Experimental and Computational Studies on Stepwise [3+2]-Cycloadditions of Diaryldiazomethanes with Electron-Deficient Dimethyl (E)- and (Z)-2,3-Dicyanobutenedioates

Article abstract of DOI:10.1002/ejoc.201800837

Electron-deficient dimethyl (E)- and (Z)-2,3-dicyanobutenedioates (dimethyl dicyanofumarate and dicyanomaleate, respectively) react with diaryldiazomethanes at room temperature under spontaneous evolution of N₂. The type of the products strongly depends on the structure of the diazo compound. Whereas diphenyldiazomethane and its bis(4-methoxy) derivative yield cyclopropanes, sterically crowded 5-diazo-5H-dibenzo[a,d]cycloheptane derivatives afford either the dimer of the carbene formed via N₂ elimination or the adduct of the carbene onto the starting diazo compound. The course of the studied reactions is rationalized by stepwise mechanisms initiated by the formation of a C–N bond. A cascade of reactions leads to the corresponding cyclopropanes or to release of a carbene species. The formation of a pyrazole via [3+2]-cycloaddition (32CA) is observed only in reactions with (trimethylsilyl)diazomethane, which behaves similar to the parent diazomethane. The proposed mechanisms were analyzed computationally using the DFT method.

Full text of DOI:10.1002/ejoc.201800837

A Journal of
Accepted Article
Title: Experimental and Computational Studies on Stepwise Reactions
of Diaryldiazomethanes with Electron-Deficient Dimethyl (E)- and
(Z)-2,3-Dicyanobutenedioates
Authors: Grzegorz Mloston, Małgorzata Celeda, Radomir Jasiński, and
Heinz Heimgartner
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201800837
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European Journal of Organic Chemistry
10.1002/ejoc.201800837
Experimental and Computational Studies on Stepwise [3+2]-
Cycloadditions of Diaryldiazomethanes with Electron-
Deficient Dimethyl (E)- and (Z)-2,3-Dicyanobutenedioates
Grzegorz Mlostoń,*[a] Małgorzata Celeda,[a] Radomir Jasiński,[b] and Heinz
Heimgartner[c]
Dedicated to Professor Janusz Zakrzewski (University of Łódź) on the occasion of his
7
0th birthday
Abstract
Electron-deficient dimethyl (E)- and (Z)-2,3-dicyanobutenedioates (dimethyl
dicyanofumarate and dicyanomaleate, respectively) react with diaryldiazomethanes at
room temperature under spontaneous evolution of N . The type of the products strongly
2
depends on the structure of the diazocompound. Whereas diphenyldiazomethane and its
bis(4-methoxy) derivative yield cyclopropanes, sterically crowded 5-diazo-5H-
dibenzo[a,d]cycloheptane derivatives afford either the dimer of the carbene formed via
2
N elimination or the adduct of the carbene onto the starting diazo compound. The
course of the studied reactions is rationalized by stepwise mechanisms initiated by the
formation of a C–N bond. A cascade of reactions leads to the corresponding
cyclopropanes or to release of a carbene species. The formation of a pyrazole via [3+2]-
cycloaddition (32CA) is observed only in reactions with (trimethylsilyl)diazomethane,
which behaves similar to the parent diazomethane. The proposed mechanisms were
analyzed computationally using the DFT method.
_
_____________________
[
a]
Department of Organic and Applied Chemistry, University of Łódź,
Tamka 12, PL-91-403 Łódź, Poland
E-mail: grzegorz.mloston@chemia.uni.lodz.pl
http://www.chemia.uni.lodz.pl/professors/gmloston.html
[
[
b]
Institute of Organic Chemistry and Technology, Cracow University of Technology,
Warszawska 24, PL-31-155 Cracow, Poland
c]
Department of Chemistry, University of Zurich, Winterthurerstrasse 190,
CH-8057 Zurich, Switzerland
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European Journal of Organic Chemistry
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Keywords:
Diaryldiazomethanes,
electron-deficient
ethylenes,
(trimethylsilyl)diazomethane, reaction mechanism, cyclopropanation, DFT
calculations
Introduction
The [3+2]-cycloadditions (32CA) of activated ethylenes with diazomethanes as three
atom components (TACs) constitute an important method for the formation of
pyrazolines.[1,2] In the presence of a metal catalyst or under photolytical conditions, the
initial decomposition of the diazo compounds leads to a reactive carbenoid or carbene,
respectively, which reacts with the C=C bond to give the corresponding cyclopropane
derivative.[3a] In a recent publication, efficient cyclopropanations of some electron
deficient ethenes using unstable aryl diazomethanes generated under ‘continous flow
conditions’ were described.[3b] An attractive, photocatalytical approach based on the
usage of the blue-led light, which has extensively been developed in recent years,[3c]
was also successfully applied for cyclopropanation rections of some electron rich
ethylenes using diversely substituted alkyl diazoarylacetates.[3d] These formal [2+1]-
cycloadditions occur stereoselectively or non-stereoselectively depending on the
properties of the reactive intermediate.
In a series of recent studies, electron-deficient dimethyl (E)- and (Z)-2,3-
dicyanobutenedioates 1 (dicyanofumarate (E)-1 and dicyanomaleate (Z)-1) were
described as superior components of 32CAs with TACs such as thiocarbonyl S-
methanides, azomethine ylides, and diazoalkanes.[4] Whereas the reaction with
diazomethane (2a) afforded mixtures of pyrazole derivatives and stereoisomeric
cyclopropanes,[5] reactions with 2-diazopropane (2b) led to exclusive formation of the
[
6]
corresponding cyclopropanes in a stereoselective manner (Scheme 1).
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European Journal of Organic Chemistry
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Scheme 1. Reactions of diazoalkanes 2a,b with dimethyl dicyanofumarate ((E)-1) (ref.
[5-6])
.
