Homo-Diels-Alder reaction of a very inactive diene, bicyclo[2,2,1]hepta-2, 5-diene, with the most active dienophile, 4-phenyl-1,2,4-triazolin-3,5-dione. Solvent, temperature, and high pressure influence on the reaction rate

Article abstract of DOI:10.1002/poc.3054

Solvent, temperature, and high pressure influence on the rate constant of homo-Diels-Alder cycloaddition reactions of the very active hetero-dienophile, 4-phenyl-1,2,4-triazolin-3,5-dione (1), with the very inactive unconjugated diene, bicyclo[2,2,1]hepta

Full text of DOI:10.1002/poc.3054

Research Article
Received: 27 June 2012,
Revised: 31 August 2012,
Accepted: 2 September 2012,
Published online in Wiley Online Library: 2012
(wileyonlinelibrary.com) DOI: 10.1002/poc.3054
Homo-Diels–Alder reaction of a very inactive
diene, bicyclo[2,2,1]hepta-2,5-diene, with the
most active dienophile, 4-phenyl-1,2,4-
triazolin-3,5-dione. Solvent, temperature, and
high pressure influence on the reaction rate
Vladimir D. Kiseleva*, Ilzida I. Shakirovaa, Dmitry A. Kornilova,
Helen A. Kashaevaa, Lubov N. Potapovaa and Alexander I. Konovalova
Solvent, temperature, and high pressure influence on the rate constant of homo-Diels–Alder cycloaddition reactions
of the very active hetero-dienophile, 4-phenyl-1,2,4-triazolin-3,5-dione (1), with the very inactive unconjugated
diene, bicyclo[2,2,1]hepta-2,5-diene (2), and of 1 with some substituted anthracenes have been studied. The rate
constants change amounts to about seven orders of magnitude: from 3.95.10ꢀ3 for reaction (1+2) to 12200 L molꢀ1
s
ꢀ1 for reaction of 1 with 9,10-dimethylanthracene (4e) in toluene solution at 298 K. A comparison of the reactivity
(ln k2) and the heat of reactions (Δr-nH) of maleic anhydride, tetracyanoethylene and of 1 with several dienes has
been performed. The heat of reaction (1+2) is ꢀ218 ꢁ 2kJ molꢀ1, of 1 with 9,10-dimethylanthracene ꢀ117.8 ꢁ 0.7 kJ
molꢀ1, and of 1 with 9,10-dimethoxyanthracene ꢀ91.6 ꢁ0.2 kJ molꢀ1. From these data, it follows that the exothermicity
of reaction (1+2) is higher than that with 1,3-butadiene. However, the heat of reaction of 9,10-dimethylanthracene with 1
(ꢀ117.8 kJ molꢀ1) is nearly the same as that found for the reaction with the structural C=C counterpart, N-phenylmalei-
mide (ꢀ117.0 kJ molꢀ1). Since the energy of the N=N bond is considerably lower (418 kJ/bond) than that of the C=C bond
(611 kJ/bond), it was proposed that this difference in the bond energy can generate a lower barrier of activation in the
Diels–Alder cycloaddition reaction with 1. Linear correlation (R = 0.94) of the solvent effect on the rate
constants of reaction (1+2) and on the heat of solution of 1 has been observed. The ratio of the volume of
¼
¼
activation (ΔV ) and the volume of reaction (ΔVr-n) of the homo-Diels–Alder reaction (1+2) is considered as
“normal”: ΔV /ΔVr-n = ꢀ25.1/ꢀ30.95 = 0.81. Copyright © 2012 John Wiley & Sons, Ltd.
