Conjugated nitroalkenes in cycloaddition reactions. Part 2.+ diels-alder reactions of E-2-aryl-1-cyano-1-nitroethenes with cyclopentadiene

Article abstract of DOI:10.1135/cccc2009562

The reaction between E-2-aryl-1-cyano-1-nitroethenes 1a-1e and cyclopentadiene (2) occurs according to the carbodiene [4+2]cycloaddition scheme and leads to the corresponding 6-endo-aryl-5-endo-cyano-5-exo-nitronorbornenes 3a-3e and 6-exo-aryl-5-exo-cyano-5-endonitronorbornenes 4a-4e as the only reaction products. The attempts to detect the products of heterodiene [4+2]cycloaddition in the reaction environment were not successful.

Full text of DOI:10.1135/cccc2009562

Diels–Alder Reactions of E-2-Aryl-1-cyano-1-nitroethenes
919
CONJUGATED NITROALKENES IN CYCLOADDITION REACTIONS.
PART 2.+ DIELS–ALDER REACTIONS OF
E-2-ARYL-1-CYANO-1-NITROETHENES WITH CYCLOPENTADIENE
1,
2
3
Radomir JASIŃSKI *, Michał RZYMAN and Andrzej BARAŃSKI
Institute of Organic Chemistry and Technology, Cracow University of Technology,
1
2
31-155 Kraków, Poland; e-mail: radomir@chemia.pk.edu.pl, m.rzyman@gmail.com,
pcbarans@chemia.pk.edu.pl
3
Received December 10, 2009
Accepted June 3, 2010
Published online September 9, 2010
The reaction between E-2-aryl-1-cyano-1-nitroethenes 1a–1e and cyclopentadiene (2) occurs
according to the carbodiene [4+2]cycloaddition scheme and leads to the corresponding
6-endo-aryl-5-endo-cyano-5-exo-nitronorbornenes 3a–3e and 6-exo-aryl-5-exo-cyano-5-endo-
nitronorbornenes 4a–4e as the only reaction products. The attempts to detect the products
of heterodiene [4+2]cycloaddition in the reaction environment were not successful.
Keywords: [4+2]Cycloaddition; Nitroalkenes; Cyclopentadiene; Nitronorbornenes; Diels–
Alder reaction; Carbocycles; Heterocycles.
Reactions of simple nitroalkenes with conjugated dienes proceed according
to the carbodiene [4+2]cycloaddition scheme2,3. However, in the case of ex-
tremely electrophilic nitroalkenes, the carbodiene [4+2]cycloaddition (CDA)
may compete with the heterodiene [4+2]cycloaddition (HDA), in which
one of the N=O bonds of the nitro group participates in the reaction as
a part of the 4π electron system3–5. For example, HDA products are formed
in addition to the CDA products in the reactions of 2,3-dimethylbutadiene
and cyclohexa-1,3-diene with 1-nitro-1-(phenylsulfonyl)ethene6,7. Compe-
tition between these pathways is an interesting issue both from the practi-
cal and theoretical perspective. Therefore, while continuing our studies on
the reactivity of nitroalkenes in [4+2] π-electron cycloadditions1,8–10, in the
present work we have focused on structures of the products formed in the
reaction between E-2-aryl-1-cyano-1-nitroethenes 1a–1e and cyclopenta-
+ For Part 1, see ref.1
Collect. Czech. Chem. Commun. 2010, Vol. 75, No. 9, pp. 919–929
© 2010 Institute of Organic Chemistry and Biochemistry
doi:10.1135/cccc2009562

