Structure and photochemical behaviour of 3-azido-acrylophenones: A matrix isolation infrared spectroscopy study

Article abstract of DOI:10.1016/j.tet.2011.07.084

(Z)-3-Azido-3-methoxycarbonyl-2-chloro-acrylophenone (MACBP) has been synthesized, isolated in low temperature argon and xenon matrices and studied by FTIR spectroscopy, complemented by DFT(B3LYP)/6-311++G(d,p) calculations. The molecule was characterized both structurally and spectroscopically, and its photochemistry used to probe the mechanism of photo-induced conversion of 3-azido-acrylophenones into oxazoles. In situ UV irradiation (λ = 235 nm) of matrix-isolated MACBP yielded as primary photoproduct a 2H-azirine, which undergoes subsequent photoisomerization to methyl 4-chloro-5-phenyl-1,3-oxazole- 2-carboxylate. In a competitive process, a ketenimine is also formed upon photolysis of MACBP. The reported results indicate that this ketenimine must be formed from the starting 3-azido-acrylophenone via a Curtius type concerted rearrangement.

Full text of DOI:10.1016/j.tet.2011.07.084

Tetrahedron 67 (2011) 7794e7804
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Structure and photochemical behaviour of 3-azido-acrylophenones: a matrix
isolation infrared spectroscopy study
a
a
a
,
,
a,
*
Susy Lopes , Claudio M. Nunes , Andrea Gomez-Zavaglia b, Teresa M.V.D. Pinho e Melo a , Rui Fausto
*
ꢀ
ꢀ
a Department of Chemistry, University of Coimbra, P-3004-535 Coimbra, Portugal
Centro de Investigacion y Desarrollo en Criotecnología de Alimentos (Conicet La Plata, UNLP), RA-1900, Argentina
b
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 April 2011
Received in revised form 22 July 2011
Accepted 26 July 2011
Available online 31 July 2011
(Z)-3-Azido-3-methoxycarbonyl-2-chloro-acrylophenone (MACBP) has been synthesized, isolated in low
temperature argon and xenon matrices and studied by FTIR spectroscopy, complemented by DFT(B3LYP)/
6-311þþG(d,p) calculations. The molecule was characterized both structurally and spectroscopically, and
its photochemistry used to probe the mechanism of photo-induced conversion of 3-azido-acryl-
ophenones into oxazoles. In situ UV irradiation (
l
¼ 235 nm) of matrix-isolated MACBP yielded as pri-
mary photoproduct a 2H-azirine, which undergoes subsequent photoisomerization to methyl 4-chloro-5-
phenyl-1,3-oxazole-2-carboxylate. In a competitive process, a ketenimine is also formed upon photolysis
of MACBP. The reported results indicate that this ketenimine must be formed from the starting 3-azido-
acrylophenone via a Curtius type concerted rearrangement.
Keywords:
3-Azido-acrylophenones
2H-Azirines
Oxazoles
Ketenimines
Ó 2011 Elsevier Ltd. All rights reserved.
Matrix isolation
IR spectroscopy
DFT(B3LYP)/6-311þþG(d,p) calculations
Conformational analysis
Photochemistry
1. Introduction
The mechanisms for the decomposition of azides through
thermal and/or photochemical treatment and the intermediates
Molecules containing the azido-moiety (eN3) are energy-rich
and flexible chemical systems that have enjoyed increased in-
terest over the years. Organic azides constitute a versatile class of
compounds used as building blocks in organic synthesis,1e5 with
particular relevance in peptide and bioorganic chemistry.6e14 The
copper(I)-catalyzed Huisgen azideealkyne 1,3-dipolar cycloaddi-
tion forming triazoles,7,10,11,14e20 known as the ‘click reaction’, is
one of the most effective ways to make connections between
structures that bear a wide variety of functional groups. This re-
action has found applications in a wide variety of research areas, for
example, in materials science and drug design.10,11,14
It is well known that azides are also explosive substances that
decomposewith thereleaseofnitrogenthroughtheslightestinputof
external energy.4 However, in spite of their explosive properties, the
industrial interest in organic azide compounds extends to a number
of areas, as they have applications in polymer synthesis9,13,16,17 and
light-induced activation of polymer surfaces,21,22 as photo resistors
forlithography,23,24 inphotoaffinitylabelingbiologicalmethods,25,26
or as energetic additives for solid propellants.27,28
formed have been extensively investigated. Nevertheless, there are
still many cases where general consensus in relation to the precise
mechanism involved in these processes could not been obtained.