Keywords: activation volume; bicyclo[2,2,1]hepta-2,5-diene; heat of reaction; homo-Diels–Alder reaction; rate constants;
reaction volume; 4-phenyl-1,2,4-triazolin-3,5-dione
hepta-2,5-diene (2) (ꢀ218 kJ molꢀ1, this work) and with 1,3-butadi-
INTRODUCTION
[5]
ene (ꢀ189 kJ molꢀ1), one can expect a much higher activity of
Norbornadiene or bicyclo[2,2,1]hepta-2,5-diene (2) is not a
conjugated 1,3-diene, but can enter into a (2p+2p +2p) homo-
Diels–Alder cycloaddition reactions with some active dieno-
philes. The rate constants of Diels–Alder reactions with common
electronic demand “diene-donor + dienophile-acceptor” can be
estimated for reactants with C=C bonds using values of the
ionization potential of the diene (IP), the electron affinity of the
dienophile (EA), and the energy balance of breaking and forming
bonds, reflected in the value of the enthalpy of reaction (Δr-n H),
and taking into account the interatomic distance in the diene,
RC(1)-C(4), conditioning the degree of overlap of the reaction
orbitals.[1,2] From this analysis, it follows that the influence of
the energy of stabilization by orbital interactions on the change
of reaction rates is about 50%, the influence of the enthalpy of
reaction and the interatomic distance in the diene is about 25 %
from the total energy change. Taking into account the values
of the IP of bicyclo-2,5-heptadiene (8.69 eV)[3] and 1,3-butadiene
(9.03 eV), [2] the interatomic distance in diene 2 (RC(2)-C(6) =246pm),[4]
and 1,3-butadiene (RC(1)-C(4) =290pm),2 the enthalpies of Diels–Alder
reactions of 4-phenyl-1,2,4-triazolin-3,5-dione with bicyclo[2,2,1]
diene 2 in comparison with 1,3-butadiene. The rate constants
of the reaction of diene 2 with 4-phenyl-1,2,4-triazolin-3,
5-dione (1) and with tetracyanoethylene (6) do not meet these
expectations (Table 1). Bicyclo[2,2,1]hepta-2,5-diene (2) is about
three orders of magnitude less active in homo-Diels–Alder reaction
than 1,3-butadiene in Diels–Alder reactions with the same
dienophiles.
One of the reasons for the low reactivity of homo-diene 2 in
homo-Diels–Alder reaction can be the difficulty of a concerted
formation of the new C3–C5 cyclopropane bond in adduct 3. This
* Correspondence to: Vladimir D. Kiselev, Department of Physical Chemistry,
Butlerov Institute of Chemistry, Kazan Federal University, Kremlevskaya str.
18, Kazan, 420008, Russian Federation.
E-mail: vkiselev.ksu@gmail.com
a V. D. Kiselev, I. I. Shakirova, 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. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.

V. D. KISELEV ET AL.
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Copyright © 2012 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. (2012)

HOMO-DIELS–ALDER REACTION
was analyzed by the known absorption coefficient (e532
is 171 in
bond forms in homo-Diels–Alder reaction (1+2) from the large
distance (246 pm), almost on 100 pm longer than C2–C3 in the
diene moiety of the activated complex of common Diels–Alder
reactions. At the present time, the rate constant of reaction 2
with maleic anhydride (10) is still unknown.
nm
[20]
1,4-dioxane at 298 K).
Dienes 4a–g were prepared and purified as
All solvents were purified and dried by known
[9]
described before.
