Reactivity of 4-Phenyl-1,2,4-triazoline-3,5-dione and Diethylazocarboxylate in [4+2]-Cycloaddition and Ene Reactions: Solvent, Temperature, and High-Pressure Influence on the Reaction Rate

Article abstract of DOI:10.1002/kin.20908

We have studied the solvent, temperature, and pressure influences on the reaction rates of cyclic and acyclic N=N bonds in the Diels-Alder and ene reactions. The transfer from N-phenylmaleimide (9) to a structural analogue, 4-phenyl-1,2,4-triazoline-3,5-dione (2), is accompanied by the rate increase in five to six orders of magnitude in the Diels-Alder reactions with cyclopentadiene (4) and 9,10-dimethylanthracene (5), whereas the transfer from dimethyl fumarate (10) to diethyl azodicarboxylate (1) increases only in one to two orders of magnitude. The ratio of the reaction rate constants (2 + 4)/(1 + 4) is very large (5.2 × 10⁷) and almost the same (5.3 × 10⁷) as in the ene reactions with tetramethylethylene (7), (2 + 7)/(1 + 7). It has been observed that the N=N bond in reagent 2 has strong electrophilic, and its N-N moiety in the transition state has nucleophilic properties, which results from the analysis of the solvation enthalpy transfer of reagents, activated complex, and adduct in the Diels-Alder reaction of 2 with anthracene 22.

Full text of DOI:10.1002/kin.20908

Reactivity of 4-Phenyl-1,2,4-
triazoline-3,5-dione and
Diethylazocarboxylate in
[4+2]-Cycloaddition and
Ene Reactions: Solvent,
Temperature, and
High-Pressure Influence on
the Reaction Rate
VLADIMIR D. KISELEV,1 DMITRY A. KORNILOV,1 ILZIDA I. LEKOMTSEVA,1
ALEXANDER I. KONOVALOV2
1Department of Chemistry, Butlerov Institute of Chemistry, Kazan Federal University, Kazan, 420008, Russian Federation
2Arbuzov Institute of Organic and Physical Chemistry, Russian Academy of Sciences, Kazan, 420088, Russian Federation
Received 28 October 2014; revised 14 January 2015; 30 January 2015; accepted 4 February 2015
DOI 10.1002/kin.20908
Published online 25 February 2015 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: We have studied the solvent, temperature, and pressure influences on the reaction
rates of cyclic and acyclic N=N bonds in the Diels–Alder and ene reactions. The transfer
from N-phenylmaleimide (9) to a structural analogue, 4-phenyl-1,2,4-triazoline-3,5-dione (2),
is accompanied by the rate increase in five to six orders of magnitude in the Diels–Alder
reactions with cyclopentadiene (4) and 9,10-dimethylanthracene (5), whereas the transfer from
dimethyl fumarate (10) to diethyl azodicarboxylate (1) increases only in one to two orders
of magnitude. The ratio of the reaction rate constants (2 + 4)/(1 + 4) is very large (5.2 ×
107) and almost the same (5.3 × 107) as in the ene reactions with tetramethylethylene (7),
Correspondence to: Vladimir D. Kiselev; e-mail: vkiselev.ksu@
gmail.com.
Contract grant sponsor: Russian Foundation for Basic Research
(Project No 12–03–00029).
Contract grant sponsor: Ministry of Science and Education of
the Russian Federation (Joint Program “Fundamental Research and
High Education”.)
Contract grant sponsor: U.S. Civilian Research and Development
Foundation.
C
ꢀ
2015 Wiley Periodicals, Inc.
Contract grant number: REC 007.

290
KISELEV ET AL.
(2 + 7)/(1 + 7). It has been observed that the N=N bond in reagent 2 has strong electrophilic,
and its N–N moiety in the transition state has nucleophilic properties, which results from the
analysis of the solvation enthalpy transfer of reagents, activated complex, and adduct in the
ꢀC
Diels–Alder reaction of 2 with anthracene 22. 2015 Wiley Periodicals, Inc. Int J Chem Kinet
47: 289–301, 2015
ily violated for asymmetric reagents, but the difference
in the energy of the synchronous and asynchronous
states can be small [1–4,7–10]. A calculated one-step
two-stage mechanism has been proposed for the ene re-
actions [14]. The conservation of stereospecificity and
the lack of the rate acceleration of two stage reactions
in polar solvents can be explained by the shorter time
between the formation of the first and second bond
in the transition state, than the time required for the
rotation around the C–C bond with violation of stere-
ospecificity and for the solvent reorganization [3,4].
Only in few cases, the lifetime of an intermediate is
sufficient to capture, for example, methanol molecules
from the solution in the DAR [15], in violation of stere-
ospecificity [11,16,17] and in the ene reactions with 4-
substituted-1,2,4-triazoline-3,5-diones [18–20]. Data
on the lifetime of bipolar intermediates can be ex-
pected from the femtosecond time-resolved investiga-
tions [1,2].
