Highly strained 2,3-bridged 2H-azirines at the borderline of closed-shell molecules

Article abstract of DOI:10.1002/chem.201002474

Substituted 1-azidocyclopentenes and 1-azidocyclohexenes were photolyzed to generate 2,3-bridged 2H-azirines. In the case of bridgehead azirines with a six-membered carbocycle, detection by NMR spectroscopic analysis was possible, whereas even kinetically

Full text of DOI:10.1002/chem.201002474

DOI: 10.1002/chem.201002474
Highly Strained 2,3-Bridged 2H-Azirines at the Borderline of Closed-Shell
Molecules**
Klaus Banert,*[a] Barbara Meier,[a] Enrico Penk,[a] Biswajit Saha,[a]
Ernst-Ulrich Wꢀrthwein,*[b] Stefan Grimme,*[b] Tobias Rꢀffer,[c]
Dieter Schaarschmidt,[c] and Heinrich Lang[c]
Dedicated to Professor Stefan Spange on the occasion of his 60th birthday
Abstract: Substituted 1-azidocyclopen-
tenes and 1-azidocyclohexenes were
photolyzed to generate 2,3-bridged 2H-
azirines. In the case of bridgehead azir-
ines with a six-membered carbocycle,
detection by NMR spectroscopic analy-
sis was possible, whereas even kineti-
cally stabilized bridgehead azirines
with a five-membered ring could not
be characterized by low-temperature
NMR spectroscopic analysis. Thus, a
recent report on the latter heterocycles
was corrected. Depending on the sub-
stitution pattern, irradiation of 1-azido-
cyclopentenes either led to products
that can be explained on the basis of
short-lived 2,3-bridged 2H-azirines, or
gave secondary products generated
from triplet nitrenes. The diverse pho-
toreactivity of 2,3-bridged 2H-azirines
was also studied by quantum chemical
methods (DFT, CCSD(T), CASSCF-
AHCTUNGTRENN(GUN 6,6)) with respect to the singlet and
Keywords: azides
·
nitrogen
triplet energy surfaces. The ring-open-
ing processes leading to the corre-
sponding vinyl nitrenes were identified
as key steps for the observed reactivity.
heterocycles · photolysis · quantum
chemical calculations
intermediates
·
reactive
Introduction
(Scheme 1).[1–3] The stability of the heterocycle 2a allowed
this compound to be to distilled in vacuo.[4] In contrast to
such stability, the more strained compounds of type 2 with a
six-[4a,5,6] or a five-membered[6] ring were produced only in
situ; the existence of such species were proved by trapping
reactions such as addition of nucleophiles at the C=N bond.
However, the bridgehead azirines 2b–d could be generated
in solution by irradiation of 1b–d at low temperature and
have recently been detected in 83–91% yield by IR and
NMR spectroscopic analysis even at ambient temperature.[7]
Especially in the presence of wet organic solvents, subse-
quent reaction led to the formation of the dimers 3b–f, and
products with R=H were quickly oxidized to the corre-
sponding pyrazines 4b, 4c, and 4e when oxygen was not rig-
orously excluded. Nevertheless, highly strained azirines such
as 2b–d can undergo addition and cycloaddition reactions
that are not possible for simple 2H-azirines. For example, 2-
cyanoaziridines 5b–d were available from 2b–d by addition
of hydrogen cyanide at ꢀ508C, whereas less strained azir-
ines did not react with this reagent even at 908C and long
reaction times. Tricyclic compounds 6b and 6c were gener-
ated by 1,3-dipolar exo cycloaddition of diazomethane at 2b
and 2c, respectively, and characterized at low temperature
because cycloreversion took place at temperatures above
Strained compounds are of special interest because of their
increased energy content and the enhanced reactivity that
frequently results from this. The 2,3-bridged 2H-azirines 2
include considerable ring strain and can be easily generated
by photolysis or thermolysis of cyclic vinyl azides
1
[a] Prof. K. Banert, Dr. B. Meier, Dr. E. Penk, Dr. B. Saha
Organic Chemistry, Chemnitz University of Technology
Strasse der Nationen 62
09111 Chemnitz (Germany)
Fax : (+49)371-531-21229
E-mail: klaus.banert@chemie.tu-chemnitz.de
[b] Prof. E.-U. Wꢀrthwein, Prof. S. Grimme
Institute of Organic Chemistry, University of Mꢀnster
Corrensstrasse 40, 48149 Mꢀnster (Germany)
[c] Dr. T. Rꢀffer, D. Schaarschmidt, Prof. H. Lang
Inorganic Chemistry, Chemnitz University of Technology
Strasse der Nationen 62
09111 Chemnitz (Germany)
[**] Reactions of unsaturated azides, Part 25; for Part 24, see: K. Banert,
M. Hagedorn, J. Wutke, P. Eccorchard, D. Schaarschmidt, H. Lang,
Chem. Commun. 2010, 46, 4058–4060.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002474.
1128
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Chem. Eur. J. 2011, 17, 1128 – 1136

FULL PAPER
bered carbocycle were isolated
and characterized by elementa-
ry analyses as well as on the
basis of IR, 1H, and 13C NMR
spectroscopic data.[9] The unex-
pectedly high stability of 11 and
12 may be caused by kinetic
stabilization due to the sterical-
ly demanding substituents. For
example, comparison of the
properties of 2c with those of
2d shows that an additional
methyl group at the bridgehead
increases the kinetic stability of
the azirines significantly.[7]
Herein, we report the synthe-
ses of 1-azidocyclopentenes
with sterically bulky substitu-
ents and on our attempts to
generate and detect kinetically
stabilized bridgehead azirines
of type 2 (n=0). Furthermore,
a
structural corrigendum of
compound 11 is presented. Fi-
nally, through the use of addi-
tional photochemical reactions
of cyclic vinyl azides and by
quantum chemical calculations,
we show that highly strained
2,3-bridged azirines are located
at the borderline of closed-shell
Scheme 1. Synthesis and reaction behavior of 2,3-bridged 2H-azirines.
