Study of the diels-alder and retro-diels-alder reaction between furan derivatives and maleimide for the creation of new materials

Article abstract of DOI:10.1039/c5ra01185j

The Diels-Alder reaction leads to a mixture of two diastereomers, one called endo and the other one exo. The cyclo-reversion temperature of the first one is lower than the exo adduct and the ratio between endo and exo adducts varies according to the substituents of the Diels-Alder partners and experimental parameters. Therefore, the influence of some reaction parameters such as the substituents of furan and maleimide derivatives, the reaction temperature and the presence of a nucleophile on the endo/exo Diels-Alder ratio and/or the retro-Diels-Alder reaction have been studied. For instance, furan and maleimide derivatives with electron withdrawing substituents induced the creation of the endo adduct preferentially. Also the presence of a far electron withdrawing substituent on furan and/or an electron attracting mesomeric substituent on maleimide resulted in a faster reversibility of the endo adduct. Finally, a high temperature and the presence of a nucleophile (thiol) also induced faster retro-Diels-Alder kinetics. Moreover, it was proved that isomerization from the endo to the exo diastereomer is preceded by a retro-Diels-Alder reaction of the endo adduct. The presence of a nucleophile in the mixture confirmed this result. This study allowed the highlighting of different parameters of the Diels-Alder reaction to obtain as much endo adduct as possible, and a fast and/or full retro-Diels-Alder reaction of this adduct.

Full text of DOI:10.1039/c5ra01185j

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Page 1 of 25
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DOI: 10.1039/C5RA01185J
IELS-ALDER AND RETRO-DIELS-ALDER REACTION BETWEEN FURAN
STUDY OF THE
D
DERIVATIVES AND MALEIMIDE FOR THE CREATION OF NEW MATERIALS
a
a
c
a
b
a
V. Froidevaux , M. Borne , E. Laborbe , R. Auvergne , A. Gandini , B. Boutevin
a
UMR 5253, Institut Charles Gerhardt, équipe IAM, 8 rue de l’école Normale, 34296 Montpellier, France
b
INP, Institut National Polytechnique, 46 avenue Félix Viallet, 38031 Grenoble, France
Hutchinson SA centre de recherche, Rue Gustave Nourry, 45120 ChâletteꢀSurꢀLoing, France
c
Corresponding author: bernard.boutevin@enscm.fr
Abstract
The DielsꢀAlder reaction leads to a mixture of two diastereomers, one called endo and the other one exo. The
cycloꢀreversion temperature of the first one is lower than the exo adduct and the ratio between endo and exo
adduct varies according to substituents of DielsꢀAlder partners and experimental parameters. Therefore, the
influence of some reaction parameters such as the substituents of furan and maleimide derivatives, the reaction
temperature and the presence of a nucleophile on the endo/exo DielsꢀAlder ratio and/or the retroDielsꢀAlder
reaction have been studied. For instance, furan and maleimide derivatives with electro withdrawing substituents
induced the creation of the endo adduct preferentially. Also the presence of far electron withdrawing substituent
on furan and/or an electron attracting mesomeric on maleimide implied a faster reversibility of the endo adduct.
Finally, a high temperature and the presence of a nucleophile (thiol) also induced a faster retroDielsꢀalder
kinetic. Moreover, it was proved that isomerization from endo to exo diastereomer is preceded by a retroDielsꢀ
Alder of the endo adduct. The presence of a nucleophile in the mixture confirmed this result.
This study allowed highlighting different parameters of DielsꢀAlder reaction to obtain more endo adduct as
possible, a fast and/or full retroDielsꢀAlder of this one.
Keywords
retroDielsꢀAlder kinetic, Michael addition, Fast unprotection, Maleimide derivatives, Furan
derivatives
Highlight
-The more the substituent of furan and maleimide derivatives are electron withdrawing, the
more the endo adduct is obtained;
-The NMR analysis is the best method to determine a retroDielsꢀAlder temperature;
-When the substituent of the furan derivative is far electron withdrawing the retroDielsꢀAlder
of endo adduct is fast;
When the substituent of the maleimide derivative is electron attracting mesomeric the
retroDielsꢀAlder of endo adduct is fast;
Prior to endo to exo diasteromer isomerization, the retroDielsꢀAlder reaction of the endo
adduct occurred;
The presence of a nucleophile proves the isomerization hypothesis and allows a faster
-
-
-
unprotection of DielsꢀAlder adducts.
1

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1
Introduction
Selfꢀhealing » materials and materials which present a postꢀcrosslinking on demand
1
ꢀ3
«
represent a huge interest for scientists. In both cases, stimuli reversible reactions and
controlled reactions (or bonds) are used. Temperature, radiation, O , UV, Electron beam, Xꢀ
2
ray, water, pH etc. are commonly used as external stimuli. For this study, the temperature,
4
which is the easier way for industrialization, has been chosen. Systems as urethane group ,
5
6
7ꢀ10
11, 12
nitroso group , ester group , ionene complex , azlactoneꢀphenol adduct
and, lastly, the
allow the formation of reversible covalent bonds under high
temperature conditions. DielsꢀAlder system was chosen for this project, in spite of existing
13ꢀ18
DielsꢀAlder adduct
14, 19ꢀ21
studies
already proved its significance for the formation of reversible covalent bonds formation
or for the synthesis of new polymers/materials having intermittent life. Among them, thermoꢀ
, because this is the most famous and the easier system. The DielsꢀAlder reaction
2
2, 23
2
4
25
sensitive surfactants , selfꢀhealing materials , high and mild temperature reversible
26
27
materials and materials having a controlled crosslinking reaction can be found. This study
on DielsꢀAlder reaction will help to improve the control of crosslinking reaction during
materials formation.