For example, it is considered that, in general, the release of mo-
lecular nitrogen is accompanied by the formation of a nitrene in-
termediate, which then undergoes further reactions, including
isomerization to ketenimines, cyclization to azirines, CeH bond
insertion or C]C bond addition.29e39 However, even in the case of
the common synthetic approach for preparing 2H-azirines from
photolysis or thermolysis of vinyl azides the precise mechanism of
the reaction has been questioned, in particular in relation to the
involvement of the nitrene intermediate in the process (vs con-
certed rearrangement).32e39
Recently,40 we reported that the thermolysis of 3-azido-3-
methoxycarbonyl-2-halo-acrylophenones (e.g., 1, in Scheme 1)
produces the corresponding 2-benzoyl-2-halo-2H-azirine-3-carbo-
xylates (e.g., 2), which undergo ring expansion to give 4-halo-5-
phenyl-1,3-oxazole-2-carboxylates (e.g., 6) and not the isomeric
isoxazoles (e.g., 4). Since oxazoles are obtained in high yield (>90%)
we could conclude that only the reaction pathway B is observed. In
fact, the formation of the vinylnitrene intermediates (e.g., 3) should
lead to the competitive formation of the corresponding isoxazoles. It
* Corresponding authors. E-mail addresses: tmelo@ci.uc.pt(T.M.V.D. PinhoeMelo),
rfausto@ci.uc.pt (R. Fausto).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.07.084

S. Lopes et al. / Tetrahedron 67 (2011) 7794e7804
7795
should be emphasized that the stereochemistry of the 3-azido-
acrylophenones (the structure of the bromo derivative was estab-
lished as Z using X-ray crystallography,41 bearing the azide and
benzoyl groups trans) precludes their direct conversion into iso-
xazoles by a concerted process. However, the formation of 2-benzoyl-
2H-azirines from the vinyl azides in a concerted manner is possible.
The production of the oxazole via nitrile ylide intermediates was
initially not expected,41 since it is generally accepted that 2H-azirines
react preferentially upon thermal excitation through cleavage of the
CeN bond, the required route to the nitrene species,42e51 whereas
thermal cleavage of the CeC bond is less common.45
O
O
Ph
Ph
N
N
A
Δ
MeO2C
Cl
MeO2C
Cl
3
4
N
N3
MeO2C
Cl
COPh
Cl
Δ
- N2
MeO2C
COPh
2
1
Cl
Cl
N
CO2Me
N
Δ
Ph
O
Ph
CO2Me
B
O
5
6
Scheme 1.
On the other hand, photochemical excitation of 2H-azirines
leads most frequently to the CeC bond cleavage of the azirine ring,
yielding the corresponding nitrile ylides (e.g., 5),46e53 which are the
expected intermediates to the oxazole production. However, pho-
tochemical processes involving cleavage of the CeN bond have also
been observed upon photolysis of substituted azirines bearing
electron-withdrawing substituents in the ring.48e51,54,55
Fig. 1. Low energy conformers of
1
(MACBP), optimized at the DFT(B3LYP)/6-
In the present study, (Z)-3-azido-3-methoxycarbonyl-2-chloro-
acrylophenone (1) (or methyl (Z)-2-azido-3-chloro-3-benzoy-
lpropenoate, MACBP; Fig. 1) has been chosen as target to further
investigation of the mechanism of conversion of 3-azido-acryl-
ophenones into oxazoles. The compound was synthesized and then
isolated in cryogenic matrices (Ar, Xe), where its conformational
preferences and UV-induced photochemistry were investigated by
infrared spectroscopy, supported by DFT calculations. The matrix
isolation technique was selected to carry out this study because in
a matrix the reactions are cage-confined, since molecular diffusion is
inhibited, and no reactions involving molecules initially located in
different matrix sites can occur. Hence, only unimolecular reactions
are expected to take place in relation with the molecule of the initial
reactant, a characteristic that introduces a very useful simplification
in the study of photochemical reactivity, in particular in the charac-
terization of the associated reaction mechanisms.
