Diels-Alder reaction rate in the solid state and the evidence of the location of molecular complexes between the reagents on the reaction pathway

Article abstract of DOI:10.1002/kin.21434

The rate of reactions in the solid phase with uniform grinding of crystals of dienes, anthracene, and 9,10-dimethylanthracene, with dienophiles, tetracyanoethylene, N-phenylmaleimide, and 4-phenyl-1,2,4-triazoline-3,5-dione, has been studied. It was shown

Full text of DOI:10.1002/kin.21434

Received: 1 June 2020
DOI: 10.1002/kin.21434
Revised: 13 August 2020
Accepted: 10 September 2020
A RT I C L E
Diels-Alder reaction rate in the solid state and the evidence
of the location of molecular complexes between the
reagents on the reaction pathway
Vladimir D. Kiselev
Alexey A. Shulyatiev
Anastasia O. Kolesnikova
Alexander E. Klimovitskii
Ildar F. Dinikaev
Dmitry A. Kornilov
The A. M. Butlerov Chemical Institute,
Kazan Federal University, Kazan,
Russian Federation
Abstract
The rate of reactions in the solid phase with uniform grinding of crystals
of dienes, anthracene, and 9,10-dimethylanthracene, with dienophiles, tetra-
cyanoethylene, N-phenylmaleimide, and 4-phenyl-1,2,4-triazoline-3,5-dione, has
been studied. It was shown that, despite the high difference in the reac-
tion rates in solution, the rates of these reactions in the solid phase are
much closer. For anthracene-tetracyanoethylene and 9,10-dimethylanthracene-
tetracyanoethylene pairs, it was concluded that their intermolecular complexes
are on the reaction pathway.
Correspondence
Vladimir D. Kiselev, The A. M. Butlerov
ChemicalInstitute, Kazan FederalUniver-
sity, Kremlevskaya str., 18, 420008, Kazan,
RussianFederation.
Email:vkiselev.ksu@gmail.com
Fundinginformation
Ministryof Education andScience of
theRussian Federation, Grant/Award
Number:4.6223.2017/9.10
K E Y WO R D S
Diels-Alder reaction, kinetics, molecular complex, solid state
1
INTRODUCTION
reagents is grinded. Reactions between the solid reactants
also proceed successfully when a small amount (5-10% by
weight) of a suitable solvent is added.2─7 However, in these
examples the rapid and successful reaction is often due to
the high concentration of both reagents in the liquid phase.
The study of the reaction rate is of particular inter-
est when both reagents and products are in the solid
phase. If we fix the conditions for the reactions (the grind-
ing rate of the reaction mixture, the shape and mate-
rial of the dishes, temperature), we can notice what the
additional factors can affect the rate of such reactions.
In contrast with reactions in solution, it is necessary in
the solid phase to tear out the molecules from the sur-
face of the crystals with grinding, and this procedure is
determined by the cohesive energy of the reagent crys-
tals. The usual methods of monitoring the progress of the
reaction adopted in solution are relevant here only for
slow processes.2,4,8 In this case, the course of the reac-
tion in solution during the analysis can be neglected. Mon-
itoring of the rate of Diels-Alder reactions in the solid
phase with the participation of active reagents such as
The reactions in the solid phase proceeding with a high
yield should be attributed to the most successful examples
of green chemistry. This opens up the possibility of quanti-
tative conversion without using a solvent, catalysts or high
pressure. It can be argued that among many organic reac-
tions in solution, the kinetics of the Diels-Alder reactions
is most thoroughly studied, particularly due to an absence
of by-products.1 It is clear that a simple mixing of solid
reagents does not lead to success, since the reaction rate
in the solid phase sharply depends on the diffusion of the
reagent molecules.2 However, a manual mixing of solid
reagents with a pestle in a mortar or mechanization of this
process in ball mills leads to the destruction of crystals, a
sharp increase in the contact surface, renewal of contacts
during mixing, and promotion of interaction between the
reagents with increased pressure in the space between the
pestle (ball) and the mortar.2─5 There are several exam-
ples of successful implementations of the Diels-Alder reac-
tion without solvent, when a mixture of solid and liquid
Int J Chem Kinet. 2020;1–6.
wileyonlinelibrary.com/journal/kin
© 2020 Wiley Periodicals LLC
1