Indeed, the DielsꢀAlder cycloaddition reaction would benefit from being upgraded on
blocking and unblocking temperature, particularly by studying the influence of reactant
substituents on the DielsꢀAlder and retroDielsꢀalder reaction, to upgrade adduct
stereochemistry and the kinetic of the reaction, respectively. The selectivity of the DielsꢀAlder
28
reaction between maleimide and maleic anhydride has already been studied. Also, some
study has been done on the influence of both furan and maleimides derivatives. However,
those results were used to study the endergonic and exergonic behavior of the DielsꢀAlder
2
9
reaction . Moreover, those results were obtained by computerized simulation, and only few
examples have been experimentally studied. It is also important to notice that the reaction
between furan and maleimide are often concerted cycloaddition, but due to nucleophilic
character of furan derivatives, the reaction with pꢀelectron deficient components, as
maleimide, may proceed via asynchronous transition state and in extremely case, a stepwise
3
0, 31
zwitterionic mechanism is also possible
.
It is interesting to go further on structure of products used for the Dielsꢀalder reaction and on
the characterisation of the cycloꢀreversion reaction. The main goal of this publication is to
establish the precise rules of furan and maleimides derivatives substituents on DielsꢀAlder and
retroDielsꢀAlder reactions.
2

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2
Results and discussion
It is well known that the DielsꢀAlder (DA) reaction leads to a mixture of two diastereomers
compounds, one call endo (the kinetic compound) and the other exo (the thermodynamic
13, 32
compound)
(Scheme 1). For simplicity the adduct formed after the DA is called as
followed: A for Adduct, then endo or exo depending on the adduct obtained (if there is
nothing after A, it means that it is a mixture), then F and M for the Furan and the Maleimide
derivatives, respectively.
Scheme 1: Diastereomers obtained after DielsꢀAlder reaction between furan (F) and
maleimide (M)
During the reverse reaction, called retroDielsꢀAlder (rDA), the endo compound is first
33
unblocked , i.e. at lower temperature than the exo compound. As a consequence, the
parameters affecting the diastereoselectivity (structure, experimental conditions, solvent,
temperatures, etc…) have to be controlled to upgrade the obtaining of endo adduct, giving
thus the possibility to break it at average temperature and give the maleimide double bond’s
reactivity back. Some studies deal with the DielsꢀAlder between furan and maleimide
derivatives but there is a lack of precision about the control of all these parameters. First, a
complete modeling study has been carried out to get all the information to control them. For
1, 13, 14, 34
that, the maleimide/furan derivatives system
is studied thanks to existing compound
(commercial, c) or synthetized compound (s) (Table 1).
All products were chosen to have the widest diversity of partners for the DA reaction, with
various characteristics (electronic and hindering effect). It is established that the DA reaction
is a spontaneous reaction between a diene, i.e. furan derivatives, and a dienophile, i.e.
maleimide derivatives. Furan derivatives are described, in the literature, as very good dienes
for the DA reaction. Indeed, the oxygen atom enables the relocation of diene double bonds
and increases the reactivity of these doublets. For maleimide derivatives, electrons attracting
mesomeric substituents (imides) on both sides of the double bond allow increasing the
reactivity regarding dienes. In this way, furan derivatives and maleimides react quickly
together at room temperature (20°C). They are also known to give low retroDielsꢀAlder
3
5
temperature (~110°C) .
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Table 1: Structures of different commercial (c) or synthesized (s) furan (F) and maleimide
M) derivatives
(
Blocking
agent
R2
R3
Mass yield (%)
R1
Maleimide
Mass yield (%)
H
1
F (c)
/
1
M (c)
/
H
H
F2 (s)
F3 (s)
95
87
M (s)
57
56
2
3
M (s)
H
F4 (s)
F5 (s)
6
99
M (s)
62
64
4
H
H
86
M (s)
5
F (c)
/
6
M (s)
74
72
H
H
7
F (c)
/
7
M (s)
F8 (s)
95
88
9
F (s)
It is worth to note that the presence of substituents on those partners can facilitate the reaction
(attracting inductive substituent on the diene and electron attracting mesomeric substituent on
3
6ꢀ41
dienophile
) or destabilize the final adduct (hindering substituent). Indeed, it’s known that
the addition of substituent on both partners can have a real influence on the ratio between the
endo and the exo compounds, but also on the stability of the adduct obtained after Dielsꢀ
1 2 3
Alder. That is why some R substituents on maleimide, R /R on furan derivatives are added.
These substituents could be electro donating, electro withdrawing, electron attracting and
donating mesomeric or hindering substituent. Thus, DielsꢀAlder compounds can be formed
with some pairs of F and M. The results are separated in two groups, the first one with
various furan derivatives (Table 2) and the second one with various of maleimide (Table 3).