311þþG(d,p) level of theory, with atom numbering. The picture was made using the
Ortep-3 for Windows program (Farrugia, L. J. J. Appl. Cryst. 1997, 30, 565). Atoms color
code: carbon, hydrogen: black; nitrogen: blue; chlorine: green; oxygen, red.
2. Computational methods
A systematic preliminary conformational exploration of the
MACBP potential energy surface (PES) was performed using the
semi-empirical PM3 method56,57 and the HyperChem Conforma-
tional Search module (CyberChem, Inc.Ó 2004).58 These calcula-
tions provided a quick assessment of the main features of the
conformational space of the molecule, which were later on taken
into account in the subsequent analysis performed at higher level of
theory. Taking into account the high flexibility of the MACBP mol-
ecule, a random search appeared as the most appropriate way to
perform the conformational search.59e61 The program generates
starting conformations for energy minimization using a random
variation of the conformationally relevant dihedral angles obtained
for previously located minima.60,61 The method searches on until
no new minima are generated. The same approach was used in the
structural studies performed for the conformationally flexible
photoproducts of MACBP, specifically the azirine (MBCAC), and
ketenimine (CBMK) photoproducts. For all three compounds
30,000 initial structures were generated by varying the relevant
dihedral angles, and the structures corresponding to the 25 (18 for
CBMK) lowest energy unique minima were saved and used as input
geometries for the subsequent higher level calculations undertaken
with Gaussian 0362 at the DFT level of theory, using the split
As it will be shown in detail in this paper, upon photolysis
(l >235 nm) matrix-isolated 1 (MACBP) evolves to the corre-
sponding azirine 2 (MBCAC), which subsequently undergoes ring
expansion to the oxazole 6 (MCPOC). An additional photoproduct,
C-chloro-C-benzoyl-N-methoxycarbonylketenimine 7 (CBMK) was
also observed. It is proposed that it results from a Curtius type
concerted rearrangement of the starting 3-azido-acrylophenone 1,
since the alternative pathway involving the initial formation of the
2H-azirine would require the generation of vinyl nitrene 3. The
expected vinyl nitrene cyclization product, the corresponding iso-
xazole, was not observed, which supports the conclusion that the
nitrene species is not involved in this conversion.

7796
S. Lopes et al. / Tetrahedron 67 (2011) 7794e7804
valence triple-
z
6-311þþG(d,p) basis set63 and the three-parameter
The calculated optimized geometries for the three most stable
conformers are given in Table S1 (Supplementary data). In the de-
scription of the structures of the conformers, the position of the
azide (eN¼N^N), carboxylic ester (eCOOCH3) and benzoyl
(eC6H5C]O) groups will be considered in relation to the central
C1]C2 double bond.
Rotation around the N3eC2 bond defines the relative orientation
of the azide group in relation to the C2]C1 double bond. The cal-
culations show that the orientation of the azide moiety allows us to
divide the conformers into two groups: one where the azide is in an
almost trans orientation, to which the three lower energy con-
formers belong (I, II, and III, where the N4]N3eC2]C1 dihedral
angle is ꢁ171.0ꢀ, ꢁ160.1, and þ155ꢀ, respectively), and the other
where the azide in a nearly cis orientation (N4]N3eC2]C1 di-
hedral angle of ca. ꢂ40ꢀ), to which the higher energy conformers
(IVeVII) belong. The higher energy of conformers IVeVII can then
be associated with unfavorable interactions between the closely
located azide and chlorine substituents in these forms.
