Reactivity Variation of Tetracyanoethylene and 4-Phenyl-1,2,4-Triazoline-3,5-Dione in Cycloaddition Reactions in Solutions

Article abstract of DOI:10.1007/s10953-019-00846-6

The reasons for the very high reactivity and variability of reactivity of two dienophiles, tetracyanoethylene (1) and 4-phenyl-1,2,4-triazoline-3,5-dione (2), in the Diels–Alder reactions were considered. The data on the rate of reactions with anthracene (3), benzanthracene (4) and dibenzanthracene (5) in 14 solvents over a range of temperatures and high pressures, data on the change in the enthalpy of solvation of reagents, transition state, and adducts in the forward and backward reactions, and the enthalpies of these reactions in solution were obtained. Strong π-acceptor dienophile 1 has sharply reduced reactivity in reactions in π-donor aromatic solvents. It was observed that the π-acceptor properties of dienophile 1 disappear upon passage to the transition state and adduct. Large solvent effects on the reaction rate can be predicted for all types of reactions involving tetracyanoethylene. Very high reactivity of dienophiles 1 and, especially, 2 can be useful to catch such carcinogenic impurities such as 3–5 and neutralize them by transformation into less dangerous adducts.

Full text of DOI:10.1007/s10953-019-00846-6

Journal of Solution Chemistry
https://doi.org/10.1007/s10953-019-00846-6
Reactivity Variation of Tetracyanoethylene
and 4‑Phenyl‑1,2,4‑Triazoline‑3,5‑Dione in Cycloaddition
Reactions in Solutions
1
1
1
1
Vladimir D. Kiselev · Dmitry A. Kornilov · Oleg V. Anikin · Alexey A. Shulyatiev ·
2
Alexander I. Konovalov
Received: 7 May 2018 / Accepted: 7 October 2018
©
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
The reasons for the very high reactivity and variability of reactivity of two dienophiles, tet-
racyanoethylene (1) and 4-phenyl-1,2,4-triazoline-3,5-dione (2), in the Diels–Alder reac-
tions were considered. The data on the rate of reactions with anthracene (3), benzanthra-
cene (4) and dibenzanthracene (5) in 14 solvents over a range of temperatures and high
pressures, data on the change in the enthalpy of solvation of reagents, transition state,
and adducts in the forward and backward reactions, and the enthalpies of these reactions
in solution were obtained. Strong π-acceptor dienophile 1 has sharply reduced reactivity
in reactions in π-donor aromatic solvents. It was observed that the π-acceptor properties
of dienophile 1 disappear upon passage to the transition state and adduct. Large solvent
eꢀects on the reaction rate can be predicted for all types of reactions involving tetracya-
noethylene. Very high reactivity of dienophiles 1 and, especially, 2 can be useful to catch
such carcinogenic impurities such as 3–5 and neutralize them by transformation into less
dangerous adducts.
Keywords High pressure · Diels–Alder reaction · Kinetics · Transition state · Enthalpy
1
Introduction
An increased interest in the Diels–Alder reaction (DAR) is due to its enormous potential
in laboratory and industrial synthesis of a variety of cyclic products [1, 2]. This has led
to research to obtain the most reliable quantitative information on the eꢀect of various
internal and external factors (catalyst, solvent, temperature, high pressure) on the reaction
rate, equilibrium and selectivity [2–4]. Huge amounts of accumulated experimental data
have served as a basis for a number of theoretical models of cycloaddition reactions [3–6].