4

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Table 2: Results on endo/exo ratio adduct of DielsꢀAlder reaction between Nꢀ
methylmaleimide (M ) or Nꢀdodecylmaleimide (M ) and furan derivatives (F)
Adduct Solvent endo (%)* exo (%)*
1
2
F
M
R (%)
94
T (°C)
23
AF
AF
AF
AF
AF
AF
AF
AF
AF
AF
AF
AF
AF
AF
1
2
3
4
5
6
7
9
1
2
3
4
5
6
M
1
M
1
M
1
M
1
M
1
M
1
M
1
M
1
M
2
M
2
M
2
M
2
M
2
M
2
F
1
2
3
4
5
6
7
9
1
2
3
4
5
6
M
1
1
1
1
1
1
1
1
2
2
2
2
2
2
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
THF
77
71
73
70
71
61
87
73
64
62
62
63
64
60
23
29
27
30
28
39
13
27
36
38
38
37
36
40
F
F
F
F
F
F
F
F
F
F
F
F
F
M
M
M
M
M
M
M
M
M
M
M
M
M
92
23
96
23
90
23
95
23
95
23
98
23
99
23
85
23
83
23
THF
87
23
THF
92
23
THF
88
23
THF
93
23
THF
1
*
Determined by H NMR
1
After an H NMR analysis of all DielsꢀAlder adducts, for example the AF
1
M
1
adduct (Figure
2
), the ratio endo/exo is determined for each one. To calculate these ratios, the peaks
corresponding to endo or exo adduct are used. To determine them, the system is analyzed with
the Karplus equation (Equation 1) on vicinal protons H and H . The Karplus rule links the
angle between the germinal protons and their constant coupling . Thus, the theoretical
c
d
4
2
coupling constant of protons H
c
should be higher for the endo diastereomer. The comparison
3
3
between theoretical ( JHcHd = 5.5Hz) and experimental ( JHcHd = 5.8Hz) constant coupling
value for Aendo
F
1
M
1
enabled attributing peaks for the endo and exo H
c
proton (Figure 1,
Figure 2). The angle between the two germinal protons of AexoF M
1 1
adduct (83.8°), was to
close from the Karplus limit to afford accurate results. Nevertheless, the value obtained, from
the Karplus equation, for the exo adduct indicated that the coupling constant should be lower
compare to the endo adduct and near to a 0 Hz coupling constant. Thus, the experimental
value of 1,6Hz confirmed the theoretical one (Figure 1, Figure 2). Therefore, the integration
of c and c1 peaks, allowed attributing all other peaks by integration comparison (called x for
the endo adduct and x1 for the exo adduct). Moreover, the endo/exo ratio is determined thanks
to c and c1 integrations, which give the percentage of exo or endo adduct.
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3
J = 8.5 cos2 Θꢀ 0.28 for 0° < Θ < 90°
3
and J = 9.5 cos2 Θ ꢀ 0.28 for 90° < Θ < 180°
Equation 1: Karplus equation
Figure 1: Molecular representation of Aendo
1 1 c
F M , determination of the angle between H
3
and H protons and experimental and theoretical coupling constant determination JHcHd
d
Table 3: Results on endo/exo ratio adducts (AF
furfuryl acetate (F ) and maleimide derivatives (M)
R (%) T (°C) Solvent endo (%)* exo (%)*
1 x
M ) of DielsꢀAlder reaction between
1
Adduct
F
M
AF M1
96
94
92
90
89
91
93
88
23
23
23
23
23
23
23
23
DCM
Ether
THF
77
100
64
65
55
67
76
75
23
0
1
F
1
M
1
A
endo
F
1
M
1
AF M2
F1 M2
36
35
45
33
24
25
1
AF
1
M
3
F
1
M
3
DCM
DCM
DCM
DCM
DCM
AF M4
F1 M4
1
AF
1
M
5
F
1
M
5
AF M6
F1 M6
1
AF
1
M
7
F
1
M
7
1
*
Determined by H NMR
The DielsꢀAlder reaction occurs efficiently between all furan and maleimide derivatives.
Moreover, the ratio is always higher for the endo adduct which is the kinetic product, but
when the temperature is increased the ratio of the exo adduct is increased at the same time.
The ratio could be increased for the endo adduct if the substituent is electron withdrawing for
the furan and/or the maleimide derivative. The increase of the ratio in favor of one of the
diastereomer compare to the other is well known but the interest of this work is principally
axed on the rDA of each adduct and the effect of some parameters on it.
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1
Figure 2: H NMR spectrum of the AF M adduct
1
1
2.1 Characterization of the retroDiels-Alder reaction
33
According to literature, the endo compound is unblocked at a lower temperature than the
4
3, 44
1
45, 46
33, 47
exo compound. IR
, H NMR
and DSC
analysis are generally used for the study of
1 1
maleimide unblocking (retroDielsꢀAlder) reaction. The DSC was performed on the AF M
adduct and the spectrum (Figure 3) and showed an endothermic peak starting at 80°C with a
decrease until 110°C (region I, rDA endo) corresponding to the endo’s unblocking reaction.
Then an other endothermic peak begins at 110°C until a maximum point at 130°C (region II,
33
rDA exo) corresponding to the exo’s unblocking reaction .
1 1
Figure 3: DSC Analysis of Nꢀmethylmaleimide (M )/ furfuryl acetate (F ) DielsꢀAlder
adduct
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1 1
The DSC analysis was also performed on the adduct AendoF M and the analysis showed one
endothermic peak with a maximum point at 120°C corresponding to the unblocking reaction
of the compound Aendo . Then an isothermal analysis, of the same compound, at 70°C
1 1
F M
during 4 hours in DSC was performed and no endothermic peak was observed, proving that
no unblocking reaction occurred.