methods. [22]
Kinetic measurements
4-Phenyl-1,2,4-triazolin-3,5-dione (1), is a heteroatomic dienophile
in the studied homo-Diels–Alder reaction, with a N=N reaction
centre instead of the common C=C bond. With various
dienes collected in Table 1, dienophile 1 has an unusually high
reactivity, exceeding tetracyanoethylene by two orders of mag-
nitude. Tetracyanoethylene (6) (EA = 2.88 eV) is the most active
p-acceptor dienophile in Diels–Alder reactions only with the
strong p-donor dienes, i.e. with anthracene and its substituted
derivatives. Extensive data on the reaction rates of 4-substituted-1,
2,4-triazolin-3,5-diones with several dienes in different solvents
have been reported, [12,13,20], but the reasons of the unusually high
reactivity of 1 have not been clarified yet. A simple linear rela-
tion, ln (k2/k1) = 8.3 - 0.85IP, was observed between IP and the
rate constants of 4-(4-nitrophenyl)-1,2,4-triazolin-3,5-dione (k2)
The rate of reaction (1+2) was determined by measuring the UV absorp-
tion of dienophile 1 at 540 nm (e540 equal 247 in toluene at 298 K). The
initial concentration of dienophile 1 was in the range (3–5)ꢂ10ꢀ3 mol Lꢀ1
and that of diene 2 (1–2)ꢂ10ꢀ1 mol Lꢀ1. Diene 2 and adduct 3 are trans-
parent in this range of spectra. The water-circulating cell-holder adapter
(type 210-2111) of the UV-spectrophotometer Hitachi-2900 with
only one-side T-control was removed and replaced by the new five-side
T-control in a water circulating box, which allowed an accuracy of ꢁ0.1 K.
The rate constants of reactions 1 with 4c were measured under similar
conditions, and with dienes 4a and 4b, the initial concentrations of
both reactants were equal (2–4)ꢂ10ꢀ3 mol Lꢀ1. Very fast reactions of 1
with 4d–4g were measured by the stopped-flow method (RX 2000 with
spectrophotometer Cary 50 Bio) in toluene solution at 298 K by monitoring
the absorption of dienes 4d–4g in the range 390–405 nm (e = 5ꢂ103 –2ꢂ104
[13]
and 4-phenyl-1,2,4-triazolin-3,5-dione (k1) with several dienes,
indicating on the p-acceptor type of 1.
L molꢀ1 cmꢀ1). The initial concentration of 1 was about 1ꢂ10ꢀ3 mol Lꢀ1
,
and the concentration of dienes 4d–4g was about 1ꢂ10ꢀ4 mol Lꢀ1. The
values of the rate constants were determined with errors of ꢁ3%, the
enthalpy of activation of ꢁ2kJmolꢀ1, and the entropy of activation of
In this work, the solvent, temperatures, and pressure influence
on the rate of the homo-Diels–Alder reaction of 4-phenyl-1,2,
4-triazolin-3,5-dione (1) with bicyclo[2,2,1]hepta-2,5-diene (2)
have been studied. The rate and the heat of reactions of some
dienes with 1 have also been measured. The values of enthalpy,
entropy, and volume of activation, the volume and enthalpy of
reactions, as well as the heats of solution of 1,7–9 in some
solvents have been also obtained (Scheme 1_).
ꢁ6Jmolꢀ1 Kꢀ1
.
The rate constants of reaction (1+2) under elevated pressure were
measured in toluene solution at 298 K with the high pressure system
using the high pressure pump “HP-500” and high pressure optical cell
“PCI-500” from Syn. Co., Ltd (Japan), adjusted to a UV-spectrophotometer
from SCINCO Co., Ltd (Korea). The photodiode array of the “SCINCO”
UV-spectrophotometer performs all new spectra in the selected time
interval.
EXPERIMENTAL
Calorimetry
Materials
The enthalpies of solution of reactants 1 and 7–9 were measured at 298 K
[16,23]
Bicyclo[2,2,1]hepta-2,5-diene (Alfa Aesar, England, 97%) was distilled
immediately before measurements: b. p. 362.5-363.5 K, n20 =1.4702.
4-Phenyl-1,2,4-triazolin-3,5-dione (Aldrich, Germany, 97%) was sublimed
before the measurements at 373 K and 100 Pa. The spectral purity of 1
with a differential calorimeter, as previously reported.