9
-Diazofluorene (2c) and diphenyldiazomethane (2d) are easily available and
both compounds are widely applied in reactions with diverse ethylene derivatives. For
example, in a recent publication, attempted 32CAs of 2c with 2-aryl-1-cyano-1-
nitroethylenes were described. Unexpectedly, instead of pyrazoles, 2,3-diazabuta-1,3-
[
7]
dienes were obtained and identified by X-ray crystallography. The formation of these
products was explained by the appearance of a zwitterionic intermediate, which after
extrusion of cyano(nitro)carbene converted into the final product. On the other hand,
both 2c and 2d were applied for the [3+2]-cycloaddition with an electron-deficient
acetylene and the corresponding pyrazoles were obtained regioselectively in good
yields.[8] In contrast to 2c and 2d, bis(4-methoxyphenyl)diazomethane (2e), 5-diazo-
1
0,11-dihydro-5H-dibenzo[a,d]cycloheptene (2f), and 5-diazo-5H-dibenzo[a,d]-
cycloheptene (2g) are rarely applied in cycloaddition reactions, and cyclopropanations
with 2e–g have not been reported yet.
In a recent publication, Domingo et al. explained the reaction of 1-diazopropan-
2
-one with 1,1-dinitroethylene,[9] which leads to a mixture of 2-acetyl-1,1-
dinitrocyclopropane and 3-acetyl-5-nitropyrazole,[10] on the basis of the Molecular
Electron Density Theory (MEDT).[11] According to these studies, the formations of both
the cyclopropane and the pyrazole derivatives occur via domino reactions with the
initial formation of 5-acetyl-3,3-dinitro-4,5-dihydro-3H-pyrazole in a [3+2]-
cycloaddition (32CA).
In the present report, the reactions of (E)-1 and (Z)-1 with diaryldiazomethanes
2
c–g will be described and the observed reaction courses will be compared with that
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European Journal of Organic Chemistry
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obtained with (trimethylsilyl)diazomethane (2h). The experimental results will be
supplemented with computational studies aimed at the elucidation of the reaction
mechanisms.
Results and discussion
The first two experiments were performed with (E)- and (Z)-1 and bis(4-
methoxyphenyl)diazomethane (2e) in CH
2
Cl solution at room temperature. The
2
initially blue-colored reaction mixture decolorized immediately and a vigorous
1
evolution of N
2
was observed in both cases. The H NMR analyses of the crude reaction
mixtures showed identical compositions of products in both experiments.
Chromatographic separation led to two isomeric products. The major one, obtained as
the more polar fraction, was identified as dimethyl trans-1,2-dicyano-3,3-bis(4-
methoxyphenyl)cyclopropane-1,2-dicarboxylate
(trans-4e,
Scheme
2).
The
configuration of this isomer was confirmed by the presence of two singlets at 3.73 and
1
3
3
.84 ppm, each for two MeO groups. In the C NMR spectrum, only one signal for
1
CN (112.2 ppm) and one signal for CO
2
Me (161.2 ppm) was present. The HNMR
spectrum of the minor, less polar cis-isomer showed three singlets at 3.71, 3.75, and
.84 ppm for MeO groups in a ration of 1:1:2. The first two signals are attributed to
MeOC groups and the last signal originates from two equivalent methyl ester
3
6
H
4
groups. The chemical shifts of the pure isomers allowed the determination of the ratio of
trans- and cis-4e in the crude reaction mixture to ca. 1.6:1.
Scheme 2. Cyclopropanation reactions of (E)- and (Z)-1 with diaryldiazomethanes 2c–e.
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European Journal of Organic Chemistry
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The next experiment was carried out using (E)-1 and 2c in CH
2
Cl solution at
2
room temperature using the substrates in a 1:1 molar ratio. The progress of the reaction
was monitored by TLC and, after 2 h, no 2c could be detected in the mixture. However,
1
the H NMR spectrum indicated that a substantial amount of (E)-1 was still present.
Apparently, 2c was partially converted into the known orange bisfluorenylidene. For
that reason, the experiment was repeated using (E)-1 and 2c in a 1:3 ratio. In that case,
1
the H NMR spectrum of the crude mixture after 2 h showed complete conversion of
(
E)-1, and two singlets for MeO groups found at 3.83 and 3.90 ppm revealed the
presence of two products in a ratio of ca. 3:1. The undesired bisfluorenylidene was
removed by trituration with CH Cl and the mother liquor, after evaporation of the
2
2
solvent, afforded a solid, which after crystallization gave the major product as a
1
crystalline material with m.p. 237–239 ºC. The H NMR spectrum showed a singlet at
3
.83 ppm for two MeO groups and a set of four multiplets characteristic for the fluorene
moiety. The 13C NMR spectrum confirmed the structure of a dimethyl 1,2-
dicyanocyclopropane-1,2-dicarboxylate. The expected trans-configuration of this
product, trans-4c (Scheme 2), was unambiguously established by X-ray crystallography
(Figure 1).
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European Journal of Organic Chemistry
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Figure 1. A view of the molecular structure of compound trans-4c. Displacement
ellipsoids are drawn at the 50% probability level. X-ray data collected at 100 K.
The mother liquor after crystallization of trans-4c was concentrated and the
semi-solid residue was subjected to preparative layer chromatography. The less polar
fraction formed colorless crystals with m.p. 222−224 ºC, which were identified, based
on spectroscopic data, as cis-4c, contaminated with an unknown product.
In a supplementary control experiment, 2c was reacted with (Z)-1, and in that
case, complete consumption of the less reactive (Z)-1 was achieved only after 24 h. The
two isomeric products trans- and cis-4c were present in the crude mixture in a ratio of
1
ca. 1:1 ( H NMR). It is worth mentioning that almost the same ratio was found in a
control spectrum registered after 2 h of the experiment, and only (Z)-1 was detected in
this spectrum.
Diphenyldiazomethane (2d) behaves differently in reactions with (E)- and (Z)-1.
The reaction with (E)-1, performed according to the protocol described for 2c, required
1
2
4 h to be complete. The H NMR spectrum of the crude reaction mixture revealed the
presence of one major product with a characteristic singlet for MeO at 3.84 ppm.
1
Careful analysis of the H NMR spectrum of the crude mixture allows estimating the
presence of the minor cyclopropane and the ratio of both diastereoisomers to ca. 9:1.