ꢀ208C to yield the allyl azides 8b/c and 9b/c. Thus, inter-
molecules and diradical triplet nitrenes.
mediates with a 1,2,3-triazabicycloACTHUNTGRNEUNG[3.1.0]hex-2-ene substruc-
ture were detected for the first time.[7] Such species were
only postulated in the known reaction of simple azirines
with diazo compounds,[8] which also resulted in the slow for-
mation of allyl azides at room temperature. Whereas alkyl-
or aryl-substituted azirines without an electron-withdrawing
group did not react with cyclopentadiene,[1b] the heterocycles
2b and 2c underwent Diels–Alder reaction under mild con-
ditions to give stereoselectively the cycloadducts 7b and 7c,
respectively.[7]
Results and Discussion
We prepared the ether 15 and the esters 16a–e from alcohol
14, which was easily accessible by reduction of the known
aldehyde 13[10] (Scheme 2). Unfortunately, we could not
detect any NMR signals arising from a 2,3-bridged azirine of
type 2 (n=0), when the azides 13, 15, or 16a–e were photo-
Even upon irradiation of 1e or 1 f at ꢀ120 and ꢀ808C, re-
spectively, and NMR spectroscopic analysis at the same tem-
peratures, it was not possible to detect 2,3-bridged 2H-azir-
ines with a five-membered ring, such as 2e or 2 f, because,
at best, only strongly broadened signals of the starting mate-
rials were observed. Nevertheless, the photolyzed solutions
yielded dimers of type 3 or the corresponding aromatic pyr-
azine derivatives 4 after thawing.[7] Thus, intermediates 2e
and 2 f are plausible, but too short lived to allow observation
by spectroscopic methods in solution. Surprisingly, synthesis
of bridgehead azirine 11 by simply heating a solution of
vinyl azide 10 in chloroform to reflux was reported recent-
ly.[9] Treatment of 11 with zinc and ammonium chloride in
boiling tetrahydrofuran was claimed to yield the product 12.
Both 2,3-bridged 2H-azirines 11 and 12 with a five-mem-
Scheme 2. Synthesis of 1-azidocyclopentenes with substituents in posi-
tion 2.
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K. Banert, E.-U. Wꢀrthwein, S. Grimme et al.
lyzed in chloroform at ꢀ508C. Even irradiation at ꢀ1208C
in deuterated dichlorofluoromethane and NMR spectro-
scopic analysis at the same temperature led to the same neg-
ative result, which is in sharp contrast to the formation and
stability of the highly strained azirines 11 and 12. We
assume that the ring strain of bridgehead azirines of type 2
(n=1) is considerably lower if compared with that of ring-
contracted compounds 2 (n=0). Whereas in the latter case
the kinetic stabilization by the bulky substituents was not
sufficient, the same substituents are able to raise the stabili-
ty of the former azirines significantly.[11]
Our results cast doubt upon the claimed structures of the
products 11 and 12.[9] Furthermore, we estimated the
13C NMR spectroscopic data that would result from these
structures on the basis of other 2,3-bridged 2H-azirines[7,11]
and 2-chloro-2H-azirines[12] with a carbonyl group of an
ester or a ketone in position 2. Thus, we calculated shift
values of d=50–60 ppm for C-1 of 11 and 12, whereas shifts
of d=85.48 and 87.49 ppm, respectively, were reported.[9]
Moreover, azirines[1b] and especially bridgehead azirines,[7]
show a characteristic IR absorption band, for example, at
1743 cmꢀ1 for 2c, that is missing in the case of 11. These dis-
crepancies encouraged us to prepare azide 10 and its secon-
dary product 11 so that the structure of the latter could be
clarified. After heating 10 in boiling chloroform, a substance
was isolated in 67% yield that was identical to previously
Scheme 3. Generation of nitrile 18 from azide 10.
1
described 11[9] according to its melting point as well as its H
and 13C NMR spectroscopic data. In the 13C,1H long-range
correlation 2D-NMR spectrum optimized to geminal and vi-
cinal coupling with J=5 Hz, however, the cross-signal of
bridgehead carbon C-5 (d=166 ppm) and 3-H was missing,
and a clear correlation between the carbon at d=166 ppm
and the CH2 protons was found. This is not compatible with
the structure of 11, and the carbon signal with d=166 ppm
cannot be assigned to either the C=N unit or the carbonyl
group of 11. Instead, it should result from an sp2-hybridized
carbon at position 2 of a 1,3-dioxolane ring. Therefore, we
considered the isomeric alternative structure 18, which in-
cludes a nitrile group (with d=114 ppm) and a push–pull
substituted olefin that was in agreement with all spectro-
scopic data (Scheme 3). Final concerns about the structure
of 18 were resolved by single-crystal X-ray diffraction analy-
sis (Figure 1). Compound 18 was also formed by photolysis
of 10 in chloroform (maximum yield ca. 40%) along with
several other products. However, even at low temperature,
irradiation of 10 did not lead to a set of NMR signals that
were consistent with the corresponding 2,3-bridged azirine.
