4-Phenyl-1,2,4-triazoline-3,5-dione in the ene reactions with cyclohexene, 1-hexene and 2,3-dimethyl-2-butene. the heat of reaction and the influence of temperature and pressure on the reaction rate

Article abstract of DOI:10.1002/poc.3277

The values of the enthalpy (53.3; 51.3; 20.0 kJ mol^-1), entropy (-106; -122; -144 J mol^-1K^-1), and volume of activation (-29.1; -31.0; -cm³ mol^-1), the reaction volume (-25.0; -26.6; -cm³ m

Full text of DOI:10.1002/poc.3277

Research Article
Received: 30 October 2013,
Revised: 12 December 2013,
Accepted: 15 December 2013,
Published online in Wiley Online Library: 17 January 2014
(wileyonlinelibrary.com) DOI: 10.1002/poc.3277
4-Phenyl-1,2,4-triazoline-3,5-dione in the ene
reactions with cyclohexene, 1-hexene and 2,
3-dimethyl-2-butene. The heat of reaction and
the influence of temperature and pressure
on the reaction rate
Vladimir D. Kiseleva*, Dmitry A. Kornilova, Helen A. Kashaevaa,
Lyubov N. Potapovaa and Alexander I. Konovalova
The values of the enthalpy (53.3; 51.3; 20.0 kJ molꢀ1), entropy (ꢀ106; ꢀ122; ꢀ144 J molꢀ1Kꢀ1), and volume of activation
(ꢀ29.1; ꢀ31.0; ꢀcm3 molꢀ1), the reaction volume (ꢀ25.0; ꢀ26.6; ꢀcm3 molꢀ1) and reaction enthalpy (ꢀ155.9; ꢀ158.2;
ꢀ150.2 kJ molꢀ1) have been obtained for the first time for the ene reactions of 4-phenyl-1,2, 4-triazoline-3,5-dione 1, with
cyclohexene 4, 1-hexene 6, and with 2,3-dimethyl-2-butene 8, respectively. The ratio of the values of the activation volume to
the reaction volume (ΔV≠corr/ΔVr ꢀ n) in the ene reactions under study, 1 + 4→5 and 1 + 6→7, appeared to be the same, namely
1.16. The large negative values of the entropy and the volume of activation of studied reactions 1 + 4→ 5 and 1 + 6→7 better
correspond to the cyclic structure of the activated complex at the stage determining the reaction rate. The equilibrium con-
stants of these ene reactions can be estimated as exceeding 1018 L molꢀ1, and these reactions can be considered irreversible.
Copyright © 2014 John Wiley & Sons, Ltd.
Keywords: activation volume; cyclohexene; 2,3-dimethylbytene-2; ene-reaction; enthalpy reaction; 1-hexene; high pressure effect; 4-
phenyl-1,2,4-triazolin-3,5-dione; rate constant
whereas the rate of reaction with 1 is 5–6 orders of magnitude
INTRODUCTION
higher than that of the C = C analogue.[12] The relatively high
There are some papers dealing with the study of behavior of
4-substituted 1,2,4-triazoline-3,5-dione (TAD) in thermal and
photochemical [4 + 2]-, [3 + 2]-, [2 + 2]-cycloaddition, ene and
curing reactions.[1–11] An extensive study of the products
structure, the reaction rate and the solvent effect on the reaction
thermal stability of the [4+ 2]-cycloaddition adducts coupled with
the increased reaction rate allowed TAD to participate in many
thermal DARs with usual dienes, as well as with very inactive
dienes such as hexachlorocyclopentadiene,[2,3] norbornadiene[2,12]
and even with naphthalene and methyl-substituted naphtha-
lene.[13] 9,10-Diphenylanthracene 2 does not normally react with
all known dienophiles possessing the C = C bond due to the
complete steric shielding of the 9,10-diene reaction centers.