*
Vladimir D. Kiselev
vkiselev.ksu@gmail.com
1
2
Laboratory of High Pressure Chemistry, Butlerov Institute of Chemistry, Kazan Federal University,
Kremlevskaya Str., 18, Kazan, Russian Federation 420008
Arbuzov Institute of Organic and Physical Chemistry, Kazan Scientiꢁc Center of Russian
Academy of Sciences, Kazan, Russian Federation 420088
Vol.:(0123
1
456789)
3

Journal of Solution Chemistry
The presence of an active diene or dienophile allows trapping of another reagent (dieno-
phile/diene) in solution, polymers and biological objects. Among the dienophiles, the most
active are tetracyanoethylene (1) and 4-phenyl-1,2,4-triazoline-3,5-dione (2), which are
capable of fast reactions with a variety of dienes. Such reactions are often in the frame
of “click-chemistry” [7]. There is an opportunity to use DAR to catch such carcinogenic
impurities as anthracenes, benzanthracenes, dibenzanthracenes and to neutralize them by
transformation into less dangerous adducts. Some questions remained unanswered, such
as why dienophile 2 with moderate electron aꢂnity (E ) may be more active in DAR with
A
some dienes and vice versa with other dienes than tetracyanoethylene 1 with the highest
π-electron aꢂnity. The reactivity of “diene—π-donor, dienophile—π-acceptor” systems
with C=C bonds in the DARs depends on the ionization potential (IP ) of the dienes, elec-
D
tron aꢂnity (E ) of the dienophiles [(IP −E )/eV], energy balance of the breaking and
A
D
A
−
1
forming of bonds (ΔH /kJ·mol ) and the 1–4 distance of reaction centers in 1,3-dienes
1–4
r–n
(
R
/Å), (Eq. 1, correlation coeꢂcient, r=0.972; n=93) [3]:
log k = − 28.81 + 316.3∕(IP − E ) − 69.9 ⋅ R1−4∕(IPD − E ) − 0.054 ⋅ ΔH
r−n
1
0
2
D
A
A
(
1)
For this reason, trans-1,3-butadiene (R1 =3.66 Å) should be 8 orders of magnitude less
–4
active than cis-1,3-butadiene (R =2.88 Å). Hence it follows that the maximum of reac-
1–4
tivity in the DAR should be observed for the planar cis-1,3-dienes with a minimum value
of R . The energy balance of the reacting C=C bonds always aꢀects the value of acti-
1
–4
vation energy, but rarely governs the relative rate of the forward cyclization reaction [3].
−1
−1
Since the entropy values of the DARs are usually in the range −(120–150) J·mol ·K
≠
−1
(
298·ΔS ~− 40 kJ·mol ), the reaction is thermodynamically favorable if the exothermic-
−
1
ity of the DAR is higher than − 40 kJ·mol .
The relation (1) allows one to predict the activity of reagents with C=C bonds, since
here the IP and E parameters are usually proportional to the charges at the reaction cent-
D
A
ers. The balance between the energies of rupture of C=C bonds in the reagents and for-
mation of new C–C bonds in the products equals the enthalpy of the DAR. The changes
in enthalpy values mainly reꢃect the diꢀerence in the conjugation energy of reacting
C=C π-bonds [3]. The π-acceptor properties of 2 are rather moderate (E ~1.0 eV) [8],
A
and appreciably inferior to the dienophile of 1 (E =2.88 eV) [9]. On the other hand, the
A
−1
N=N bond disruption energy (418) is much less than that of the C=C bond (611 kJ·mol ),
which can be the main reason of a very high activity of 2. In some cases the DARs can
proceed rather fast, but with very low conversion, so it is important to consider both the
kinetic and equilibrium parameters [3]. In this work the diꢀerences in the solvation enthal-
pies of reagents, transition state (TS) and adducts in the DAR of 1 and 2 with anthracene
(
3), 1,2-benzanthracene (4) and 1,2,5,6-dibenzanthracene (5) were determined on the basis
of kinetic and thermochemical data (Scheme 1 of reactions).
2
Experimental Section
2
.1 Chemicals Used
Tetracyanoethylene (1) and 4-phenyl-1,2,4-triazoline-3,5-dione (2) (Aldrich, 97%) were
sublimated at 100 °C and 10 Pa before the measurements. White snow-like 1 had m.p.