After evaporation of the solvent, the residue was crystallized from methanol yielding a
colorless crystalline product. The tentatively proposed structure trans-4d was elucidated
1
3
by comparison of the NMR data with those of trans-4c. For example, the C signal for
two CN groups in trans-4c is located at 111.0 ppm and in trans-4d at 112.0 ppm. In
addition, the signals for the CO Me groups appear at 160.0/54.8 and 161.1/54.6 ppm,
2
respectively. In addition, the trans-configuration of this isomer was confirmed by the
appearance of only one signal for an aromatic C atom (135.2 ppm) and three signals of
non-equivalent, aromatic CH-units at 129.6, 129.3, and 127.9 ppm.
The reaction of (Z)-1 with 2d was much faster and complete decolorization of
1
the diazo compound was observed after 3 h. The H NMR spectrum of the crude
mixture allowed establishing the presence of the major cyclopropane cis-4d,
accompanied by a small admixture of the above-described trans-4d in a ratio of ca.
9
0:10. The chromatographic separation of the ‘cyclopropane fraction’ followed by
fractional crystallization allowed to isolate pure cis-4d. In that case, its configuration
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European Journal of Organic Chemistry
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was unambiguously confirmed by the appearance of two signals for aromatic C atoms
(136.6 and 133.4 ppm) and six CH-signals for two non-equivalent aromatic rings.
In comparison with 2c–e, the reactivity of 2f and 2g towards (E)- and (Z)-1
differ dramatically. Whereas in the reaction of (E)-1 with 2f the blue color of the
solution vanished after 4 h, the analogous experiment with 2g required 72 h. In both
1
cases, after decolorization of the reaction solutions, the H NMR spectra of the crude
mixtures evidenced the presence of the non-converted (E)-1. After workup, in each case
a colorless solid was isolated, and the spectroscopic data, supported by elemental
analyses, allowed to identify them as ethylene 5[12] and azine 6,[13] respectively (Figure
2
). Whereas compound 5 can be considered as the dimer of the carbene generated from
f, product 6 may result from the reaction of the in situ-generated analogous carbene
2
with its precursor, i.e. 2g. The same products were formed in reactions of (Z)-1 with 2f
and 2g, respectively.
Figure 2. Products 5 and 6 obtained in reactions of (E)-1 with diazocompounds 2f and
2
g, respectively.
In extension of the study and in order to compare the reaction courses of (E)-1
with diazomethane (2a)[5] and diaryldiazomethanes 2c–e, (trimethylsilyl)diazomethane
2h) was also tested. The experiment with (E)-1 and a two-fold excess of 2h was
performed in THF at room temperature. After 30 min, tetrabutylammonium fluoride
TBAF) was added to remove the TMS group from the products formed. The crude
(
(
reaction mixture was separated by column chromatography, which afforded only one
1
product isolated as a crystalline material. The H NMR spectrum showed three signals
located at 4.00, 4.22 and 7.79 ppm with an intensity ratio of 3:3:2. The 13C NMR
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European Journal of Organic Chemistry
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spectrum suggested the presence of only one ester group by the signal at 158.1 ppm.
These data did neither correspond with the expected structures of a pyrazoline
derivative nor with that of cyclopropane derivatives. Finally, the structure of this
product was determined by X-ray crystallography (Fig. 3), which showed that the
compound was identical with the already reported methyl 4-cyano-1-methylpyrazole-5-
carboxylate (7, Scheme 3).[5,14] The analogous experiment performed with (Z)-1 led to
the same product. Elimination of methyl cyanoformate from the dimethyl pyrazole
dicarboxylate analogous to 8, obtained after [3+2]-cycloaddition of (E)-1 with the
parent diazomethane, which also led to product 7, was carefully studied by Huisgen.[5a]
Scheme 3. Reaction sequence via [3+2]-cycloaddition, desilylation and elimination
leading to pyrazole 7.
Figure 3. A view of the molecular structure of compound 7. Displacement ellipsoids
are drawn at the 50% probability level. X-ray data collected at 100 K.
The reported reactions of 9-diazofluorene (2c) and diphenyldiazomethane (2d)
with the electron-deficient tetracyanoethylene (TCNE) offer a straightforward access to
the corresponding cyclopropanes.[15,16] The supposed mechanism for their formation
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European Journal of Organic Chemistry
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involves the initial [3+2]-cycloaddition leading to pyrazolines, which upon heating were
converted into cyclopropanes via N elimination. The same mechanism was believed to
2
occur in reactions aimed at the determination of reaction rates.[17,18] In the present study,
the use of two isomeric ethylene derivatives, namely (E)-1 and (Z)-1 in reactions with
2
c–e allows getting more information about the reaction mechanism. Despite the fact
that the reactions were performed under mild conditions, a spontaneous evolution of N
2
1
was observed. Moreover, the H NMR control evidenced the presence of cyclopropanes
4
as the only products over the entire reaction time. Based on these observations and
loss of the stereochemical integrity, we postulate that these reactions follow a stepwise
zwitterionic or diradical mechanism (Scheme 4).
Scheme 4. Proposed, stepwise reaction mechanism for the formation of cyclopropanes
5.
In order to obtain an additional support for the proposed molecular mechanism
of the formation of stereoisomeric cyclopropanes in reactions of (E)-1 and (Z)-1 with
c–e, theoretical DFT studies were performed. Firstly, we analyzed global and local
2
conceptual DFT reactivity indices of the reagents. It is generally known[19] that 32CAs
require nucleophilic activation of the molecule of one of the addends, and electrophilic
activation of the second component, in order to favor the reaction through a polar
process.[20] Notably, this type of activation is also observed in the series of reactions
between 1 and diaryldiazomethanes 2c−e. In particular, (E)-1 is characterized by high
global electrophilicity, in excess of 4 eV. For the same molecule the global
nucleophilicity index was calculated to the value of only 1.03 eV. Therefore, it should
be classified to the group of strong electrophiles within the reactivity scale proposed by
Domingo.[21] On the other hand, calculated global electrophilicities of
diaryldiazomethanes 2c−e are in the range of 1.30−1.84 eV, whereas global
nucleophilicities are in excess of 3.5 eV (Table 1). Based on these data, one can assume
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European Journal of Organic Chemistry
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0
that the initial step of theses reactions is the interaction between one of the equivalent
C=C atoms of (E)-1 and the more nucleophilic, activated centre of the diazo compound.