Unexpectedly, the reaction 1 + 2→3 takes place even at room
temperature at 1,4-atoms of diene 2, which are less active, but
sterically accessible (s 1).[14]
of [4 + 2]-cycloaddition of
1 with various 1,3-dienes was
presented.[1–5] Practically, at the same time, the ene synthesis
of 1 with various alkenes was initiated.[5–11] N-allylurazole
derivatives, the ene reaction products, are usually formed when
the allyl hydrogen atom is present in alkene as well as α-shift
of the double bond in the product is possible.[9–11]
For the ene reactions of TAD with alkenes, the presence of inter-
mediates such as an aziridinium imide, 1,2-, 1,4-dipolar intermedi-
ates[9,11,15,16] and biradical has been proposed.[17] However, the
effect of the solvent polarity on the rate of the ene reactions with
1 in most cases is very small, and an increased rate is usually
observed in the low-polar media.[8] According to the correlation of
* Correspondence to: V. D. Kiselev, Department of Physical Chemistry, Butlerov
Institute of Chemistry, Kazan Federal University, Kremlevskaya str. 18, Kazan,
420008, Russian Federation.
The high activity of the substituted TAD is rationalized by the
enhanced easiness of breaking endocyclic π-N = N bond.[12] The
acceptor properties and exothermicity of the Diels–Alder
reactions (DAR) between 4-phenyl-1,2,4-triazoline-3,5-dione 1
and the C = C analogue, 4-phenylmaleimide, are comparable,
E-mail:vkiselev.ksu@gmail.com
a V. D. Kiselev, D. A. Kornilov, H. A. Kashaeva, L. N. Potapova, A. I. Konovalov
Department of Physical Chemistry, Butlerov Institute of Chemistry, Kazan
Federal University, Kremlevskaya str. 18, Kazan, 420008, Russian Federation
J. Phys. Org. Chem. 2014, 27 401–406
Copyright © 2014 John Wiley & Sons, Ltd.

V. D. KISELEV ET AL.
and 100 Pa before the measurements. The spectral purity of 1 was analyzed
in accordance with the known absorption coefficients (ε532 nm = 171 in
1,4-dioxane at 25 °C,[3] ε540 nm = 245 in toluene) and by means of
spectrophotometric titration of 1 in the fast reaction with scintillating
anthracene. The structures of the reaction products 5, 7 and 9 were
proved earlier.[7] All solvents were purified by known methods.[26]
Kinetic measurements
The rate of reactions 1+4→5 and 1+6→7 was determined by measuring
the UV absorption of enophile 1 at 530–540 nm (the UV-spectrophotometer
Hitachi-2900), where other compounds are transparent. The initial concentra-
tion of 1 was in the range of (3–5) · 10ꢀ3 mol Lꢀ1 and that of 4 and 6ꢀ
(1–2) · 10ꢀ1 mol Lꢀ1. Temperature accuracy was 0.1 °C. The rate of fast
reaction 1+8→9 was measured by the stopped-flow method (RX 2000 with
spectrophotometer Cary 50 Bio) in benzene solution at 23.3 °C and 40.1 °C
and in 1,2-dichloroethane solution at 23.5 °C by monitoring the absorption of
1 at 540 nm. The concentration of 1 stock solutions was 8.13ꢁ10ꢀ3 mol Lꢀ1
and that of 8 was 8.86ꢁ10ꢀ2 mol Lꢀ1. The values of the rate constants were
determined with errors of 3%, the enthalpy of activation ꢀ 2 kJ molꢀ1 and
Scheme 1. 4-Phenyl-1,2,4-triazoline-3,5-dione 1 in the Diels–Alder reaction
at 1.4-atoms of 9.10-diphenylanthracene 2, and in the ene reactions with
cyclohexene 4, 1-hexene 6 and 2,3-dimethyl-2-butene 8
the entropy of activation ꢀ 6 J molꢀ1 Kꢀ1
.
