAM. To avoid possible thermal decomposition of the
alkoxyamine, all experiments were carried out at 328 K (see SI,
section 5).
4
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ARTICLE
It should be noted that although one could expect an aldonitrone
proton signal (e) (see Figure 3a) to be the best option for
monitoring the 1,3-dipolar cycloaddition kinetics, it overlapped in
most cases with the signals of vinylic protons. For example, the
NMR spectra of the reaction mixture of 2 and NIPAM before and
after 300 min of incubation at 328 K are displayed in Figure 3a.
On the other hand, growing signals e’ and a’ of the forming
cycloadduct have a low signal-to-noise ratio because their
multiplet structures yield low-quality kinetic curves with high
uncertainty of approximation. Fortunately, the proton of the
phenylacetic moiety (j) was found to be sensitive to the reaction
despite being far outside of the reaction center. Recently, it was
demonstrated that this proton is located at 5.23–5.25 ppm for a
styrene cycloadduct but at 5.13–5.14 ppm for the initial
alkoxyamine (see SI in ref. [26]). Consequently, for the
measurements of reaction kinetics we chose the line at 5.1 ppm.
Conclusion
A detailed study of 1,3-dipolar cycloaddition reactions of 11
different vinyl monomers to 3-imidazoline-3-oxide-1-oxyl was
conducted by EPR spectroscopy. An effective approach was
developed for the measurement of the rate constants in the case
of similar values of hyperfine coupling constants between the
initial and newly formed nitroxides and was applied to the
studied systems. It was shown that the rate constants of the 1,3-
dipolar cycloaddition to the nitroxide vary within one order of
magnitude depending on the structure of the monomers. It was
found that the rate constants for the 1,3-dipolar cycloaddition to
an alkoxyamine at 328 K are close to the ones for the addition to
the nitroxide. Methyl vinyl ketone manifested the highest rates of
cycloaddition to both the alkoxyamine and nitroxide. All the
above-mentioned details of the mechanism should help to
further optimize the NMP of various monomers.
Experimental Section
Chemicals
The following substances were used as received: toluene (Reachem,
99.9%), NIPAM (Sigma-Aldrich, 99%), n-butyl acrylate (Sigma-Aldrich,
99%), n-butyl vinyl ether (Sigma-Aldrich, 98%), methyl methacrylate
(Acros Organics, 99%), acrylonitrile (Sigma-Aldrich, 99%), vinyl acetate
(Sigma-Aldrich, 99%), and 1-vinylimidazole (Sigma-Aldrich, 99%).
Styrene (Sigma-Aldrich, 99%) and 4-vinylpyridine (Acros Organics, 95%)
were additionally purified by distillation. Nitroxide 1 was synthesized as
described in ref. [40], and alkoxyamine 2 was obtained according to ref.
[26]. Monomer 3 was synthesized as described below.
Measurements of EPR kinetics of 1,3-dipolar cycloaddition of a
monomer to 1
A sample of a reaction mixture containing the nitroxide and a monomer in
toluene in a glass capillary was placed in an oil or water thermostat
preheated to a desired temperature and was incubated for selected
periods (2–60 min depending on the conversion rate). The concentration
of the nitroxide was 0.02 mM, and the concentration of monomers was 1
M except for n-butyl acrylate and methyl vinyl ketone (0.5 and 0.1 M,
respectively). EPR spectra were recorded on a Bruker Elexsys E540
spectrometer at room temperature for various incubation periods. The
parameters of EPR measurements were as follows: mw frequency = 9.87
Figure 3. NMR spectra of a mixture of NIPAM and 2 before (bottom) and after
(top) the cycloaddition reaction (a), and the kinetics of 1,3-dipolar cycloaddition
of n-butyl acrylate (black), acrylonitrile (red), NIPAM (blue) (b), or methyl vinyl
ketone (c) to 2 at 328 K (solid curves: approximation; inset: linearization).
5
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1
GHz, mw power = 2 mW, modulation amplitude = 0.3 mT, the number of
points = 1024, conversion time = 20 ms, and sweep width = 6 mT.