O
O
O
O
f
f1
O
O
O
g
e
e1
d1
O
O
a3
a
DA
a2
N
O
N
h
O
N
h
O
f3
rDA
b
d
c
c1
O
O
O
F1
M1
AendoF1M1
AexoF1
M1
1
Figure 4: H NMR Analysis of the capsule content after an isothermal DSC of the
A
F
endo 1
M
1
adduct during 4 hours in DMSO at 70°C
1
After this isothermal experiment in DSC, a H NMR of the capsule content was done and the
characteristic peaks of furan F
1
and maleimide M
1
appeared on the spectra, implying that a
retroDielsꢀAlder of Aendo
F
1
M
1
occurred (Figure 4). Moreover, the peaks of adduct AexoF M
1 1
appeared at 5.1 (c1), 4.35 (f1) and 3 ppm (e1 and c1), confirming an isomerization of the endo
1
adduct. Thus, the H NMR showed that the starting material (Aendo
F
1
M
1
) has been unblocked
and the free maleimide M
1
undergoes through a DA reaction with F
1 1 1
to give AexoF M
adduct.
8

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In the literature, the DielsꢀAlder temperature is generally determined by DSC analysis and the
4
7
retroDielsꢀAlder temperature corresponds to the endothermic peak’s maximum. However,
the retroDielsꢀAlder of the endo adduct is actually lower. Thanks to the last experiment, DSC
analysis is not an appropriate analytical tool to precisely detect DielsꢀAlder or retroDielsꢀ
1
Alder reactions temperature. In this study the H NMR is used as analytical tool to obtain
precise results on endo and exo compounds, on DielsꢀAlder ratio and on retroDielsꢀAlder
temperatures.
A model study of the DielsꢀAlder and retroDielsꢀAlder reaction for all adduct of table 4. As
an example the reaction between of Nꢀdodecylmaleimide (M ) and cyclohexancarboxylate
2
1
furfuryl (F
5
) has been carried out in thermal H NMR (Figure 5).
O
O
O
O
O
f
O
f1
O
O
O
f2
e
e1
d1
a
DA
O
N
O
N
a1
N
b
1
1
O
11
1
1
d
s
rDA
c
s
c1
a2
F5
s
O
O
O
M2
A
Endo
F
5
M
2
Exo 5 2
A F M
1
Figure 5: Temperature H NMR spectrum overlaying of the DielsꢀAlder and retroDielsꢀ
Alder reaction of adducts AF
5
M
2
Looking at the endo (f) and exo (f1) adduct peaks, or furan (f2) and maleimide (a1) reagents,
the stoichiometry variation of endoꢀexo adducts and furanꢀmaleimide reagent can be
observed.
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2
The temperature was plotted as function of the peak intensity with the methyl (s) of M as
reference (Figure 6).
1
Figure 6: Proton peak intensity of adduct Aexo
F
5
M
2
♦
, AendoF
M
5 2
■
and F
5
▲of H NMR
analysis versus temperature
Figure 6 shows three distinct regions: the first one (I), from 40 to 60°C, where quantities of
AendoF5M and A F M increase and at the same time furfuryl acetate quantity decreases,
2
exo
5
2
corresponding to the DielsꢀAlder reaction; the second one (II), from 60 to 85°C, where the
adduct quantity decreases and Aexo adduct amount increases, meaning that the
retroDielsꢀAlder reaction of AendoF5M adduct occurred from T>60°C, and at the same time
endo
A F
5
M
2
5 2
F M
2
the creation (DA) of Aexo
until 75°C and then decreases until 85°C, which is due to the retroDielsꢀAlder of Aendo
and the DielsꢀAlder of Aexo adduct, respectively. This transformation of endo to exo
5 2 5
F M keeps going. In this region the quantity of F slowly increases
5 2
F M
5 2
F M
adduct is not a simple isomerization. These variations of each adduct ratio and the
modification of F quantity show that prior to transformation from endo to exo form a
5
retroDielsꢀAlder of the endo adduct occurs. In the third region (III), from 85 to 120°C, the
5 2 5 2 5
quantities of AendoF M and AexoF M decrease while the F quantity increases. This result
proves that the rDA of A F M starts up to 85°C and whilst the retroDielsꢀAlder of
exo
5
2
endo 5 2
A F M keeps going.
1
From H NMR, it is possible to determine, by 10 °C in our case, the rDA temperature. This
method was applied to determine rDA temperature of a few furan derivatives. Results from
Table 4 just allow showing the influence of R
the substituent is electro withdrawing (AF
facilitated; on the contrary if the substituent is an electron attracting mesomeric effect
AF ) the DA reaction doesn’t occurred as expected. The mesomeric form of the
2
substituent added on the furan derivatives. If
1
M
2
to AF , and AF ), DA and rDA are
7
M
2
9 2
M
(
8 2
M
8
compound F blocks the doublet of the furan cycle which can not participate to the DA
cyclization.
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All those results show and confirm that the endo compound as to be mostly obtained to reach
average unblocking temperatures.
Table 4: Variation of retroDielsꢀAlder temperature of exo and endo adduct as a function of
blocking agents used with Nꢀdodecylmaleimide (M
2
)
retroDiels-Alder temperature(°C)
Blocking Agent
endo
exo
AF
AF
AF
2
4
6
M
2
M
2
M
2
Between 55 and 65
AF M2
7
Between 85 and 95
AF
AF
AF
9
1
3
M
2
M
2
M
2
Between 65 and 75
AF M2
5
AF
8
M
2
No DielsꢀAlder before 120°C
Yet, these results do not allow determining unblocking kinetic which is an important aspect
for the creation of material. That is why a complete study of some parameters effect on the
retroDielsꢀAlder kinetic, i.e. temperature, presence of a nucleophile and the partners’
backbone, has been carried out.
