Stable but chimeric antiaromatic 1H-azirines? A threefold reinvestigation Dedicated to Professor Holger Butensch?n on the occasion of his 60th birthday

Article abstract of DOI:10.1016/j.tetlet.2013.08.122

Three different reports on the syntheses of isolable 1H-azirines 6, 15, and 21 were reinvestigated. Instead of the claimed heterocyclic product 6, the isomeric thiazole derivative 7 has been isolated now with nearly identical yield. In the case of the asserted bicyclic 1H-azirine 15, the corrected structure includes the isomeric 3-aminomaleimide moiety of 18. A mechanism to explain the formation of this substance is suggested. The isolation of the antiaromatic compound 21 has also to be rejected. Thus, 1H-azirines keep their classification as very elusive high-energy intermediates.

Full text of DOI:10.1016/j.tetlet.2013.08.122

Tetrahedron Letters 54 (2013) 6185–6188
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Stable but chimeric antiaromatic 1H-azirines? A threefold
reinvestigation
⇑
Klaus Banert , Sandra Bochmann, Manfred Hagedorn, Frank Richter
Organic Chemistry, Chemnitz University of Technology, Strasse der Nationen 62, 09111 Chemnitz, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 3 July 2013
Revised 31 July 2013
Accepted 28 August 2013
Available online 5 September 2013
Three different reports on the syntheses of isolable 1H-azirines 6, 15, and 21 were reinvestigated. Instead
of the claimed heterocyclic product 6, the isomeric thiazole derivative 7 has been isolated now with
nearly identical yield. In the case of the asserted bicyclic 1H-azirine 15, the corrected structure includes
the isomeric 3-aminomaleimide moiety of 18. A mechanism to explain the formation of this substance is
suggested. The isolation of the antiaromatic compound 21 has also to be rejected. Thus, 1H-azirines keep
their classification as very elusive high-energy intermediates.
Dedicated to Professor Holger Butenschön
on the occasion of his 60th birthday
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Antiaromatic heterocycles
Maleimides
NMR spectroscopy
Structural corrigendum
Thiazoles
Introduction
indoles,8 oxazoles,9 isoquinolines,10 ketenimines,11 nitriles,12 or
anilines.13 Furthermore, many quantum chemical calculations, that
Strained compounds are of special interest because of their in-
creased energy content and enhanced reactivity, which frequently
results from this. For 1H-azirines 1 and 2H-azirines 2, it is obvious
that both types of heterocycles include a considerable ring strain
(Scheme 1). However, the properties of 1 and 2 are quite different.1
A great number of 2H-azirines 2, especially those with R1 – H,
were isolated and characterized by spectroscopic methods in solu-
tion or even by X-ray crystallographic structure determination.
Although compounds of type 2 are highly reactive, the 2H-azirine
unit has been found in a few natural products.2 On the other hand,
only six examples of short-lived 1H-azirines 3a–c were photo-
chemically generated and detected at very low temperatures by
IR and UV spectroscopy.3 Most probably, the electron-withdrawing
property of the cyano group diminishes the antiaromatic character
of the 1H-azirine structure and increases the relative stability of
3a–c. Thus, attempts to isolate or observe the parent compound
(1 with R1 = R2 = R3 = H) by cycloaddition4 or cyclorevision5 ap-
proaches and by using argon-matrix isolation technique were
unsuccessful and yielded unsubstituted 2 and other isomeric spe-
cies. Elusive 1H-azirine intermediates were merely postulated in
several other reactions, which finally led to 2H-azirines,6 pyrroles,7
analyzed the energy content,14 the molecular geometry,15 the
nitrogen inversion barrier,16 the basicity,17 and the vibrational fre-
quencies and IR intensities18 of the parent 1H-azirine 1, have been
published. All experimental and theoretical results emphasize the
properties of the antiaromatic heterocycles 1 as short-lived inter-
mediates, which cannot be isolated at room temperature.
Nevertheless, three quite different studies to generate and iso-
late 1H-azirines were reported.19–21 We have reinvestigated the
corresponding reactions and present our results.
R1
N
CN
N
N
R2
R3
R3
R2
R1
R2
R1
1
2
3a−c
a R1 = R2 = H
b R1 = NPh2, R2 = P(O)Ph2
c R1 = OMe, OiPr, OCHMeEt,
OCHEt2, R2 = H
⇑
Corresponding author. Tel.: +49 37153131463; fax: +49 37153121229.
E-mail address: klaus.banert@chemie.tu-chemnitz.de (K. Banert).
Scheme 1. Structures of 1H-azirines and 2H-azirines.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.08.122