The generation of 18 from 10 can be explained by invok-
ing the help of intermediate 17, because ring contraction of
3-azidocycloalk-2-en-1-ones to yield 2-cyanocycloalkanones
is a well-known transformation.[3b,13] After deprotonation,
cleavage of the four-membered ring, and reprotonation,
compound 17 should afford the final product 18. Alterna-
tively, keto–enol tautomerism of 10 may produce the 1-azi-
docyclopentadiene 19, which should undergo ring fission to
furnish 1-cyanobuta-1,3-diene 20.[14] This intermediate will
tautomerize to give the ketone 18.
Figure 1. Molecular structure of 18 determined by single-crystal X-ray
diffraction analysis.
When solutions of azides 21a[5f] or 21b[5f,6] were irradiated
in anhydrous CD2Cl2 at ꢀ808C or in anhydrous CDCl3 at
ꢀ508C with a mercury high-pressure lamp, bridgehead azir-
ines 22a (22%) and 22b (65%), respectively, were formed
(Scheme 4). These compounds could be characterized even
1
at ambient temperature by H and 13C NMR spectroscopic
analysis and on the basis of their IR spectra. However, pho-
tolysis of the starting material 21c[6] in chloroform at ꢀ508C
did not lead to 2H-azirine 22c or to dimeric compounds sim-
ilar to 3 f. Surprisingly, quite different products, namely, ni-
trile 23c and azo compound 24c, were generated and isolat-
ed by chromatography in 41 and 15% yield, respectively.
The structure of 23c was established not only by the usual
spectroscopic methods, but also by hydrolysis to produce the
1130
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2,3-Bridged 2H-Azirines
FULL PAPER
Scheme 4. Photolysis of azides 21a–c.
enamine 27c and the diketone 28c. The former compound
was prepared for comparison through the Staudinger reac-
tion of azide 21c followed by hydrolysis of the resulting imi-
nophosphorane 26c. When a solution of 21b in chloroform
was irradiated at ꢀ508C, in the presence of the photosensi-
tizer benzophenone, no azirine 22b could be detected, and
the dimeric compound 23b as well as a trace amount of 24b
were observed instead. Attempts to purify 23b by chroma-
tography resulted in a rearrangement reaction, and the spiro
compound 25b was isolated in 30% yield (based on 21b).
The product 25b was characterized not only by the usual
spectroscopic methods, but also by single-crystal X-ray dif-
fraction analysis, which confirmed the structure and espe-
cially the stereochemistry (Figure 2).
A possible mechanism for the formation of compounds
23b, 23c, 24b, 24c, and 25b assumes the initial formation of
3
3
the triplet nitrenes 29b and 29c, which is probably induced
either by the ketone unit of 21c as an intramolecular photo-
sensitizer[15] or, in the case of azide 21b, by the sensitizer
3
benzophenone (Scheme 5). Dimerization of 29 can lead to
Figure 2. The molecular structure of compound 25b.
the formation of side product 24. Processes that lead to the
creation of azo compounds are well known for aromatic
azides and the corresponding triplet nitrenes.[16,17] Another
3
dimerization reaction of 29 can generate the diradical 30,
It is remarkable that quite different products result from
simple photolysis of homologous azides 21b and 21c. Be-
cause we were not able to detect intermediates such as 22c
and 329 directly, quantum chemical calculations were per-
formed to further analyze the reaction.
which gives rise to nitrile 23 by ring cleavage. In the case of
23b, the isomerization to yield spiro compound 25b can be
explained by keto–enol tautomerism to intermediate 31b,
followed by an intramolecular nucleophilic addition.
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K. Banert, E.-U. Wꢀrthwein, S. Grimme et al.
3
azirine triplet structure 22c is,
as expected, high in energy
(59.5 (44.8) kcalmolꢀ1). Passing
a transition state for the ring-
opening
reaction
i.e.,
(69.5
10.0
(56.4) kcalmolꢀ1,
(11.6) kcalmolꢀ1 more than the
azirine triplet) a very favorable
triplet vinyl nitrene 29c struc-
3
ture with a relative energy of
ꢀ3.6
(ꢀ6.0) kcalmolꢀ1
was
found.
The calculations for the six-
membered ring systems 22b
predict
a different reactivity.
Two closed-shell conformers
(twist-boat and twist-chair) for
the bicyclic azirine system 22b
could be localized, the latter
being 7.4 (4.6) kcalmolꢀ1 lower
in energy (Erel =0.0 kcalmolꢀ1).
Scheme 5. Postulated mechanisms to explain the formation of 23b, 23c, 24b, 24c, and 25b.
Quantum chemical calculations: To study the nature of the
intermediates resulting from irradiation of the azides and
their dependence on the ring size, quantum chemical calcu-
lations were performed. At first, DFT calculations using the
PBE/TZVP and PBE/TZVPP method (Turbomole 6.0)[18]
and B3LYP/6-31+G(d) (Gaussian 03)[19] were performed to
localize the appropriate structures on the energy hypersur-
face. Then, to appropriately treat both open-shell as well as
closed-shell species, CASSCFACTHNUTRGENUGN(6,6) calculations using the 6-
31G(d) basis set, as implemented in the Gaussian 03 pack-
age of programs, were used. In the CASSCF treatment of
22c, the correlated orbitals refer to the s and p bonds of the
three-membered ring and their anti-bonding counterparts
(for a typical plot see the Supporting Information). For the
3
triplet species 29c the active space consists of the orbitals
ꢀ
of the s and p bonds of the C N-part of the molecule, the
corresponding antibonding combinations, and the formally
singly occupied lp orbitals that result from the broken sigma
molecular orbital.
