Ene reaction between 4-phenyl-1,2,4-triazoline-3,5-dione and hex-1-ene. The enthalpies, entropies, and volumes of activation and reaction in solution

Article abstract of DOI:10.1007/s11172-014-0424-y

The rates of an ene reaction between 4-phenyl-1,2,4-triazoline-3,5-dione and hex-1-ene were studied in a temperature range of 15-40 °C and in a pressure range of 1-2013 bar. The enthalpy of reaction in 1,2-dichloroethane (-158.2±1.0 kJ mol^-1), the enthalpy (51.3±0.5 kJ mol^-1), entropy (122±2 J mol^-1 K^-1), and volume of activation (-31.0±1.0 cm³ mol^-1), and the volume of this reaction (-26.6±0.3 cm³ mol^-1) were determined. The high exothermic effect of the reaction suggests its irreversibility.

Full text of DOI:10.1007/s11172-014-0424-y

Russian Chemical Bulletin, International Edition, Vol. 63, No. 1, pp. 280—283, January, 2014
280
Brief Communications
Ene reaction between 4ꢀphenylꢀ1,2,4ꢀtriazolineꢀ3,5ꢀdione and hexꢀ1ꢀene.
The enthalpies, entropies, and volumes of activation and reaction in solution
V. D. Kiselev,a H. A. Kashaeva,a L. N. Potapova,a D. A. Kornilov,a and A. I. Konovalovb
aA. M. Butlerov Chemical Institute, Kazan Federal University,
18 ul. Kremlyovskaya, 420008 Kazan, Russian Federation.
Fax: +7 (843) 292 7278. Eꢀmail: vkiselev.ksu@gmail.com
bA. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan Research Center of the Russian Academy of Sciences,
8 ul. Akad. Arbuzova, 420088 Kazan, Russian Federation.
Fax: +7 (843) 273 1872
The rates of an ene reaction between 4ꢀphenylꢀ1,2,4ꢀtriazolineꢀ3,5ꢀdione and hexꢀ1ꢀene were
studied in a temperature range of 15—40 C and in a pressure range of 1—2013 bar. The enthalpy
of reaction in 1,2ꢀdichloroethane (–158.2 1.0 kJ mol–1), the enthalpy (51.3 0.5 kJ mol–1),
entropy (122 2 J mol–1 K–1), and volume of activation (–31.0 1.0 cm3 mol–1), and the
volume of this reaction (–26.6 0.3 cm3 mol–1) were determined. The high exothermic effect of
the reaction suggests its irreversibility.
Key words: ene reaction, 4ꢀphenylꢀ1,2,4ꢀtriazolineꢀ3,5ꢀdione, hexꢀ1ꢀene, kinetics, thermoꢀ
chemistry, high pressure.
Scheme 1
The Alder ene reactions occur between an alkene reacꢀ
tant having an allylic H atom and enophiles containing
C=C, C=O, or C=N bonds activated by electronꢀwithꢀ
drawing substituents1 (Scheme 1).
The use of various catalysts2—5 provides great scope for
the synthesis of many potentially useful (including optiꢀ
cally active) products. In the absence of catalysts, heating
(~100 C) is required to initiate these reactions, which
makes most kinetic data uncertain.
The Diels—Alder reaction The Alder ene reaction
A is diene, B is dienophile, C is ene, and D is enophile.
The double N=N bond in 4ꢀphenylꢀ1,2,4ꢀtriazolineꢀ
3,5ꢀdione (1) is reactive in [4+2], [3+2], and [2+2] cycloꢀ
additions as well as in the Alder ene reaction.6,7 The much
better reactivity of compound 1, which is ~105 times highꢀ
er than that of its structural analog Nꢀphenylmaleimide
with the C=C bond, has been attributed8 to the lower
dissociation energy of the N=N bond (418 kJ mol–1) comꢀ
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 0280—0283, January, 2014.
1066ꢀ5285/14/6301ꢀ0280 © 2014 Springer Science+Business Media, Inc.

