Kinetics of the [4+2] cycloaddition of cyclopentadiene with (E)-2-aryl-1-cyano-1-nitroethenes

Article abstract of DOI:10.1007/s00706-012-0735-3

The electrophilicity of (E)-2-aryl-1-cyano-1-nitroethenes is not sufficient to induce a zwitterionic course of their [4+2] cycloaddition to cyclopentadiene. The one-step mechanism of these reactions is indicated by the activation parameters, and the substituent and solvent effects on the reaction. The Author(s) 2012.

Full text of DOI:10.1007/s00706-012-0735-3

Monatsh Chem (2012) 143:895–899
DOI 10.1007/s00706-012-0735-3
ORIGINAL PAPER
Kinetics of the [4+2] cycloaddition of cyclopentadiene
with (E)-2-aryl-1-cyano-1-nitroethenes
Radomir Jasi n´ ski
Andrzej Bara n´ ski
•
Magdalena Kwiatkowska
•
Received: 18 August 2011 / Accepted: 9 February 2012 / Published online: 2 March 2012
Ó The Author(s) 2012. This article is published with open access at Springerlink.com
Abstract The electrophilicity of (E)-2-aryl-1-cyano-1-
nitroethenes is not sufficient to induce a zwitterionic course
of their [4?2] cycloaddition to cyclopentadiene. The one-
step mechanism of these reactions is indicated by the
activation parameters, and the substituent and solvent
effects on the reaction.
However, stereospecificity cannot be regarded as sufficient
evidence of the concertedness [10–12]. Nevertheless, it
cannot be ruled out that the reaction proceeds by a zwitter-
ionic mechanism [10, 13–15] (paths C and D), while
preserving the original nitroalkene configuration in cyc-
loadducts, when the activation barrier for the carbocyclic
ring closure in postulated intermediates is lower than the
corresponding barrier for rotation around the C–C bond of
the cyanonitroethyl moiety. The aim of this work was to
evaluate the nature of the transition complexes and to con-
firm or exclude the presence of zwitterionic intermediates on
the reaction paths. For this purpose, the kinetics of the title
reaction were investigated, and the substituent and solvent
effects as well as activation parameters were determined.
Keywords [4?2] Cycloaddition ꢀ Nitroalkene ꢀ
Kinetics ꢀ Mechanism
Introduction
For the last decade, reactions of conjugated nitroalkenes
have aroused our considerable interest [1–9], and we have
exerted considerable effort to examine their reactivity in
thermally allowed cycloaddition reactions. Especially the
Results and discussion
[
4?2] cycloaddition reactions of cyclopentadiene (1) with
strong electrophilic (E)-2-aryl-1-cyano-1-nitroethenes 2a–2g
were investigated [4–6, 9].
Kinetic studies (Table 1) showed that the rate constants of
the reactions leading to the nitronorbornenes 3a–3g and
4a–4g (Scheme 1) increase with the nitroalkene electro-
philicity. In particular, the rate constants k and k for the
By means of HPLC and NOESY experiments [6], it has
been unequivocally established that this reaction leads to the
corresponding 6-endo-aryl-5-endo-cyano-5-exo-nitronor-
bornenes 3a–3g and 6-exo-aryl-5-exo-cyano-5-endo-
nitronorbornenes 4a–4g as the only reaction products. The
stereospecific reaction course suggests that, independently
of the nitroalkene electrophilicity, this reaction occurs
according to a concerted mechanism (paths A and B).
A
B
reactions involving the least electrophilic (E)-2-(p-
methoxyphenyl)-1-cyano-1-nitroethene (2a) (x = 3.14 eV
-
[5]) are 0.08 9 10 dm mol
3
3
-1 -1
-3
s
and 1.66 9 10
3
dm mol
-1 -1
s , while those for the reactions with the most
electrophilic (E)-2-(p-methoxycarbonylphenyl)-1-cyano-1-
nitroethene (2g) (x = 3.80 eV [5]) are 13.35 9
-
3
3
dm mol
-1 -1
-3
and 83.42 9 10 dm mol
3
-1 -1
s ,
10
s
respectively. We investigated the substituent effect on the
reaction using classical correlation analysis, seeking the
best match between substrate reactivity (log k and log k )
R. Jasi n´ ski (&) ꢀ M. Kwiatkowska ꢀ A. Bara n´ ski
Institute of Organic Chemistry and Technology,
Cracow University of Technology, Warszawska 24,
A
B
?
and the values of the Hammett constants r , r , r , r [16]
p
R
I
of the substituents. The best linear correlations (R = 0.995
for the reaction 1 1 2 ? 3 and 0.991 for the reaction
31-155 Cracow, Poland
e-mail: radomir@chemia.pk.edu.pl
123