2
00–201 °C (lit [10] 201 °C) and red 2 had m.p.165–170 °C, with decomposition (lit [11]
65–170 °C, decomp.). The purity of 2 was analyzed by the known absorption coeꢂcient
1
1
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Journal of Solution Chemistry
Scheme 1 Scheme of reactions
in toluene (ε
=245), 1,2-dichloroethane (ε
=178), and in 1,4-dioxane (ε
=171
5
40 nm
540 nm
532 nm
−
1
−1
L·mol ·cm ) [12–14]. Anthracene (3), 1,2-benzanthracene (4) and 1,2,5,6-dibenzanthra-
cene (5) (Aldrich, 99%) were used as purchased. All solvents were dried and puriꢁed by
1
3

Journal of Solution Chemistry
known methods [15]. The UV spectra of solutions of reagents 1–5 in these solvents remained
unchanged for at least one day.
2
.2 Kinetic Measurements at Ambient Pressure
The temperature in the cell of the UV-spectrophotometer (Hitachi U-2900, Japan) was sta-
ble within ± 0.1 °C. The square-proꢁled sample cuvette (1×1×4 cm) made up of quartz was
sealed with a ground glass stopper. The reaction rates with dienophile 2 were determined by
following the absorption of dienophile 2 at 530–560 nm, and those of reactions with dieno-
phile 1 were studied at 380–405 nm following the change in the absorption of the dienes 3–5.
The initial concentrations of dienophile 2 was 15–50 times lower than that of dienes 3–5. The
reaction rates with tetracyanoethylene in aromatic solvents were measured according to the
change of the molecular complex’s absorption at 450–550 nm.
The decomposition reactions 6→1+3 and 7→1+4 were carried out in the pres-
−
1 −1
ence of quadricyclane (12), which rapidly (k =104.4 L·mol ·s ) and irreversibly
2
−1
(
ΔH =−236.6 kJ·mol ) [16] binds dienophile 1. The values of the rate constants were
r–n
−1
determined with errors not exceeding ± 3%, the enthalpies of activation ± 2 kJ·mol , and the
−1
−1
entropies of activation ± 6 J·mol ·K .
2
.3 Kinetic Measurements Under Elevated Pressure
The pressure eꢀect on the rate of the reaction 2+4→10 was studied in toluene at 25 °C
using the pressure multiplier (HP-500, Japan), a quartz cell with variable volume (PCI-500,
Japan) and UV spectrophotometer (SCINCO S-3100, Korea). The observed activation volume
≠
exp
−1
(
ΔV ) of the reaction 2+4→10 (C =C =0.0103 mol·L ) was calculated on the basis of
04 02
the rate constants measured at ambient pressure and 1000 bar, applying the previously pro-
posed [17] relationship (2).
ꢀ
ꢁ
−3
−3
ꢀlnk∕ꢀP)T, P = 1 = (1.15 ± 0.03) × 10 ⋅ ln kP = 1000∕kP = 1
(
(2)
(3)
≠
exp
ΔV
= −R ⋅ T ⋅ (ꢀlnk∕ꢀP)T,P=1
≠
The corrected value of activation volume (ΔV ) [18] of the reaction 2+4→10 was
corr
determined by Eqs. 3 and 4 taking into account the isothermal compressibility coeꢂcient
−
5
−1
of toluene (β =9.19 × 10 bar ) [19]:
T
≠
≠
ΔV = ΔV + ꢀ ⋅ R ⋅ T
T
corr
exp
(4)
2
.4 Reaction Volumes in Solution
For reaction 2+4→10, the total volume of solution with reagents 2, 4 and adduct 10 can be
expressed by Eqs. 5–7:
ꢀ
ꢁ
ꢀ
ꢁ
V
= V + c − cꢁꢂ,t ⋅ V + c − cꢁꢂ,t ⋅ V + c ⋅ V
ꢁꢂ
(5)
(
t)
s
0,ꢀ
ꢀ
0,ꢃ
ꢃ
ꢁꢂ,t
ꢀ
ꢁ = V(t=0)
+ cꢂꢃ,t ⋅ ΔVr−n
V
= [V + (c ⋅ V + c ⋅ V )] + c ⋅ V − V − V