The rate constants of reactions 1 + 4→5 and 1 + 6→7 under elevated
pressure were measured in toluene solution at 25 °С with the high
pressure system, using the high pressure-pump “HP-500” and the high
pressure optical cell “PCI-500” produced by Syn. Co., Ltd (Japan), adjusted
to our UV-spectrophotometer (SCINCO Co., Ltd, Seoul, Korea).
the solvent effect on the rate of the ene reactions with 1 (lnk(1))[8,9,11]
and the DARs with 1 (lnk(2))[12], the following equation was obtained:
lnk(1) = (0.9 0.1)lnk(2) +1.5; R=0.974, N=6. Very similar solvent
effect on the rate constants of these reactions generates the slope
close to unity.
Calorimetry
In comparison with the DARs, the ene reactions with the C = C
bond in enophile usually possess higher activation energy
(about 100–150 kJ molꢀ1), while the activation entropies are near
the same (about ꢀ120 – ꢀ170 J molꢀ1Kꢀ1).[18] The concerted
mechanism for the retro-ene reaction was proposed on the base
of Arrhenius parameter as the firmly established one.[19,20] The
same mechanism should be considered for the direct ene-
reaction.
Due to their low activity, only few ene reactions with the C = C
and C = O reaction centers in the enophiles have been studied
under the high pressure and at elevated temperatures.[21–25]
The necessity to recalculate the values of the activation volume
within the range of temperatures from the high to room one
causes the additional errors.[21–25]
It is interesting to compare the activation parameters of the
ene reactions and the Diels–Alder ones with 1. It should be noted
that the pressure effect on the rate of alkenes ene reactions with
TAD has not been studied. 4-Phenyl-1,2,4-triazoline-3,5-dione 1 is
very convenient for such measurements in the reactions 1 + 4→5
and 1 + 6→7 at 25 °C. Taking into account everything above, we
studied the effect of temperature and pressure on the rate of ene
reactions and compared the values of enthalpy, entropy and
volume of activation, enthalpy and volume of reactions
1 + 4→5, 1 + 6→7 and 1 + 8→9 with those for the DAR. Due to
a very fast reaction rate, the pressure effect on the reaction
1 + 8→9 was not studied.
The enthalpies of all the studied reactions in solution were measured at
25 °C by calorimetric method, described previously.[12] The solid sample
(20–35 mg) of 1 was added to the solution of cyclohexene 4 in toluene
(150 mL, с4 = 1.0 mol Lꢀ1). Four consecutive measurements for reaction
1+ 4→5 provided: ꢀ138.0, ꢀ137.8, ꢀ136.8 and ꢀ137.8 kJ molꢀ1. With
the heat of solution of solid 1 in toluene (18.3 kJ molꢀ1), the enthalpy of
reaction 1+ 4→5 in toluene solution is ꢀ155.9 0.4 kJ molꢀ1. Three consec-
utive measurements of the heat of reaction 1+ 6→7 in 1,2-dichloroethane
solution provided: ꢀ136.1, ꢀ135.1 and ꢀ137.8 kJ molꢀ1. With the heat of
solution of solid 1 in 1,2-dichloroethane (21.9 kJ molꢀ1), the enthalpy of
reaction 1 + 6→7 in 1,2-dichloroethane solution is ꢀ158.2 1.0 kJ molꢀ1
.
The heat of reaction 1+ 8→9 was measured in toluene (ꢀ121.9; ꢀ120.3
and ꢀ120.6 kJ molꢀ1) as well as in 1,2-dichloroethane (ꢀ127.1; ꢀ130.2
and ꢀ127.4 kJ molꢀ1). Taking into account the heat of solution of solid 1,
the enthalpy of reaction 1 + 8→9 is ꢀ139.2 0.6 kJ molꢀ1 in toluene and
ꢀ150.2 1.4 kJ molꢀ1 in 1,2-dichloroethane, respectively.
Volume parameters
The volume of activation, ΔV≠, was calculated using the dependence lnkp
vs P, where kp is the rate constant of reactions 1 + 4→5 and 1 + 6→7
carried out under pressure P. Reactions 1 + 8→9 were very fast for such
measurements.