The analysis of the local reactivity in molecules 2c−e shows clearly that the more active
center is located on the terminal nitrogen atom of the diazo function. In consequence,
the analyzed process will be initiated by the attack of this atom onto the C=C bond of
(
E)-1. Analogous distribution of local reactivity can be interpreted for the reaction of
Z)-1 with 2c−e.
(
Table 1. Global and local electronic properties for diazomethanes 2c−g.
Global properties
 [eV]  [eV] N [eV]
Local properties

[eV]
P-
C
P
-
N
N
C
[eV]
0.83
N [eV]
N
2
c
−3.67
−3.36
−3.02
−3.33
−3.36
–5.96
–5.85
3.67
3.70
3.52
3.69
3.41
4.27
4.58
1.84
1.52
1.30
1.51
1.65
4.16
3.73
3.61
3.91
4.34
3.94
4.06
1.03
0.98
0.23
0.23
0.15
0.24
0.17
0.39
0.41
0.38
0.40
0.34
1.41
1.62
1.65
1.57
1.39
2
d
0.91
0.64
0.93
0.68
2
2
e
f
2
g
(
(
E)-1
Z)-1
Next, the detailed possible pathways of transformations of the addends were
analyzed. This study was performed for the model process (E)-1+2c. The DFT
calculation showed that the formations of isomeric cyclopropanes 4c proceed following
a multi-step mechanism presented in Scheme 4 and Fig. 4.
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European Journal of Organic Chemistry
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1
Figure 4. Views of key critical structures for reactions 1+2c according to B3LYP/6-
1++G(d)(PCM) computational study
3
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European Journal of Organic Chemistry
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2
In particular, the conversion of the reaction system (E)-1+2c into trans-4c is a domino
process, occurring via a labile 5-membered heterocyclic intermediate, a 4,5-dihydro-3H-
pyrazole (32CA, Scheme 5), which might be considered as a formal [3+2]-cycloadduct.
The first step of this conversion (path A in Scheme 5) requires a Gibbs free energy of
activation calculated to 35.5 kcal/mol and leads to the cycloadduct trans-32CA, which
is evidently unstable from the thermodynamic point of view. Therefore, two competitive
transformations of this intermediate are possible: (a) a [3+2]-cycloelimination
(
cycloreversion) reaction leading to 2c and (E)-1; (b) irreversible nitrogen extrusion
path B in Scheme 5), which proceeds via a single transition state, yielding trans-4c.
(
Both processes proceed via 5-membered transition states (TSE2 and TSE3,
respectively), the nature of which was confirmed by vibrational analysis and IRC
calculations. It is worth mentioning that the nitrogen extrusion from pyrazoline ring
requires rather high energy of activation (Figure 5). This interpretation would require a
diastereoselective course of the analyzed reaction. However, the performed experiments
led to the formation of two isomeric cyclopropanes cis- and trans-4c. For this reason, a
competitive mechanism for the conversion of the addends should be considered. In
particular, primary interaction in the reaction system (E)-1+2c may lead via transition
state TSE1 to the intermediate (E)-ZW (path C in Scheme 5), whose zwitterionic nature
was confirmed by the GEDT analysis (Fig. 5).
Figure 5. Gibbs free energy profiles for the reaction (E)-1+2c according
to B3LYP/6-31++G(d)(PCM) computational study
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European Journal of Organic Chemistry
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3
The (E)-ZW is characterized by an ‘extended’ conformation, which can’t
undergo the ring closure. A similar type of intermediate was identified recently on the
basis of comprehensive theoretical and experimental studies for the reaction between 2c
and 2-phenyl-1-cyano-1-nitroethene.[7] In these types of zwitterionic intermediates,
rotation around the C–C bond in the ethylene derivative substructure is likely and was
confirmed experimentally in reactions performed with electron-rich thiocarbonyl S-
methanides and both isomers of 1.[22] Consequently, the isomeric (Z)-ZW zwitterion is
formed and its dissociation leads to a mixture of (Z)-1 and 2c, which may further be
converted into cis-4c via TSZ2 and TSZ3 in analogy to the transformation (E)-
1+2c→trans-4c. Thus, the performed DFT study fully explained the domino mechanism
of the studied reactions leading to mixtures of diastereoisomeric cyclopropanes starting
with (E)-1 or (Z)-1 as the required electron deficient ethylene derivative. This
mechanism, which explains the formation of isomeric cyclopropanes, accompanied by
spontaneous evolution of N , is outlined in Scheme 5.
2
Scheme 5. Domino reaction mechanism for the formation of isomeric cyclopropanes 4c
in the light of B3LYP/6-31++G(d)(PCM) computational study.
The stepwise mechanism with the initial attack of the N-terminus of
diazofluorene (2c) onto the activated C=C bond with subsequent elimination of a
carbene species leads to the formation of 2,3-diaza-1,3-butadienes has been described in
ref. [7]. The same interpretation can be applied to explain the results obtained with 2f
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European Journal of Organic Chemistry
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4
and 2g. In particular, analysis of local nucleophilicities suggests that reactions are
initiated by the analogous attack of the more nucleophilic, terminal nitrogen atom of the
diazo fragment (Table 1). This type of the attack leads to the formation of the reactive
intermediates of type 10 (Scheme 6). However in these cases, the intermediates 10
undergo dissoziation leading to betaines 11 and carbenes 12 with no competitive
conversion leading to cyclopropanes via 5-membered heterocyclic intermediates. The
latter intermediates, depending on the structure and, therefore, on the reactivity of the
diazo compound 2, undergo either dimerization yielding ethene 5 or addition onto the
less reactive diazo group of the less reactive 2g to give azine 6. The betaine 11 most
likely eliminates spontaneously N
2
releasing the starting ethylene derivative 1.
Formally, the formation of 11 can be understood as an in-situ diazo-transfer reaction.
Scheme 6. Proposed reaction mechanism for the formation of carbenes 12, ethane 5 and
azine 6.