The carboxylic ester group can be arranged in a cis (O8]C7eC2]
C1 w0ꢀ) or trans (O8]C7eC2]C1 w180ꢀ) orientation towards the
central double bond, while the benzoyl group exhibits a quasi-
planar configuration and assumes a nearly perpendicular geome-
try in relation to the main molecular plane in all conformers.
Among the three most stable conformers (see Fig. 1), conformer I
presents the unique arrangement of its carboxylic ester in the trans
orientation relative to the C2]C1 bond, while in II and III this group
adopts the cis arrangement. For the higher energy conformers, the
carboxylic ester group is cis in conformers IV and VI and trans in
conformers V and VII.
B3LYP density functional.64,65 Structures were optimized using the
Geometry Direct Inversion of the Iterative Subspace (GDIIS)
method.66,67 Transition states for conformational interconversions
were determined at the same level of approximation, with help of
the synchronous transit-guided quasi-Newton (STQN) method.68
In order to assist the analysis of the experimental infrared (IR)
spectra, vibrational frequencies and IR intensities were also calcu-
lated at the same level of theory. The computed harmonic fre-
quencies were scaled down by a single factor (0.978) to correct
them for the effects of basis set limitations, neglected part of
electron correlation and anharmonicity effects. The nature of sta-
tionary points on the potential energy surface was checked through
the analysis of the corresponding Hessian matrix.
Normal coordinate analysis was undertaken in the internal co-
ordinates space, as described by Schachtschneider and Mortimer69
and the optimized geometries and harmonic force constants
resulting from the DFT(B3LYP)/6-311þþG(d,p) calculations. The
internal coordinates used in this analysis were defined following
the recommendations of Pulay et al.70
3. Results and discussion
3.1. Molecular structures and relative energies of MACBP
conformers
MACBP has four conformationally relevant rotational axes de-
fined by the N4]N3eC2]C1, O8]C7eC2]C1, C16eC14eC1]C2, and
C
17eC16eC14eC1 dihedral angles. The s-cis conformation of
a methyl ester group (C10eO9eC7]O8 equal to w0ꢀ) is well-
known71e73 to be considerably more stable than the s-trans one
(C10eO9eC7]O8 equal to w180ꢀ) and only structures with the first
type of arrangement were taken into account (s-trans methyl ester
minima can be expected to be at least 25 kJ molꢁ1 higher in energy
than the s-cis forms71e73). After re-optimizing at the DFT(B3LYP)/6-
311þþG(d,p) level of theory the structures obtained from the
preliminary semi-empirical random conformational search, 14
different minima were found on the PES of the molecule with rel-
ative energies within 19 kJ molꢁ1. These minima correspond to
seven pairs of equivalent-by-symmetry conformers, all of them
belonging to the C1 symmetry point group. Table 1 displays the
predicted relative energies (including zero-point corrections) of
these conformers.
According to calculations, repulsive interactions between the
azide and carboxylic ester groups should be considered the main
factor determining the relative stability of the three lower energy
conformers of MACBP. In conformer I, it is the carbonyl oxygen (O8),
which interacts with the azide group, whereas in conformers II and
III the interaction involves the methoxyl oxygen atom (O9). The
calculated atomic polar tensor (APT) charges74 for the three con-
formers are shown in Table 2. From this table, it can be noticed that
O9 is more negatively charged than O8, so that the N3/O9 elec-
trostatic repulsion in II and III is more important than the N3/O8
electrostatic repulsion in I. In addition, the N3/O8/9 contact dis-
tances decrease in the order I (285.8 pm) > II (276.7 pm)>III
(275.5 pm), also contributing to make the N3/O8/9 repulsive
electrostatic interaction more important in the order III>II>I.