The DAR of azodicarboxylates with conjugated di-
enes [21–23] and ene reactions with olefins [24,25]
as well as both types of these reactions [23,26] has
been thoroughly studied. As it was established, the
ene reactions can be considered as additive substi-
tution with allylic hydrogen addition to the nitrogen
atom accompanied by the obligated shift of the C=C
double bond in olefins [25–30]. Numerous reactions
were carried out in the mild conditions with the very
active N=N bond of 4-substituted-1,2,4-triazoline-3,5-
diones. The large kinetic isotope effect was observed in
INTRODUCTION
It is widely accepted that the Diels–Alder reactions
(DAR) have commonly concerted, “aromatic” tran-
sition state with synchronous formation of both new
C–C bonds. However, at the present time the pos-
sibility of implementing of all options of the DAR
mechanism has been considered. The one-step one-
stage synchronous mechanism of the bonds formation
has been proven for the nonpolar DAR of symmet-
rical reagents as cyclopentadiene and ethylene with
formation and decay of norbornene [1–4], as well as
polar DAR with the large charge separation, such as
a reaction of tetracyanoethylene with cyclopentadi-
ene [4] and with 9,10-dimethylanthracene [5]. For the
latter reaction between very strong π-donor diene and
the strongest π-acceptor dienophile, the intermolec-
ular interaction energy in a molecular complex be-
tween reagents is more than the subsequent activation
energy for this reaction. Therefore, the observed ac-
tivation energy is negative. This corresponds to the
fact that the geometry of the molecular complex is
similar to the geometry of the transition state of this
reaction [5]. Note that the solvent effect on the rate
of this polar reaction is determined by the hard de-
activation of tetracyanoethylene in π-donor solvents.
The rate constants (25°C) and dissolution enthalpies
of solid tetracyanoethylene in trichloromethane, ace-
tonitrile, benzene, toluene, o-xylene, and mesitylene
are 260 × 103, 43.5 × 103, 8.5 × 103, 2.8 × 103,
0.82 × 103, 0.30 × 103 L mol−1 s−1 and 24.5, 15.2,
14.9, 9.7, 1.4, and –2.7 kJ mol−1, respectively [5,6].
Overall, for all DAR involving asymmetric dienes and
dienophiles should be expected for the asynchronous
types of the bond formations via one-step two-stage
biradical or bipolar transition states [3,7–10], and two-
stage zwitterionic intermediate, when asymmetry of
reagents is very large [4,10–12], and even via the ionic
mechanism, as in the DAR of cyclopentadiene with
the iminium cation [4]. The basis for the selection
of the DAR mechanism is ab initio calculations, sub-
stituent effects on the rate, kinetic isotope effects, con-
servation/violation of stereospecificity, solvent effect
on the rate, the values of entropy and volume of acti-
vation [3,4,9,10,12,13]. Most calculations have shown
that the synchronous formation of new bonds is eas-
,3-CD -butene-2 [30],
the ene reactions with cis-2-CH3
3
and the subsequent studies [15,18,31–35] confirmed
the formation of aziridinium imide during the first re-
action step. The conservation and violation of the reac-
tion stereospecificity of E,E-, E,Z-, and Z, Z-hexa-2,4-
dienes with 4-phenyl-1,2,4-triazoline-3,5-dione were
discussed in detail in several papers [18,36]. Currently,
there is a wide variety of the experimental data which
confirm different versions of these reactions mecha-
nism: aziridinium imide (A), dipolar or biradical (B),
and concerted (C) ones (Scheme 1). The A and B types
result from the completion of the reaction under the
addition of methanol molecules, used as the solvent
or cosolvent in these reactions. In the case of reac-
tions of active dienes with diethyl azodicarboxylate (1)
or 4-phenyl-1,2,4-triazoline-3,5-dione (2), the major
International Journal of Chemical Kinetics DOI 10.1002/kin.20908

REACTIVITY of 4-PHENYL-1,2,4-TRIAZOLINE-3,5-DIONE AND DIETHYLAZOCARBOXYLATE
291
R
O
R
R
O
O
N
N
N
N
H
O
O
N
O
N
N
N
H
N
H
Aziridinium imide
A
Dipolar
B
Concerted
C
Scheme 1 Three versions of the ene reaction mechanism: aziridinium imide (A), dipolar or biradical (B), and concerted (C).
products are the [4+2]-cycloadducts with their stere-
ospecificity preserved. In the case of less active di-
enes such as 1,3-cyclohexadiene [37] and Z,Z-1,4-
disubstituted-1,3-butadienes [18,19,33], the ene reac-
tions of 1 successfully compete with the DAR.
The series of 4-substituted-1,2,4-triazoline-3,5-
diones has been studied with a number of dienes. All
these dienophiles have the little difference in the reac-
tion rates, which corresponds to the weak influence of
4-R substituents on the activity of the N=N reaction
center [38,39].
The extensive study of the solvent influence on the
rate of the DAR and ene reactions with 1 and 2 shows
that the solvent polarity is not the main reason of reac-
tivity changes [15,18,31–35,38–41]. Great similarity
of the solvent influence in both DAR and ene-reactions
with 1 (R = 0.99), as well as with 2 (R = 0.99) has been
observed [40,41]. However, there is a large difference
of the solvent effect on the reaction rates with 1 when
compared with 2.
solvents were purified by standard methods [42]. Al-
most quantitative yields of adducts 15, 17, 19, and their
structures were described previously [38,39,43].
Adduct 14. 1H NMR, “Bruker Avance,” 400 MHz,
DCCl3, 25°C, δ, ppm: 1.26 (t, 3J(H,H) = 7.1 Hz, 6H,
2CH3), 1.71–1.73 (m, 2H, CH2), 2.95 (m, 2H, 2CH),
4.18 (q, 3J(H,H) = 7.1 Hz, 4H, 2CH2), 6.42–6.55 (m,
2H, 2CH) [44]∗. Adduct 16: mp 153–154°C; MS (70
eV), m/z calcd. for M 16 + Na+: 402.8; found: 403.0.