CCSD(T)/6-31+G(d) single-point calculations were fur-
ther utilized to verify the CASSCF
ary points were checked by frequency analyses. The DFT as
well as the CASSCF(6,6) and CCSD(T) relative energies
ACHTUNGTNER(NUNG 6,6) results. All station-
AHCTUNGTRENNUNG
qualitatively agree reasonably well (see the Supporting In-
formation, Tables S1 and S2). In the following discussions
CASSCF values are used with the CCSD(T) data given in
parentheses.
Scheme 6. Results of the quantum chemical calculations for the ring-
opening reactions of compounds 22c and 22b. Singlet and triplet elec-
tronic states were considered. The relative energies [kcalmolꢀ1] given
refer to the CASSCFACHTNUTGRNEUNG(6,6)/6-31G(d) calculations. Data in parenthesis in
For the five-membered system, we were able to identify
minima on the singlet hypersurface for the azirine 22c
(Erel =0.0 kcalmolꢀ1) as well as for the (open shell) vinyl ni-
trene structure 29c (3.7 kcalmolꢀ1 (5.1 at CCSD(T)),
Scheme 6). The transition state for the respective ring-open-
ing process has a calculated energy of 7.3 (12.0) kcalmolꢀ1
relative to 22c, thus making this reaction very facile. The
the case of the triplet reactions refer to the reaction energies on the trip-
let hypersurface.
No minimum for a singlet vinyl nitrene structure 29b upon
ꢀ
widening of the C N-bond was found. Two energy-rich trip-
let azirine conformers for 322b (59.9 (62.0) and 60.2
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2,3-Bridged 2H-Azirines
FULL PAPER
(62.6) kcalmolꢀ1) were found, which open through a transi-
tion state with an energy of 86.2 (72.6) kcalmolꢀ1 to produce
a minimum structure for the corresponding triplet vinyl ni-
tivity detector and DB-1 column (30 m). HRMS (ESI) spectra were re-
corded with an Applied Biosystems Mariner 5229 mass spectrometer or a
Bruker micrOTOF-QII spectrometer. Elemental analyses were per-
formed with a Vario EL elemental analyzer from Elementar Analysen-
systeme GmbH Hanau or with a Vario Micro Cube from Elementar. Ele-
mental analyses of explosive azides and highly unstable azirines could
not be performed. TLC was performed with Macherey–Nagel Polygram
SIL G/UV254 polyester sheets. Flash column chromatography was per-
formed with 32–63 mm silica gel.
3
trene 29b with a relative energy of 8.1 (10.3) kcalmolꢀ1.
We interpret these results to be a consequence of the dif-
ferent ring strain in both systems. Thus, in the case of the
six-membered ring system, the less pronounced ring strain
significantly disfavors the vinyl nitrene structure, making the
2,3-bridged azirine in its two conformers the only minima
on the singlet hypersurface. In contrast, in the case of the
five-membered ring system, the azirine structure is less
stable due to higher ring strain and leads easily to a singlet
vinyl nitrene structure of comparable energy (3.7 (5.1) kcal
molꢀ1).
The (photochemical) singlet–triplet excitation energies
were calculated to be of comparable energy, with a signifi-
cantly higher barrier for ring opening in the case of the six-
membered ring system. For the five-membered ring system,
the triplet vinyl nitrene is calculated to correspond to the
global minimum (3.6 (6.0) kcalmolꢀ1 better than the singlet
azirine), whereas the six-membered triplet nitrene is predict-
ed to be less stable than the corresponding singlet azirine by
8.1 (10.3) kcalmolꢀ1.
General procedure for photolysis: Irradiation was conducted by using a
high-pressure mercury lamp (TQ150, Heraeus GmbH) supplied with
glass equipment and an ethanol cryostat (ꢀ50 to ꢀ808C) or cooling with
a mixture of 2-methylbutane and liquid nitrogen (ꢀ1208C). Most of
these photolyses were monitored by NMR spectroscopic analysis utilizing
anhydrous CDCl3 (ꢀ508C), CD2Cl2 (ꢀ808C), or CDCl2F[21] (ꢀ1208C). A
solution of the appropriate starting material was irradiated in an NMR
tube. To exclude oxygen, the solution was flushed with argon in an ultra-
sonic bath prior to irradiation. Dioxane was used as a standard to deter-
1
mine yields based on H NMR spectroscopic data.
Single-crystal X-ray diffraction analysis: Data were collected with an
Oxford Gemini S diffractometer at 110 K using MoKa (l=0.71073 ꢂ)
(18) and CuKa (l=1.54184 ꢂ) (25b) radiation. The structures were
solved by direct methods and refined by full-matrix least-square proce-
dures on F2.[22] All non-hydrogen atoms were refined aniosotropically and
a riding model was employed in the refinement of the hydrogen atom po-
sitions. CCDC-790143 (18) and 790144 (25b) contain the supplementary
crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
(2-Azidocyclopent-1-en-1-yl)methanol (14): At 08C, a solution of NaBH4
(125 mg, 3.3 mmol) in water (0.4 mL) was added dropwise to a solution
of 13[10] (361 mg, 2.63 mmol) in THF (5 mL) and Et2O (5 mL). The mix-
ture was continuously stirred for 1 h at 08C and for 24 h at RT. There-
after, the solvents were removed under reduced pressure, and the residue
was treated with saturated aqueous NaCl (10 mL). This mixture was ex-
tracted with Et2O (3ꢃ5 mL), and the combined layers were washed with
water (5 mL) and dried over MgSO4. After removal of the solvent under
reduced pressure, 14 remained as a yellow oil (134 mg, 37%). 1H NMR
(CDCl3): d=1.67 (brs, 1H; OH), 1.96 (m, 2H; CH2), 2.46 (m, 2H; CH2),
2.61 (m, 2H; CH2), 4.13 ppm (s, 2H; CH2OH); 13C NMR (CDCl3): d=
20.25 (t, CH2), 31.08 (t, CH2), 32.13 (t, CH2), 58.00 (t, CH2OH), 125.72
(s), 132.89 ppm (s); IR (CDCl3): n˜ =3612 (OH), 2113 (N3), 1280 cmꢀ1
(N3).