8
96
R. Jasi n´ ski et al.
Table 1 Results of kinetic measurements of [4?2] cycloaddition of cyclopentadiene with (E)-2-aryl-1-cyano-1-nitroethenes 2a–2g
3
3
9 10 /
3
9 10 /
Nitroalkene
Nr R
Solvent [E
S
(30)]
T/°C ktotal 9 10 /
Isomer ratio k
c [3]/[4]
A
k
dm mol
B
R
SD
3
dm mol
-1 -1
3
dm mol
-1 -1
s
3
-1 -1
s
s
r
p
x/eV
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
a
OCH
3
-0.27 3.14 Nitromethane (46.3)
-0.17 3.31 Nitromethane (46.3)
0.00 3.42 Nitromethane (46.3)
0.06 3.50 Nitromethane (46.3)
0.23 3.68 Nitromethane (46.3)
0.23 3.71 Nitromethane (46.3)
0.45 3.80 Nitromethane (46.3)
0.45 3.80 Nitromethane (46.3)
0.45 3.80 Nitromethane (46.3)
0.45 3.80 Dichloroethane (41.9)
0.45 3.80 Dichloromethane (41.1)
0.45 3.80 Chloroform (39.1)
0.45 3.80 Chloroform (39.1)
0.45 3.80 Chloroform (39.1)
25
25
25
25
25
25
5
1.74
3.28
0.05
0.09
0.12
0.12
0.14
0.14
0.15
0.15
0.16
0.12
0.12
0.10
0.11
0.12
0.11
0.08
0.27
0.67
0.78
2.76
3.09
4.58
7.82
13.35
3.69
3.56
0.57
1.00
2.17
0.74
1.66
3.01
0.998 0.01
0.998 0.02
0.998 0.02
0.997 0.03
0.998 0.03
0.998 0.04
0.998 0.05
0.998 0.05
0.997 0.07
0.999 0.04
0.999 0.05
0.999 0.03
0.997 0.05
0.999 0.04
0.997 0.07
b CH
3
c
H
6.23
5.56
d F
7.26
6.48
e
f
Cl
Br
22.46
25.13
35.15
59.99
96.77
34.46
33.23
6.26
19.70
22.04
30.57
52.17
83.42
30.77
29.67
5.69
g
g
g
g
g
g
g
g
g
COOCH
COOCH
COOCH
COOCH
COOCH
COOCH
COOCH
COOCH
COOCH
3
3
3
3
3
3
3
3
3
15
25
25
25
5
15
25
10.05
20.28
7.45
9.05
18.11
6.71
0.45 3.80 Tetrachloromethane (32.5) 25
Scheme 1
CN
B
A
NO2
C H -R
H
6
4
NO2
CN
4a-4g
H
3a-3g C H -R
6
4
NC
NO2
+
R-C H4
6
1
2a-2g
+
+
O
D
C
O
-
-
N
N
O
O
R-C H
R-C H4
6
4
6
CN
CN
+
+
NO2
CN
CN
CN
C H -R
H
2
6
4
CN
NO
8a-8g C H -R
R-C H
R-C H4
6
4
6
N
H
N
O
-
O
7a-7g
6
4
O
a-6g
-
O
6
5a-5g
R = p-OCH3 (a), p-CH3 (b), H (c),p-F (d), p-Cl (e), p-Br (f), p-COOCH3 (g)
1
1 2 ? 4) were obtained with the Hammett constants rp
Fig. 1). This means that the substituent effect is trans-
the reaction 1 1 2 ? 4) are almost two times higher than
those for concerted [2?4] cycloadditions of cyclopentadiene
with (E)-2-aryl-1-nitroethenes [2], while still being within
the range typical of one-step cycloadditions [16]. Further-
more, the positive sign of the q constant shows that charge
transfer within the transition complexes of both competing
reactions occurs from the cyclopentadiene substructure
towards the nitroalkene substructure. Moreover, the rela-
(
ferred to the reaction centers via both induction and
resonance [17], and the transition complexes are similar
regardless of the nitroalkene electrophilicity. These rela-
tionships are expressed by the Hammett Eqs. 1 and 2.
log k ¼ 2:93 ꢀ r ꢁ 3:20 ðR ¼ 0:995Þ
log kB ¼ 2:31 ꢀ rp ꢁ 2:20 ðR ¼ 0:991Þ
ð1Þ
ð2Þ
A
p
tionships between log kA and log k and the nitroalkene
B
It is noteworthy that the values of the reaction constants q
in Eqs. 1 and 2 (2.93 for the reaction 1 1 2 ? 3 and 2.31 for
global electrophilicity indexes x are similar. In this case,
however, the correlations are slightly lower.
1
23