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KISELEV et al.
S C H E M E
1
Reagents and products of the Diels-Alder reaction (1-8) as well as compounds (9, 10, 13), which form molecular
π,π-complexes (11, 12, 14, 15), but do not form the reaction products
9,10-dimethylanthracene, 2,5-diphenylisobenzofuran, 4-
phenyl-1,2,4-triazoline-3,5-dione, tetracyanoethylene, and
some others is very difficult. During the analysis of samples
of such mixtures in solution, the reactions usually proceed
completely. Therefore, the rate of such solid-phase reac-
tions can be monitored during analysis either in solution
at very low concentrations or in the solid phase (IR; solid
phase NMR; XRD).
In this report, we studied the rates of a number of reac-
tions in the solid phase at room temperature (∼ 25◦C), as
well as the molecular complexes (MC) formation between
the solid reagents (Scheme 1).
(2) and 4-phenyl-1,2,4-triazoline-3,5-dione (7) were
sublimated in vacuum (100-110◦C, 10 Pa), and N-
phenylmaleimide (4) was purified by crystallization
from a hexane-benzene (5:1) mixture. Anthracene (1a)
and 9,10-dimethylanthracene (1b) were used without
additional purification. All solvents were purified by
standard methods.10
The rates of the following reactions were studied:
(I) anthracene (1a), mp 216◦C, and tetracyanoethylene
(2), mp 200◦C, with the formation of adduct 3, mp
260◦C (decomp.)11; (II) 9,10-dimethylanthracene (1b), mp
184◦C, and N-phenylmaleimide (4), mp 90◦C, with the
formation of adduct 5, mp 273◦C [Ref. 11]; (III) 9,10-
dimethylanthracene (1b) and tetracyanoethylene (2) with
the formation of adduct (6), mp. 249–250◦C, decomp.12;
(IV) anthracene (1a) and 4-phenyl-1,2,4-triazoline-3,5-
dione (7), mp. 165◦C (decomp.) with the formation of
adduct (8), mp 220-222◦C.13
2
EXPERIMENTAL
2.1
Reagents and products
All reagents were purchased (Sigma-Aldrich, Germany)
and had known9 melting points. Tetracyanoethylene
Hexamethylbenzene (9), mp 165-165.5◦C,14 tetrachloro-
1,4-benzoquinone (10) (chloranil), mp 288–290◦C,15 and