920
Jasiński, Rzyman, Barański:
diene (2). In the dienophiles selected for our research, steric hindrance is
very similar, but the dienophiles electrophilicity is different due to the pres-
ence of the substituent R in the phenyl ring. This is proved by the elec-
tronic chemical potentials (µ) and the electrophilicity power (ω), calculated
by us according to the expressions recommended by Domingo et al.11,12
.
Assuming the concerted reaction mechanism, we could expect six
cycloadducts in the reactions studied (Scheme 1). The aim of the present
work was to determine experimentally which of the theoretically possible
pathways were really favored. Previously1, we carried out detailed analysis
of the A–F pathways of 1d + 2 reaction using B3LYP/6-31G(d) calculations.
SCHEME 1
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Diels–Alder Reactions of E-2-Aryl-1-cyano-1-nitroethenes
921
These calculations suggest that the reaction on kinetically most favorable
path leading finally to endo-nitronorbornene 4d proceeds according to a
two step mechanism (in nitromethane ∆G≠ = 27.5 kcal/mol). The initially
formed 4-cyano-5-phenyl-2-oxa-3-azabicyclo[4.3.0]nona-3,8-diene-3-oxide
(5d) is converted to endo-nitronorbornene 4d by [3.3]-sigmatropic shift. On
less preferred paths B, D, E and F (∆G≠ = 28.0, 28.5, 37.9 and 37.4 kcal/mol,
respectively) the reaction proceeds according to a one step mechanism.
RESULTS AND DISCUSSION
All the cycloadditions studied in this work were carried out at room tem-
perature, using a triple stoichiometric excess of cyclopentadiene (2) relative
to the dienophiles and nitromethane as solvents. When the reaction was
complete, the solvent and the excess of cyclopentadiene were removed in
vacuum and the semi-solid residue was analysed by HPLC. It was found
that independently of the nature of the substituent R, the reaction afforded
only two products, whose retention times were different from those of the
starting materials. For example, the reaction of methyl E-4-(2-cyano-2-nitro-
ethenyl)benzoate (1a) with cyclopentadiene (2) led to the products with
the retention times of 19.7 and 30.0 min (Fig. 1). The ∆ω index11 suggests
the cycloaddition of these substrates should have the highest polar charac-
ter. The same types of products appeared in the reaction medium, when the
reaction was carried out at 0 °C or in other solvents of various polarity
FIG. 1
The chromatogram of the reaction mixture of methyl E-4-(2-cyano-2-nitroethenyl)benzoate
(1a) with cyclopentadiene (2)
Collect. Czech. Chem. Commun. 2010, Vol. 75, No. 9, pp. 919–929