Conclusion
Clearly, ring strain plays a dominant role when cyclic vinyl
azides lose dinitrogen to generate the corresponding 2,3-
bridged 2H-azirines or short-lived nitrenes. Based on this
fact, and strongly supported by quantum chemical calcula-
tions, the formation of quite different products upon photol-
ysis of homologous 3-azidocycloalk-2-enones 21b and 21c
can be explained. However, the substitution pattern of the
vinyl azides is also important, as shown by comparison of
the different reactions 1e,f!3e,f, 10!18, and 21c!23c+
24c. The reaction mechanism postulated for these transfor-
mations, and recent studies on similar thermal or photo-
chemical reactions,[3,5–7,20] demonstrate the manifold chemis-
try of cyclic vinyl azides.
(2-Azidocyclopent-1-enyl)methyl triphenylmethyl ether (15): Trityl chlo-
ride (130.3 mg, 0.467 mmol) was added to a solution of 14 (50 mg,
0.359 mmol) in a 1:1:1 mixture (1 mL) of pyridine, THF, and CH2Cl2 and
stirred at RT for 48 h. After removal of the solvent under reduced pres-
sure, the residue was purified by flash chromatography (silica gel; Et2O/
hexane, 1:10) to give 15 as a light-yellow oil (122 mg, 89%). 1H NMR
(CDCl3): d=1.99 (m, 2H; CH2), 2.60 (m, 4H; 2ꢃCH2), 3.70 (s, 2H;
CH2O), 7.24–7.36 (m, 9H; ArH), 7.51 ppm (m, 6H; ArH); 13C NMR
(CDCl3): d=20.18 (t, CH2), 30.95 (t, CH2), 32.56 (t, CH2), 59.10 (t,
CH2O), 86.59 (s, CPh3), 124.30 (s), 126.85 (d, ArC), 127.67 (d, ArC),
128.67 (d, ArC), 132.31 (s), 144.12 ppm (s); IR (CDCl3): n˜ =2100 cmꢀ1
(N3).
Experimental Section
Caution! Care must be taken in handling azides, which are explosive. Es-
pecially, neat azides can lead to large explosions on friction, impact, or
heating.
General procedure for the synthesis of esters 16a–e: At 08C, the corre-
sponding acyl chloride (0.719 mmol) was added dropwise to a solution of
14 (50 mg, 0.359 mmol) and triethylamine (72.8 mg, 99.9 mL, 0.719 mmol)
in CH2Cl2 (2 mL). The mixture was continuously stirred at RT for 16 h.
Thereafter, the solvent was removed under reduced pressure, and the res-
idue was purified by flash chromatography (silica gel; Et2O/hexane, 1:10)
to give the ester 16 as a yellowish oil.
Instrumentation and measurement: Melting points were determined with
a Pentakon Dresden Boetius apparatus. FTIR spectra were recorded
with a Bruker IFS 28 FTIR spectrophotometer. IR measurements were
made on solutions in KBr cuvettes or as KBr pellets. 1H NMR spectra
were recorded with Varian Gemini 2000 or Unity Inova 400 spectrome-
ters operating at 300 and 400 MHz, respectively. By using the same spec-
trometers, 13C NMR data were recorded at 75 and 100 MHz. NMR sig-
nals were referenced to TMS (d=0 ppm) or solvent signals and recalcu-
lated relative to TMS. The multiplicities of 13C NMR signals were deter-
mined with the aid of DEPT135 experiments. GC-MS spectra were ac-
(2-Azidocyclopent-1-enyl)methyl 2-methylpropanoate (16c): Yield 77%;
3
1H NMR (CDCl3): d=1.14 (d, J=7.0 Hz, 6H; CH3), 1.95 (m, 2H; CH2),
3
2.39 (m, 2H; CH2), 2.53 (sept., J=7.0 Hz, 1H; CH), 2.60 (m, 2H; CH2),
4.57 ppm (s, 2H; CH2O); 13C NMR (CDCl3): d=18.95 (q, CH3), 20.11 (t,
quired with
a Shimadzu quadrupole mass spectrometer (EI, 70 eV)
linked to a Shimadzu GC-17A gas chromatograph with thermal conduc-
CH2), 30.99 (t, CH2), 32.13 (t, CH2), 33.92 (d, CH), 58.85 (t, CH2O),
Chem. Eur. J. 2011, 17, 1128 – 1136
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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1133

K. Banert, E.-U. Wꢀrthwein, S. Grimme et al.
121.24 (s), 135.27 (s), 176.95 ppm (s, C=O); IR (CDCl3): n˜ =2104 (N3),
CH2), 23.31 (t, CH2), 38.15 (t, CH2), 140.01 (s), 161.62 (s), 201.82 ppm (s,
C=O).