The reaction volume (ΔVr ꢀ n) was calculated using Eqn (1)[12]
:
1=d ¼ 1=dðt¼0Þ þ cAdd ΔVrꢀn=1000d
(1)
ðtÞ
;t
ðt¼0Þ
EXPERIMENTAL
Here, cAdd,t is the current concentration of 5 or 7 adduct, d(t=0) and dt
are the initial and current densities of solution of the reaction mixture,
ΔVr ꢀ n is the reaction volume. The vibration densimeter manufactured
by Anton Paar, model DSA 5000M, was employed for the measurements
of the solution density at 25 0.002 °C. The densimeter was calibrated
with water and air, in accordance with the manual.
Materials
Cyclohexene 4, 1-hexene 6 (Acros, 99 + %) and 2,3-dimethyl-2-butene
8
(Sigma-Aldrich >98%) were used as purchased. 4-Phenyl-1,2,
4-triazoline-3,5-dione 1 (Aldrich, Germany, 97%) was sublimed at 100 °C
wileyonlinelibrary.com/journal/poc
Copyright © 2014 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2014, 27 401–406

4-PHENYL-1,2,4-TRIAZOLINE-3,5-DIONE IN THE ENE REACTIONS
Table 2. The adiabatic ionization potentials of some acyclic
alkenes (IPad/eV) and the rate constants of the ene reactions
RESULTS AND DISCUSSIONS
alkenes with 1 (lnk2/k2 in L molꢀ1 ꢀ1) in dichloromethane
s
Kinetic measurements at ambient pressure
at 23.5 °C
The rate constants of reactions 1 + 4→5, 1 + 6→5 and 1 + 8→9
in temperature range as well as the activation parameters are
summarized in Table 1.
No
Alkene
IPad
lnk2
It can be noted that the rate constants and activation param-
eters for the ene-reactions of 1 + 4→5 and 1 + 6→7 are nearly
the same. The reaction rate of 1 with tetramethylethylene 8 is
higher on four orders of magnitude, mainly due to a very low
enthalpy of activation. All the kinetic data obtained for the ene
synthesis with 1 are more consistent with the asymmetric forma-
tion of new C–N and N–H bonds, which does not preclude their
concerted formation.[5] The large and negative values of the
activation entropy in the studied ene reactions (Table 1) are
similar to those of the Diels–Alder reaction[12,27] and correspond
to a highly ordered transition state.
It is interesting to find out what determines the change of
alkenes activity in the ene reactions with 1. The values of alkenes
ionization potentials of (IP/eV),[28] and their reaction rate
constants (lnk2) with 1 in dichloromethane,[9] are collected in
Table 2.
1.
2.
3.
4.
5.
6.
7.
2,3-Dimethylbutene-2
2-Methylbutene-2
cis-Hexene-3
8.27
8.68
8.95
9.11
9.10
9.24
9.46
5.81а
1.87
ꢀ0.115
ꢀ0.967
ꢀ1.71
ꢀ1.64
ꢀ4.61
cis-Butene-2
trans-Butene-2
2-Methylpropene
Hexene-1
аData of this work in 1,2-dichloroethane at 23.5 °C.
Kinetic measurements under high pressure
The experimental values of the activation volume were calcu-
lated from Eqn (2). The rate constants of the under pressure re-
actions were obtained by admitting the initial concentrations
of reagents as independent of the pressure.