1H NMR, 400 MHz, DCCl3, 25°C, δ: 0.99 (t, 3J(H,H)
= 7.1 Hz, 6H, 2CH3); 2.66 (s, 6H, 2CH3); 3.86–3.94
(m, 4H, 2CH2); 7.17–7.57 (m, 8H, aromatic). These
data and the absence of N–H signal are in agreement
with the reactions 1 + 4 → 14 and 1 + 5 → 16.
Apparatus and Procedures
Kinetic Measurements. At normal pressure the rate of
reactions 1 + 4 → 14, 1 + 5 → 16, and 1 + 7 → 21
were determined by measuring the UV absorption
of reagents equimolar solutions (3–5) × 10−2 M at
400–405 nm (spectrophotometer Hitachi-2900), where
adducts 14, 16, and 21 are transparent. The rate of fast
reactions 2 + 4 → 15 and 2 + 7 → 20 was measured
by the stopped-flow method (RX 2000, the spectropho-
tometer Cary 50 Bio). A very fast reaction 2 + 4 → 15
[k2 = (1.6 0.35) × 105 L mol−1 s−1] was measured by
the change of 2 absorption in toluene, 24.5°C at 550 nm
with equimolar concentrations of 2 and 4 (0.004 mol
In this paper, we discuss some possible reasons for
the differences in the solvent, temperature, and high hy-
drostatic pressure influences on the rate of the acyclic
and cyclic N=N and C=C reaction centers in the DAR
and ene reactions (Scheme 2) on the basis of new ex-
perimental data.
EXPERIMENTAL
Materials
L
−1). The rate of reaction 2 + 7 → 20 was measured
by the stopped-flow method in 1,2-dichloroethane at
23.5°C, k2 = 335 9 L mol−1
s
−1, and in benzene at
−1, and at 40.1°C,
All chemicals were purchased (Aldrich) and were used
without further purification, except of 2. 4-Phenyl-
1,2,4-triazoline-3,5-dione (2) was purified by sublima-
tion at 100°C and 10 Pa, mp 165–170°C (decomp.),
λmax (ε) = 540 (248) in benzene; 527 (186) in diox-
ane; 540 nm (245) in toluene. Cyclopentadiene (4) was
obtained by the cracking of the crystalline dimer and
was repeatedly distilled before the measurements. The
23.5°C, k2 = 55.6 0.5 L mol−1
s
90.5 1.3 L mol−1 s−1
.
The rate constants of reactions 1 + 4 → 14 and
1 + 5 → 16 under elevated pressure were measured in
∗
The same structure of adduct 14 has been previously suggested
on the basis of the adduct transformation into bicyclo[2.1.0]pentane.
International Journal of Chemical Kinetics DOI 10.1002/kin.20908

292
KISELEV ET AL.
Scheme 2 Reagents with C=C and N=N reacting bonds in the DA and ene reactions.
a toluene solution at 25oС, using the high-pressure op-
tical cell “PCI-500” produced by Syn (Japan), adjusted
to the UV-spectrophotometer SCINCO.
kp is the rate constant of reactions 1 + 4 → 14 or
1 + 5 → 16 carried out under pressure P. It is important
ࣔ
to compare the values of activation (ꢀV ) and reaction
ࣔ
(ꢀr−nV) volumes, ꢀV /ꢀr−nV. Here, we use the more
accurate method of determination of ꢀr−nV. As an
example, for the reaction 1 + 4 → 14, the total volume
of solution with reagents 1, 4 and adduct 14 can be
ࣔ
Volume Parameters. The volumes of activation, ꢀV ,
were calculated using the dependence lnkp vs. P, where
International Journal of Chemical Kinetics DOI 10.1002/kin.20908

REACTIVITY of 4-PHENYL-1,2,4-TRIAZOLINE-3,5-DIONE AND DIETHYLAZOCARBOXYLATE
293
O
N
N
O
Ph
N
+2
22
23
Scheme 3 The DA reaction of anthracene (22) with 4-phenyl-1,2,4-triazoline-3,5-dione (2) with adduct (23) formation.
0.14 kJ mol−1, and the heat of reaction 1 + 4→14
expressed by Eqs. (1) and (2):
is –112.4
3.3 kJ mol−1. The heat of reaction of
2 kJ mol−1
in benzene and
V(t) = Vs + (c0,1 − c14,t ) · V1 + (c0,4 − c14,t ) · V4
2 + 4 → 15 is –134
–135 2 kJ mol−1 in 1,2-dichloroethane solution.
+ c14,t · V14
(1)
Differences in the Solvation Enthalpies. The transfer
of solvation enthalpies of reagents (࢚trH(2+22)), adduct
(࢚trH(23)), and activated complex (࢚trHAC) of the DA
reaction 2 + 22→23 was calculated using benzene as
the reference solvent [Eqs. (4)–(6)]:
V(t) = [Vs + (c0,1 · V1 + c0,4 · V4)]
+ c14,t · (V14 − V1 − V4)
= V(t=0) + c14,t · ꢀr−n
V
(2)
∂trH(2+22) = (ꢀsolH(2) + ꢀsolH(22)
)
S
−(ꢀsolH(2) + ꢀsolH(22)
)
(4)
(5)
(6)
benzene
1/d(t) = 1/d(t=0) + c14,t · ꢀr−nV/1000d(t=0) (3)
Equation (3), derived from Eq. (2), is more conve-
nient to measure the density of the reaction mixture.