922
Jasiński, Rzyman, Barański:
(Table I). The compounds were effectively separated by semipreparative
HPLC, which yielded individual compounds in an analytically pure form
(Table II).
The IR spectra of the reaction products show the absorption bands, which
confirm13 the presence of cyano, nitro and methoxycarbonyl groups, a
bridging methylene group, a double bond, and a para-substituted benzene
ring (Table III). However, there are no bands confirming the presence of
six-membered cyclic nitronates that usually occur in the range of 1570–
1610 cm–1 (refs5,14–16). Similarly, the UV spectra do not show the presence
of a derivative of 1,2-oxazine-N-oxide with a substituent at the carbon atom
of >C=N(O)O– moiety capable of conjugation with the C≡N bond. This is
confirmed by the absence of a band in the 260–320 nm range14. A strong
band (log ε > 4.1) at ~235 nm is observed instead, which is most likely re-
lated to the π → π* transitions in the para-substituted benzene ring13,17
.
Therefore, the pathways leading to the products of heterodiene [4+2]cyclo-
addition (paths C–F) can be excluded.
The mass spectra of both compounds are nearly identical. The molecular
weight of the molecular ions (M•+) is equal to the sum of molecular weights
of the substrates 1a and 2 (m/z = 298), which allowed us to assign the mo-
lecular formula of C16H14N2O4. The formula is consistent with the results of
elemental analysis.
TABLE I
Selectivity of reaction between compounds 1a–1e and 2
Condition
Isomer ratio
Reaction
[3]/[4]
t, °C
Solvent
1a + 2
1a + 2
1a + 2
1a + 2
1b + 2
1c + 2
1d + 2
1e + 2
25
0
nitromethane
nitromethane
THF
0.19
0.17
0.18
0.14
0.15
0.15
0.14
0.08
25
25
25
25
25
25
dioxane
nitromethane
nitromethane
nitromethane
nitromethane
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Diels–Alder Reactions of E-2-Aryl-1-cyano-1-nitroethenes
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924
Jasiński, Rzyman, Barański:
1
The H NMR spectra of both compounds are very similar. Nevertheless,
the proton chemical shifts and their coupling constants (Table IV) clearly
indicate arylnorbornene structures18–20. As expected, the resonance signals
of H8 and H7 protons of methylene bridge occur at higher fields than the
aromatic ring protons, which appear at the lowest ones. The H4 proton sig-
nal in both isomers occurs in a field slightly lower than that of the H1 pro-
ton. This is caused by location of the H4 protons in vicinity of the CN and
NO2 groups, which cause larger deshielding than the aromatic ring does in
vicinity of the H1 proton. For the major isomer 4a, the doublet from the
H6 proton is at δ 3.91 ppm, while for the minor isomer 3a, at δ 4.39 ppm.
It is probably caused by the anisotropy of double bond in the norbornene
ring18,19; H6 seems to be closer to the endo proton than to the exo one. The
protons of the vinylidene moiety of both isomers have similar chemical
shifts as well.
1
On the basis of the IR, UV, MS and H NMR spectra, it was possible to
assign the structure of methyl (6-cyano-6-nitronorbornen-5-yl)benzoate to
the products obtained. However, their stereochemistry could not be as-
signed from those spectra. This was determined from the NOESY experi-
ments.
In the NOESY spectrum of the minor compound (Fig. 2), a correlation sig-
nal occurs at the intersection of projection lines of chemical shifts of H6
and H7 protons on the spectrum diagonal, which proves their spatial prox-
TABLE III
Essential IR data for compounds 3a–3e and 4a–4e (cm–1
)
Compound
ν(CN)
ν(R)
ν(NO2)
δ(CH2)
δ(HC=CH)
δ(Ar)
3a
4a
3b
4b
3c
4c
3d
4d
3e
4e
2256 vw
2255 w
–
1729 s
1724 s
1011 m
1011 m
1012 m
1015 m
–
1556 s, 1338 m
1567 s, 1342 m
1559 s, 1384 m
1567 s, 1330 s
1572 s, 1336 s
1568 s, 1332 m
1560 s, 1340 s
1562 s, 1341 s
1560 s, 1337 s
1560 s, 1333 m
1459 w
1458 w
1460 m
1456 w
1459 w
1456 m
1459 w
1454 w
1459 m
1459 m
715 m
719 m
727 m
727 m
728 m
728 m
739 m
735 m
737 m
728 m
853 m
858 m
840 m
839 m
842 m
840 m
696 m
695 s
2249 w
–
2250 w
–
2254 w
–
–
1031 m
1032 m
844 m
838 m
2253 w
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Diels–Alder Reactions of E-2-Aryl-1-cyano-1-nitroethenes
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926
Jasiński, Rzyman, Barański:
imity and suggests the exo-position of the H6 proton in the molecule. In-
deed, according to the B3LYP/6-31G(d) calculations1, the H6–H7 proton
distance in the 3a isomer does not exceed 2.4 Å, while in the 4a isomer,
it is more than 3.7 Å. The absence of the correlation signal between the
H7 proton and the ortho protons of the phenyl ring proves that the ring is
in the endo-position. Therefore, a structure of methyl (6-endo-cyano-6-exo-
nitronorbornen-5-yl)benzoate (3a) can be assigned to the compound. In
the NOESY spectrum of the major product, a correlation signal between the
H6 and H7 protons is not observed. This fact indicates that the H6 proton
is located below the plane of the carbocyclic ring. Due to the presence of
the correlation signal between the H7 proton and the ortho protons of the
phenyl ring, a structure of methyl (6-exo-cyano-6-endo-nitronorbornen-5-yl)-
benzoate (4a) can be assigned to the compound.
The configuration of the CN and NO2 groups in 3a and 4a was confirmed
by HPLC analysis of the products of their retro [4+2]cycloaddition reac-
tions, which were carried out in boiling nitromethane. It was found that
both reaction mixtures, apart from cyclopentadiene oligomers and the un-
FIG. 2
The NOESY correlations for nitronorbornenes 3a and 4a
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Diels–Alder Reactions of E-2-Aryl-1-cyano-1-nitroethenes
927
reacted substrates, contained methyl E-4-(2-cyano-2-nitroethenyl)benzoate
(1a) and no methyl Z-4-(2-cyano-2-nitroethenyl)benzoate (9a). The latter
should be found, if the substrates used in the retro reactions had the structure
of methyl (6-exo-cyano-6-endo-nitronorbornen-5-yl)benzoate (10a) or methyl
(6-endo-cyano-6-exo-nitronorbornen-5-yl)benzoate (11a).
The [4+2]cycloaddition reactions that involve the E-2-aryl-1-cyano-1-
nitroethenes 1b–1e proceed analogously. Regardless of their
electrophilicity, only the carbodiene [4+2]cycloaddition products were
formed in each case. Their physical properties are listed in Tables II–IV.
Thus, our experimental observations indicate that the reaction between
E-2-aryl-1-cyano-1-nitroethenes 1a–1e and cyclopentadiene (2) leads to
6-endo-aryl-5-endo-cyano-5-exo-nitronorbornenes 3a–3e and 6-exo-aryl-
5-exo-cyano-5-endo-nitronorbornenes 4a–4e as the final reaction products.
According to the B3LYP/6-31G(d) calculations, the reaction path leading
finally to endo-nitronorbornenes 3a–3e should be the most favored one
from the kinetic point of view, independently on polarity of the reaction
medium. The experimental results confirm this prediction (Table I). How-
ever, it was not possible to prove the existence of HDA intermediates dur-
ing conversion of substrates into endo-nitronorbornenes. All attempts to
identify heterocyclic intermediates in the reaction medium (1H NMR, HPLC)
were not successful. Therefore, it has to be assumed that the reactions of
E-2-aryl-1-cyano-1-nitroethenes with cyclopentadiene are proceeding ac-
cording to carbodiene cycloaddition. However, the mechanism of the
cycloaddition itself is a separate issue. Despite large electrophilicity of the
dienophiles, the reaction on both competing pathways proceeds with re-
tention of stereochemical configuration of nitroalkenes. This fact suggests
concerted mechanism of the reaction. Nevertheless, we are aware that final
confirmation of this mechanism can be acquired only from kinetic mea-
surements.
Collect. Czech. Chem. Commun. 2010, Vol. 75, No. 9, pp. 919–929