The presented explanation can also be applied for the reaction course observed
in the case of diazofluorene (2c). Apparently, this diazo compound reacts with 1 via two
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European Journal of Organic Chemistry
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5
competitive reaction pathways, and both a C–C and a C–N bond are formed in the initial
step, leading finally to cyclopropanes 4c and to bisfluorenylidene, respectively.
The comparison with the results obtained with diazomethanes 2c–g shows that
diaryldiazomethanes 2c–e and 2f,g react with 1 via stepwise mechanisms but the
structure of the final products depends on the type of TACs used in the reaction. An
alternative SET (single electron transfer)-mechanism initiated by the electron transfer
from the electron-rich diazo compound to the electron-deficient ethylene acceptor
should be also considered, but the products formed via the in situ generated carbene
support the presented zwitterionic or diradical interpretation.
A plausible reaction mechanism leading to 7, obtained in a rather low yield from
the reaction of 2h with (E)- and (Z)-1 followed by desilylation is formulated in Scheme
3
. In that case, in contrast to diaryldiazomethanes, the initial step comprises the [3+2]-
cycloaddition leading to the five-membered cycloadduct, which after 1,3-H shift and
methylation’ with 2h forms the intermediate 8. The latter, without isolation, is
‘
converted into 7 via desilylation and elimination of methyl cyanoformate. The same
product was reported for the reaction of excess 2a and (E)-1 in ethereal solution
(
Scheme 1), and its formation was explained based on a similar mechanistic
interpretation.[5]
It is worth mentioning that the reactions of 2h with tetracyanoethylene and other
electron-deficient ethenes lead to the corresponding cyclopropanes, and no pyrazole
derivatives were reported.[23] Unfortunately, no experimental data are available in this
publication. Nevertheless, based on our results, a similar, stepwise mechanism can be
formulated for all presented cases related to the synthesis of cyclopropanes in reactions
with strongly electron deficient teracyanethylene.
3
. Conclusions
The preparation of cyclopropanes bearing electron-withdrawing substituents and
especially of so called (D-A) cylopropanes (donor-acceptor substituted cyclopropanes)
is of current interest[24,25]; the most frequently applied methods for their synthesis are
metal-catalyzed or metal-free reactions of α-diazocarbonyl compounds with aryl
substituted alkenes. The present study showed that the metal-free reactions of both
dimethyl dicyanofumarate (E)-1 and dicyanomaleate (Z)-1 with diazomethanes 2 lead to
different products depending on the type of the used diazo compound. Apparently, these
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European Journal of Organic Chemistry
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reactions take place via domino processes, which are initialized by a 32CA reaction
between the aryldiazomethane and the electrophilic ethylene, yielding the corresponding
1-pyrazoline. The subsequent reactions, which determine the type of the final products,
depend on both electronic and steric factors. Whereas diphenyldiazomethanes 2c–e
afford mixtures of isomeric cyclopropanes exclusively, the sterically crowded diazo
compounds 2f–g react with 1 to give products resulting from the intermediacy of the in
situ generated cyclic carbenes. Thus, in the reaction with the more reactive 2f, the dimer
5
was obtained, but in the case of the very slow reaction with 2g, the extruded carbene
species reacts with the diazo precursor present in the reaction mixture forming the azine
. The postulated domino reactions of diazo compounds 2c–e with electron deficient,
E/Z-isomeric ethylenes 1 are supported by DFT calculations. On the other hand,
trimethylsilyl)diazomethane (2h), in analogy to the parent diazomethane (2a)[5] reacts
with 1 via [3+2]-cycloaddition leading to five-membered pyrazole derivatives, with no
evolution of N . In the case of the silylated [3+2]-cycloadduct, however, it undergoes
6
(
2
further conversions upon treatment with TBAF solution. In both cases, elimination of
methyl cyanoformate from the initially formed 4,5-dihydro-1H-pyrazole dicarboxylate,
leading to aromatization of the obtained products, was observed.
Experimental Section
General: All solvents were dried over appropriate drying agents and distilled before
1
13
use. The H and C NMR spectra were measured on a Bruker Avance III instrument
600 and 150 MHz, respectively), using the solvent (CDCl , residual CHCl ) signal as
(
3
3
reference. The following abbreviations are used to describe peak patterns where
appropriate: br (broad), s (singlet), d (doublet), t (triplet), q (quartet), sept (septet), m
(multiplet), dd (doublet  doublet), and td (triplet  doublet). Coupling constants (J) are
reported in Hertz (Hz). The IR spectra (KBr pellets) were recorded on a Nexus FT-IR
spectrophotometer. The elemental analyses were determined on a Vario Micro Cube.
Flash column chromatography (FCC) was carried out using Silica gel 60 (Sigma-
Aldrich, 230–400 mesh). Melting points were determined in a capillary using a Stewart®
SMP30 apparatus.
Starting materials: (E)- and (Z)-Dimethyl 2,3-dicyanobutenodioates ((E)-1 and (Z)-1)
were obtained according to published procedures starting with methyl dicyanoacetate
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European Journal of Organic Chemistry
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and HCl-free thionyl chloride.[26]
Bis(4-methoxyphenyl)diazomethane (2e),[27]
diphenyldiazomethane (2d),[28] 9-diazofluorene (2c),[28] 5-diazo-10,11-dihydro-5H-
dibenzo[a,d]cycloheptene (2f),[28] and 5-diazo-5H-dibenzo[a,d]cycloheptene (2g),[28]
were prepared from the corresponding hydrazones by oxidation with activated
manganese dioxide (MnO
2
)[27] or with yellow mercury oxide (HgO).[29] Hydrazones of
bis(4-methoxyphenyl)ketone,
9H-fluorenone, and 10,11-dihydro-5H-
dibenzo[a,d]cycloheptenone were efficiently prepared from the corresponding
thioketones[30] by treatment with hydrazine hydrate in ethanolic solutions; all reactions
were completed after 10–30 min at room temperature and the required hydrazones were
obtained in 80–90% yield. Trimethysilyl diazomethane (2h) and other reagents used in
the study were commercially available substances.
Reactions of diaryldiazomethanes 2 with (E)- or (Z)-dimethyl dicyanofumarates 1.