ꢁ
(
t)
s
0,ꢀ
ꢀ
0,ꢁ
ꢁ
ꢂꢃ,t
ꢂꢃ
ꢀ
(
6)
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3

Journal of Solution Chemistry
1
∕d(t) = 1∕d(t=0) + cꢀꢁ,t ⋅ ΔVr−n∕1000 ⋅ d(t=0)
(7)
Equation 7, derived from Eq. 6, is more convenient for determining the density of the reac-
tion mixture. Here, V and V are the solution volumes at the beginning and during the
(
t=0)
(t)
reaction; V is the solvent volume; V , V , and V are the partial molar volumes of com-
s
2
4
10
pounds 2, 4, and 10; c , c , and c are the initial molar concentrations of reagents 2 and
0
,2 0,4
10,t
4
, and the current concentration of adduct 10; ΔV is the reaction volume. The current
r–n
concentration of adduct 10 was calculated from the kinetic data. Linear dependences 1/d
(t)
versus c were observed for three measurements up to 95% conversion. A vibration densi-
10
−6
tometer (DSA 5000 M, Austria) was employed to measure the solution density (± 2×10
−
3
g·cm ) at 25± 0.002 °C.
2
.5 Calorimetric Experiment
The enthalpies of solution of the compounds were measured at 25 °C by the calorimetric method
described previously [20]. The accuracy of the calorimeter was veriꢁed by comparing the heat
−1
of dissolution of dry potassium chloride in water at 25 °C, 17.5± 0.2 kJ·mol . The results are
−1
in a good agreement with the recommended data (17.51± 0.008 kJ·mol ) [21]. The solid sam-
ple (30–50 mg) was added to a solvent (150 mL). Usually 3–4 consecutive dissolutions of each
compound were carried out with a subsequent calibration of the calorimeter. The enthalpies of
reactions were measured at 25 °C by addition of 20–40 mg of freshly sublimed 1 or 2 to the
solution of the excess of diene, taking into account the heat of dissolving of 1 or 2. The heat of
slow reaction 2+4→10 in 1,2-dichloroethane at 25 °C was measured using a “TAM III” preci-
sion solution calorimeter (USA), working in semi-adiabatic mode, using a 25 mL glass calo-
rimetric vessel equipped with a gold stirrer, a Joule heater, and a thermistor. The calorimetric
−1
vessel was initially ꢁlled with 25 mL of a 0.0334 mol·L solution of 4. After about 1 h of ther-
−1
mostatting and heater calibrations, a known volume of solution 2 (0.191 mol·L ) was added in
portions of 60–80 μL from the electronically operated microsyringe.
2
.6 Diferences in Solvation Enthalpies
The transfer of solvation enthalpies of reagents (δΔH
), adducts (δΔH
) and
solv, add
solv,reag
transition state (δΔH
) of the reactions 1+3→6; 6→1+3; 1+4→7 and 2+4→10
solv, TS
were calculated using acetonitrile as the reference solvent (Eqs. 8–10):
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ΔHsolv,ꢀꢁꢂꢃ
=
ΔHdissolv,ꢀꢁꢂꢃ S − ΔH
dissolv,ꢀꢁꢂꢃ
(8)
(9)
acetonitrile
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ΔHsolv,ꢀꢁꢁ
=
ΔHdissolv,ꢀꢁꢁ S − ΔH
dissolv,ꢀꢁꢁ acetonitrile
≠
≠
ꢀ
ΔHsolv,ꢀꢁ = ꢀΔHsolv,ꢂꢃꢄꢅ + ΔHS − ΔH
(10)
acetonitrile
3
Results and Discussion
A set of data on the rate constants and the activation parameters of the forward (1+3→6)
and backward (6→1+3) DARs in 14 solvents are collected in Table 1.