As it can be seen from Fig. 1, the definite increase of the ene
reactions rate with that of π-donor properties of alkenes has
been observed. Similar linear relations lnk2 vs IP of the dienes
are typical for the DARs.[27]
The enthalpies of the studied ene reactions (from ꢀ139 to ꢀ158
kJ molꢀ1) are more exothermic than those of the Diels–Alder
reactions of 1 with substituted anthracenes (from ꢀ50 kJ molꢀ1
to ꢀ118 kJ molꢀ1), and 1 with cyclopentadiene (ꢀ134 kJ molꢀ1),
but less exothermic than in reaction of 1 with 1,3-butadiene
(ꢀ201 kJ molꢀ1) and norbornadiene (ꢀ218 kJ molꢀ1).[12]
ΔV≠
¼ ꢀRT ∂lnðkPÞ=∂P
(2)
app
The corrected value (ΔVc≠orr) of the activation volume can be
calculated taking into account the compression of the solvent[29]
:
ΔV≠
¼ ꢀRT ∂lnðkPÞ=∂P þ ðn ꢀ 1ÞRTβ (3)
corr
T
Here, kP is the rate constant of the reaction under pressure calcu-
lated without the correction of the concentration value (m, c, x)
Table 1. The rate constants (k2/L molꢀ1 sꢀ1), enthalpy (ΔH≠/kJ molꢀ1), entropy (ΔS≠/J molꢀ1 Kꢀ1) and free energy (ΔG≠/kJ molꢀ1
of activation for the ene reactions 1 + 4 → 5; 1 + 6 → 7 and 1 + 8 → 9
)
b
T/°С
k2
R a
N
ΔH≠
ΔS≠
ΔG≠
Reaction 1 + 4 → 5 in toluenꢀe3
15.0
20.0
25.0
25.0
26.0
40.0
3.84 · 10ꢀ3
5.51 · 10ꢀ3
8.07 · 10
0.9999
0.9999
0.9999
0.9999
0.9999
0.9997
76
54
51
24
95
60
53.3
ꢀ106
84.9
7.91 · 10ꢀ3
8.58 · 10ꢀ3
24.3 · 10ꢀ3
Reaction 1 + 6 → 7 in toluenꢀe3
15.0
25.0
25.0
25.0
25.0
40.0
1.15 · 10ꢀ3
2.33 · 10
0.9999
0.9998
0.9999
0.9997
0.9999
0.9998
19
15
22
21
17
15
2.35 · 10ꢀ3
2.32 · 10ꢀ3
2.37 · 10ꢀ3
6.90 · 10ꢀ3
51.8
20.0
ꢀ122
ꢀ144
88.1
62.9
Reaction 1 + 8 → 9
c
23.3
40.1
23.5
55.6 0.5
≥0.9998 c
0.9999 c
≥63 c
≥43 c
≥15
c
90.5 1.3
d
d
335
9
≥0.9998 d
aR is the correlation coefficient.
bN is the number of points in reaction run.
cThe mean values of ten repeated measurements of reaction rate in benzene;
d
The mean values of ten repeated measurements in 1,2-dichloroethane.
J. Phys. Org. Chem. 2014, 27 401–406
Copyright © 2014 John Wiley & Sons, Ltd.
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V. D. KISELEV ET AL.
Figure 1. The dependence of the rate constant (lnk2) of the ene reac-
tions 4-phenyl-1,2,4-triazoline-2,5-dione with alkenes on their ionization
potentials (IP/eV). The numbering of the points corresponds to Table 2
Figure 2. Pressure effect on the rate of reaction 1+4→5 in toluene at 25 °C:
1 – 1; 2 – 219; 3 – 336; 4 – 594; 5 – 783; 6 – 1034; 7 – 1296; 8 –1578; 9 – 1793
and 10 – 1987 bar
under pressure P, and (n ꢀ 1)RTβT is the correction item, where n is
the reaction order, and βT = ∂ln(d)/∂P is the solvent compressibility
coefficient. The experimental values of the rate constants under
pressure are summarized in Table 3.
The rate of reaction 1 + 4 → 5 was followed by the change of
the absorption of enophile 1 at the comparable stock concentra-
tions of the reagents: c0,1 = 0.0103 and c0,4 = 0.0202 mol Lꢀ1
,
(Fig. 2). The stock concentrations of reagents c0,1 = 0.0061 and
c
0,6 = 0.0622 mol Lꢀ1 were used for the slower rate of reaction
1 + 6 → 7 (Fig. 3). All the reaction mixtures were prepared from
the stock solutions of reagents to decrease the errors in ratio
Ln(kP/kP=1).