1729 cmꢀ1 (-CO2-).
Photolysis of 13, 15, and 16: Solutions of 15 or 16c (ca. 0.1 mmol) in
CDCl2F (0.6 mL) were irradiated at ꢀ1208C, and degradation of the
starting materials were monitored by NMR spectroscopic analysis at the
same temperature. No set of product signals could be detected, and this
negative result was also found when solutions of the azides 13 or
16a,b,d,e (see the Supporting Information) in CDCl3 were photolyzed at
ꢀ508C.
2-[(Z)-3-Cyanoproylidene]-3,cis-6-dimethyl-1-oxa-4-azaspiroACTHUNTGRNEUNG[4.5]dec-3-
1
en-7-one (25b): Yield 30%; m.p. 87–888C; H NMR (CDCl3): d=0.76 (d,
J=6.8 Hz, 3H; Me at C-6), 1.75 (brd, J=14.4 Hz, 1H; 10-Heq), 1.96–2.04
(m, 2H; 9-H), 2.13 (s, 3H; Me at C-3), 2.29 (brtd, J=13.2, 5.6 Hz, 1H;
10-Hax), 2.33–2.55 (m, 6H; 8-H, 2’-H, 3’-H), 2.97 (q, J=6.8 Hz, 1H; 6-H),
4.80 ppm (m, 1H; 1’-H); 13C NMR (CDCl3): d=6.00 (q, Me-C-6), 14.02
(q, Me-C-3), 16.99 (t, C-2’ or C-3’), 21.88 (t, C-9), 21.70 (t, C-2’ or C-3’),
35.33 (t, C-10), 40.51 (t, C-8), 52.09 (d, C-6), 95.65 (d, C-1’), 113.71 (s, C-
5), 119.23 (s, CN), 156.03 (s, C-2 or C-3), 162.96 (s, C-2 or C-3),
208.25 ppm (s, C-7); IR (CCl4): n˜ =2249 (CN), 1723 cmꢀ1 (C=O); elemen-
tal analysis calcd (%) for C14H18N2O2 (246): C 68.27, H 7.37, N 11.37;
found: C 67.76, H 7.27, N 11.07.The assignment of the signals was con-
firmed by NOE, HSQC, and HMBC experiments. Especially, the Z con-
figuration was proved by NOE experiments with irradiation at the fre-
quencies of 1-H’ and Me-C-3.
2,4-Dichloro-4-dioxolan-2-ylidene-3-oxobutanenitrile (18): As described
previously,[9] a solution of 10 in chloroform was heated to reflux for 2 h.
After concentration of the solution under reduced pressure, crystalliza-
tion started on cooling. The resulting crystals were washed with n-
hexane/ethyl acetate (9:1) to give the colorless compound 18 (67%). The
melting point (126–1278C) as well as the 1H and 13C NMR spectroscopic
data measured in [D6]acetone were identical with the data described for
11.[9] 1H NMR (CDCl3): d=4.73 (m, 2H; CH2O), 4.88 (m, 2H; CH2O),
5.60 ppm (s, 1H; CHCl); 13C NMR (CDCl3): d=45.10 (d, CHCl), 67.08
(t, CH2O), 70.56 (t, CH2O), 86.15 (s, CCl), 113.39 (s, CN), 168.31 (s,
X-ray crystal data for 25b: C14H18N2O2; M=246.30 gmolꢀ1; crystal di-
mensions 0.30ꢃ0.30ꢃ0.08 mm; T=110 K; monoclinic; P21/n, a=
OCO), 175.55 ppm (s, C=O); IR (KBr): n˜ =1588 (C=O), 1088 cmꢀ1
.
7.2881(2);
b=7.8659(2),
c=22.7567(5) ꢂ,
b=95.321(2)8,
V=
X-ray crystal data for 18: C7H5Cl2NO3; M=222.02 gmolꢀ1; crystal dimen-
1298.96(6) ꢂ3; Z=4; 1calcd =1.259 gcmꢀ3; m=0.685 mmꢀ1; q range=5.95–
62.498; reflections collected: 4401, independent: 2050 (Rint =0.0209), R1 =
0.0397, wR2 =0.1066 [I>2s(I)].
¯
sions 0.45ꢃ0.22ꢃ0.04 mm; T=110 K; triclinic; P1; a=5.2694(4), b=
8.2771(6), c=10.1159(9) ꢂ, a=95.829(6), b=99.430(7), g=101.196(6)8;
V=422.89(6) ꢂ3; Z=2; 1calcd =1.744 gcmꢀ3; m=0.736 mmꢀ1; q range=
3.04–26.008; reflections collected: 2692, independent: 1639 (Rint =0.0172),
R1 =0.0337, wR2 =0.0867 [I>2s(I)].
Photolysis of 21a: A solution of 21a[5f] (10 mg, 0.073 mmol) in CD2Cl2
(0.7 mL) was irradiated at ꢀ808C, and the reaction was monitored by
NMR spectroscopic analysis at the same temperature. After 30 min, com-
pound 22a was generated in 22% yield whereas 47% of 21a remained
unchanged. The highly unstable azirine 22a decomposed on extended ir-
radiation.
Photolysis of 21c: A solution of 21c[6] (700 mg, 5.10 mmol) in chloroform
(ca. 30 mL) was irradiated at ꢀ508C for about 23 h. Thereafter, the sol-
vent was removed under reduced pressure, and the oily residue was sepa-
rated by flash chromatography (silica gel; Et2O/CH2Cl2, 1:1) to give first
24c (85 mg, 15%) as reddish-brown crystals, and then 23c as a yellow oil
(227 mg, 41%).