General procedure: A solution of 194 mg (1 mmol) of 1 and 3 mmol of the respective
2
in 2 mL of dichloromethane was stirred magnetically at room temperature until the
characteristic violet color of the starting diazo compound vanished. Only in the case of
2
c, the fast reaction with (Z)-1 was completed after 30 min using only 1.2 mmol of the
1
diazo compound. After evaporation of the solvent, crude mixtures were analyzed by H
NMR spectroscopy and subsequently separated chromatographically on plates coated
with silica gel (PLC). Typically, a mixture of petroleum ether and ethyl acetate (4:1)
was used as an eluent. The isolated ‘cyclopropane fraction’ with R ca. 0.5 was
f
additionally separated either by repeated chromatography or by fractional
crystallization. The ratios of isomeric trans- and cis-cyclopropanes 4c–e formed in
1
experiments with diaryl diazomethanes 2c–e were determined based on the H NMR
analysis of the initially isolated ‘cyclopropane fractions’. After separation, analytically
pure samples were obtained by crystallization.
Reaction with Bis(4-methoxyphenyl)diazomethane (2e): Experiment A: molar ratio
(E)-1/2e = 1:1.2; reaction time: 30 min; trans-4e : cis-4e = ca. 85:15. Experiment B:
molar ratio (Z)-1/2e = 1:1.2; reaction time: 20 min; trans-4e : cis-4e = ca. 60:40.
trans-Dimethyl
1,2-Dicyano-3,3-bis(4-methoxyphenyl)cyclopropane-1,2-
dicarboxylate (trans-4e): Less polar fraction, colorless crystals, m.p. 178–179 °C (from
1
MeOH). H NMR (600 MHz, CDCl
3
): δ = 7.37, 6.84 (2d, JH,H = 6.0 Hz, 8 arom. CH),
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European Journal of Organic Chemistry
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8
1
3
3
1
5
1
8
.84, 3.73 (2s, 6 H, 2 OCH
3
) ppm. C NMR (150 MHz, CDCl
3
): δ = 161.2 (2 C=O),
59.7 (2 arom. C), 128.6, 114.8 (8 arom. CH), 127.5 (2 arom. C), 112.2 (2 CN), 55.5,
4.6 (2 OCH ), 52.6, 36.3 (2 C) ppm. IR (KBr): υ = 2962 (w), 2837 (w), 2252 (C≡N),
752 (vs, C=O), 1609 (m), 1513 (s), 1466 and 1436 (m), 1247 (vs., br. O–C), 1026 (s),
3
–
1
89 (m), 838 (m), 582 and 571 (m) cm . EA for C23
H
20
N
2
6
O (420.41): calcd. C 65.71,
H 4.79, N 6.66; found C 65.5, H 4.80, N 6.69.
cis-Dimethyl 1,2-Dicyano-3,3-bis(4-methoxyphenyl)cyclopropane-1,2-dicarboxylate
1
(
cis-4e): Colorless crystals, m.p. 108–111 °C (from MeOH). H NMR (600 MHz,
CDCl
3
): δ = 7.48, 6.90 (2d, JH,H = 6.0 Hz, 4 arom. CH), 7.33, 6.76 (2d, JH,H = 6.0 Hz, 4
1
3
arom. CH), 3.84 (s, 6 H, 2 OCH
3
) ppm. C NMR (CDCl
59.3 (2 arom. C), 129.6, 129.2, 114.9, 113.9 (8 arom. CH), 129.1, 125.7 (2 arom. C),
13.2 (2 CN), 55.2, 55.1 (2 OCH ), 54.4 (2 OCH -ester), 53.9 (C), 37.2 (2 C) ppm. IR
3
): δ = 160.9 (2 OCH ), 159.9,
3
1
1
3
3
(
(
(
KBr): υ = 2837 (w), 2248 (w), 1759 (vs, C=O), 1609 (m), 1514 (vs), 1458 and 1437
m), 1254 (vs, br, O–C), 1176 (m), 1149 (m), 1035 (s), 910 (m), 843 (m), 592 (m), 568
–
1
m) cm . EA for C23
H N
20 2
6
O (420.41): calcd. C 65.71, H 4.79, N 6.66; found C 65.57,
H 4.92, N 6.70.
Reaction with 9-Diazofluorene (2c): Experiment A: molar ratio (E)-1/2c = 1:3;
reaction time: 24 h; trans-4c : cis-4c = ca. 65:35. Experiment B: molar ratio (Z)-1/2c =
1
:3; reaction time: 3 h; trans-4c : cis-4c = ca. 74:26.
trans-Dimethyl
2,3-Dicyanospiro[cyclopropane-1,9'-fluorene]-2,3-dicarboxylate
1
(
trans-4c): Colorless crystals, m.p. 236–238 °C (from MeOH)). H NMR (CDCl
3
): δ =
1
3
7
.84–7.35 (4m, 8 arom. CH), 3.83 (s, 6 H, 2 OCH
3
) ppm. C NMR (CDCl
3
): δ = 161.0
(2 C=O), 141.7, 134.4 (4 arom. C), 130.4, 128.0, 123.8, 120.7 (8 arom. CH), 111.0 (2
CN), 54.8 (2 OCH ), 45.0 (C), 34.9 (2 C) ppm. IR (KBr): υ = 2246 (w, C≡N), 1755 (vs,
3
–
1
C=O), 1454 (m), 1249 (s, O–C), 742 (m), 653 (w) cm . EA for C21
H
14
N
2
4
O (358.35):
calcd. C 70.39, H 3.94, N 7.82; found C 70.44, H 4.01, N 7.82.
cis-Dimethyl 2,3-Dicyanospiro[cyclopropane-1,9'-fluorene]-2,3-dicarboxylate (cis-
1
4
c): Colorless crystals, m.p. 222–224 °C (from MeOH). H NMR (CDCl
3
): δ = 7.99–
13
7
.25 (5m, 8 arom. CH), 3.90 (s, 6 H, 2 OCH
3
) ppm. C NMR (CDCl
3
): δ = 159.7 (2
C=O), 142.4, 141.2, 134.8, 132.7 (4 arom. C), 130.4, 130.1, 127.9, 127.1, 125.1, 120.2,
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European Journal of Organic Chemistry
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9
1
20.22, 120.17 (8 arom. CH), 111.9 (2 CN), 54.6 (2 OCH
3
), 47.1 (C), 35.8 (2C) ppm.