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Journal of Solution Chemistry
The rate constants of the forward reaction have decreased by almost 1500 times from
1
,1,2,2-tetrachloroethane to the nonpolar mesitylene. This is mainly determined by a
change in the activation enthalpy. Reduced reaction rates are observed in the oxygen-
containing n-donor solvents (entries 1–4) and in π-donor alkylbenzenes (entries 11–14).
In chloroalkanes (entries 6–9, Table 1) the reaction rates are increased. Note, that the
rate constants of the adduct’s decay (6 → 1+3) and the activation enthalpies in all these
solvents diꢀer insigniꢁcantly (Table 1). It can be presumed that the solvent inꢃuence on
the enthalpies of the reagents and the transition state for the direct DAR 1 + 3→6 are
signiꢁcantly diꢀerent, in contrast to the retro reaction 6 → 1 + 3. In order to carry out a
quantitative analysis of the change in the TS enthalpy, the thermochemical data on the
change in the dissolution enthalpies of the reagents and reaction products in addition to
the kinetic data are needed. These data, as well as the enthalpies of the reaction in the
studied solvents are collected in Table 2.
The reliability of the dissolution heats (ΔH
) and reaction heats (ΔH ) in the
r–n
dissol
studied solvents (Table 2) can be checked using the Hess law by calculating the standard
enthalpy of reaction 1 + 3 → 6 (ΔH ) (Eq. 11):
r–n, (st.st.)
ΔHr−n, (st.st.) = ΔHr−n(S) + ΔHdissol(ꢀ+ꢁ)(S) − ΔHdissolꢂ(S)
(11)
The data were not obtained in aromatic solvents (entries 10–14) due to the very low solu-
−
1
bility of adduct 6. Very close values of ΔH
(− 40.5± 0.6 kJ·mol , entries 1–9,
r–n, (st.st.)
Table 2) conꢁrm the reliability of the obtained parameters. The dissolution heat of the
adduct 6 can be calculated from Eq. 11 when the values of ΔH (S) and ΔH
r–n
dissol (1+2)
(
S) are known. In the series of solvents (entries 6–10) the dissolution heat of 1 is close to
−1
its fusion heat, 24.9 kJ·mol [25], and the reaction rates are increased in these solvents.
The correlations between the change in the enthalpy of solvation of 1 (entries 10–14) and
the enthalpy of formation of the π,π-complexes [26], the partial molar volumes of 1 [24],
the charge transfer energy of the band of these π,π-complexes with the solvents, and the
changes of the DAR rates [3, 6, 24] (all r>0.95) are clearly manifested. Similar changes
in the solvation enthalpy of the reagents (δH
) and the adduct (δH
) are observed
solv 6
solv 1+3
in the solvents (entries 1–10, Table 2) with an angular coeꢂcient (0.85) close to unity,
r=0.98, n=10 (Fig. 1, line a). The strong speciꢁc interaction of 1 with benzene, toluene,
o-xylene and mesitylene disappears upon transition to the adduct 6.
The changes in the enthalpies of solvation of TS are calculated (Eqs. 12 and 13) for
the forward (δΔH
) and reverse (δΔH
) DARs (Table 2):
solv TS (1)
solv TS (−1)
≠
≠
ꢀ
ΔHsolv TS (1) = ΔH (S) − ΔH
+ ꢀΔHsolv (ꢀ+ꢁ)
(12)
(13)
(1)
(1)acetonitrile
≠
≠
ꢀ
ΔHsolv TS (−1) = ΔH (S) − ΔH
+ ꢀΔHsolv (ꢀ)
(−1)
(−1)acetonitrile
In all solvents the changes in enthalpy of solvation of TS (δΔH
) for direct 1+3→6
solv TS
and reverse 6→1+3 reactions coincide within the total error of its determination
−
1
(
± 6 kJ·mol ). Attention should be paid at the increased stabilization of the TS in H-donor
solvents, 1,1,2,2-tetrachloroethane and trichloromethane (Table 2). The enthalpy of the TS
is insensitive to the π-donor properties of the solvents, in contrast to that of the reagents
(
entries 10–14, Table 2). Very close kinetic parameters for retro-DAR in the all studied
media correspond to a loss of the π-acceptor properties on the way from tetracyanoethylene
1) to the TS and adduct 6 of reaction 1+3→6.