Here, V(t = 0) and V(t) are the solution volumes at the
beginning and during the reaction; Vs is the solvent vol-
ume; V1, V4, and V14 are the partial molar volumes of
compounds 1, 4, and 14; c0,1, c0,4, and c14,t are the initial
molar concentrations of reagents 1 and 4, and the cur-
rent concentration of adduct 14; ꢀr−nV is the reaction
volume. The current concentration of adduct 14 can
be calculated from the kinetic data or from the change
of the optical density of the same solution in the den-
sitometer and spectrophotometer. Linear dependences
1/d(t) vs. c14 were observed for two measurements up
to 95% conversion (Fig. 2). The vibration densime-
ter, model DSA 5000M, was employed to measure the
solution density at 25 0.002°C.
∂trH(23) = (ꢀsolH(23))S − (ꢀsolH(23)
)
benzene
∂trHAC = ∂trH(2+22) + ꢀHS# − ꢀHb#enzene
Here, ꢀsolH(2), ꢀsolH(22), and ꢀsolH(23) are the solu-
tion enthalpies of compounds 2, 22, and 23, and ꢀH is
ࣔ
the enthalpy of activation of the reaction 2 + 22 → 23
in solvent (S) and in benzene.
RESULTS AND DISCUSSION
cis, trans-Dienophile Reactivity of N=N and
C=C Bonds in the DA Reactions
4-Phenyl-1,2,4-triazoline-3,5-dione (2) is very active
in the DAR with most of dienes [37–39,43,46]. For
this reason, it is very convenient to study the heat of
reactions, but is difficult to study the reaction rates at
ambient and elevated pressure. Recently, the rates and
reaction enthalpies of 15 dienes with 2 have been dis-
cussed [46]. The structure of the products of ene reac-
tions between 2 and various alkenes and cycloalkenes
was described [15,18,26–29,31–35,40,41,47]. A cis-
Isomer of substance 1 is unknown. The reaction rates
with 1 are usually very low. Useful information can be
Calorimetric Experiment. The enthalpies of solution
of compounds 2, 22, and 23 were measured at 25°C
by the calorimetric method described previously [45].
The solid sample (30–50 mg) was added to the solvent
(150 mL). Usually three to four consecutive dissolu-
tions of each compound were carried out with the sub-
sequent calibration of the calorimeter. The heat of reac-
tion 1 + 4 → 14 was measured in 1,2-dichloroethane
by the addition of 1 (40–60 mg) to the solution of
4 (150 mL, 0.1 M). The heat of the solution of 1 is
International Journal of Chemical Kinetics DOI 10.1002/kin.20908

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KISELEV ET AL.
Table I Rate Constant (k2, L mol−1 s−1) of the DA Reactions of Dienophiles (1–3, 8–12) with Cyclopentadiene (4) and
with 9,10-Dimethylanthracene (5), and the Ene Reactions with Trimethylethylene (6) and with Tetramethylethylenes
(7), the Ratio of Their Rates (ki/kj), and Enthalpies of Reactions in Solution (ꢀr−nH, kJ mol−1) at 25°C
Cyclopentadienea (4)
9,10-Dimethylanthracenea (5)
Dienophile
k2
ki/kj
−ꢀr−n
H
k2
ki/kj
−ꢀr−n
H
trans-1,2- Dicyanoethylene (12) 8.1 × 10−4
12:13 = 0.89
10:11 = 120
126b
−
1.39 × 10−4
1.31 × 10−4
6.70 × 10−6
5.64 × 10−9
12:13 = 1.06
102b
−
cis-1,2- Dicyanoethylene (13)
Dimethyl fumarate (10)
Dimethyl maleate (11)
Maleic anhydride (8)
N-Phenylmaleimide (9)
trans-Diethyl azodicar- (1)
boxylate
9.1 × 10−4
7.4 × 10−4
6.3 × 10−6
−
10:11 = 1200
−
−
−
5.56 × 10−2 8:10 = 7.5 × 103
7.05 × 10−2 b 9:10 = 9.5 × 103
3.22 × 10−2 c 2:9 = 2.3 × 106
129b
142b
112c
1.13 × 10−2 8:10 = 1.7 × 103
3.0 × 10−2 b 9:10 = 4.5 × 103
1.57 × 10−3 c 2:9 = 4.1 × 105
104b
117b
−
3.03 × 10−3 d
4:5 = 2.7
4-Phenyl-1,2,4-triazoline- (2)
3,5-dione
1.6 × 105 d
2:1 = 5.2 × 107
134c
113b
−
1.22 × 104 e
9.3 × 104 b
2:1 = 7.8 × 106
118e
88b
4:5 = 13.1
2:3 = 370
Tetracyanoethylene (3)
4.3 × 102
2:3 = 0.13
2,3-Dimethyl-2-butene (7)
Enophile
2-Methyl-2-butene (6)
trans-Diethyl azodicar- (1)
boxylate
9.15 × 10−5
7/6 = 0.4
6.25 × 10−6
(60oC)f
/23.3°C)c
−
3.5 × 10−5
/60°Cc
4-Phenyl-1,2,4-triazoline- (2)
8.3(24°C)f
2:1 = 2.6 × 105
−
335(23.3°C)g 2:1 = 5.3 × 107
150c
3,5-dione
24.3 (60°C)f
7/6 = 40
aFrom [11,49]. bFrom 48. cThis work, in 1,2-dichloroethane. dThis work, in toluene. eFrom 46. fFrom 41. gFrom 47.
obtained from the comparison of the kinetic parame-
ters of the DAR of C=C and N=N reaction centers
of dienophiles 1–3, 8–13 with cyclopentadiene (4) and
9,10- dimethylanthracene (5), as well as those of ene re-
actions with 2-methyl-2-butene (6) and 2,3-dimethyl-
2-butene (7) (Scheme 2 and Table I).
ties of 9,10-dimethylanthracene give the higher rate
of reaction with such strong π-acceptors, as tetracya-
noethylene. If the π-acceptor properties of dienophiles
are weak, the stabilization energy is rather low and
the activation barrier is largely controlled by the en-
ergy balance of the old bonds rupture and new bonds
formation. Hence, in such systems the less conjugated
cyclopentadiene should be a more active diene.