KISELEV et al.
3
TA B L E 1 The content (%) of dienes in reactions 1a + 2 → 3 and 1b + 4 → 5 during grinding of an equimolar mixture of solid reagents in
an agate mortar at 25oС
Reaction 1a + 2 → 3
Reaction 1b + 4 → 5
Run 1
Run 1
Run 2
Run 2
Time (min)
1а
100
64.5
50.7
14.1
12.4
4.4
Time (min)
1а
100
75.8
27.3
26.2
15.4
9.5
Time (min)
1b
100
99.2
93.4
42.4
20.7
13.1
Time (min)
1b
100
100
0
0
1
8
10
17
27
0
20
33
42
50
60
0
15
30
50
70
–
5
14
21
29
40
94.5
6.2
4.7
–
naphthalene (13), mp 79–80◦C,9 were used as purchased.
The Diels-Alder reactions between tetracyanoethylene (2)
or chloranil (10) with hexamethylbenzene (9) or with
naphthalene (13) do not proceed due to the high conjuga-
tion energy in these reagents.1 The formation of brightly
colored stable molecular complexes (11, 12, 14, 15) in solu-
tion occurs instantly.16
The Diels-Alder reactions were carried out by grind-
ing equimolar amounts (0.005 mol) of solid reagents in
an agate mortar with an agate pestle equipped with a
“scraper” made of curved thin copper sheet with a shape
similar to that of a mortar. This allowed the powder to be
removed from the walls to the bottom of the mortar dur-
ing the mixing. Grinding was carried out with the constant
uniform force at a speed of 2 revolutions per second. The
slow reaction 1b + 4 → 5 was additionally carried out in
a ball mill (“Vibrator-GM9458,” Germany) with two stain-
less steel balls with a diameter of 10 mm and an oscillation
frequency of 25 Hz at room temperature.
TA B L E 2 The content of adduct 6 in the reaction 1b + 2 → 6 in
the selected samples during the grinding of equimolar mixture of
reagents. The contents were calculated from the change in the
intensity of bands 1b (1619 cm−1) and adduct 6 (784 cm−1) (A);
bands of 1b (1365 cm−1) and adduct 6 (784 cm−1) (B); bands of 2
(2227 cm−1) and adduct 6 (784 cm−1) (C)
Grinding time (min)
0
5
11
16
(А)
0
53
78
88
(В)
0
50
80
93
(С)
0
53
76
92
3
RESULTS AND DISCUSSION
The observed reaction rate of 1a + 2 → 3 and 1b + 4 → 5
with continuous uniform grinding of the equimolar mix-
ture of solid reagents was determined by monitoring the
content of diene 1a and diene 1b in the selected samples
(Table 1).
Since the reaction 1b + 2 → 6 in the solution proceeds
instantly, its rate in the solid phase was determined from
the change in the intensity of the absorption bands in the
selected samples (Table 2).
All the rate constants of the studied reactions in 1,2-
dichloroethane were known1: this value for the reaction 1a
+ 2 → 3 is 3.0, for the reaction 1b + 4 → 5 is 0.03, for the
reaction 1a + 7 → 8 is 0.13, and for the reaction 1b + 2
→ 6 is 89 000 L⋅mol−1⋅s−1.1 Due to the enormous absorp-
tion coefficient (ε) of diene 1a (ε 6456 at 341 nm; 8696 at
378) and diene 1b (ε 6305 at 359.5 nm; 9176 cm−1⋅M−1 at
400.5 nm, both in 1,2-dichloroethane), the UV monitoring
of dienes 1a and 1b absorption in solution is possible at
the concentration range 10−5-10−4 M. During the prepa-
ration of the solution and the time of analysis (3-5 min,
“Hitachi U2900,” Japan), the error due to the additional
reaction 1a + 2 → 3 and 1a + 7 → 8 does not exceed 5%,
for reaction 1b + 4 → 5 is less than 1%, and reaction 1b
+ 2 → 6 proceeds completely. Therefore, the reaction rate
1b + 2 → 6 was monitored by IR spectroscopy (Vertex 70
FTIR spectrometer, Bruker, Germany) by the changing of
the band intensities of reagents and adduct. Unfortunately,
the intense absorption of C≡N groups of tetracyanoethy-
lene (2227 cm−1), almost completely disappears in adducts
3 and 6.
It should be noted that the ratio of the rate constants and
ratio of half-life time for the reactions 1b + 2 → 6 and 1a +
2 → 3 in 1,2-dichloroethane is 3 × 104.1 These solid-phase
reactions were carried out both under the same conditions.
However, the half-life times in the solid phase differ only by
about two times. Apparently, in the solid phase the rate of
these reactions is substantially influenced by the diffusion
kinetics, so the difference in chemical reactivity is much
less pronounced. The slow reaction 1b + 4 → 5 was addi-
tionally carried out in the ball mill (“Vibrator-GM9458,”
Germany) at a frequency of 25 Hz at room temperature
(Figure 2).
As can be seen from a comparison of Figures 1 and 2, the
reaction 1b + 4 → 5 proceeds in a vibration mill about four
times faster compared to the grinding in an agate mortar. In