IR (KBr): υ = 2955 (w), 2243 (C≡N), 1767 and 1750 (vs, C=O), 1452 and 1433 (s),
274 and 1244 (vs, br. O–C), 1147 (m), 1087 (w), 936 (w), 900 (w), 733 (vs), 653 (m)
1
–
1
cm . EA for C21
.91, N 7.73.
H
14
N
2
4
O (358.35): calcd. C 70.39, H 3.94, N 7.82; found C 70.26, H
3
Reaction with Diphenyldiazomethane (2d): Experiment A: molar ratio (E)-1/2d =
1
1
:3; reaction time: 24 h; trans-4d : cis-4d = 57:43. Experiment B: molar ratio (Z)-1/2d =
:3; reaction time: 3 h; trans-4d : cis-4d = 55:45.
trans-Dimethyl 1,2-Dicyano-3,3-diphenylcyclopropane-1,2-dicarboxylate (trans-
1
4
d): Colorless crystals, m.p. 221–223 °C (from MeOH). H NMR (600 MHz, CDCl
3
): δ
=
7.50–7.26 (3m, 10 arom. CH), 3.84 (s, 6 H, 2 OCH
3
) ppm. 13C NMR (150 MHz,
): δ = 161.2 (2 C=O), 135.2 (2 arom. C), 129.6, 129.3, 127.9 (10 arom. CH),
), 53.5 (C), 36.1 (2 C) ppm. IR (KBr) υ = 2255 (w, C≡N),
761 and 1746 (vs, C=O), 1496 (m), 1454 and 1429 (m), 1306 (m), 1233 (vs, O–C),
CDCl
3
1
1
1
12.1 (2 CN), 54.8 (2 OCH
3
–
1
106 (m), 945 (w), 884 (w), 751 (m), 707 (s), 578 (m) cm . EA for C21
H
16
N
2
O
4
(360.36): calcd. C 69.99, H 4.47, N 7.77; found C 70.27, H 4.54, N 7.50.
cis-Dimethyl 1,2-Dicyano-3,3-diphenylcyclopropane-1,2-dicarboxylate (cis-4d):
1
Colorless crystals, m.p. 146–148 °C (from MeOH). H NMR (600 MHz, CDCl
3
): δ =
) ppm. 13C NMR (150 MHz,
): δ = 160.8 (2 C=O), 136.6, 133.4 (2 arom. C), 129.5, 129.3, 129.2, 128.6, 128.5,
), 36.8 (2 C) ppm. IR
KBr): υ = 2243 (w, C≡N), 1774 and 1761 (vs, C=O), 1451 (m), 1432 (m), 1268 (m),
7
.62–7.20 (6m, 10 arom. CH), 3.83 (s, 6 H, 2 OCH
3
CDCl
3
1
28.4, 128.0 (10 arom. CH), 113.0 (2 CN), 54.7 (C), 54.4 (2 OCH
3
(
–
1
1
240 (s, br., O–C), 1144 (m), 1027 (w), 907 (w), 709 (m) cm . EA for C21
H
16
N
2
O
4
(360.36): calcd. C 69.99, H 4.47, N 7.77; found C 69.81, H 4.54, N 7.52.
Reaction with 5-Diazo-10,11-dihydro-5H-dibenzo[a,d]cycloheptene (2f): An
experiment was performed only with (E)-1; molar ratio (E)-1/2f = 1:3; reaction time 4 h.
A colorless solid precipitated from the CH
colorless product were separated and crystallized from MeOH with a small admixture of
CH Cl
2
Cl
2
solution. After filtration, 190 mg of a
2
2
.
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European Journal of Organic Chemistry
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0
5
,5’-Bi(10,11-dihydro-5H-dibenzo[a,d]cycloheptenylidene) (5): Colorless crystals,
m.p. 315–318 °C (from MeOH/CH Cl
); [ref.[12a] m.p. 291–292.5 °C]. 1H NMR
CDCl ): δ = 7.13–6.78 (3m, 16 arom. CH), 3.77–3.71 (m, 4 H, 2 CH ), 3.08–3.03 (m, 4
H, 2 CH ): δ = 140.9, 138.4 (8 arom. C), 140.0 (2 Csp2), 129.1,
28.8, 126.7, 125.4 (16 arom. CH), 32.9 (4 CH ) ppm. EA for C30 24 (384.51): calcd. C
3.71, H 6.29; found C 93.60, H 6.38.
2
2
(
3
2
13
2
) ppm. C NMR (CDCl
3
1
9
2
H
Reaction with 5-Diazo-5H-dibenzo[a,d]cycloheptene (2g): An experiment was
performed only with (E)-1; molar ratio (E)-1/2g = 1:3; reaction time 72 h. The crude
product was purified chromatographically on plates coated with silica gel using
petroleum ether/CH
2
Cl mixture (1:1) as an eluent.
2
1
,2-Bis(10,11-dihydro-5H-dibenzo[a,d][7]annulen-5-ylidene)hydrazine (6): Yellow
Cl
) [ref.[13c] m.p. 224–226 °C]. H NMR
1
needles, m.p. 212–214 °C (from MeOH/CH
2
2
(CDCl
3
): δ = 7.66 (br., 2 H), 7.55 (br., 4 H), 7.40–7.30 (m, br., 8 H), 6.94 (br., 2 H),
6
.74 (br. 4 H) ppm. IR (KBr): υ = 1647 (w, C=N), 1578 (m), 1325 (m), 954 and 942
–
1
(
w), 800 (vs), 778 (vs), 729 (m) cm . EA for C30
H
20
2
N (408.49): calcd. C 88.21, H
4
.93, N 6.86; found C 87.79, H 5.01, N 6.92.
Reaction with (Trimethylsilyl)diazomethane (2h): Experiment A: molar ratio (E)-
/2h = 1:3 ; reaction time: 30 min. Experiment B: molar ratio (Z)-1/2h = 1:3; reaction
time: 30 min. From both experiments only one product 7 in comparable yields of ca.
5% was isolated after filtration through a short chromatographic column filled with
silica gel. Attempted aqueous work-up was not successful and no products were found
in the separated, organic phase (CH Cl ).