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Jasiński, Rzyman, Barański:
EXPERIMENTAL
General
For reaction testing, a HPLC Knauer apparatus equipped with a UV-Vis detector and
a Lichrospher 100-5 RP18 column (4 × 250 mm) was applied. Methanol–water or THF–water
mixtures were used as eluents. For separation of the cycloadducts from post-reaction
mixtures, a semipreparative HPLC equipped with a Lichrospher 100-10 RP18 column (16 ×
250 mm) was applied, and methanol–water (65:35 v/v) was used as an eluent at the flow
rate 10 ml/min. Melting points were determined on a Boetius apparatus and are not cor-
rected. IR spectra were recorded on a Bio-Rad 175C spectrophotometer in KBr pellets. UV
spectra were taken with a StellarNet EPP-2000C spectrometer in methanol. The mass spectra
were obtained using a Finningan 955 apparatus, operated at 70 eV ionization energy and
100–130 °C ion source temperature. 1H NMR spectra were taken on a Bruker AMX 500 spec-
trometer using CDCl3 as a solvent and TMS as an internal reference. The E-2-aryl-1-cyano-
1-nitroethenes 1a–1e and cyclopentadiene (2) were prepared according to the method
described in literature21,22
.
Synthesis of 6-Aryl-5-cyano-5-nitronorbornenes
A mixture of nitroalkene 1 (4 mmol) and freshly distilled cyclopentadiene (2) (12 mmol) in
5 ml of dry nitromethane was stirred at room temperature for 24 h. The excess of cyclo-
pentadiene and the solvent were then evaporated to dryness in vacuo and the residue was
separated by semipreparative HPLC. Evaporation of the eluent from the obtained fractions
gave pure 6-aryl-5-cyano-5-nitronorbornenes. The products were recrystallized from ethanol.
Their physical constants are given in Tables II–IV.
We wish to express our thanks to Prof. V. M. Berestovitskaya from Herzen Russian State
Pedagogical University in St. Petersburg for valuable discussions during the synthesis of
E-2-aryl-1-cyanonitroethenes, as well as to Dr E. Schneler and Dr P. Suder from Jagiellonian
University in Cracow for measuring 1H NMR and mass spectra. This work was supported by Polish
Ministry of Science and Higher Education (Grant No. C2/293/DS/09).
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