(E)-5-(2-Methyl-3-oxocyclopent-1-enylimino)-4-oxohexanenitrile (23c):
1H NMR (CDCl3): d=1.49 (t, 5J=1.7 Hz, 3H; 2’-Me), 1.99 (s, 3H; 6-H),
2.55 (m, 2H; 4’-H or 5’-H), 2.62 (m, 2H; 4’-H or 5’-H), 2.66 (t, J=7.1 Hz,
2H; 2-H), 3.33 ppm (t, J=7.1 Hz, 2H; 3-H). The E configuration was
confirmed by NOE experiments. 13C NMR (CDCl3): d=7.17 (q), 11.61 (t,
C-2), 15.38 (q), 27.61 (t), 32.71 (t), 33.57 (t), 118.74 (s), 120.79 (s), 162.26
(s), 172.67 (s), 196.00 (s, C=O), 205.47 ppm (s, C=O); IR (CDCl3): n˜ =
1700 cmꢀ1 (C=O); GC-MS: m/z (%): 218 (5) [M]+, 136 (56), 95 (77), 67
(100), 53 (32), 41 (69), 39 (49); HRMS (ESI): m/z: calcd for C12H15N2O2:
219.1128 [M+H+]; found: 219.1098.
7-AzabicycloACHTUNGTRENNUNG
1.79–2.30 (m, 4H), 2.89 (s, 1H), 3.30 (m, 1H; 5-H), 3.47 ppm (m, 1H; 5-
H).
Photolysis of 21b: A solution of 21b[5f,6] (50 mg, 0.33 mmol) in CDCl3
(0.7 mL) was irradiated at ꢀ508C for 4.5 h. The product 22b was generat-
ed in 65% yield.
1-Methyl-7-azabicycloACHTUNGTRENNUNG
[4.1.0]hept-6-en-2-one (22b): 1H NMR (CDCl3):
d=1.53 (s, 3H), 1.58–2.65 (m, 4H), 3.15 (m, 1H; 5-H), 3.36 ppm (m, 1H;
5-H); 13C NMR (CDCl3): d=13.73 (q), 18.77 (t), 26.91 (t), 39.89 (t), 41.28
(s, C-1), 174.25 (s, C-6), 207.38 ppm (s, C=O); IR (CDCl3): n˜ =1763 (C=
N), 1696 cmꢀ1 (C=O).
(E)-Bis(2-methyl-3-oxocyclopent-1-enyl)-diazene (24c): M.p. 157–1658C
(Et2O/CH2Cl2); 1H NMR (CDCl3): d=2.22 (t, 5J=1.9 Hz, 3H), 2.60 (m,
2H), 2.79 ppm (m, 2H); 13C NMR (CDCl3): d=8.27 (q), 22.98 (t), 33.95
(t), 146.19 (s), 173.33 (s), 208.47 ppm (s, C=O); IR (CDCl3): n˜ =
1686 cmꢀ1 (C=O); GC-MS: m/z (%): 218 (43) [M]+, 55 (64), 54 (92), 53
(88), 52 (85), 41 (93), 39 (100); HRMS (ESI): m/z: calcd for C12H15N2O2:
219.1128 [M+H+]; found: 219.1103.
Photolysis of 21b in the presence of benzophenone: A solution of 21b
(150 mg, 1.0 mmol) and benzophenone (180 mg, 1.0 mmol) in chloroform
(ca. 2.1 mL) was irradiated at ꢀ508C for about 7 h. Thereafter, the sol-
vent was removed under reduced pressure, and the oily residue was sepa-
rated by flash chromatography (silica gel; Et2O/CH2Cl2, 1:1) to give first
a red fraction, which was composed of benzophenone and trace amounts
of 24b. The second fraction gave 23b as a yellow oil. On attempts to
purify 23b by repeated chromatography (silica gel; n-hexane/ethyl ace-
tate, 9:1), the rearrangement product 25b was formed. After crystalliza-
tion (n-hexane/ethyl acetate, 9:1), product 25b was isolated as a white
solid. When 21b was irradiated in CDCl3 without benzophenone but in
the presence of acetone, acetophenone, or thioxanthone, either 22b was
generated exclusively or mixtures of 22b, 23b, and 24b were obtained.
Hydrolysis of 23c: At 08C, aqueous HCl (1m, 5 mL) was added to 23c
(236 mg, 1.08 mmol). The mixture was continuously stirred for 48 h at RT
and then extracted with Et2O. The organic layer was washed with water,
dried over MgSO4, and concentrated under reduced pressure to give 28c
as a yellow oil (74 mg, 55%). The acidic aqueous phase (see above) was
made alkaline with diluted sodium hydroxide and extracted repeatedly
with CHCl3. The combined organic layers were washed with water, dried
over MgSO4, and concentrated under reduced pressure to afford 27c as a
white solid (13 mg, 11%), which was purified by recrystallization from
CHCl3/hexane.
3-Amino-2-methylcyclopent-2-enone (27c): M.p. 131–1358C; 1H NMR
(CDCl3): d=1.57 (t, 5J=1.4 Hz, 3H), 2.37 (m, 2H), 2.49 (m, 2H),
5.05 ppm (brs, NH2); 13C NMR (CDCl3): d=5.98 (q), 26.47 (t), 33.19 (t),
109.24 (s, C-2), 171.90 (s, C-3), 203.98 ppm (s, C=O); IR (CDCl3): n˜ =
3521, 3419 (NH2), 1601 cmꢀ1 (C=O); elemental analysis calcd (%) for
C6H9NO (111): C 64.84, H 8.16, N 12.60; found: C 64.42, H 7.97, N 12.51.