2.2 Influence of reaction parameters on the retroDiels-Alder kinetic
The aim of this part is to show the link between the DielsꢀAlder partners’ backbone, the ratio
endo/exo and the unblocking kinetic.
It is first important to increase the ratio in favor of the endo compound so as to obtain a
system with an average unblocking temperature (70°C). According to starting diene and
dienophile used for the DielsꢀAlder reaction, the endo or exo adduct could be favored. Some
studies have been done about the influence of solvent and pressure on DielsꢀAlder conversion
4
8
and enantiomeric yield. However, these works showed that the increase of endo content is
partially improved and principally depends on the partner backbone.
The use of a catalyst could also improve the ratio in favor of the endo adduct. For instance,
4
9ꢀ51
some metallic catalysts have been described in the literature
and allow favoring this
selectivity. However, Gandini and Belgacem showed that the addition of a Brönsted acid or
a Lewis acid as catalyst in presence of furan derivatives with a CH in position of the furan
cycle, implies a polymerization of it. Moreover, the addition of a catalyst affects the endo/exo
52
α
2
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ratio, but not the unblocking kinetic. In a nutshell, the use of a catalyst in our system is
impossible.
It is well known that a decrease of the temperature during the protection (DA) increases the
formation of the endo adduct and on the contrary, an increase of it implies an increase of the
exo adduct formation. Thus, the DielsꢀAlder reaction has to be carried out at low temperature.
Nevertheless, it can be stated the lower the temperature, the longer the blocking time and the
34
longer the reaction time, the more the exo adduct is formed . Therefore, it is significant to
find the best compromise between temperature and blocking kinetic. In the project, the room
temperature (20°C) has been chosen because it gives an endo/exo ratio largely in favor of the
endo adduct and the protection kinetic (one week) is acceptable. Moreover, each
diene/dienophile couple shows its own rDA temperature. It is therefore important to control if
the unblocking temperature is not too high for the application. Indeed, the lower the
temperature the better the material will be. Afterwards, the influence of the temperature on the
unblocking reaction and the presence of nucleophilic reagent have been studied even if in all
chemical reactions, the higher the temperature, the faster the kinetic.
Thus, work in ease conditions (20°C, without catalyst, long time) to obtain major endo adduct
was chosen.
2.2.1 Kinetic study of retroDielsꢀAlder reaction at different temperature
The DielsꢀAlder reaction is a balance between a blocked and an unblocked form; it is thus
expected to reach a plateau of the ratio during the retroDielsꢀAlder reaction. This ratio will
depend upon the temperature.
1 1
To study the influence of the temperature, the compound AF M has been used and analyzed
1
by H NMR at three different temperatures (Figure 7).
Figure 7: Evolution of Aendo
F
1
M
1
adduct mole number during the retroDielsꢀAlder reaction
1
at different temperatures (60°C
▲, 70°C
♦
and 80°C
■
) obtained by H NMR.
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Figure 7 shows that the higher the temperature the faster the mole number of Aendo
decreases, which is coherent according to kinetic laws. Moreover, this analysis shows that,
after 290min, the Aendo adduct still remains and the unblocking reaction is, as predicted,
1 1
F M
1 1
F M
not complete. Indeed, even if the unblocking is the main reaction at high temperature, a small
amount of both endo and exo adduct are, nevertheless, formed (Figure 10).The same results
are observed for Aendo
The equilibrium constant K can be calculated at each temperature (Table 5) and plotting ln (n)
versus f (t) allows obtaining straight lines with a slope being ꢀ k (Figure 8), where = 1 in
1 2
F M adduct.
α
α
our case (equivalent number). The reaction has, thus, an order of one. The retroDielsꢀAlder
reaction rate constant could be determined for each couple enabling to compare them (Table
6).
Table 5: Equilibrium constants for Aendo
F
1
M
1
and Aendo
F
1
M
2
at each temperature
T (°C)
60°C
1,17.10
2,37.10
70°C
80°C
ꢀ
5
4
ꢀ4
ꢀ3
K
AendoF1M1
1,17.10
3,18.10
ꢀ
ꢀ3
ꢀ3
KAendoF1M2
3,56.10
3,58.10
1 1
Figure 8: Rate constant determination of AendoF M adduct at different temperatures
The unblocking reactions were carried out at different temperatures. It is therefore possible to
plot ln (k) versus f (1/T), from which the slope is equal to –Ea/R. Activation energy of the
ꢀ1
1 1
unblocking reaction of AendoF M obtained is 156,7 kj.mol .
1 1
Table 6: Summarize of all reaction rate constants obtained for AendoF M
T (°C)
60
70
80
3
ꢀ1
k*10 (min )
2,8
17,1
71,1
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Table 6 shows that the higher the temperature, the higher the rate constant. An increase of
0°C implies an approximately five times increase of the rate constant, which is significant.
With these rate values, activation parameters were determined from Eyring equation
Equation 2).