4
KISELEV et al.
F I G U R E 1 Change in the content of
diene 1a in the reaction 1a + 2 → 3 (points
for the run 1 and points for the run 2) and
diene 1b in reaction 1b + 4 → 5 (points for
the run 1 and points for the run 2) during
the grinding of equimolar mixtures [Color
figure can be viewed at
wileyonlinelibrary.com]
F I G U R E 2 Change in the content of
9,10-dimethylanthracene (points for the
run 1 and points for the run 2) during the
grinding of equimolar mixtures 1b and 4 in a
vibrational mill [Color figure can be viewed
at wileyonlinelibrary.com]
both cases, an induction period is observed, whose nature
is unknown.
crystals of tetracyanoethylene (2, electron affinity, EA,
2.88 eV) quickly leads to the formation of stable brightly
colored red (15) or green (12) solid complexes, respec-
tively. The maxima of the absorption bands of these solid
complexes, determined from the reflection spectra (Jasco
650 spectrophotometer, Japan), are closely consistent with
the data available in tetrachloromethane.16 The transition
of molecular complexes 12 and 15 into reaction adducts is
impossible due to the high conjugation energy and
the complete equilibrium shift towards reagents.1
Similar complexes (14, 11) were observed during the
The reaction 1а + 7 → 8 between the colorless crys-
tals of anthracene (1a) and the red one of 4-phenyl-1,2,4-
triazoline-3,5-dione (7) was carried out in a vibration mill
(Figure 3).
The grinding of two solid reagents can lead to the
appearance of brightly colored molecular complexes in
the solid phase. We observed that the grinding of colorless
crystals of naphthalene (13, ionization potential, IP, 8.15)
or hexamethylbenzene (9, IP 7.85 eV) with the colorless

KISELEV et al.
5
F I G U R E 3 Change in the proportion of the red 4-phenyl-1,2,4-triazoline-3,5-dione (7) during the reaction 1а + 7 → 8 in a ball mill after
4, 9, and 14 min [Color figure can be viewed at wileyonlinelibrary.com]
F I G U R E 4 Adverse (A) and favorable
(B) geometry of the molecular complex for
the reactions between 1a or 1b and 2 [Color
figure can be viewed at
wileyonlinelibrary.com]
A
B
grinding of these π-donors with π-acceptor, tetrachloro-
1,4-benzoquinone (10). Similar colored complexes are also
rapidly formed between dienes 1a or 1b with chloranil (10).
It turned out that color is absent during the whole time
of grinding the solid reagents with pronounced π,π-donor-
acceptor properties,1,16 such as anthracene (1a), IP 7.33 eV,
with tetracyanoethylene (2), EA 2.88 eV, and even 9,10-
dimethylanthracene (1b), IP 7.04 eV, with 2.
If the stable orientation of molecular complexes between
the molecules of anthracenes 1a, 1b and tetracyanoethy-
lene 2 did not coincide with the geometry of the transition
state of these reactions (Figure 4A), then one would expect
the accumulation of a fraction of these colored complexes
in the solid phase during the grinding. With a close cor-
respondence between the structures of the molecular and
activated complex (Figure 4B), one should expect a small
activation energy upon the transition of molecular com-
plexes to adduct.
Therefore, the absence of colored molecular complexes
1a-2 or 1b-2 during the grinding of the reactants can be
explained by the fast conversion of molecular complexes
(b, Figure 4) into adducts 3 and 6. The passing of reaction
1b + 2 → 6 in solution through the intermediate formation
of a molecular complex between the reactants has been
previously proved.18
4
CONCLUSION
The reaction rate constant 1a +2 → 3 in 1,2-dichloroethane
solution is 125 times greater than in reaction 1b + 4 →
5.1 As can be seen from the data obtained, the rates of
these reactions in the solid phase differ only by about four
times. For the reactions 1b + 2 → 6 and 1a + 2 → 3 in
1,2-dichloroethane solution, the ratio of reaction rates is
enormous, 3 × 104 times,1 but in the solid phase the dif-
ference is less than two times. Since the reaction rate in
the solid phase is largely determined by the energy of the
release of reagent molecules from the crystals (ΔНsubl), it
can be assumed that the order of the activity of the reagents
for reactions in the solid phase may not coincide and even
invert compared with reactions in solution.
The absence of coloration of molecular complexes in the
solid phase for reactions 1b + 2 → 6 and 1a + 2 → 3 can
be explained by their rapid transition to the adducts, due to
the close geometry of the molecular complex and activated
complex.
AC K N OW L E D G E M E N T
This work was supported by the Ministry of Educa-
tion and Science of the Russian Federation (Project No
4.6223.2017/9.10).

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KISELEV et al.
C O N F L I C T O F I N T E R E S T
The authors hereby confirm they have no conflict of
interest.
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AO, Dinikaev IF, Shulyatiev AA, Klimovitskii AE,
Kornilov DA. Diels-Alder reaction rate in the solid
state and the evidence of the location of molecular
complexes between the reagents on the reaction
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https://doi.org/10.1002/kin.21434