Table 1
Table 2
DFT(B3LYP)/6-311þþG(D,P) calculated relative energies (
D
E0/kJ molꢁ1), including
DFT(B3LYP)/6e311þþG(D,P) calculated atomic polar tensor (APT) charges on atoms
zero-point vibrational contributions, and predicted relative populations for the
conformers of MACBP
(with hydrogens summed into heavy atoms) for conformers I, II, and III of MACBPa
Atom
APT charges/e
a
Conformer
D
E0
Population (%)
I
II
III
0.218
0.176
ꢁ0.882
1.130
ꢁ0.616
ꢁ0.298
1.233
ꢁ0.729
ꢁ0.897
0.482
T¼298 K
75.3
15.3
8.5
0.4
0.3
T¼323 K
72.4
16.6
9.7
0.6
0.4
C1
C2
N3
N4
N5
Cl6
C7
O8
O9
C10
C14
O15
C16
C17
C18
C19
C20
C21
0.117
0.249
0.182
0.205
I
II
0.00
3.95
5.40
12.73
14.07
15.26
18.71
ꢁ0.905
ꢁ0.883
III
IV
V
VI
VII
1.164
1.127
ꢁ0.618
ꢁ0.314
1.196
ꢁ0.746
ꢁ0.818
0.465
ꢁ0.617
ꢁ0.304
1.236
ꢁ0.715
ꢁ0.911
0.483
0.2
w0.0
0.2
0.1
a
See Fig. 1 and Fig. S1 (Supplementary data) for structures of the conformers.
1.208
1.210
1.192
ꢁ0.789
ꢁ0.339
0.068
ꢁ0.051
ꢁ0.800
ꢁ0.342
0.110
ꢁ0.052
ꢁ0.798
ꢁ0.332
0.103
According to the calculations, conformers II and III are 3.95 and
5.40 kJ molꢁ1 higher in energy than the most stable conformer I
(Fig. 1). The remaining forms (conformers IVeVII; Fig. S1 in the
Supplementary data) have calculated relative energies at least
12 kJ molꢁ1 higher than that of conformer I, and as a whole are
predicted to constitute less than 1.0% of the total conformational
population in gas phase at room temperature (see Table 1).
ꢁ0.046
0.061
0.061
0.058
ꢁ0.052
0.104
ꢁ0.055
0.065
ꢁ0.052
0.058
See Fig. 1 for atom numbering. 1 e¼1.60217646ꢃ10ꢁ19 ꢀC.
a

S. Lopes et al. / Tetrahedron 67 (2011) 7794e7804
7797
It is interesting to point out that the crystal structure de-
1.0
0.5
termined by X-ray diffraction of the analogous bromo-substituted
compound, methyl (Z)-2-azido-3-bromo-3-benzoyl-propenoate41
bears great similarity to the second stable conformer (II) of
MACBP. In the bromo-substituted molecule, the N4]N3eC2]C1,
O8]C7eC2]C1 and C16eC14eC1]C2 dihedral angles were found to
be ꢁ162.7(6)ꢀ, 14.4(10)ꢀ, and ꢁ102.0(10)ꢀ, which can be compared
with the values for the same dihedral angles calculated for con-
former II of MACBP (ꢁ160.1ꢀ, 19.6ꢀ, and ꢁ106.6ꢀ, respectively).
Argon (T=10 K)
0.0
1.0
22001800
22001800
22001800
22001800
1600
1600
1600
1600
1400
1400
1400
1400
1200
1200
1200
1200
1000
800
600
Xenon (T=20 K)
3.2. Matrix isolation infrared spectra of as-deposited matrices
0.5
0.0
As shown in the previous section, the calculations predicted
three experimentally relevant conformers of MACBP in gas phase:
conformers I, II and III, with estimated relative energies of 0.0, 3.95
and 5.40 kJ molꢁ1, respectively. At the sublimation temperature
used to prepare the cryogenic matrices (T¼323 K) the estimated gas
phase equilibrium Boltzmann populations are 72.4%, 16.6%, and
9.7% (see Table 1). The combined populations of the higher energy
conformers (IVeVII) are 1.3% and therefore these forms are of no
practical interest for our study.