(125.76 MHz, CDCl3): δC = 147.1 (dm, JCF = 247 Hz; C-(2,6)), 144.3
1
2
1
(ddm, JCF = 262, JCF = 18 Hz; C-(3,5)), 120.8 (q, JCF = 275 Hz; CF3),
2
2
119.0 (t, JCF = 20 Hz; C-(1)), 109.6 (qtd, JCF = 35 Hz, 2JCF = 13 Hz, 3JCF
 4 Hz; C-(4)), 35.8 (t, JCF = 3 Hz; Cα), 29.5 (d, JCF  1.5 Hz; Cβ). 19F
4
5
NMR measurement of the kinetics for the 1,3-dipolar cycloaddition
of monomers to alkoxyamine 2
4
NMR (282.36 MHz, CDCl3, C6F6 (δ = −162.9)): δF −57.5 (t, JFF = 22, 3F;
CF3), −132.7 to −132.9 (m, 2F; F-(2,6)), −140.4 to −140.9 (m, 2F; F-(3,5))
ppm. IR (film, νmax, cm−1): 2972(vw) (CH), 1645(s) (ArF), 1608(w),
1479(vs) (ArF), 1429(m), 1398(m), 1329(vs) (CF3), 1259(m), 1184(s),
1149(s) (CF3), 1059(w), 982(vs) (CF), 949(m), 831(s) (C-S), 715(s) (C-S),
624(m), (CBr) 571(vw), 509(vw), 478(vw), 441(vw). UV (hexane, λmax, nm
(log ε)): 224 (3.75), 276 (3.88).MS (70 eV) m/z calcd for C9H4BrF7S:
355.9100; found: 355.9098; elemental analysis calcd (%) for C9H4BrF7S:
C 30.27; H 1.13; Br 22.38; F 37.24; S 8.98; found: C 30.06; H 1.11; Br
22.08; F 37.78; S 8.75.
A pseudo-two-dimensional 1H NMR spectrum was recorded using a
Bruker Avance 200 spectrometer equipped with a BVT 3300 temperature
control unit. The first axis was 1H NMR spectra, and the second axis was
time. The concentration of 2 was 30 mM, and the concentration of a
monomer was 0.4 M, except for methyl vinyl ketone (30 mM). Toluene-d8
served as a solvent (0.525 mL, δH 2.13 ppm). All experiments were
conducted at 328 K. Kinetic data were obtained as a dependency of
signal integral intensity on time. For measurement purposes, the
resonance of α-H of the ester group was selected (δ = 5.1 ppm) due to
the absence of overlapping signals. Recently, it was reported that this
signal changes its position during the reaction under study (see SI in ref.
[26]). This signal is also affected by the homolysis reaction, but at 328 K,
homolysis is negligible.
1‐(Ethenylsulfanyl)‐2,3,5,6‐tetrafluoro‐4‐(trifluoromethyl)benzene (4-
perfluorotolyl vinyl sulfide)
To a solution of 1-[(2-bromoethyl)sulfanyl]-2,3,5,6-tetrafluoro-4-(trifluoro-
methyl)benzene (4.45 g, 12.46 mmol) in dioxane (40 mL), 9.40 g of a
40% aqueous NaOH solution (94.00 mmol) was added. Into the resulting
two-phase system, 2.01 g (9.56 mmol) of tetraethylammonium bromide
was introduced with stirring. The resultant mixture was next stirred at
50°C, with monitoring of the reaction progress via 19F NMR until the
starting compound fully reacted. The reaction ended in 48 h. After cooling
to room temperature, the product mixture was poured into 400 mL of
water and allowed to settle. The bottom organic layer was separated and
placed in 11 mL of ca. 10% HCl, shaken, and allowed to settle again. In
this way, 2.97 g (86%) of the crude target product was isolated. The
resulting product was dried over CaCl2 and then sublimated in the
presence of hydroquinone at 80−82°C under a pressure of 0.5 mmHg.
Thus, 2.41 g of the pure target compound was obtained.