6-(2-Methyl-3-oxocyclohex-1-enylimino)-5-oxoheptanenitrile
(23b):
1H NMR (CDCl3): d=1.49 (t, J=1.7 Hz, 3H; CH3), 1.93 (s, 3H; CH3),
1.96–2.11 (m, 4H; CH2), 2.37 (m, 2H; CH2), 2.46 (m, 4H; CH2),
3.10 ppm (t, J=7.0 Hz, 2H; CH2); 13C NMR (CDCl3): d=9.40 (q, CH3),
14.92 (q, CH3), 16.56 (t, CH2), 19.54 (t, CH2), 21.84 (t, CH2), 28.42 (t,
CH2), 34.82 (t, CH2), 36.99 (t, CH2), 114.97 (s), 119.12 (s), 161.30 (s),
161.71 (s), 197.90 (s, C=O), 199.17 ppm (s, C=O); IR (CCl4): n˜ =2245
(CꢁN), 1706 (C=O), 1644 cmꢀ1 (C=N).
1
4,5-Dioxohexanenitrile (28c): H NMR (CDCl3): d=2.36 (s, 3H), 2.60 (t,
J=7.1 Hz, 2H), 3.14 ppm (t, J=7.1 Hz, 2H); 13C NMR (CDCl3): d=
(E)-Bis(2-methyl-3-oxocyclohex-1-enyl)diazene (24b): 1H NMR (CDCl3):
d=2.05 (m, 4H; CH2), 2.30 (t, J=1.8 Hz, 6H; CH3), 2.56 (m, 4H; CH2),
2.62 ppm (m, 4H; CH2); 13C NMR (CDCl3): d=9.99 (q, CH3), 20.96 (t,
ꢁ
10.97 (t, C-2), 23.47 (q, C-6), 31.92 (t, C-3), 118.39 (s, C N), 194.53 (s, C=
O), 195.87 ppm (s, C=O); IR (CCl4): n˜ =1721 cmꢀ1 (C=O); elemental
1134
www.chemeurj.org
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 1128 – 1136

2,3-Bridged 2H-Azirines
FULL PAPER
analysis calcd (%) for C6H7NO2 (125): C 57.59, H 5.64, N 11.19; found: C
57.55, H 5.75, N 11.07.
1548; c) B. Rose, D. Schollmeyer, H. Meier, Liebigs Ann. 1997, 409–
412.
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N-(2-Methyl-3-oxocyclopent-1-enyl)-triphenyl-iminophosphorane (26c):
At 08C under a nitrogen atmosphere and with stirring, a solution of tri-
phenylphoshine (383 mg, 1.46 mmol) in anhydrous Et2O (ca. 20 mL) was
added dropwise to
a solution of 21c (200 mg, 1.46 mmol) in Et2O
(15 mL). The mixture was stirred at RT for 2 h. Thereafter, the solvent
was removed under reduced pressure to give 26c as a yellow solid
(542 mg, 100%), which was purified by recrystallization from CHCl3/
Et2O to yield a white solid. M.p. 195–1998C; 1H NMR (CDCl3): d=1.86
(s, 3H), 1.94 (m, 2H), 2.20 (m, 2H), 7.45–7.79 ppm (m, 15H; Ph);
3
13C NMR (CDCl3): d=7.41 (q), 31.37 (t and d, J
G
(t, C-4), 119.79 (s and d, 3J
13 Hz; m-Ph), 129.54 (s and d, J
(P,C)=10 Hz; o-Ph), 132.29 (d and d, 4J
ACHTUNGTRENNUNG
(P,C)=22 Hz; C-2), 128.73 (d and d, 3J
ACHUTGTNRNEUNG AHCUTNGTREN(NUGN P,C)=
1
AHCTUNGTRENNUNG
G
1), 205.22 ppm (s, C=O); IR (CDCl3): n˜ =1555 (C=O), 1419 cmꢀ1 (N=P);
elemental analysis calcd (%) for C24H22NO2P (387): C 77.61, H 5.97, N
3.77; found: C 77.68, H 6.16, N 3.66.
Synthesis of 27c by hydrolysis of 26c: At 08C, aqueous HCl (1m, 20 mL)
was added to 26c (542 mg, 1.46 mmol). The mixture was stirred for 48 h
at RT and then washed repeatedly with Et2O. The acidic aqueous phase
was made alkaline with concentrated sodium hydroxide and extracted re-
peatedly with ethyl acetate. The combined organic layers were dried over
MgSO4 and concentrated under reduced pressure. The residue was re-
crystallized from CHCl3/hexane to give 27c as a white solid (31 mg,
19%), which was identical to 27c obtained from hydrolysis of 23c.
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Acknowledgements
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We thank Dr. M. Hagedorn and Dr. N. Ramezanian for assistance with
NMR spectroscopy and with the manuscript. Financial support of the
Alexander von Humboldt Foundation and especially a fellowship for B.S.
are gratefully acknowledged. D.S. is thankful for a fellowship of the
Fonds der Chemischen Industrie. E.U.W. and S.G. thank the DFG (SFB
424 and 858) and the Fonds der Chemischen Industrie (Frankfurt) for fi-
nancial support.
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K. Banert, E.-U. Wꢀrthwein, S. Grimme et al.
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Received: August 26, 2010
Published online: December 3, 2010
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