1000
800
600
600
600
300
280
Simulated: 72.4% I
+ 26.3% II
50
0
Another important piece of information for interpretation of the
matrix-isolation experimental spectroscopic results described in
this section is the knowledge of the energy barriers for in-
terconversion between the conformers whose abundance in the
gas phase prior to deposition is significant. This is because when
a higher energy conformer is separated from a lower energy form
by a small barrier (of a few kJ molꢁ1) the higher energy form can be
converted into the lower energy form during matrix deposition,
since the thermal energy available in the gaseous beam might be
enough to allow surpassing of the barrier during the landing of the
molecules onto the cold substrate of the cryostat (conformational
cooling effect51,75,76e80). The energy barrier between III/II was
indeed found to be extremely low, amounting only to ca.
0.03 kJ molꢁ1 (1.4 kJ molꢁ1 in the opposite direction), indicating
that conformer III shall relax into conformer II during matrix de-
position. On the other hand, the energy barrier separating forms I
and II is w15 kJ molꢁ1 (for II/I; ca. 19 kJ molꢁ1 in the reverse
direction). With an energy barrier of this magnitude, the II/I
conversion cannot occur during deposition of the matrices at the
deposition temperatures used (10e20 K),51,75,76e80 so that we can
expect experimental observation of both conformers I and II in the
matrices, the latter with a population equal to the sum of the
populations of conformers II and III in the gas phase before de-
position. The expected I:II population ratio in the matrices is then
72.4:26.3, i.e., nearly 3:1.
Fig. 2 shows the infrared spectra of MACBP isolated in both solid
argon and xenon (as-deposited matrices; nozzle temperature 50 ꢀC,
substrate temperature: argon, 10 K, xenon, 20 K), together with the
calculated spectra for conformers I and II and the simulated spec-
trum of the predicted conformational mixture in the matrices as-
suming the relative abundances equal to 72.4:26.3 (calculated
infrared spectrum of conformer III is included in the
Supplementary data; Fig. S2). In the simulated spectra, bands were
represented by Lorentzian functions centered at the calculated
wavenumbers (scaled by 0.978) and with fwhm (full width at half
maximum) equal to 2 cmꢁ1 (see Experimental section for meth-
odology used in the matrix isolation experiments).
1000
800
1200
600
Conformer I
Conformer II
300
0
1000
800
Wavenumber / cm-1
Fig. 2. Top (two panels): infrared spectra of MACBP isolated in solid argon and xenon
(as-deposited matrices; temperature of the deposited vapor: 323 K; substrate tem-
perature during deposition: argon, 10 K, xenon, 20 K); Bottom: DFT(B3LYP)/6-
311þþG(d,p) calculated infrared spectra of MACBP conformers shown as stick spec-
tra (wavenumbers scaled by 0.978): I (open triangles,O) and II (black circles, C);
Middle: simulated spectrum of the expected gas phase equilibrium conformational
mixture at 323 K, built by adding the calculated spectrum of conformers I and II with
intensities scaled by their predicted populations (72.4% for conformer I and 26.3% for
conformer II; see text). In the simulated spectra, bands were represented by Lorentzian
functions centered at the calculated wavenumbers (scaled by 0.978) and with fwhm
(full width at half maximum) equal to 2 cmꢁ1
.
from normal mode analysis carried out for the three MACBP con-
formers are presented in Tables S3eS5 (Supplementary data).
Due to the similarity of the spectra of the two experimentally
relevant conformers (I and II), secure assignment of bands to
a unique conformer is not possible, except in the 1270e1230 cmꢁ1
spectral range, where the calculations predict a different pattern for
the spectral profile of the two forms (see Fig. 2). Besides the azide
anti-symmetric stretching band observed as an intense band at
w2130 cmꢁ1 (both in argon and xenon), the bands in the
1270e1230 cmꢁ1 range are the most intense of the spectra. These
bands are due to the
the stretching of the adjacent CeC bond; notated as
Table 3) and 14eC16 (designated as CeCPh in Table 3) stretching
n
CeO ester (also with some contribution from
n
CeCa E in
nC
n
modes. The higher frequency band, observed in argon matrix at
1267/1261 cmꢁ1 (1268/1262 cmꢁ1 in xenon) is due to the ester
n
CeO stretching in conformer I, which is predicted to occur at
The spectra obtained in argon and xenon matrices look very
similar and they are generally well reproduced by the simulated
spectrum. The proposed assignments for the fundamental bands
are given in Table 3. MACBP has 72 fundamental vibrations, all of
them active in the infrared. The definition of the internal co-
ordinates adopted in the performed vibrational analysis is provided
in Table S2 (Supplementary data). The calculated wavenumbers,
infrared intensities, and potential energy distributions resulting
1251 cmꢁ1 with an intensity of 324.0 km molꢁ1 by the calculations.