(
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Journal of Solution Chemistry
Fig. 1 Comparison of the changes in solvation enthalpy of tetracyanoethylene and anthracene (δΔH
solv
)
relative to that of adduct (δΔH
), (ꢁlled blue diamond, δΔH
= 0.85·δΔH
+2.05;
solv (1+3)
(1+3)
solv
6
solv
6
r = 0.9643, entries 1–8), and (δΔH
) relative to that of 4-phenyl-1,2,4-triazoline-3,5-dione and
solv (1+3)
anthracene (δΔH
), (ꢁlled red triangle, δΔH
=0.83·δΔH
+0.82; R=0.9769, entries
solv (2+3)
solv (2+3)
solv (1+3)
1
–10). The solvent numerical designators are the same as in Table 2 (Color ꢁgure online)
Similar measurements were also performed for the DAR of anthracene (3) with 4-phe-
nyl-1,2,4-triazoline-3,5-dione (2) (Table 3).
The changes in the enthalpy of solvation of reagents 2 and 3 (entries 1–9, Table 3)
are proportional to those of reagents 1 and 3 (Fig. 1, line b, slope=0.88, r=0.98). The
moderate π-acceptor properties of 2 are clearly manifested through a weak change in its
enthalpy of dissolution and slight changes of the rate of reaction 2+3→9 in aromatic sol-
vents (entries 10–14, Table 3). The increased stabilization of the TS in reaction 2+3→9
is observed again in H-donor solvents (entries 7, 9, Table 3). In all solvents the reaction
−
1
2
+3→9 is more exothermic than reaction 1+3→6 by about 20 kJ·mol . The decay rate
of adduct 9 has not been studied, since the half-life of adduct 9 decomposition at 25 °C is
estimated to be more than 2 years. Data for the reaction of tetracyanoethylene with 1,2-ben-
zanthracene (1+4→7) are collected in Table 4.
It can be expected that the diꢀerence in enthalpy of reaction 2+3→9 and 1+3→6
−
1
(
about 20 kJ·mol , Table 3) is due to the diꢀerence in the balance of the bond energies
of dienophiles 1 and 2 in the DAR with any diene. An increase in the conjugation energy
of diene 4 compared with 3 [27] leads to the fact that the equilibrium constant of reac-
tion 1+4→7 (Table 4) is almost 3 orders of magnitude lower than that for the reaction
1
+3→6.
The reaction rate of tetracyanoethylene with dibenzanthracene 1+5→8 was not deter-
mined because of the very low stability of adduct 8 in solution. It is interesting to note
that the rates of reactions 2+4→10 and 1+4→7 in acetonitrile are very close. How-
ever, due to the same reasons the reaction 2+4→10 in π-donor toluene proceeds already
−3
−1 −1
3
0 times faster than 1+4→7. The rate of reaction 2+5→11 (5.30×10 L·mol ·s ,
Table 4) is signiꢁcantly less than those of reactions 2+4→10 (0.165) and 2+3→9 (1.55
−
1 −1
L·mol ·s ) in accordance with the change in conjugation energy of these dienes [27].