Ene reaction of 4ꢀphenylꢀ1,2,4ꢀtriazolineꢀ3,5ꢀdione Russ.Chem.Bull., Int.Ed., Vol. 63, No. 1, January, 2014
281
pared to that of the C=C bond (611 kJ mol–1). Alkenes
containing an allylic proton with the possibility of shifting
their C=C bond readily and quantitatively react with diꢀ
Table 1. The rate constants k2 of the reaction
(see Scheme 2) in toluene under elevated hydroꢀ
static pressures P
azene 1 in the ene synthesis. In this context, such reacꢀ
P/bar
k2/L mol–1 s–1
ln(kp/kp=1)
tions can be classified as "click chemistry" processes.
A reaction of diazene 1 with hexꢀ1ꢀene (2) only yields
product 3 with a proven6 structure (Scheme 2).
1
0.00204
0.00285
0.00369
0.00447
0.00544
0.00671
0.00840
0.01050
0.01360
0
214
431
628
816
1025
1313
1618
2013
0.336
0.592
0.784
0.980
1.190
1.415
1.638
1.898
Scheme 2
The activation parameters (enthalpy, entropy, and
Gibbs energy) of the reaction (see Scheme 2) have
the following values: H = 51.8 1.0 kJ mol–1, S
=
= –122 3 J mol–1 K–1, and G = 88.1 1.0 kJ mol–1
The rate constants at elevated pressures are given
in Table 1.
.
The volume of activation Vcorr corrected for the comꢀ
pressibility of the solvent is –31.0 1.1 cm3 mol–1. The
volume of reaction (Vr–n = –26.6 0.3 cm3 mol–1) is
appreciably smaller than the volume of activation. It should
be noted that close values of these parameters have been
obtained14 for a reaction of diazene 1 with cyclohexene
(Vcorr = –29.1 0.5 cm3 mol–1, Vr–n = –25.0 0.4 cm3
mol–1). The reactions of compound 1 with hexꢀ1ꢀene and
cyclohexene show the same ratio of the volume parameꢀ
ters (Vcorr/Vr–n = 1.16). Since the solvent only slightꢀ
ly affects the rate of ene reactions, which are rapid in
nonpolar media,15 one can rule out the electrostriction of
the solvent in the solvation of the activated complex. The
data obtained (Vcorr/Vr–n > 1) suggest that the activatꢀ
ed complex has a smaller volume than the reaction prodꢀ
uct, probably because the cyclic transition state is more
ordered than the linear structure of adduct 3. This concluꢀ
sion agrees with the previous data for ene reactions involvꢀ
ing the C=O bond of an enophile:12,13 Vcorr/Vr–n
(converted to 25 C) = 1.1—1.3, which is also greater
than unity.
The mechanism of these reactions can further be eluꢀ
cidated by analyzing the ratio of the volume of activation
to the volume of reaction (V/Vr–n). High hydrostatic
pressure has been shown9—13 to influence the rate of highꢀ
temperature ene reactions involving the C=C and C=O
bonds in an enophile, so experimental V values meaꢀ
sured at high temperatures should be converted to stanꢀ
dard conditions (25 C, 1 bar). The V/Vr–n ratio can be
used to compare its values for the ene and Diels—Alder
reactions (for the latter, the cyclic mechanism of the tranꢀ
sition state has been reliably proved). The V and Vr–n
values themselves allow estimation of an increase in the
rate and equilibrium constants under the highꢀpressure
conditions of ene reactions.
The high rate of the reaction enables one to obtain
reliable values of the enthalpy of reaction in solution and
compare them with known values8 for the Diels—Alder
reaction.
Data on the effects of temperature and pressure on the
rate of the reaction (see Scheme 2) are lacking in the
literature. The rate constants k2 obtained for the reaction
in toluene under normal pressure are tabulated below.
The reaction studied is highly exothermic
(–158.2 1.0 kJ mol–1) and hence can be believed to ocꢀ
cur irreversibly under normal conditions.
Experimental
T/C
k2
/L mol–1 s–1
0.00114
0.00237
0.00233
0.00235
0.00232
0.00690
Hexꢀ1ꢀene (99+%, Acros) was used as purchased. 4ꢀPheꢀ
nylꢀ1,2,4ꢀtriazolineꢀ3,5ꢀdione (97%, Aldrich, Germany) was
sublimed at 100 C and 100 Pa. The structure of product 3 had
been proved earlier.6
15
25
The rate of the reaction in toluene was determined from the
changes in the absorption band of reactant 1 with max = 540 nm
40
(
= 245 L mol–1 cm–1); compounds 2 and 3 are optically
540