6186
K. Banert et al. / Tetrahedron Letters 54 (2013) 6185–6188
Me
N
S
N
Ph
H
N
K2CO3
(1.5 equ.)
Me
Ph
Me
N
6
H
N
Me
Ph
NCS
Br
Br
DMI
120 °C
H
N
S
5
N
Ph
C
Me
Me
4
S
Ph
N
NCS
0 °C, 24 h
32%
Me
7
8
S
Me
Me
Me
K2CO3
(1.5 equ.)
H2N
N
Ph
Me
N
N
N
S
N
N
Me
S
Ph
Br
S
+
Br
S
Ph
Ph
Br
Ph
N
Me
CHCl3
rt, 5 h
75%
DMI
120 °C
1.5 h
N
NH HBr
Me
Me
10
59%
12
8%
9
11
Scheme 2. Cylization products of thiourea 5 and isothiourea 10.
Thus, the high energy content of the molecule should exclude the
isolation of 15.
Results and discussion
When we repeated the reaction of 13 with 2-(cyclohex-1-
enyl)ethylamine as described,20 we obtained two substances,
which indicated 1H NMR and 13C NMR spectroscopic data that
were identical with those reported for 14 and 15. However, some
additional NMR experiments showed that the structure of 15 has
to be excluded. The 1H NMR spectrum of this minor product exhib-
its a vicinal coupling within the CH2–NH moiety, which allows to
distinguish between both CH2–N units. But another vicinal cou-
pling between NH and the ring proton 5-H cannot be found even
when the measurement is performed in d6-DMSO instead of CDCl3.
Moreover, the minor product shows three quarternary 13C NMR
signals at low field (d = 172.5, 167.3, 149.0), which indicated
long-range correlations to CH2–N protons that are incompatible
with the reported structure of 15. Our HMBC experiments, opti-
mized for J(13C,1H) = 8 Hz, detected coupling between the protons
of the CH2–N unit and the carbon atoms with d = 167.3 and 172.5,
as well as correlation between the CH2 protons of the CH2–NH moi-
ety and the carbon atom with d = 149.0. These and other results
from 2D NMR spectroscopy23 led to the idea that the assigned
structure of 15 has to be corrected by the isomeric 3-aminomalei-
mide structure 18. Furthermore, we noticed that the UV and IR
spectroscopic data, which were reported for 15,20 are similar to
those published29a for compound 16. The later heterocycle is easily
accessible by heating the alkyne 17 with an excess of cyclohexyl-
amine.29 Thus, we measured the 13C NMR spectrum of 16, and
especially the d values assigned to the four carbon signals of the
maleimide ring (d = 172.8, 167.6, 147.7, 83.3) bear resemblance
to the data, which were reported for the butenolide unit of 15. Fi-
nally, we treated 17 with 2-(cyclohex-1-enyl)ethylamine to syn-
thesize the heterocycle 18. In every respect,23 this compound
was identical with the product claimed to possess the structure
of 15.
Recently, Narasaka and co-workers described the formation of
1H-azirine 6 in 28% yield, besides trace amounts of thiazole 7, by
treating thiourea 5 with potassium carbonate in 1,3-dimethyl-2-
imidazolidinone (DMI) at 120 °C (Scheme 2).19 Unfortunately, no
information on the spectroscopic data or the properties of the
antiaromatic heterocycle 6 was given. Moreover, hints to the syn-
thesis of the new compound 5 are also missing completely. Thus,
we subjected the known22 isothiocyanate 4 to N-benzylmethyl-
amine, which led to the desired thiourea 5 in good yield.23 After
treating 5 with potassium carbonate in DMI at 120 °C, we obtained
the aromatic heterocycle 7 with 27.5% yield. But no other side
products that might have a 1H-azirine structure like 6 were de-
tected. The thiazole 7 could be conveniently prepared for compar-
ison when allenyl isothiocyanate (8)24 was reacted with N-
benzylmethylamine. Alkylation of known25 N-benzyl-N-meth-
ylthiourea with the help of dibromide 9 did not produce 5, how-
ever, the hydrogen bromide salt of isothiourea 10 was formed
instead. This outcome is not surprising because S-alkylation of
thioureas is well known.26 Treatment of 10 with potassium car-
bonate in DMI at 120 °C led to thiazoles 11 and 12. The formation
of the side product 12 might tentatively be explained by the decay
of DMI and the condensation of the resulting formaldehyde with
11.27 For comparison, the known28 thiazole 11 was also prepared
by Hantzsch synthesis from chloroacetone and N-benzyl-N-meth-
ylthiourea in 92% yield.23
We assume that Narasaka and co-workers19 erroneously as-
signed the structure of the antiaromatic compound 6 to the iso-
meric substance 7 (see the comparable yields) or perhaps to the
thiazole 11.
Quite recently, Chen, Wang, and coworkers reported on the
reaction of the 2(5H)-furanone 13 with 2-(cyclohex-1-enyl)ethyla-
mine in the presence of potassium fluoride, which afforded the
simple substitution product 14 and the 1H-azirine 15 after separa-
tion by chromatography (Scheme 3).20 Both products were charac-
terized by melting points, elemental analysis, and UV, IR, MS, 1H
NMR, and 13C NMR spectroscopic data. In the case of 15, a mecha-
nism was offered to explain the formation of this unique nitrogen
heterocycle.
Subjection of halo-substituted butenolides of type 13 to amines
is well known to give not only trivial mono-substitution30
compounds, such as 14, but also several other products,30d,31 for
example, maleic acid monoamide monoesters32 or isomaleimides.33
To the best of our knowledge, however, maleimides like 18 were
never reported as a result of these transformations.34 A simple
mechanism to explain the genesis of 18 might include substitution
of the methoxy group in 13 by the attack of the primary amine at the
carbonyl group and formation of a cis-3-aminocarbonyl-acrolein
We suppose that the bicyclic structure of 15 combines unfavor-
able antiaromatic electronic properties with extraordinary ring
strain, which significantly exceeds that of monocyclic azirines.