The rate constants of reaction 1 + 4 → 5 under pressure were
calculated on the basis of the absorption coefficient ε1 in toluene
(245 L molꢀ1 cmꢀ1, λmax = 540 nm) and its alterations under
pressure: ε1,P (540 nm) = 245 + 0.0428P
–
3.83 10ꢀ6 P2,
Figure 3. Pressure effect on the rate of reaction 1 + 6→7 in toluene at
25 °C: 1 – 1; 2 – 214; 3 – 431; 4 – 628; 5 – 816; 6 – 1025; 7 – 1313; 8 –1618
and 9 – 2013 bar
R = 0.9998, N = 5. The dependence of the reaction rate on the
pressure (line a, Fig. 4) was described by polynomial Eqn (4) as
well as logarithmically (5).
LnðkP=kP¼1Þ ¼ ꢀ0; 0299 þ 1:269·10ꢀ3P ꢀ 1:62·10ꢀ7P2; (4)
R² ¼ 0; 9990; N ¼ 10
According to Eqns (4) and (2), the experimental value of activa-
tion volume, ΔVa≠pp, is equal to ꢀ31.3 0.5 cm3 molꢀ1. The
corrected value of ΔV≠corr was calculated according to Eqn (3) with
the compressibility coefficient of toluene, βT = 90.10ꢀ6 bar[30] and
is equal to ꢀ29.1 0.5 cm3 molꢀ1. In accordance with the inde-
pendent logarithmic relation (5), the values of ΔV≠app and ΔVc≠orr
are calculated as ꢀ30.7 0.9 and ꢀ28.5 0.9 cm3molꢀ1, respec-
tively. Relation (4) has the better correlation coefficient than the
logarithmic one (5). However, Eqn (4), in contrast to Eqn (5), im-
plies the existence of the false peak at P = 3920 bar, and the reac-
tion rate [Ln(kP/kP=1)] under higher pressure cannot be predicted.
It should be noted that the difference in two values of ΔV≠ at P = 1
is within experimental error. Equation (5) is applicable for the de-
termination of the activation volume and is also useful when
choosing the conditions of the reaction under pressure. For in-
stance, Eqn (5) predicts value kP/kP=1 equal to 206 at 10 kbar
and 660 at 15 kbar, respectively.
LnðkP=kP¼1Þ ¼ 3:535Ln½ð2844 þ PÞ=2844ꢂ; R² ¼ 0; 9976; (5)
N ¼ 10
Table 3. Pressure (P/bar) effect on the rate constant
(k2/L molꢀ1 sꢀ1
)
of the reactions 1 + 4→5 and
1 + 6→7 in toluene at 25 °C
Reaction 1 + 4 → 5
Reaction 1 + 6 → 7
P
k2
Ln(kP/kP=1
)
P
k2
Ln(kP/kP=1)
1
0.00778
0.00972
0.0110
0.0152
0.0188
0.0233
0.0297
0.0385
0.0438
0.0488
0
1
0.00204
0
219
336
594
783
1034
1296
1578
1793
1987
0.223
0.349
0.667
0.883
1.097
1.341
1.599
1.729
1.836
214 0.00285
431 0.00369
628 0.00447
816 0.00544
1025 0.00671
1313 0.0084
1618 0.0105
2013 0.0136
0.336
0.592
0.784
0.980
1.190
1.415
1.638
1.898
The rate of slower reaction 1 + 6→7 under pressure was stud-
ied with account of the pseudo first-order conditions (Fig. 3). The
pressure effect on the reaction rate (Fig. 4, line b) was described
by polynomial Eqn (6).
LnðkP=kP¼1Þ ¼ 0; 030 þ 1:338·10ꢀ3P ꢀ 2:08·10ꢀ7P2; (6)
R² ¼ 0; 9990; N ¼ 9
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J. Phys. Org. Chem. 2014, 27 401–406

4-PHENYL-1,2,4-TRIAZOLINE-3,5-DIONE IN THE ENE REACTIONS
Figure 6. Relation dꢀ1 (cm3 gꢀ1) vs c7,t (mol Lꢀ1) for reaction 1 + 6→7
Figure 4. Correlation ln(kp/kp=1) vs P for ene reaction 1 + 4→5 (a) and
for reaction 1 + 6→7 (b) in toluene at 25 °С. For clarity, the line b shifted
up on the value of 0.5
in toluene at 25 0.002 °C. 1.