1
(
ꢁ
1
log ꢀ ꢃ =ꢄ−8162,3 ∗ + 19,382
ꢂ
ꢂ
Equation 2: Eyring equation obtain from the plot f(1/T) = log(k/T)
‡
The activation enthalpy (
ꢀ
H ) was estimated from the plot of logarithm of the quotient of rate
k and temperature versus the reciprocal of temperature (1/T), while the entropie of activation
‡
(ꢀ
S ) was determined from the intercepts with the y axe. The activation enthalpy found is
16.21 kcal/mol and the entropy activation is ꢀ8.6 cal/molK. This last value is far from typical
53
value for concerted [2+4] cycloaddition reactions. For instance, Jasiński et al. found that the
reaction between cyclopentadiene and Eꢀ2ꢀphenylnitroethene was ꢀ28.6 cal/molK activation
entropy and 17.3 kcal/mol activation enthalpy. They proved that it was a concerted
mechanism. In this case, the activation enthalpy and entropy values are close to the value
obtained for the cycloaddition of 1,1ꢀdimethoxyꢀ1,3ꢀbutadiene and tetracyanoethylene, which
‡
‡
54
is known to occur via zwitterionic mechanism (ꢀH = 10.8 kcal/mol, ꢀS = ꢀ6.2 cal/molK) .
For the retroDielsꢀAlder of AendoF1M , it seems that a switterionic mechanism actually
1
happened in the rateꢀdetermining step. Then, after unblocking reaction study, the elimination
of one reagent after rDA has been investigated so as to verify if the reaction between a
nucleophile and maleimide (Michael addition) is possible in situ from the rDA and if there is
an impact on it’s kinetic.
2.2.2 Influence of the presence of a nucleophile agent during rDA reaction
To do this study a thiolate is added in the mixture (a good nucleophile for the Michael
5
5
reaction), which could reacts with the maleimide after unblocking reaction and avoid a reꢀ
blocking of the maleimide’s double bond. The Michael reaction shows a faster kinetic than
5
6
the DA reaction kinetic, the maleimide should thus react with the thiolate preferentially
Figure 9). It shows that the mole number of Aendo decreases faster in presence of
(
1 1
F M
thiophenol. As a consequence, the retroDielsꢀAlder reaction kinetic is faster when one of the
partners is consumed at the same time as the unblocking reaction.
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1
Figure 9: Mole number of Aexo
F
1
M
1
as function of time, obtained by H NMR, during the
or without thiophenol
retroDielsꢀAlder at 70°C (left) and 80°C (right) with
♦
■
Moreover, during the unblocking reaction, the nucleophile enables consuming the unblocked
maleimide and thus changing the balance on the unblocking reaction. Indeed, the retroDielsꢀ
Alder reaction is complete whereas, without a nucleophile, there is the balance reaction
(Figure 10). This result confirms the assumption that the formation of exo adduct from the
endo adduct is necessarily preceded by a retroDielsꢀalder of it. Indeed, with the thiophenol, no
formation of exo adducts occurred, the maleimide’s double bound is released and reacts, in
situ, with the thiol avoiding it’s creation. Moreover it was proved that the reaction of thiolate
with the maleimide in situ of the unblocking reaction is possible.
All these results have allowed to set rules about the improvement of the unblocking reaction
kinetic: increasing the temperature and consuming one of the partners after the unblocking
reaction; and to confirm that a retroDielsꢀAlder occurs before the transformation from endo to
exo adduct.
1
Figure 10: Mole number of A F M as function of time, obtained by H NMR, during the
exo
1
1
retroDielsꢀAlder of AendoF M at different temperatures
1
1
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This work has been done at the same concentration and, as it is known, the kinetic depends on
the concentration of all starting materials. So the influence of the ratio and the concentration
on the reaction kinetic has been investigated.
2.2.3 Influence of the endo/exo ratio and of the starting material concentration
The influence of the ratio between endo and exo adduct on the unblocking reaction of
1
AendoF1M was studied by H NMR with a constant concentration of starting materials
1
endo
(A F
1
M
1
/AexoF
1
M
1
) and a varying ratio (Figure 11). The variation of the peak intensity of
f 1 1
H protons of AendoF M adduct at 70°C was plotted against time.
Figure 11: Effect of the endo/exo ratio (100%
♦, 77%
▲, 61%
■
) (left) and the
concentration (30mg/ml and 50mg/ml ) (right) on the rDA kinetic at 70°C
♦
■
To compare all three curves of the Figure 11, as before, the rate constants (k) are used. The
ꢀ1
results show that whatever the ratio, the rate constants (k) are all equal to 0,017 min . As a
conclusion, the ratio has no effect on the unblocking reaction kinetic. Also, with or without
exo adduct in the mixture, the retroDielsꢀAlder reaction kinetic remains the same.
1 1
Afterwards, the variation of the AendoF M adduct concentration on the rate constant (k) was
studied (Figure 11) and shows that the kinetic is the same whatever the starting concentration.
The reaction kinetic is concentration dependent, and it seems less probable that the
concentration hasn’t got an influence on the unblocking kinetic. In this case, the unblocking
reaction is not affected by the concentration because this last is too low.
In this study, only the compound AF
be dependent on the couple furan derivatives/maleimide used for the DielsꢀAlder reaction.
Thus, in this last part, the influence of substituents R of the maleimide and R /R of the furan
derivatives on the DielsꢀAlder reaction was investigated.
1 1
M was used. However, the unblocking reaction should
1
2
3
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2.2.4 Influence of the couple furan derivatives/maleimide
The product used is significant for the DielsꢀAlder reaction. Indeed, the addition of an
electron donating mesomeric substituent on the diene and the addition of an electron attracting
mesomeric substituent on the dienophile, increase considerably the blocking reaction kinetic
and the creation of endo adduct.
Several furan and maleimide derivatives were thus synthesized and used to determine the
influence of substituent added on diastereoselectivity and on rDA kinetic. It will allow
deducing rules about blocking reaction between furan and maleimide derivatives.