Samples
were weighed into a small stainless-steel cylinder, both polished sides
of which were covered by thin (0.1 mm) rings of Teflon seals and sealed
by a screwed cap. The mass of the cylinders filled with diethyl ether
Scheme 1. (4pi+2pi) Diels-Alder reaction of 4-phenyl-1,2,4-triazolin-3,5-dione (1) with substituted anthracenes (4a–g) and (2pi+2pi+2pi) homo-Diels-
Alder cycloaddition of 1 with norbornadiene (2), and some pi-acceptor compounds (6–11).
J. Phys. Org. Chem. (2012)
Copyright © 2012 John Wiley & Sons, Ltd.
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V. D. KISELEV ET AL.
proved to be constant after 24 h, ensuring the tightness of the container.
After equilibrium of temperature, these Teflon seals were cut out by a
razor. The accuracy of the calorimetric measurements was verified by
determining the enthalpy of dissolution of dry potassium chloride in
water at 298 K. The result (17.4 ꢁ 0.2 kJ molꢀ1) is in agreement with the
[24]
published value of (17.514 ꢁ 0.008 kJ molꢀ1).
For all solutions, 3–5
measurements of sequentially dissolving samples were carried out.
The total uncertainty of the measurements did not exceed ꢁ2%. No
concentration dependences of the heat of solution were observed.
The heat of reaction (1+2) was measured by addition of solid 1
(25–35 mg) to a toluene solution of diene 2 (1.5–2.0 mol Lꢀ1), and for
reactions (1+4e) and (1+4f) by addition of 35–45 mg of solid 1 to a 1,2-
dichloroethane solution of diene (10ꢀ2 mol Lꢀ1). The values of enthal-
pies of reactions were calculated taking into account the heat of
solution of solid 1 in toluene (18.3 kJ molꢀ1) and in 1,2-dichloroethane
(21.9 kJ molꢀ1).
Volume parameters
¼
The volume of activation, ΔV , was calculated from the dependence lnkp
versus P, were kp is the rate constant of reaction (1+2) under pressure P.
The reaction volume is usually calculated by the difference of the partial
molar volumes (PMV) of adduct and reactants with a total error of ꢁ1–2
cm3molꢀ1. This error can be sharply decreased using a kinetic method.
The vibration densimeter manufactured by Anton Paar, model DSA
5000 M, was used for the measurements at 298 ꢁ 0.002 K. The
densimeter was calibrated with water and air following the instructions.
RESULTS AND DISCUSSIONS
Figure 1. Linear correlations of the enthalpy of solvation of tetracya-
noethylene (6) (dsolvH6 = ΔsolH6 (Si) - ΔsolH6 (o-xylene) with that of
tetrachloro-1,4-benzoquinone (7), 1,3,5-trinitrobenzene (8), 4-phenyl-1,2,4-
triazolin-3,5-dione (1), maleic anhydride (10), 1,4-benzoquinone (9)
Kinetic measurements at ambient pressure
The cause of еру outrageously high reactivity of dienophile 1 is
still unknown.[12,13,20] The very high reactivity of tetracya-
noethylene (6) [9,10,17] and of the dianhydride of ethylenetetracarbo-
nic acid (11)[25–27] as dienophiles in Diels–Alder reactions,
conditioned by the high value of p-EA of 6 (ЕА = 2.88 eV), and the
high strain energy of the C=C bond in dienophile 11. All efforts to
isolate the compound 11 have failed.[25–27] In this work, it was
assumed that the range of p-acceptor properties can be estimated
by the difference of the heat of solution in p-donor solvents, e.g. in
substituted benzenes. The data obtained were collected in Table 2.
As it follows from Fig. 1, the changes of the heat of solution of the
p-acceptors studied in p-donor solvents arrange the acceptor prop-
erties in the following sequence: tetracyanoethylene (6), 100%,
blank, correlation coefficient, r = 1; tetrachloro-1,4-benzoquinone
(7), 43%, r = 0.97; 1,3,5-trinitrobenzene (8), 24%, r = 0.99; 4-
phenyl-1,2,4-triazolin-3,5-dione (1), 19%, r = 0.94; maleic anhy-
dride (10), 15%, r = 0.99); 1,4-benzoquinone (9), 6%, r = 0.95.