Kinetics of the [4?2] cycloaddition of cyclopentadiene
897
p
Fig. 1 Plot of log k versus Hammett constants r and global electrophilicity index x for the [4?2] cycloaddition of cyclopentadiene with (E)-2-
aryl-1-cyano-1-nitroethenes 2a–2g (at 25 °C in nitromethane)
log k ¼ 3:04 ꢀ x ꢁ 13:64 ðR ¼ 0:986Þ
ð3Þ
ð4Þ
A
log k ¼ 2:37 ꢀ x ꢁ 10:34 ðR ¼ 0:971Þ
B
In order to gain more insight into the nature of the
transition complexes, we studied the effect of solvent
polarity on the reaction kinetics. This was done for the
reaction of the most electrophilic nitroalkene 2g (Table 1).
The cycloadditions 1 1 2g ? 3g and 1 1 2g ? 4g were
found to occur more rapidly in polar than in non-polar
solvents. This suggests that the transition complexes of the
reactions tested are more polar than the substrates. This is
consistent with the magnitude of dipole moments
(
l = 7.36/8.54 D) of the respective critical structures
located on the reaction potential energy hypersurface
using B3LYP/6–31 g(d) calculations [5].
To obtain a quantitative description of the solvent effect
on the reaction rate, we studied correlations between sub-
strate reactivity and the solvent polarity constants such as
dielectric constant e, dipole moment l, Berson constant X,
T
Fig. 2 Plot of log k versus Reichardt–Dimroth E (30) constants for
and Reichardt-Dimroth E (30) constant [18, 19]. The best
T
the [4?2] cycloaddition of cyclopentadiene with (E)-2-(p-methoxy-
carbonylphenyl)-1-cyano-1-nitroethene (2g) (at 25 °C)
linear correlations were obtained for the E (30) constants
T
(
Eqs. 5, 6) (Fig. 2).
log k ¼ 0:09 ꢀ E ꢁ 6:08 ðR ¼ 0:987Þ
ð5Þ
ð6Þ
A
T
Hence, the nature of the Hammett and Reichardt–Dim-
roth relationships suggests that the reactions 1 1 2 ? 3
and 1 1 2 ? 4 do not proceed according to a zwitterionic
mechanism [21] (paths C and D). The values of activation
parameters (Table 2) lead to similar conclusions. These
parameters were determined from the rate constants of the
reactions 1 1 2g ? 3g and 1 1 2g ? 4g at 5, 15, and
25 °C using the Eyring equation in the form:
log k ¼ 0:08 ꢀ E ꢁ 4:76 ðR ¼ 0:994Þ
B
T
These data prove that solvent polarity affects the rate of
1
1 2g cycloaddition to a much higher extent than that for
the reaction involving (E)-2-phenyl-1-nitroethene [2].
However, the sensitivity constants in Eqs. 5 and 6 do not
exceed 0.1 and are below the typical range observed for
ionic reactions [18, 20].
123