d
ꢀ1 = ꢀ0.030935C7 +1.162857;
R2 = 0.9997; C01 = 6.986 · 10ꢀ3 mol Lꢀ1; C06 = 0.12495 mol Lꢀ1. For clarity,
the line 1 shifted down on the value of 0.0002; 2. dꢀ1 = ꢀ0.031638 C7
+1.163056; R2 = 0.9997; C01 = 7.172·10ꢀ3 mol Lꢀ1; C06 = 0.1246 mol Lꢀ1
3. dꢀ1 = ꢀ0.030409 C7 +1.162612; R2 = 0.9994; C01 = 6.90·10ꢀ3 mol Lꢀ1
;
;
The values of ΔV≠app (ꢀ33.2 cm3molꢀ1) and ΔVc≠orr (ꢀ31.0
cm3molꢀ1) for the ene reaction 1 + 6→7 were obtained from
these data.
C
06 = 0.1114 mol Lꢀ1
It is important to compare the volume of activation and that of
reaction. In this work, we used a more convenient method,
namely, Eqn (1), to determine the reaction volume. Linear depen-
dences 1/d(t) vs c5,t or c7,t have been obtained for up to 100% of
conversion for reactions 1 + 4 → 5 (Fig. 5) and 1 + 6 → 7 (Fig. 6).
The mean value of the 1 + 4 → 5 reaction volume, ΔVr ꢀ n, is
ꢀ25.0 0.4 cm3 molꢀ1, and the ratio ΔV≠corr/ΔVr ꢀ n is ꢀ29.1/
ꢀ25.0 equal to 1.16. For the ene reaction 1 + 6 → 7, the same
ratio was obtained: ꢀ31.0/ꢀ26.6 = 1.16. This suggests that the
volume of the activated complex of reactions 1 + 4 → 5 and
1 + 6 → 7 in toluene is smaller by 16% than that of the products
5 and 7. The data available,[8,9,11] showing the weak influence of
the solvent polarity on the reaction rate, allow to exclude the
solvent electrostriction in the solvation shell of the activated
complex as the main cause resulting in this ratio: ΔV≠corr/ΔVr ꢀ n
1. This “abnormal” ratio (ΔV≠corr/ΔVr ꢀ n > 1) in the isopolar DARs
was explained by different possibilities of the solvent molecules
to fit the large steric hindered structures of the cyclic activated
complex and cyclic adducts.[31] A more suitable explanation of
the results obtained (ΔV≠corr/ΔVr ꢀ n >1) for studied ene reactions
1 + 4 → 5 and 1 + 6 → 7 can result from the assumption of the
formation of more compact cyclic structure of the activated
complex when compared to the noncyclical structure of reaction
products 5 and 7. This conclusion is in agreement with the
results of earlier investigations in the field of the ene reactions
of alkenes and enophiles with the C = O and C ≡ C reaction
centers,[24,25,32] where ratio ΔV≠/ΔVrꢀ n, recalculated for 25 °C, was
in the range of 1.1–1.3. The ratio for the ene reaction with diethyl
azodicarboxylate needs to be tested.[32] Van der Waals molar
volumes were calculated for the model ene reaction of propylene
(35.1 cm3molꢀ1) and ethylene (25.6 cm3molꢀ1) with the formation
of the activated complex in this ene reaction (54.6 cm3molꢀ1) and
the product, 1-pentene, (55.8 cm3molꢀ1).[33,34] The activated
complex should be more compact than the product of this ene
reaction, which follows from these data of ab initial calculations
(ΔV≠W/ΔVrꢀ n,W =1.15).[33,34]
CONCLUSIONS
To sum up everything above, we can say that we obtained the
values of the enthalpy, entropy and volume of activation, the reac-
tion volume and the enthalpy of ene-reactions of 4-phenyl-1,2,
4-triazoline-3,5-dione with cyclohexene, 1-hexene and 2,3-dimethyl-
2-butene. It can be assumed that the large negative values of the
entropy (ΔS≠ =ꢀ106 and ꢀ122 J molꢀ1 Kꢀ1) and volume of activa-
tion (ΔV≠corr =ꢀ26.6 and ꢀ29.1cm3 molꢀ1) of the 1+4→5 and
1+6→7studied reactions are in good agreement with the assump-
tion that the activated complex on the stage determining the ene
reaction rate has the cyclic structure. Very small solvent polarity effect
on the ene reaction rate,[8,9,11] similar to the solvent effect in the
DARs,[3,12,27] excludes the charge generation in the transition state.