282
Russ.Chem.Bull., Int.Ed., Vol. 63, No. 1, January, 2014
Kiselev et al.
transparent at this wavelength. The data obtained in a temperaꢀ
ture range of 15—40 C are given above. The rates of the reaction
under elevated pressure were measured as described earlier.8
ln(D0/Dt)
1.6
9
3
The volume of reaction at 25 C (Vr–n = –26.6 0.3 cm3 mol–1
)
8
2
was calculated from the relationship between the specific volꢀ
ume of the working solution of reactants 1 and 2 and the concenꢀ
tration of adduct 3 during the reaction (Eq. (1)). The plot of 1/dt
vs. C3,t keeps linear up to the 100% conversion (Fig. 1).
5
7
1.2
0.8
0.4
6
4
1
1/dt = 1/dt=0 + C3,tVr–n/(1000dt=0).
(1)
For all measurements of the reaction rates under elevated
pressure, working solutions of reactants 1 (20.2 mmol L–1) and 2
(262 mmol L–1) were prepared and mixed in equal volumes
(1 mL each). The kinetic curves of the reaction for different
elevated pressures are shown in Fig. 2. The effect of the pressure
on the rate constant is illustrated in Table 1. The experimental
plot of ln(kp/kp=1) vs. P (Fig. 3) obeys equation (2).
500 1000 1500 2000 2500 3000
t/s
Fig. 2. Kinetic curves for the reaction (see Scheme 2) at 25 C
and P = 1 (1), 214 (2), 431 (3), 628 (4), 816 (5), 1025 (6),
1313 (7), 1618 (8), and 2013 bar (9).
ln(kp/kp=1) = 0.00133938P – 0.00000021P2
(R2 = 0.9989).
(2)
ln(kp/kp=1
1.8
)
The apparent volume of activation (V
=
1.6
1.2
0.8
0.4
app
= –33.2 1.1 cm3 mol–1) at P = 1 bar was calculated by formula (3).
Vapp = –RTln(kP)/P.
(3)
The corrected volume of activation (V
=
corr
= –31.0 1.1 cm3 mol–1) was calculated with allowance for the
compressibility of toluene under pressure (4).
Vcorr = –RTln(kP)/P + (n – 1)RT ,
(4)
500
1000
1500
2000
P/bar
T
Fig. 3. Plot of ln(kp/kp=1) vs. P for the reaction (see Scheme 2) in
toluene at 25 C.
where kP is the reaction rate constant at pressure P while considꢀ
ering the solvent incompressible, (n – 1)RT is the correction
T
for the compressibility of the solvent, n is the order of the reacꢀ
tion, and  = ln(d)/P is the compressibility of the solvent
T
The enthalpy of the reaction (see Scheme 2) in 1,2ꢀdichloꢀ
roethane at 25 C was measured by calorimetry as described earꢀ
lier.8 Crystals of compound 1 (25—35 mg) were added to a solution
of hexꢀ1ꢀene in 1,2ꢀdichloroethane (150 mL, C2 = 1.0 mol L–1).
Three successive measurements of the heat effect gave –136.1,
–135.1, and –137.8 kJ mol–1. The enthalpy of dissolution of
a solid sample of 1 in 1,2ꢀdichloroethane makes a contribution of
21.9 kJ mol–1, so the average enthalpy of the reaction (see
Scheme 2) in solution is –158.2 1.0 kJ mol–1. This exothermic
effect is appreciably greater than those obtained for the Diels—Alꢀ
(90•10–6 bar–1 for toluene at 25 C).16
d
–1/cm3 g–1
1.1631
1.1630
1.1629
1.1628
1.1627
1.1626
1.1625
1.1624
2
1
der reactions of compound 1 with anthracene (–97 kJ mol–1
)
and cyclopentadiene (–134 kJ mol–1), yet being lower than
the enthalpy of a reaction of compound 1 with butaꢀ1,3ꢀdiene
(–201 kJ mol–1).8
3
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 12ꢀ03ꢀ00029a).
0.001 0.002 0.003 0.004 0.005
C3/mol L–1
Fig. 1. Plots of d–1 vs. C3 (see Eq. (1)) for the reaction (see
Scheme 2) in toluene at 25 0.002 C: (1) d–1 = –0.030935C3 +
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Received May 29, 2013