In addition, the influence of p-donor solvents on the rate con-
stants (Table 2) comes in a similar order: tetracyanoethylene,
100%, blank, r = 1; 4-phenyl-1,2,4-triazolin-3,5-dione, 45%,
r = 0.997; maleic anhydride, 34%, r = 0.967). This means that the
acceptor properties of dienophiles 1 and 10 are moderate and
Table 2. Enthalpies of solution (ΔsolH/kJ molꢀ1) of some p-acceptors and the rate constants (k2/L molꢀ1 sꢀ1) of the Diels–Alder
reactions in p-donor solvents at 298 K
p-Acceptor
o-Xylene
solH
Toluene
Benzene
solH
Chlorobenzene
Δ
k2
Δ
solH
k2
Δ
k2
ΔsolH
k2
Tetracyanoethylene
Tetrachloro-1,4-benzoquinone
s-Trinitrobenzene
4-Phenyl-1,2,4-triazolin-3,5-dione
Maleic anhydride
1,4-Benzoquinone
1.4
9.2
5.9
18.0
15.1
16.3
0.061a
9.7
12.6
7.5
18.3
16.4
16.4
0.13a
14.9
16.7
9.6
20.6
16.9
17.1
0.38a
–
23.0
18.0
10.9
21.8
18.4
17.6
1.82a
–
–
–
–
–
–
–
b
0.21
0.33b
0.019d
–
0.52b
0.022d
1.01b
0.045d
–
0.017c
–
[10,11]
aWith anthracene, Ref.
bWith anthracene, Ref.
cThis work.
.
[12]
.
dWith 9,10-dimethylanthracene, Ref.
.
[16]
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Copyright © 2012 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. (2012)

HOMO-DIELS–ALDER REACTION
rather similar, and they cannot be the reason of the very high
differences in reactivity {log (k1/k10) ꢃ 4–6, Table 1}. Note, that
in the Diels–Alder reactions with moderate p-donor dienes, e.g.
substituted butadiene, 4-phenyl-1,2,4-triazolin-3,5-dione is more
active than tetracyanoethylene (Table 1).
The solvent influence on the heat of solution of reactants
1 and 4a as well as on the rate of the Diels–Alder reaction
(1+4a) is collected in Table 3.
The solvent effect on the rate constant was studied for the
[21]
reaction of 1 with 1,2,3,4-tetrachlorocyclopentadiene,
and
turned out to be very similar to that in the reaction (1+4a),
Table 3, (r = 0.987, n =9, slope 0.925). A comparison of the solvent
effect on the rate (ln k2) and the heat of solution of diene
(ΔsolH4a) leads to the conclusion that the differences in diene
4a solvation do not influence the reaction rate at all (r = 0.15).
This is unlike the differences of the dienophile (1) solvation,
where a linear correlation (r = 0.94, n = 12) of ln k2 versus ΔsolH1
was observed (Fig. 2).
Figure 2. Linear correlation between the enthalpies of solution (ΔsolH1)
of 4-phenyl-1,2,4-triazolin-3,5-dione (1) in some solvents and the rate
constants, (ln k2) of reaction (1+2). The solvent numeration is as in
Table 3
It means that the changes in enthalpy of solvation of dieno-
phile 1 (dsolvH1) in the studied solvents differ sharply from the
change of the dienophile 1 solvation in the activated complex.
¼
Table 4. Rate constants (k2/L molꢀ1 sꢀ1), enthalpy (ΔH /kJ
¼
¼
molꢀ1), entropy (ΔS /J molꢀ1 Kꢀ1), and Gibbs energy (ΔG /kJ
molꢀ1) of activation for the Diels–Alder reaction (1+2) in
toluene
The same conclusion was made before in the reaction of maleic
[16]
anhydride with 9,10-dimethylanthracene.