1
3

Journal of Solution Chemistry
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3

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1
3

Journal of Solution Chemistry
1
.1564
.15635
.1563
.15625
.1562
.15615
.1561
.15605
1
1
1
1
1
1
1
3
1
2
0.012
0
0.002
0.004
0.006
0.008
0.01
0.014
−1
3
−1
Fig. 2 The ratio between the speciꢁc volume of the solution (d /cm ·g ) and the concentration of the
−
1
−1
−3
adduct (C /mol·L ) in the reaction 2+4→10. 1 d =−(0.019443± 0.000155)·C +(1.158067± 0.0
1
0
10
3
−1
−1
0
0
00005); ∆V =−16.8 cm ·mol ; C =C =5.00×10 mol·L ; r=0.9995. Curve 1 is shifted down by
r–n
2
4
3
−1
−
−1
3
−1
.0017 cm ·g . 2 d =−(0.021016± 0.000069)·C +(1.156329± 0.0000006); ∆V =−18.2 cm ·mol
;
1
0
r–n
2
−1
−2
−1
−1
C =1.005×10 mol·L , C =1.002×10 mol·L ; r=0.9998. 3 d =−(0.021187± 0.000038)·C
+
2
4
r–n
10
3
−1
−2
−1
−2
−1
(
1.156349± 0.0000003); ∆V =18.3 cm ·mol ; C =1.005×10 mol·L , C =1.004×10 mol·L
;
2
4
r=0.9999
The pressure eꢀect on the rate of reaction 1+3→6 has been described previ-
ously [3, 16, 24]. The rate of reaction 2+4→10 was studied at ambient pressure
−
2
−2
−1 −1
[
k =(2.04± 0.02)×10 ] and at 1000 bar [k =(5.61± 0.07)×10 L·mol ·s ] in toluene
2 2
≠
3
−1
at 25 °C. The value of ∆V is −28.8± 0.8 cm ·mol (Eqs. 2 and 3) and the corrected
exp
≠
corr
3
−1
value (Eq. 4) of ΔV is −26.5± 1.0 cm ·mol . The value of the reaction volume (∆V
)
r–n
has been determined from three measurements (Fig. 2).
3
−1
The mean value of ∆V is −17.8± 0.6 cm ·mol . Note that the volume of the TS in
r–n
3
−1
the reaction 2+4→10 is 8.7 cm ·mol is more compact than the volume of adduct 10.
≠
The ‘anomalies’, when ΔV ∕V
> 1, for the isopolar DARs are due to the diꢂculties
corr
r−n
for the solvent molecules to approach the branched structures of the adducts [28].
4
Conclusions
The data obtained make it possible to explain the reasons for the sharp diꢀerence in the
reactivity of tetracyanoethylene (1) and 4-phenyl-1,2,4-triazoline-3,5-dione (2) in the
DARs with diꢀerent dienes in diꢀerent solvents. For the reaction of pronounced π-donor
diene such as anthracene (3) (IP =7.40 eV) with a strong π-acceptor 1 (E =2.88 eV),
D
A
the contribution of intermolecular stabilization energy is the determining parameter of the
increased reaction rate with 1 compared with 2 (Table 3). There is a clear inversion of the
reactivity for reagents 1 and 2 in the DARs with the less active dienes. The dienophile 2 is
a moderate π-acceptor (E ~1.0 eV), but disruption of N=N bonds is easier compared to
A
the C=C bond in dienophile 1. Tetracyanoethylene sharply loses its activity in aromatic
1
3

Journal of Solution Chemistry
solvents due to complex formation with a solvent. The data obtained on the solvation
enthalpy transfer of the reagents, products, and transition state clearly show that the accep-
tor properties of dienophile 1 disappear upon passage to TS and adduct. From obtained
data (Tables 1–4) we can propose that an increased solvent inꢃuence on the reaction rate
should be expected for all types of reactions involving tetracyanoethylene. The high rates
for reactions involving very active dienophiles 1 and, especially, 2 can be useful to catch
such carcinogenic impurities as 3–5 and neutralize them by transformation into less dan-
gerous adducts.
Acknowledgements We are grateful to Dr. Igor Sedov for carrying out some calorimetric measurements.
This work was supported by the Russian Foundation for Basic Research (Projects Nos. 16-03-00071 and
1
8-33-00063), the Ministry of Education and Science of Russian Federation (Project No. 4.6223.2017/9.10),
and the research grant of Kazan Federal University.
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