1
Yields and ratio of DielsꢀAlder adducts were determined by H NMR. All results are
summarized in Table 2/Table 3 and show that the backbone of R , R and R has an influence
1
2
3
on the ratio. Indeed, endo adducts are favored by the presence of electro giving substituent on
maleimide (compound M ) and by electron donating mesomeric substituent (played by the
oxygen atom on the furan backbone) coupled with electro withdrawing substituent in
position on the furan cycle (compound F ). Furan/maleimide choice couples enable favoring
or disfavoring one of the adduct and, as a consequence, to choose the best one depending on
its application. In our case, the couple F /M should be the best to obtain only the endo adduct
5
α
7
7
5
and, thus, to obtain an average unblocking temperature.
On the one hand, to study the influence of the blocking agent (furan derivatives, F) on
1
unblocking reaction kinetic of endo adduct, an H NMR monitoring was performed on the Nꢀ
methylmaleimide (M
1
) blocked with all furan derivatives. On the other hand, the influence of
1
maleimide on unblocking reaction kinetic of the endo adduct has been studied by H NMR
monitoring on each maleimide blocked with furfuryl acetate (F ) (Figure 12).
1
Figure 12: Influence of the Nꢀmethylmaleimide (M
left) and of every maleimide blocked with furfuryl acetate (F
reaction kinetic of endo adduct at 70°C
1
) blocking agent with furan derivatives
(
1
) (right) on unblocking
Since the exo adduct has no influence on unblocking reaction kinetic, it was not necessary to
purify them. Adducts mole numbers are, thus, used. The rate constant for each blocking
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and each maleimide blocked with F are summarized in the Table 7 below
1
agents used on M
1
(
M
2
has not been studied because the required amount of product for the kinetic was not fully
soluble in 0.6mL of DMSOD ).
Finally, the more the substituents R and R of the blocking agent are electro
6
2
3
1
withdrawing, the slower the retroDielsꢀAlder kinetic. Moreover the more R on maleimide is
attracting mesomeric, the faster the unblocking reaction kinetic. However, the higher the ratio
is in favor of endo adduct, the slower the retroDielsꢀAlder kinetic. It implies that the higher
endo adduct is obtained, the higher it’s thermodynamically stable and thus leads to a slower
kinetic. Nevertheless, there is no proportional behavior between unblocking kinetic and the
ratio endo/exo.
Table 7: Summarize of rate constant k for each blocking agent used on Nꢀmethylmaleimide
1 1
(M ) and each maleimide blocked with furfuryl acetate (F )
Blocking agent
F1
F2
F3
22,3
M4
F4
F5
F6
F7
F9
3
ꢀ1
k*10 (min )
Maleimide
16,3
M1
17,5
M3
12,8
M5
21,9
M6
74,1
M7
11,1
30,5
/
/
/
/
3
ꢀ1
k*10 (min )
16,3
61,4
129,1
50,0
29,7
43,0
3
Experimental part
.1 Materials and reagents
3
Furan derivatives blocking agents were obtained from commercial sources or synthesized in
laboratory. furfuryl acetate ≥98%, , 2ꢀ(2ꢀfuryl)ꢀ1,3ꢀdioxolane 98%, propionate furfuryl ≥98%,
thiophenol 97%, maleic anhydride 99%, 1ꢀmethoxyꢀ2ꢀpropylamine 95%, zinc bromide
:
≥98%, hexamethyldisilazane>99%, anhydrious sodium sulfate, aniline ≥99,5%,
99%, 4ꢀnitroaniline ≥99%, Nꢀdodecylamine ≥98%, 2ꢀmethoxyethylamine ≥99%, alcohol
furfuryl 98%, 2ꢀfuroyl chloride 95%, triethylamine ≥99%, cyclohexanecarbonyl chloride
pꢀanisidine
≥
9
8%, 3,5,5ꢀtrimethylhexanoyl chloride 98%, benzoyl chloride 99% and pivaloyl chloride
9%, 5ꢀmethylfuran 99%, 5ꢀmethylꢀ2ꢀfuraldehyde 98% were purchased from SigmaꢀAldrich.
9
Nꢀmethylmaleimide 97% and 5ꢀhydroxymethylꢀ2ꢀfuraldehyde 98% were purchased from
ABCR and used without purification.
3.2 Product Characterization
1
All products were characterized by nuclear magnetic resonance of protons ( H NMR) and
13
carbon ( C NMR). Measurements have been done thanks to a spectrometer Brucker Aspect
400MHz. Deuterium solvent used depends on the product and will be detailed in each case.
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Chemical shifts are given in (ppm) with an s for singlet, a d for doublet, a t for triplet and an
m for multiplet.
Differential Scanning Calorimetry (DSC) analyses were carried out on a NETZSCH DSC200
calorimeter. Cell constant calibration was performed using indium, nꢀoctadecane and nꢀoctane
standards. Nitrogen was used as the purge gas.
1 1
Informatics representation and calculation of AendoF M were done with MOPAC 2012
software (quantic semiꢀempirical calculation) and AM1 methods (geometrical optimization
with the convergence criteria "precise").
3.3 Synthesis of maleimide
57
The way used to synthesize maleimide model is presented in Scheme 2 .
O
O
1
2
) ZnBr2
)HMDZ, reflux
O
HO
THF
H2N R1
HN
R1
O
N
R1
O
1
hour
O
O
Mx
Scheme 2: Synthesis of maleimide model (Mx)
Maleic anhydride (1.1eq/amine function) is reacted with amine in THF (4ml/g) during 1 hour.
Amine is added drop by drop. Then ZnBr (1.4eq/amic acid function) is added under nitrogen.