The substantial reduction of the activity of dimethyl
maleate 11 as compared with the trans-isomer of 10 in
the DAR with 4 (k(10)/k(11) = 1.2 × 102) and with 5
(k(10)/k(11) = 1.2 × 103) was observed [49]. This is the
result of the steric hindrances of two ester groups of
cis-dienophile 11, what prevents the diene approach to
the dienophile 11 from the both sides [11,49]. There
are no such problems for trans-isomer 10.
In the case of small-scale and rigid linear nitrile
groups, such steric hindrance is not observed at all,
and both dienophiles 12 and 13 have comparable rates
(Table I). For cyclic planar molecules 8 and 9 both
the increase in π-acceptor properties and the ring ten-
sion energy were observed [47,48]. The DAR rates of
diene 4 with cyclic dienophiles of 8 and 9 increased
Commonly a very high activity of 2 was attributed
to its very high molecular π-acceptor properties. How-
ever, it was shown that the differences in the solvation
energy of dienophiles 2 and 8 in π-donor solvents,
such as chlorobenzene, benzene, toluene, o-xylene,
are commensurate and significantly less as compared
with that of the strong π-acceptor, tetracyanoethylene
3 [46]. This means that the π-acceptor properties of
2 and 8 are comparable and moderate. Therefore, in-
creased by five to six orders of magnitude activity of
2 when compared with the structural analogue 9 was
attributed to the reduced energy of the N=N π-bond,
as compared with the C=C π-bond [46]. On the other
hand, the acyclic N=N bond in 1 has very low ac-
tivity. It is known that cyclopentadiene is less conju-
gated and the less π-donor diene compared with 9,10-
dimethylanthracene [48]. Enhanced π-donor proper-
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295
ࣔ
ࣔ
Table II Rate Constants (k2, L mol−1s−1), the Enthalpy (ꢀH ,kJ mol− 1), Entropy (ꢀS , J mol− 1K−1), and Free
Energy (ꢀG , kJ mol−1) Activation of the DAR 1 + 4 → 14 in a Series of Solvents
ࣔ
ࣔ
ࣔ
ࣔ
Solvent
k2 (15оC)
k2 (25оC)
k2 (30оC)
ꢀH
−ꢀS
ꢀG
Toluene
Benzene
Acetonitrile
1,2-Dichloroethane
Trichloromethane
0.00179
0.00279
0.0191
0.0230
0.0739
0.00303
0.00434
0.0299
0.0322
0.0939
0.00384
0.00563
0.0353
0.0386
0.103
33.8 0.2
30.6 2.0
27.6 1.6
22.4 0.7
13.8 0.6
180
188
180
197
217
1
87.4 0.2
86.6 2.0
81.2 2.0
81.1 1.0
78.5 1.0
6
5
2
2
significantly as compared to the sterically unstressed
trans-dienophile 10: k(8)/k(10) = 7.5 × 103 and k(9)/k(10)
= 9.5 × 103. A similar ratio in the DAR with diene 5,
k(8)/k(10) = 1.7 × 103 and k(9)/k(10) = 4.5 × 103 was
observed (Table I).
with these dienes. The obtained kinetic data of reaction
1 + 4 → 14 are given in Table II.
It should be pointed out once that the solvent influ-
ence on the rate of DAR 1 + 4 → 14 (Table II) and on
the reaction rate of 1 with 2,3-dimethylbutadiene [40]
turned out to be proportional (R = 0.99). As it follows
from Table II, highly negative entropy of activation is
observed in all solutions. The contribution of entropy
The replacement of the C=C bond in N-
phenylmaleimide 9 by the N=N bond in 4-phenyl-
1,2,4-triazoline-3,5-dione 2 leads to a further dramatic
increase in the DAR rate: k(2)/k(9) = 2.3 × 106 with 4
and k(2)/k(9) = 4.1 × 105 with 5 (Table I).
ࣔ
share, TꢀS , to the value of free energy of activation
is very large and equals 60–80%.
In the DA reactions with C=C bonds, where diene
is a π-donor and dienophile is a π-acceptor, the gen-
eral proportionality between the changes of reagents
orbital properties and charges at C=C reaction centers
is usually observed [48]. Therefore, for a wide range
of diene–dienophile systems with C=C reaction cen-
ters, simple dependences of the reaction rate on the
donor–acceptor interaction energy and the DAR heat
were obtained [48].
Activation and Reaction Volumes
Large negative values of the DAR activation entropy
are often accompanied by the increased negative val-
ࣔ
ues of activation volume (ꢀV ) [50,51]. The values
ࣔ
of ꢀV were determined from the dependence of
the reaction rate constant upon the external pressure
(Table III, Fig. 1).
As it can be seen from Table I, the energy balance
of the bonds rupture and formation (reaction enthalpy)
for the DAR with structural analogues 2 and 9 is near
the same. It means that the reactivity factors observed
in the DAR with C=C bonds [48] cannot be valid for
the DAR with N=N bonds.