Synthesis
The
synthesis
of
4-perfluorotolyl
vinyl
sulfide
was
[1‐(ethenylsulfanyl)‐2,3,5,6‐tetrafluoro‐4‐(trifluoromethyl)benzene]
performed in two steps with isolation and characterization of an
intermediate product,
1‐[(2‐bromoethyl)sulfanyl]‐2,3,5,6‐tetrafluoro‐4‐(trifluoromethyl)benzene,
to ensure high purity of the target compound. Both the intermediate and
target compounds were characterized via 1H, 13C, and 19F NMR. The
NMR spectra were recorded on Bruker AV-300 [300.13 (1H) MHz, 282.40
(
19F) MHz] or Bruker DRX-500 [125.76 (13C) MHz] spectrometers for
solutions of the samples in CDCl3. NMR coupling constants (J) were
measured in Hertz. IR spectra were recorded by means of a Bruker
Vector 22 spectrophotometer from films for liquid samples. UV spectra
were obtained on a Hewlett Packard 8453 spectrophotometer. Molecular
mass and elemental composition were determined from high-resolution
3
Colorless liquid. 1H NMR (300.13 MHz, CDCl3): δH = 6.44 (ddt, JHH
=
16.5, 3JHH = 9.4, 5JHF = 1.3, 1H; Hα), 5.51 (d, 3JHH = 9.4, 1H; Hcis), 5.48 (d,
3JHH = 16.5, 1H; Htrans). 13C NMR (125.76 MHz, CDCl3): δC = 146.5 (dm,
1JCF = 248 Hz; C-(2,6)), 144.4 (ddm, JCF = 262, JCF = 17 Hz; C-(3,5)),
1
2
mass spectra acquired on
a Thermo Electron Corporation DFS
4
1
126.9 (t, JCF = 3 Hz; Cα), 121.0 (q, JCF = 274 Hz; CF3), 119.2 (s; Cβ),
119.0 (t, 2JCF = 19 Hz; C-(1)), 109.6 (qt, 2JCF = 35 Hz, 2JCF = 13 Hz; C-(4))
ppm. 19F NMR (282.36 MHz, CDCl3, C6F6 (δ = −162.9)): δF −57.5 (t, JFF
instrument (ionizing electron energy: 70 eV).
4
= 22, 3F; CF3), −133.0 to −33.2 (m, 2F; F-(2,6)), −140.8 to −141.2 (m, 2F;
F-(3,5)) ppm. IR (film, νmax, cm−1): 2879(vw) (CH), 1645(s) (ArF), 1593(s)
(C=C), 1483(vs) (ArF), 1400(m), 1329(vs) (CF3), 1269(w), 1184(s),
1151(s) (CF3), 1026(w), 982(vs) (CF), 951(m), 908(w), 831(s) (C-S),
716(s) (C-S), 660(w), 588(w), 548(vw), 484(vw), 436(vw). UV (CHCl3,
λmax, nm (log ε)): 285 (4.48). MS (70 ev) m/z calcd for C9H3F7S:
275.9838; found: 275.9837.
Scheme 2. The synthesis of 4-perfluorotolyl vinyl sulfide, which served as a
monomer.
1‐[(2‐Bromoethyl)sulfanyl]‐2,3,5,6‐tetrafluoro‐4‐(trifluoromethyl)benz
ene (3)
Acknowledgements
The kinetics measurements were supported by the Russian
Science Foundation (grant No. 20-73-00350). The synthesis of
nitroxide 1 and alkoxyamine 2 was performed under the financial
support of the Ministry of Science and Higher Education of the
Russian Federation (grant No. 14.W03.31.0034). The authors
are grateful to the Multi-Access Chemical Service Center SB
RAS for analysis and characterization of 4-perfluorotolyl vinyl
sulfide and 3.
To a solution of trifluoromethyl-tetrafluorobenzenethiol (52.86 g, 211.34
mmol) in acetonitrile (200 mL), a solution of sodium methoxide (4.2
mol/L) in methanol (55 mL) was added. The resulting thiolate solution
was added with stirring to a mixture of 1,2-dibromoethane (665.83 g,
3544.25 mmol) and acetonitrile (290 mL). The resulting mixture was
stirred at room temperature for 3 h and then incubated overnight. Dry
HBr was passed through the reaction mass to neutralize the excess of
sodium methoxide. The NaBr precipitate was filtered off, and the excess
of 1.2-dibromoethane was distilled off from the filtrate, first under normal
pressure and then in vacuum (in a water-jet pump). For the indicated
reagents’ quantities, the amount of distilled-off bromoethane was 520 g.
The residue, 66.61 g, was then distilled in vacuum (3–4 mmHg) to obtain
51.51 g (68%) of the target compound.
Keywords: alkoxyamine •1,3-dipolar cycloaddition • kinetics •
nitroxide • nitroxide-mediated polymerization
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δH = 3.51–3.43 (m, 2H; CH2), 3.43–3.35 (m, 2H; CH2) ppm. 13C NMR
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7
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Entry for the Table of Contents
Kinetics of 1,3-dipolar cycloaddition of various vinyl monomers to an aldonitrone-containing nitroxide and alkoxyamine were studied
at different temperatures by EPR and NMR spectroscopy. Among 11 studied monomers, the rate coefficient of the cycloaddition for
methyl vinyl ketone is the highest.
8
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