1
4
2
2
Methyl 4-Cyano-1-methyl-1H-pyrazole-5-carboxylate (7): Colorless crystal, m.p.
1
8
2–84 °C (from MeOH) (ref.[5] m.p. 84–86 °C). H-NMR (CDCl
3
): δ = 7.79 (1 arom.
): δ = 158.1
C=O), 141.8 (1 arom. CH), 135.3, 112.3 (2 arom. C), 96.7 (CN), 52.9 (OCH ), 40.4
NCH ) ppm. IR (KBr): υ = 3140 and 3122 (w), 2959 (w), 2240 (s, C≡N), 1733 (vs,
H, CH), 4.21 (3 H, OCH
3
), 4.00 (3 H, NCH
3
) ppm. 13C-NMR (CDCl
3
(
(
3
3
C=O), 1538 (m), 1485 and 1436 (s, br), 1274 (vs, O–C), 1116 (s), 1071 (m), 887 (m),
–
1
8
4
16 (m), 660 (m), 634 (w) 446 (w) cm . EA for C
.27, N 25.44; found C 50.61, H 4.33, N 25.59.
7
H
7
N
3
2
O (165.15): calcd. C 50.91, H
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European Journal of Organic Chemistry
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1
X-ray Crystal Structure Determination: Suitable single crystals of trans-4c and 7
were obtained by crystallization from MeOH solution. Single crystal X-ray data were
collected with a Rigaku Oxford Diffraction XtaLAB Synergy, Pilatus 300K
diffractometer (CuKα radiation, λ = 1.54178 Å, PhotonJet (Cu) X-ray Source with
mirror) at 100.0 K. Using Olex2,[32] the structure was solved with the SHELXT[33]
structure solution program using Intrinsic Phasing and refined with the SHELXL[34]
refinement package using Least Squares minimization.
Crystal data for trans-4c: C21
H
14
N
2
O
4
, M = 358.34, colorless, prism, crystal size
/c, Z = 4, 2 range for data
3
0
.497  0.177  0.169 mm , monoclinic, space group P2
1
collection 7.634 – 133.19°, a = 11.67620(10), b = 12.22460(10), c = 11.98520(10) Å, β
3
–3
=
97.3530(10)°, V = 1696.66(2) Å , T = 100.01(10) K, D
X
= 1.403 gcm , (CuKα) =
.816 mm , scan type , total reflections measured 40839, symmetry independent
reflections 2993, reflections used in refinement 2993, parameters refined 247, restraints
–
1
0
0
1
, final R
.056,  (max; min) = 0.23; –0.16 e Å .
1
[I  2(I) reflections = 0.0302, wR
[all data] = 0.0726, goodness of fit on F2
2
–
3
Crystal data for 7: C
7
H
7
N
3
O
2
, M = 165.16, colorless, plates, crystal size 0.469 
/n, Z = 4, 2 range for data collection
3
0
1
1
0
.393  0.124 mm , monoclinic, space group P2
1
1.38 – 133.11°, a = 8.35080(10), b = 6.12240(10), c = 15.8558(2) Å, β =
3
–3
01.2480(10)°, V = 795.087(19) Å , T = 100.00(10) K, D
X
= 1.380 gcm , (CuKα) =
–
1
.886 mm , scan type , total reflections measured 15946, symmetry independent
reflections 1400, reflections used in refinement 1400, parameters refined 112, restraints
0
, final R
.061,  (max; min) = 0.20; –0.18 e Å .
The crystallographic data for trans-4c and 7 have been deposited at the
Cambridge Crystallographic Data Center as supplementary publication numbers CCDC-
565893 and 1565533. These data can be obtained free of charge from the CCDC, 12
1
[I  2(I) reflections = 0.0278, wR
2
[all data] = 0.0721, goodness of fit on F2
–
3
1
1
Union Road, Cambridge CB2 1EZ, UK; fax: +44 (0) 1223 336 033; email:
deposit@ccdc.cam.ac.uk (or via http://www.ccdc.cam.ac.uk/conts/retrieving.html).
Computational details: All calculations reported in this paper were performed using
"Prometheus" infrastructure in the ‘Cyfronet’ computational center in Cracow. Hybrid
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European Journal of Organic Chemistry
10.1002/ejoc.201800837
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functional B3LYP with the 6-31++G(d), basis set included in the GAUSSIAN 09
package[34] was used. Recently we have established that the same level of theory
illustrates well the molecular mechanisms of different multistep cycloaddition
processes.[35–37] Global and local electronic properties of reactants were estimated
according to the equations recommended by Parr and Domingo.[21,38,39] For structure
optimization of the reactants and the reaction products, the Berny algorithm was
applied. The transition states were verified by diagonalization of the Hessian matrix and
by analysis of the internal reaction coordinates (IRC). The calculations were carried out
for the simulated presence of toluene or nitromethane as the reaction medium (PCM
model[40] was used). For optimized structures, the thermochemical data for the
temperature T = 298 K and pressure p = 1 atm were computed using vibrational analysis
data. Global electron density transfer (GEDT)[41] was calculated according to the
formula:
GEDT= −Σq
A
where q
A
is the net charge and the sum is taken over all the atoms of the ethylene
derivative used.
Acknowledgements The authors thank the Rector of the Łódź University for financial
support within the grant ‘Funds for basic research’. This research was supported in part
by PL-Grid Infrastructure in the regional computer center ‘Cyfronet’ in Cracow.
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Graphical abstract:
Textual abstract:
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European Journal of Organic Chemistry
10.1002/ejoc.201800837
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6
Isomeric, electron deficient dimethyl dicyanofumarate and dimethyl dicyanomaleate
react with bis(4-methoxyphenyl)diazomethane, diphenyldiazomethane and 9-
diazofluorene at room temperature in CH
2
Cl
2
solutions to give, after spontaneous
extrusion of N , mixtures of dimethyl cis- and trans-3,3-diaryl-1,2-dicyano-
2
cyclopropane-1,2-dicarboxylates. The proposed stepwise mechanism of the reaction was
rationalized on the basis of a computational study.
Key topic: Stepwise [3+2]-cycloadditions
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