The most striking
example was observed in the Diels–Alder reaction with tetracya-
noethylene where the differences of the solvation of 6 are
¼
¼
¼
Т/K
k2
R
N
ΔH
ΔS
ΔG
[29]
almost completely disappearing in the activated complex.
288
298
308
298
0.00180
0.00395
0.00763
0.99991
0.99996
0.99994
17
17
19
50.9 ꢀ120.5 86.8
Activation parameters of the homo-Diels–Alder reaction (1+2),
(Table 4), are nearly the same as those for the Diels–Alder
reaction (1+4a), (Table 2).
In a polar solvent, such as acetonitrile, the rate constant is
nearly the same as in toluene. The entropy of activation of
reaction (1+2) should be more negative due to the shielding of
one side of bicyclo-2,5-heptadiene 2 by the C7 bridge.
The rate constants of reaction (1+4a) in different solvents are
collected in Table 3. Very fast reaction rates with t1/2 <1 s were
measured by the stopped flow method in toluene at 298 K
(Table 1).The mean values of the rate constants are for reaction
(1+4d), 84.0 ꢁ 0.6 L molꢀ1 sꢀ1 (nine doubles); for (1+4e),
12180 ꢁ 140 L molꢀ1 sꢀ1 (eight doubles); for (1+4f), 13750 ꢁ 200
0.00317а 0.99992а 16а
–
–
–
аIn acetonitrile.
Lmolꢀ1 sꢀ1 (eleven doubles); and for (1+4g), 828 ꢁ 6Lmolꢀ1 sꢀ1
[20]
(13 doubles). Adducts of these reactions were analyzed before.
Steric hindrances in the reaction of 9,10-diethylanthracene 4g
with 1 reduce the rate constant by factor of 15, as compared
with reaction (1+4e). Nearly the same ratio was observed in
reactions (4g+10) and (4e+10). [6] A sharp decrease in reactivity
Table 3. Enthalpies of solution (ΔsolH/kJ molꢀ1) of anthracene (4a) and 4-phenyl-1,2,4-triazolin-3,5-dione (1) as well as rate con-
¼
¼
¼
stants (k2/L molꢀ1 sꢀ1), enthalpies (ΔH /kJ molꢀ1), entropies (ΔS /J molꢀ1 Kꢀ1), and Gibbs energies (ΔG /kJ molꢀ1) of activation
of the Diels–Alder reaction, measured in twelve solvents at 298 K
a
¼
¼
¼
No
Solvent
ΔsolH4a
ΔsolH1
k2
ΔH
-ΔS
ΔG
1
2
3
4
5
6
7
8
Tetrahydrofuran
Ethyl acetate
Methyl acetate
o-Xylene
1,4-Dioxane
Toluene
Benzonitrile
Acetonitrile
Benzene
18.2
25.1
25.0
24.0
22.6
24.8
25.6
28.0
24.7
24.6
24.9
20.8
6.7
9.0
9.9
18.0
8.8
18.3
13.6
14.2
20.6
21.8
21.9
24.4
0.028
50.2
48.5 b
46.4
46.0
43.9 b
43.1
41.8
38.5
35.1
31.8
28.0
23.4
105
106 b
113
105
117 b
109
117
125
134
138
146
155
81.5
80.1b
80.0
77.3
78.8b
75.6
76.6
75.8
75.0
72.9
71.5
69.6
b
0.057
0.070
с
0.21
b
0.094
c
0.33
0.29
0.32
c
9
0.52 ; 0.50b
c
10
11
12
Chlorobenzene
1,2-Dichloroethane
Trichloromethane
1.01
1.55
5.09
[12,28]
aFrom Ref.
.
bFrom Ref.[13]
.
cRef [12]
.