8
98
R. Jasi n´ ski et al.
Table 2 Activation parameters for [4?2] cycloaddition of cyclopentadiene with (E)-2-(p-methoxycarbonylphenyl)-1-cyano-1-nitroethene (2g)
Nitromethane
Chloroform
1
1 2g ? 3g
1 1 2g ? 4g
1 1 2g ? 3g
1 1 2g ? 4g
=
-1
DH /kJ mol
34.3
32.2
43.5
37.3
=
DS /J mol
-1 -1
K
-165.4
-157.4
-149.9
-153.2
6
6
Experimental
logðk=TÞ ¼ 10:319 þ DS =4:576 ꢁ DH =4:576T
ð7Þ
=
In particular, the activation enthalpies (DH ) were
estimated from the slopes of the plots of logarithm of the
ratio of kA or kB by the absolute temperature (T) versus
reciprocal of the absolute temperature (1/T), while the
All solvents employed for kinetic measurements were
purified by standard methods [28]. Cyclopentadiene (1)
was prepared by pyrolysis of commercially available di-
cyclopentadiene (Aldrich) at 180–200 °C, according to a
known method [29]. Before use it was distilled under
atmospheric pressure, using a 25-cm Vigreux column.
(E)-2-Aryl-1-cyano-1-nitroethenes 2a–2g were obtained
by condensation of appropriate aromatic aldehydes with
nitroacetonitrile, according to a reported procedure [30].
Their purity was confirmed by HPLC analyses.
=
entropies of activation (DS ) were determined from the
intercepts of these plots. It follows from the data in
=
Table 2 that the DH values for the cycloadditions
1
1 2g ? 3g and 1 1 2g ? 4g do not exceed
6 kJ mol . They are typical for the reactions, where the
-1
4
energy changes within the transition complexes resulting
from breaking of p bonds existing in the substrates and
formation of new r bonds present in the cycloadducts
Kinetic experiments were carried out in a glass reactor
supplied with a thermostatically controlled jacket, mag-
netic stirrer, thermometer, reflux condenser, and sampling
device. Liquid chromatography (HPLC) was done using a
Knauer apparatus, equipped with a UV–Vis detector
(k = 254 nm). For monitoring of the reaction progress a
LiChrospher 100RP column (4 9 240 mm) thermostated
at 5 °C was used, and 70% methanol at a flow rate of
=
compensate one another [16]. On the other hand, the DS
values are high and negative (-150 to -165 J
-
1
-1
K ), both in the moderately polar chloroform and
mol
the highly polar nitromethane. This is typical for one-step
cycloadditions proceeding through highly rigid transition
=
complexes [22–24]. By comparison, DS for a two-step
3
-1
cycloaddition of 1,1-dimethoxy-1,3-butadiene with tetra-
1
cyanoethene is -26 J mol
1.1 cm min was applied as the eluent.
-
-1
K
[25]. It is noticeable
that the entropy changes in the transition state are much
higher in our case than those for a similar reaction
=
Kinetic procedure
involving (E)-2-phenyl-1-nitroethene (DS = -102.5 to
The overall rate constants (ktotal) were determined by
monitoring the decrease of the HPLC peak area (A) corre-
sponding to cyclopentadiene. It was found that the product
composition was controlled kinetically, since the ratio of
the products (c = [3]/[4]) was constant throughout the
reaction course. The kinetic experiments were carried out
at 5, 15, and 25 °C up to 70–80% completion, using in each
case equimolar quantities of substrates. In definite time
-
1
-1
K
-
120 J mol
[2]). This is probably due to the
polarization and dispersion of the substrate interactions
within the transition complexes, which should be much
stronger in the case of more electrophilic (E)-2-aryl-1-
cyano-1-nitroethene than in the case of less electrophilic
(
E)-2-aryl-1-nitroethenes [26, 27]. This is indirectly con-
firmed by the depths of the pre-reaction local minima
located on the potential energy hypersurface found for
the reactions involving (E)-2-aryl-1-cyano-1-nitroethenes
3
intervals 250-mm samples were taken out of the reactor
with a microsyringe and analyzed immediately by HPLC.
In this way 15 series of measurements were completed. The
regression analysis was carried out using MATCAD 07. It
showed excellent linear relationships (R [ 0.99) between
[
5, 9] and (E)-2-aryl-1-nitroethenes [1].
1/A and time t for all kinetic runs, using the second-order
kinetic Eq. 8 of the form:
Conclusion
In summary, the electrophilicity of (E)-2-aryl-1-cyano-1-
nitroethenes is not sufficient to induce an ionic course of
their [4?2] cycloaddition to cyclopentadiene. The activa-
tion parameters and the substituent and solvent effects on
the reaction studied indicate its non-ionic mechanism.
1=A ¼ ꢁktotal ꢀ t þ const
ð8Þ
The ktotal and c values were then applied for calculation
of the rate constants k and k according to Eqs. 9 and 10:
A
B
kA ¼ c ꢀ ktotal=ðc þ 1Þ
ð9Þ
1
23

Kinetics of the [4?2] cycloaddition of cyclopentadiene
kB ¼ ktotal=ðc þ 1Þ
The second-order rate constants for different temperatures
and solvents obtained by this method are summarized in
Table 1.
899
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Acknowledgments We wish to express our thanks to Prof. V. M.
Berestovitskaya from Herzen Russian State Pedagogical University in
St. Petersburg for valuable discussions during the synthesis of (E)-2-
aryl-1-cyano-1-nitroethenes. This work was supported by the Polish
Ministry of Science and Higher Education (Grant C-2/508/BW/2010).
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