2
After solubilization, the mixture is heated at 40°C, and then HMDZ in THF (1.4eq/amic acid
function) is added drop by drop during 30 minutes. After total addition, the temperature is
raised to 70°C for 4 hours. At the end of the reaction the mixture is filtered off and the solvent
is evaporated under low pressure. Several purifications have been done and will be specified
for each product.
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3.4 Synthesis of furan derivative
Three different ways for blocking agents synthesis have been used: the first one (way 1) using
alcohol furfuryl (Scheme 3), the second one (way 2) using furoyl chloride (Scheme 4) and the
5
8
third one (way 3) using aldehydes (Scheme 5).
All substituents R /R have been chosen for their electronegativity and their hindering (Figure
3).
2
3
1
Scheme 3: Example of synthesis of
alcohol furfuryl derivatives (way 1)
Scheme 4: Synthesis of furoyl chloride
derivative F (way 2)
8
Scheme 5: Synthesis of blocking agent F using the aldehyde (way 3)
9
y
Figure 13: All furan derivatives (F ) synthesized (s)
For both way 1 and way 2: an Acid chloride (1.3eq/alcohol function) is solubilized in dry
dichloromethane (1ml/g of product). Then triethylamine (1.3eq) is added under nitrogen drop
by drop to the mixture. Amine is added drop by drop. The alcohol (1eq) is added drop by
drop, under nitrogen, at 0°C during 15 minutes. The reaction mixture is then heated at 40°C
during 5 hours. Then the mixture is filtered off and the solvent is evaporated under low
pressure. Several purifications have been done and will be specified for each product.
For way 3: an aldehyde (1eq) is solubilized in dry THF (3ml/g of product) at 0°C, under
nitrogen. Then sodium borohydride (2eq) is slowly added during 10 minutes. After 2 hours
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the reaction mixture is neutralized with HCl (2N). The product is extracted with ethyl acetate
solvent. The organic layer is dried with anhydrous magnesium sulfate and the product is
crystalized at 0°C. A pale yellow powder is obtained.
3.5 Synthesis of Diels-Alder adduct
Maleimide M (1eq) is solubilized in the solvent (DCM for M or THF for M ) (2ml/g of
1
2
product). Then the furan derivative F (1.1eq) is added. The reaction mixture is stirred for two
weeks at 23°C
.
3.6 Characterization of Diels-Alder and retroDiels-Alder reaction
1
3
.6.1 By isothermal H NMR
In an NMR tube, all reactants for DielsꢀAlder synthesis are mixed. The solvent used is DMSO
D , because of its high boiling point at 190°C, which enables to do thermal reaction directly in
6
the NMR tube. All spectrums are saved every 5 minutes with 32 acquisitions, in automatic
mode, during 1 hour by temperature stage (40, 55, 65, 75, 85, 95, 105,115 and 125°C).
3.6.2 By DSC
In a DSC capsule (sealed when solvent is used), the DielsꢀAlder adduct, solubilized or not in
DMSO, is weighted (between 5 and 10 mg). For the characterization, for blocking or
unblocking reaction, the analysis is a step up from 20°C to 150°C, 2°C/min, then a step down
to 20°C, 2°C/min. The cycle is repeated twice. For the characterization of the unblocking
reaction in solvent, the analysis is a step up, 10°C/min, until 70°C, and then the capsule is
isothermally heated at 70°C during 4 hours and then a step down to 20°C.
1
3
.6.3 Characterisation of retroDielsꢀAlder kinetic by isothermal H NMR
6
In an NMR tube, the DielsꢀAlder adduct is solubilized in 0,6ml of DMSOD . For the study in
presence of a nucleophile, a drop of thiophenol and DABCO are added. Then the tube is
placed in the NMR and is heated at constant temperature (60°C, 70°C, 80°C) during 5 hours.
A spectrum is saved every 5 minutes with 32 acquisitions, in automatic mode.
4
Conclusion
As a conclusion, this study showed a way to increase the ratio endo/exo in favor of endo
adducts in Furan derivatives/maleimide system: the DielsꢀAlder reaction has to be carried out
2 3 1
at low temperature; electro withdrawing R and R substituents, and electron donating R
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substituents should be favored. Moreover, the unblocking reaction time could be adjustable
with substituents added on DielsꢀAlder partners. The unblocking reaction is favored with
2 1
electro withdrawing R and electro attracting mesomeric R substituents. If, in the mixture,
there is also a nucleophile (thiol), the unblocking reaction is faster.
This study enabled to reꢀdemonstrate general points about the DielsꢀAlder reaction already
known, as well as to show the link between external factor and partners backbones with the
ratio endo/exo and the retroDielsꢀAlder kinetic. Consequently, it is possible to create a system
with a control of the reaction temperature, a control of the unblocking reaction kinetic and a
system which could reacts on demand with a nucleophile (release of the maleimide reactive
double bond by rDA and reaction, in situ, with nucleophile). Thanks to this study, a future
publication will aim at preparing multifunctional products so as to create new type controlled
crossꢀlinked materials containing DielsꢀAlder adducts and nucleophile where it is possible to
trigger the reaction after stimuli (submitted).
5
Acknowledgements
The authors thank the Hutchinson Company for its financial support and BOURGOGNE Cyril
for informatics calculations.
6
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STUDY OF THE DIELS-ALDER AND RETRO-DIELS-ALDER REACTION BETWEEN FURAN
DERIVATIVES AND MALEIMIDE FOR THE CREATION OF NEW MATERIALS
Graphical Abstract