J. Phys. Org. Chem. (2012)
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V. D. KISELEV ET AL.
was observed for 9-phenylanthracene and, especially, for 9,
10-diphenylanthracene, due to the almost orthogonal orienta-
tion of the planes of anthracene and the phenyl moieties. [14]
It is important to determine the ratio of the values of activa-
¼
¼
tion (ΔV ) and reaction (ΔVr-n) volumes, ΔV /ΔVr-n. Here, we use
the more accurate method of determination of ΔVr-n. The total
volume of solution with reagents 1, 2, and adduct 3 can be writ-
ten by Eqns (1) and (2):
Kinetic measurements at high pressure
À
Á
À
Á
The pressure influence on the rate constant of reaction (1+2) has
been studied in toluene at 25 ꢄC (Fig. 3, Table 5).
V
¼ Vs þ c01 ꢀ c3 ꢂV1 þ c02 ꢀ c3; ꢂV2 þ c3; tꢂV3 (1)
t
ðtÞ
; t
Â
À
ÁÃ
From the experimental dependence given by the equation: ln
(kp/kp=1) = 1.098ꢂ10ꢀ3P – 7.679ꢂ10ꢀ8P2 (r = 0.9995, n = 9) follows
that the apparent value of the volume of activation is ꢀ27.2
0.8 cm3 molꢀ1. After correction because of the compressibility
of the solvent (RTꢂb298, where b298 is the compressibility coeffi-
V
¼ Vs þ c01ꢂV1 þ c02ꢂV2 þ c3; tꢂðV3 ꢀ V1 ꢀ V2Þ
¼ Vðt¼0Þ þ c3; tꢂΔVrꢀn
ðtÞ
(2)
(3)
1=d ¼ 1=dðt¼0Þ þ c3; tꢂΔVrꢀn=1000d
ðtÞ
ðt¼0Þ
cient of toluene at 298 K, equal 90ꢂ10ꢀ6 barꢀ1
,
[30]) the value of
.
¼
activation volume, ΔV , is ꢀ25.1 ꢁ0.8 cm3 molꢀ1
Equation (3), rewritten from Eqn (2), is more convenient for the
density measurements 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 volume of solvent; V1, V2, and V3 are PMVs of
compounds 1, 2, and 3; c01, c02, and c3, t are the initial molar
concentrations of 1 and 2, and the current concentration of 3;
ΔVr-n is the reaction volume. Linear dependences 1/d(t) versus
c3 (Fig. 4) were observed for two runs up to 95% conversion.
The values of the reaction volume were calculated as
ꢀ30.81 ꢁ 0.08 and ꢀ31.09 ꢁ 0.11 cm3molꢀ1. For the large part
¼
of the studied Diels–Alder reactions, a “normal” ratio, ΔV /ΔVr-n
1, was observed.[31–33] Such value is clear, taking into account
the incomplete bond formations in the activated complex as com-
pared with the adduct. However, for nearly the same number of
¼
Diels–Alder reactions, an “abnormal” ratio, ΔV /ΔVr-n >1, was
[31–33]
observed.