Taking into account the solvent compressibility, the
corrected value of the activation volume (ꢀV corr) is
calculated from Eq. (1) [51].
ࣔ
ꢀVc#orr − RT ∂ln(kP)/∂P + (n − 1)RTβT
(7)
The rate of reaction 2 + 5 → 17 (k2 = 1.2 × 104
L mol−1s−1, toluene, 25°C, Table I) is sufficiently less
than that of the reaction 2 + 4 → 15 (k2 = (1.6
0.3) × 105 L mol−1s−1, toluene, 25°C, stopped-flow,
this work). Similarly, 9 is more active in the DA re-
action with 4 than with 5. On the other hand, 2 is
significantly more active than tetracyanoethylene 3 in
the DAR with cyclopentadiene 4 (k2/k3 = 370), but
less active in the reaction with the strong π-donor di-
ene, 9,10-dimethylanthracene 5 (k2/k3 = 0.13, Table
I). This is in agreement with the conclusion [46] that
the π-acceptor properties of dienophiles 2 and 9 are
both moderate in magnitude and considerably less than
those of tetracyanoethylene 3.
Table III Pressure (P, bar) Influence on the Rate
Constant (kp, L mol−1s−1) of Reactions 1 + 4 → 14 in
Toluene and 1 + 5 → 16 in 1,2-Dichloroethane at 25°C
Reaction 1 + 4 → 14
Reaction 1 + 5 → 16
103·kp ln(kp/kp = 1
P
103·kp ln(kp/kp = 1
)
P
)
1
3.01
5.20
7.24
10.9
16.3
0
1
1.43
1.99
2.75
3.80
4.52
6.68
0
273
492
730
971
0.546
0.877
1.284
1.691
2.130
2.586
242
429
601
749
991
1285 10.0
1567 13.1
1930 22.2
0.330
0.654
0.978
1.151
1.541
1.945
2.218
2.742
1256 25.3
1568 40.0
It is interesting to compare the activity of cyclic
dienophiles 2 and 9 in the DAR with dienes 4 and
5, and the activity of acyclic dienophiles 1 and 10
International Journal of Chemical Kinetics DOI 10.1002/kin.20908

296
KISELEV ET AL.
Figure 1 Pressure (P, bar) effect on the rate of reaction 1 + 4 → 14 in toluene at 25°C (plot 1) and 1 + 5 → 16 in
1,2-dichloroethane at 25°C (plot 2).
ࣔ
Here, βT is the toluene compressibility (80.1 × 10−6
bar−1 [52]) and n is the reaction order. The experi-
mental curve (Fig. 1) for the reaction 1 + 4 → 14 is
described by the polynomial equation (8):
cm3 mol−1 from Eq. (10) and ꢀV
is –41.0 2.5
exp
cm3 mol−1 and ꢀV corr is –39.1 2.5 cm3 mol−1 from
ࣔ
Eq. (11).
It should be noted that in accordance with the
polynomial and logarithmic equations the values of
the activation volumes at atmospheric pressure are
Ln(kp/kp=1) = −0.000000141P2 + 0.001860P;
ࣔ
close, which gives the average value of ꢀV
=
corr
R = 0.9998
(8)
–44.5 1.8 cm3 mol−1 for the reaction 1 + 4 → 14
ࣔ
and ꢀV
= –39.6 2.5 cm3 mol−1 for the reaction
1 + 5 →c1or6r . However, polynomial equations, in con-
Hence, for the DAR 1 + 4 → 14, the experimental
ࣔ
value of ꢀV
is –46.0
1.4 cm3 mol−1 and the
exp
trast to logarithmic equations, imply the existence of
corrected one is –43.8 1.4 cm3 mol−1. We also de-
scribed the experimental data of this reaction by the
logarithmic equation (9):
the false peak at P 6.6 and 6.0 kbar, whereas Eqs. (9)
and (11) can reliably determine the value of ꢀV
and predict correctly the acceleration effect at elevated
pressure.
It is useful to compare the values of activation
(ꢀV corr) and reaction (ꢀr−nV) volumes. The value
of ꢀr−nV was calculated from the dependence of the
solution density during the reaction 1 + 4 → 14 on the
adduct 14 concentration (Fig. 2).
ࣔ
exp
Ln(kp/kp=1) = 8.870 Ln[(4633 + P)/4633];
ࣔ
R = 0.9987
(9)
ࣔ
From Eq. (9), it follows that the value of ꢀV
exp
ࣔ
is –47.4 1.8 cm3 mol−1 and ꢀV
is –45.2 1.8
corr
cm3 mol−1
.
The average value of ꢀr−nV for the DA reaction
1 + 4 → 14 is equal to –35.8 0.3 cm3 mol−1. Simi-
lar measurements for the reaction 1 + 5 → 16 gave the
For the DAR 1 + 5 → 16, the experimental data
(Table III, Fig. 1) were described by polynomial (10)
and logarithmic (11) equations:
values of ꢀr−n
with the mean value –21.6
V: –21.8, –21.0, and –22.1 cm3 mol−1
0.4 cm3 mol−1. From
Ln(kp/kp=1) = −0.000000141P2 + 0.0016944P;
the data obtained, it follows that the value of activation
volume significantly exceeds the volume of the DAR
R = 0.9992
(10)
ࣔ
1 + 4 → 14, ꢀV /ꢀr−nV = –44.5/–35.8 = 1.24, and
ࣔ
even more for the DAR 1 + 5 → 16, ꢀV /ꢀr−nV = –
39.6/–21.6 = 1.83. On the other hand, for the DAR with
Ln(kp/kp=1) = 8.798 Ln[(5306 + P)/5306];
R = 0.9987
ࣔ
cyclic N=N 2 and C=C 9 bonds, the ratio ꢀV /ꢀr−n
V
differs insignificantly and is usually less or about unity.