K. Banert et al. / Tetrahedron Letters 54 (2013) 6185–6188
6187
NH2
NH2
Cl
NH
Cl
NH
MeO
Cl
Cl
MeO
H
O
O
N
O
THF, KF
THF, KF
O
HO
O
13
14 52%
H
H
N
19
H
N
H
3
2
H
H
O
O
H
O
2
5
N 5
O
N
H
Cl
15 6%
Cl
H
Cl
Cl
O
Cl
O
16
H
HO
O
O
O
HN
N
N
H
H
N
3
H
NH2
H
O
NH2
150 °C
O
2
5
N
H
H
13a
13b
13c
MeO2C
CO2Me
150 °C
48%
17
18
Scheme 3. Reaction of butenolide 13 with a primary amine.
13a, which can cyclize to generate the corresponding 5-hydroxy-
pyrrolin-2-one 13b. Subsequent shift of the C@C bond within the
five-membered ring33 and elimination of hydrogen chloride to pro-
duce 13c followed by the substitution of the last chlorine atom by a
second molecule of the primary amine can lead to maleimide 18.29a
Other orders of these steps or alternative reaction mechanisms32 are
also possible.
When we treated 13 with a great excess of 2-(cyclohex-1-
enyl)ethylamine (molar ratio 1:11), the yield of 14 significantly
diminished, while the proportion of 18 (12%–26% isolated yield) in-
creased and the new product 1923 (22%–29% yield) was also
formed. Control experiments showed that 19 but no 18 is produced
from pure 14 in the presence of 2-(cyclohex-1-enyl)ethylamine
and potassium fluoride in THF, whereas pure 19 did not lead to
18 under the same conditions.
Elguero and co-workers described flash vacuum pyrolysis (FVP)
of pyrazole derivative 20, which was claimed to yield the exclusive
product 21 at 600 °C (Scheme 4).21 Unfortunately, no yields or
properties of 21 were reported, and characterization by IR, 1H
NMR, and 13C NMR spectroscopic data was inadequate. Thus, be-
sides six signals at high field (d = 13–43), only two additional
low-field signals (d = 128.0, 110.2) were found in the 13C NMR
spectrum.21 This outcome is not compatible with the structure of
21. Furthermore, a reaction mechanism via an NÀN bond cleavage
and intermediate generation of a (2H-azirin-2-yl)methyleneamine
and a aminomethylene-2H-azirine35 was suggested to explain the
formation of 1H-azirine 21.21 However, the following question
arises: Why should the stable aromatic compound 20 undergo a
multistep rearrangement reaction to produce the strained and
antiaromatic isomer 21, which includes a high energy content.
When we repeated flash vacuum pyrolysis of pure 2036 at 500–
620 °C, we quantitatively obtained the starting compound.23 Even
at 700 °C, we got 84% recovery of 20 without detection of any
new product. At 730 °C, we observed unchanged 20 (31%) and a
similar amount of a very complicated mixture of products (39%),
whereas FVP at 775 °C and also at 850 °C led to complete decay
of 20 and formation of the multi-product mixture in low yield.
Heating of neat 20 in sealed tubes (150–410 °C for several days)
did not give any hint for the generation of 21 as well, while degra-
dation of 20 started at 410 °C.
In summary, it has been demonstrated now that not even a sin-
gle example of a stable 1H-azirine can be confirmed. In cases of the
claimed structures of 6 and 15, the corresponding isomeric com-
pounds 7 and 18 were isolated instead, whereas no antiaromatic
heterocyclic product 21 and no other product could be assigned
after thermolysis of the aromatic pyrazole 20. Thus, 1H-azirines
keep their classification as very short lived intermediates, which
can be detected only at very low temperature with the help of
the noble gas matrix isolation technique.37
Acknowledgment
Me
HN
Me
Me
We thank Dr. A. Ihle for assistance with the Letter.
FVP
400−600 °C
N
Me
N
N
?
Supplementary data
not
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.08.
122.
20
21
confirmed
Scheme 4. Postulated isomerization of pyrazole 20.

6188
K. Banert et al. / Tetrahedron Letters 54 (2013) 6185–6188
17. (a) Würthwein, E.-U. J. Org. Chem. 1984, 49, 2971–2978; (b) Mó, O.; de Paz, J. L.
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