For the studied homo-Diels–Alder reaction (1+2),
¼
the “normal” ratio ΔV /ΔVr-n =ꢀ25.1/ꢀ30.95 = 0.81 is obtained. For
reaction (1+6) in toluene at 298 K, nearly the same “normal” ratio
ΔV /ΔVr-n =ꢀ28.1/ꢀ32.9 = 0.85 was reported. [34]
¼
Calorimetric data
Obtained values of enthalpies of solution of compounds 1, 7–9
are collected in Tables 2 and 3 and were already discussed
above. The heat of reactions (1+2), (1+4e), and (1+4f) were
measured by addition of 25–40 mg of solid 1 to a solution of dienes
2 (1.5 mol Lꢀ1 in toluene), to a solution of 4e (1ꢂ10ꢀ3 molꢂLꢀ1
in toluene), and to a solution of 4f (2ꢂ10ꢀ3 mol Lꢀ1 in 1,
Figure 3. Anamorphosis of the kinetic runs of the homo-Diels–Alder
reaction (1+2) in toluene solution under high pressure at 298 K: 1–1;
2–407; 3–616; 4–757; 5–945; 6–1215; 7–1538; 8–1748 and 9–2166 bar
Table 5. Pressure (P/bar) effect on the rate constant
(k2/L molꢀ1 sꢀ1
) of the homo-Diels–Alder reaction of
4-phenyl-1,2,4-triazolin-3,5-dione with bicyclo[2,2,1]hepta-2,
5-diene, (1+2), measured in toluene at 298 K
P
103 kp
ln(kp/kp=1)
1
3.9
6.0
7.5
0
407
616
757
945
1215
1538
1748
2166
0.419
0.647
0.788
0.963
1.230
1.471
1.701
2.011
8.7
10.3
13.5
17.2
21.6
29.5
Figure 4. Relation between the specific volume (1/dt) during the reac-
tion of (1+2) in toluene solution and the concentration of adduct 3 (C3)
at 298 K. For clarity, line (■) for repeated measurement was shifted up
on 1ꢂ10ꢀ4 unity of the ordinate scale
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J. Phys. Org. Chem. (2012)

HOMO-DIELS–ALDER REACTION
2-dichloroethane). The heat of homo-cycloaddition (1+2) deserves
particular attention. From three independent measurements of
the heat of reaction (1+2) in toluene at 298 K, the following values
were obtained: ꢀ219; ꢀ214, and ꢀ220 kJ molꢀ1. The exothermicity
of reaction (1+2) is ꢀ218 ꢁ 2 kJ molꢀ1, higher than that of 1 with
substituted butadiene (Table 1). The values of the heats of
reaction (1+4e) (ꢀ118.4, ꢀ116.7, and ꢀ118.3 kJ molꢀ1) and (1+4f)
(ꢀ91.66, ꢀ 91.75, and ꢀ91.29 kJ molꢀ1) were also obtained. The lin-
ear relation between the values of the heat of reactions of the same
dienes with 4-phenyl-1,2,4-triazolin-3, 5-dione and with tetracya-
noethylene (Table 1) reflects the difference in the diene conjuga-
tion. From these data, it follows that the exothermicity of reactions
of dienes with 1 is higher than that with 6. However, it should be
noted that the heat of reaction of 9,10-dimethylanthracene (4e)
with 4-phenyl-1,2,4-triazolin-3,5-dione (ꢀ117.8 ꢁ 0.7 kJ molꢀ1) is
nearly the same as in reaction of 4e with the structural C=C counter-
part, N-phenylmaleimide (ꢀ117 kJ molꢀ1). [2] From these data, it can
be concluded that the high exothermicity of the Diels–Alder reac-
tions with 4-phenyl-1,2,4-triazolin-3,5-dione cannot be the deter-
mining factor of the very high reactivity of 1.
Acknowledgements
This work was supported by the US Civilian Research and
Development Foundation (Joint grant No REC-007), by the
Russian Federal Agency of Education (No P-2345, GK No
14.740.11.0377, GK No ОК-1/2010), and by the Russian Fund for
Basic Researches (grant No12-03-00029). We also appreciate
fruitful comments from the reviewers, which helped for us in
finalizing this manuscript.
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Since the energy of the N=N bond is considerably lower
[35]
(418 kJ/bond) than that of the C=C bond (611 kJ/bond),
this
difference in the energy of breaking bonds can generate a lower
barrier of activation in the Diels–Alder cycloaddition reactions
with 1, in spite of nearly the same acceptor properties of 1 and
maleic anhydride 10, and the same values of the heat of
reactions of the dienes with 1 and with 10. The pressure
influence on the rate constant of the homo-Diels–Alder reaction
(1+2), its volume of activation, and its volume of reaction are
similar to the “normal” (4p +2p)-cycloaddition reaction. Thereby,
an analysis of the reasons of the very low and very high reaction
rates helps to understand the nature of reactivity of different
systems in cycloaddition processes.
J. Phys. Org. Chem. (2012)
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