(11)
Thus, the reaction of 8 + 5 in acetonitrile the ratio
ࣔ
ࣔ
ꢀV /ꢀr−nV is –20.0/–23.6 = 0.85, and in the reaction
For the DAR 1 + 5 → 16, the value of ꢀV
is
2.1
exp
ࣔ
ࣔ
–42.0
2.1 cm3 mol−1 and ꢀV
is –40.1
of 9-phenylanthracene with 8 the ratio ꢀV /ꢀr−n
V
corr
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REACTIVITY of 4-PHENYL-1,2,4-TRIAZOLINE-3,5-DIONE AND DIETHYLAZOCARBOXYLATE
297
Figure 2 Relation between the specific volume (1/dt, cm3 g−1) of the reaction 1+4→14 and the adduct concentration (c14
,
mol L−1) in toluene at 25°C. 1a: 1/dt = –0.04123 c14 + 1.15908; R = 0.9999; c0,1 = 0.0221 М; c0,4 = 0.0625 М; ꢀr−nV =
–35.6 0.2 cm3 mol−1 (kinetic method). 1b: 1/dt = –0.04208c14 + 1.15918; R = 0.9999; c0,1 = 0.0221 М; c0,4 = 0.0625 М;
ꢀr−nV = –36.3 0.2 cm3 mol−1 (spectrophotometric method). For clarity, the values of dt−1 of the line 1b were displaced
upward on 0.0001 of the ordinate scale. 2a: 1/dt = –0.04096 c14 + 1.15927; R = 0.9999; c0,1 = 0.0203 М; c0,4 = 0.0717 М;
ꢀr−nV = –35.3 0.2 cm3 mol−1 (kinetic method). 2b: 1/dt = –0.04165 c14 + 1.15937; R = 0.9999; c0,1 = 0.0203 М; c0,4
=
0.0717 М; ꢀr−nV = –35.9 0.2 cm3 mol−1 (spectrophotometric method). For clarity, the values of dt−1 of the line 2b were
displaced upward on 0.0001 of the ordinate scale.
is –23.2/–27.3 = 0.84 [51]. In the reaction of 9,10-
enthalpy transfer of dienophile 2 (࢚solH(2), R =
0.10).
3. The change in the reaction rate in the solvents
ࣔ
diphenylanthracene with 2, the ratio ꢀV /ꢀr−nV is
–17.2/–15.6 = 1.10 0.10, and in the DAR of 2 with
ࣔ
bicyclo[2.2.1]hepta-2,5-diene ꢀV /ꢀr−nV is –25.1/
under study is mainly caused by the change in
ࣔ
ࣔ
activation enthalpy (ꢀꢀH = 26.9 kJ mol−1
;
–30.9 = 0.81 0.05 [46]. The ratio ꢀV /ꢀr−nV, close
Т·ꢀꢀS = 14.9 kJ mol−1).
ࣔ
to unity, corresponds to near the same volume of the
activated complex and reaction product.
4. The change in reaction enthalpy (ꢀr−nH) corre-
lates poorly with the solvation enthalpy transfer
of the reagents (࢚trH(2+22), R = 0.81) and the
adduct (࢚trH23, R = 0.61).
Differences of the Solution Enthalpies in
the DA Reaction 2 + 22 → 23
5. The solvation enthalpy transfer of the activated
complex (࢚trHAC) is determined neither by the
enthalpy transfer of dienophile (࢚trH2, R = 0.16)
nor by that of both reagents (࢚trH(22+2), R =
0.05) and is weakly correlated with the enthalpy
transfer of the adduct (࢚trH23, R = 0.80).
6. The solvation enthalpy transfer of reagents
(࢚trH(22+2)) differs greatly from the enthalpy
transfer of adduct (࢚trH23, R = 0.10).
It is useful to compare the change of the solvation
enthalpy transfer of reagents, activated complex, and
adduct in a series of solvents. Necessary kinetic and
calorimetric data for the DA reaction of anthracene 22
with 2 in 12 solvents are summarized in Table IV.
The analysis of the data in Table IV allows us to
draw the following conclusions:
1. The change of the reaction rate (lnk2) is propor-
7. The value of solvation energy for the reagents
(mainly of 2) sharply decreases with the in-
crease in the solvent acceptor number (AN) in
contrast to the activated complex and adduct
23.
ࣔ
tional to that of enthalpy (ꢀH , R = 0.96), en-
ࣔ
tropy (ꢀS , R = 0.98) of activation, as well as to
the reaction enthalpy (ꢀr−nH, R = 0.94), solva-
tion enthalpy transfer of dienophile 2 (࢚solH(2),
R = 0.94), but not to that of diene 22 (࢚solH(22)
,
R = 0.12).