In both reactions under study, namely, 1+4→5 and 1+6→7,
the value ΔV≠/ΔVrꢀ n is 1.16, which is in agreement with the previous
assumptions[24,25,32–34] that there is a more compact volume of cyclic
activated complex when compared with the noncyclical ene
products.
Figure 5. Relation dꢀ1 (cm3 gꢀ1) vs C5,t (mol Lꢀ1) for reaction 1 + 4→5 in
toluene at 25 0.002 °C. 1. dꢀ1 = ꢀ0.0296 C5,t +1.1599; R2 = 0.9996;
It was observed (Fig. 1) that the reaction rate of acyclic alkenes
with 4-phenyl-1,2,4-triazoline-3,5-dione is proportional to the
π-donor properties of alkenes.
Polynomial (4) and logarithmic (5) equations both give the
same value of the activation volume at P = 1 bar. However, only
C
01 = 4.577·10ꢀ3 mol·Lꢀ1; C04 = 5.732·10ꢀ2 mol·Lꢀ1 2. dꢀ1 = ꢀ0.0285 C5,t
+1.1600; R2 = 0.9992; C01 = 4.482·10ꢀ3 mol·Lꢀ1; C04 = 6.603·10ꢀ2 mol·Lꢀ1
3. dꢀ1 = ꢀ0.0289 C5,t +1.1595; R2 = 0.9996; C01 = 6.806·10ꢀ3 mol·Lꢀ1
;
C
04 = 3.092·10ꢀ2 mol·Lꢀ1
J. Phys. Org. Chem. 2014, 27 401–406
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/poc

V. D. KISELEV ET AL.
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1984, 49, 2910.
Eqn (5) can predict the pressure effect on the reaction rate under
high pressure.
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[13] G. W. Breton, K. A. Newton, J. Org. Chem. 2000, 65, 2863.
[14] V. D. Kiselev, I. I. Shakirova, H. A. Kashaeva, L. N. Potapova, D. A.
Kornilov, D. B. Krivolapov, A. I. Konovalov, Mendeleev Comm. 2013,
23, 235.
The values of the heat of ene reactions with 1 are reported for
the first time. High exothermicity of the studied reactions
(ꢀ155.9 for 1 + 4→ 5 in toluene; ꢀ158.2 for 1 + 6→7 in 1,
2-dichloroethane; ꢀ139.2 for 1 + 8→ 9 in toluene, and ꢀ150.2 in
1,2-dichloroethane) allows us to consider these ene reactions as
irreversible under standard conditions. The equilibrium constants
of the ene-reactions studied can be estimated as exceeding
[15] S. F. Nelsen, D. L. Kapp, J. Am. Chem. Soc. 1985, 107, 5548.
[16] M. Fujita, H. Matsushima, T. Sugimura, A. Tai, T. Okuyama, J. Am.
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1018 L molꢀ1
.
[17] D. A. Singleton, C. Hang, J. Am. Chem. Soc. 1999, 121, 11885.
[18] B. Franzus, J. Org. Chem. 1963, 28, 2954.
Acknowledgements
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The authors thank the Joint Project of the USA Civilian Research
and Development Foundation and High Education of the Russian
Federation (Project BRHE REC-007), and the Russian Foundation
for the Basic Research (Project No 12-03-00029) for the financial
support. We also highly appreciate the fruitful comments of the
reviewers, which helped us to finalize this manuscript.
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J. Phys. Org. Chem. 2014, 27 401–406