2. Normalized empirical parameters of the solvent
polarity (ETN) are in very poor correlations with
the reaction rate (lnk2, R = 0.15) and solvation
As a result, we can conclude that nitrogen atoms in
the N=N bond of dienophile 2 demonstrate electron–
acceptor properties, but the N–N moiety in the
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REACTIVITY of 4-PHENYL-1,2,4-TRIAZOLINE-3,5-DIONE AND DIETHYLAZOCARBOXYLATE
299
ࣔ
Table V Rate Constant (k2, L mol−1s−1) of the Ene Reaction of 1 with Tetramethylethylene 7, Enthalpy (ꢀH , kJ
ࣔ
mol−1) and Entropy (ꢀS , J mol−1K−1) Activation
ࣔ
ࣔ
a
Solvent
105·k2 (Т, оС)
ꢀH
−ꢀS
105k (1+7) /105k(1+6) at 60оС
Acetonitrile
0.323(25); 0.959(40);
1.74(50); 3.35(60)
0.682(25); 1.47(40);
2.19(50); 3.59(60)
1.26(25); 2.08(35);
2.58(40); 3.48(45);
4.07 (50); 6.4(60)
52.3 3.0
36.4 3.0
35.7 3.0
176 10
222 10
219 10
3.35/7.6 = 0.43
1,2-Dichloroethane
Trichloromethane
3.59/9.15 = 0.40
6.4/15.3 = 0.42
aThis work for the reaction 1 + 7 → 21 and data for the reaction 1 + 6 → 19 are obtained from 40.
activated complex and in reaction adduct 23 already
has electron–donor properties.
substituted butenes allowed to discover the huge ad-
vantages of the primary attack on the less hindered
C=C atom in alkenes with the higher charge sta-
bilization on the double-substituted adjacent carbon
atom [33–35,60,61].
Almost 100-fold unexpected decrease in the relative
activity of 1 with the higher π-donor tetramethylethene
compared with trimethylethene (Tables I and V) can
probably be explained by the appearance of large
steric hindrances that are more inherent to nonplanar
molecule 1 than to the fixed planar molecule 2.
Ene Reactions. Most of the 1,3-conjugated dienes in
the absence of sterically hindrances react with azo-
dienophiles 1 and 2via concerted [4+2]-cycloaddition.
When the s-cis-structure of 1,3-diene is distorted or un-
available, the reaction can proceed as the ene-synthesis
[26–36]. With the less active 1,3-dienes, the ene reac-
tions of 2 and especially 1 may be preferable [26].
Many questions concerning the products structure,
their regioselectivity, and the solvent influence on the
rate of ene reactions have been considered [25–36].
In the ene reaction 8 with cyclohexene, the ratio
SUMMARY
ࣔ
ꢀV /ꢀVr−n, –29.1/–25.0 is equal to 1.16 and in the
ࣔ
ene reaction 2 with 1-hexene ꢀV /ꢀVr−n –31.0/–26.6
The highest reaction rates have been observed for 4-
is also equal to 1.16 [47]. The data available showing
the weak influence of the solvent polarity on the reac-
tion rate allow us to exclude the solvent electrostriction
in the solvation shell of the activated complex as the
phenyl-1,2,4-triazoline-3,5-dione (2) in the DA reac-
tion with cyclopentadiene 4 (k2+4→ = 1.6 × 105 L
15
mol−1
s
−1) and in the ene reaction of 2 with tetram-
ethylethylene 7 (k2+7→ = 335 L mol−1
s
−1) for the
20
ࣔ
main cause resulting in the ratio, ꢀV corr/ꢀr−nV >
first time. The ratio of the rate constants of the DA reac-
tion of cyclopentadiene with 2 and with 1 in toluene at
25°C, 1.6 × 105/3.03 × 10−3 = 5.2 × 107, is very large
and near the same as in the ene reactions of tetram-
ࣔ
1. Such an “abnormal” ratio (ꢀV corr/ꢀr−nV > 1) in
the isopolar DA reactions was explained by different
possibilities of solvent molecules to penetrate through
the large steric hindered structures of the cyclic ac-
tivated complex and cyclic adducts [13]. However,
for these ene reactions an explanation of this ratio
ethylethylene with 2 and with 1, 335/6.25 × 10−6
=
5.3 × 107.
Unusually high reactivity of the endocyclic N=N
bond in 2 can be attributed to the increased elec-
trophilicity of the N=N reaction center, as well as
lower energy rupture of the π-N=N bond. From the
analysis of the differences of solvation enthalpies in
the DA reaction 2 + 22 → 23, it follows that the N=N
bond in reagent 2 shows electron–acceptor properties;
however, in the activated complex and in adduct 23, the
N–N moiety already shows electron–donor properties.
Comparison of the reactivity of the cyclic and
acyclic dienophiles with C=C and N=N bonds (Ta-
ble I) suggests that the DA reaction with 4-phenyl-
1,2,4-triazoline-3,5-dione (2) proceeds more readily
and easily than the ene reaction. For low activity of
ࣔ
(ꢀV corr/ꢀr−nV > 1) can be given on the basis of
the formation of more compact cyclic structure of the
activated complex when compared to the noncyclical
structure of reaction products. This assumption is in a
good agreement with the previous investigations of the
ene reactions of alkenes and enophiles with the C=O
ࣔ
and CࣕC reaction centers, where the ratio ꢀV /ꢀr−n
V
was in the range of 1.1–1.3 [50,56,57]. The activated
complex should be more compact than the product of
this ene reaction, as follows from the data of ab initio
ࣔ
calculations (ꢀV W/ꢀVr−n,W = 1.15) [58,59].
It is important to emphasize that the analysis of
regioselectivity of the ene reactions of 1 and 2 with
International Journal of Chemical Kinetics DOI 10.1002/kin.20908

300
KISELEV ET AL.
diethylazocarboxylate (1), conversely, a stronger de-
crease in the activity is observed in the DA reaction,
compared with the ene reaction. From the values of the
enthalpies of these reactions, it can be concluded that
these reactions are irreversible.
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International Journal of Chemical Kinetics DOI 10.1002/kin.20908

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