The middle band observed in this region (1251 cmꢁ1 in argon and
1249 cmꢁ1 in xenon) is due to the same mode in conformer II,
where it is predicted by the calculations to occur at 1238 cmꢁ1 with
an intensity of 595.2 km molꢁ1. The lower frequency band
(1240 cmꢁ1 in argon; 1237 cmꢁ1 in xenon) results from the ab-
sorption of the
nC14eC16 stretching mode in the two conformers.
This last vibration is predicted at 1231 cmꢁ1 with an intensity of

7798
S. Lopes et al. / Tetrahedron 67 (2011) 7794e7804
Table 3
Experimental (matrix-isolation) and calculated vibrational data for MACBP with vibrational assignments based on the results of normal coordinate analysisa
a
Wavenumbers (cmꢁ1 scaled by 0.978), calculated intensities (km molꢁ1).bIn the approximate description, the symbol ‘; ’ separates the description for I and II, when they
are different, while ‘,’ indicates that the approximate description for a given mode has more than one relevant contributing coordinate;
n, bond stretching, d, bending, g,
rocking, , torsion, s, symmetric, as, asymmetric, Ph, phenyl ring, E, ester; sh, shoulder, n.obs., not observed. See Table S2 (Supplementary data) for definition of internal
s
coordinates and Tables S3 and S4 (Supplementary data) for potential energy distributions.
413.1 km molꢁ1 in form I, and at 1230 cmꢁ1 with an intensity of
141.9 km molꢁ1 in form II, so that the predominant contribution to
the band is due to conformer I.
The different profiles of the spectra obtained in argon and xenon
matrices in this spectral region, in particular the different relative
intensity of the bands at 1251 cmꢁ1 (in argon) and 1249 cmꢁ1 (in
xenon), ascribable to conformer II, compared with the neighbor
bands ascribable to conformer I, clearly reveal that both conformers
I and II are present in the matrix.
the experimental spectra in relation to both the
nCeO stretching
band of form I and that assigned to 14eC16 stretching mode
n
C
(which, as stated above, has also a predominant contribution of this
latter conformer). Furthermore, the relative intensification of the
band due to conformer II is greater in the spectrum obtained in
xenon than in argon (see Fig. 2). These observations cannot be
interpreted as an indication of partial conversion of I into II during
deposition of the matrices, since the calculated barrier is large
enough to prevent this isomerization and, more importantly, if any
isomerization between these two conformers could take place (e.g.,
in case the isomerization barriers in the matrices were much
Very interestingly, compared with the simulated spectrum, the
intensity of the nCeO stretching band of form II appears larger in

S. Lopes et al. / Tetrahedron 67 (2011) 7794e7804
7799
smaller than in gas phase), this would have to occur exactly in the
CBMK
300
200
100
0
B3LYP/6-311++G(d,p)
opposite direction, i.e., the less stable conformer II would have to be
converted into the most stable form I. Moreover, this hypothetical
conversion would have to occur in larger extent during deposition
of the xenon matrix than during deposition of the argon matrix,
both because xenon is well-known to be a better matrix medium
for conformational cooling to take place than argon, and because
the temperature of the cold window of the cryostat was kept at
a higher temperature in the xenon experiments than in the argon
ones.51,75,76e80 Experimental observations show exactly the oppo-
site trend, thus requiring a different explanation.
According to the calculations, the CeO ester bond is highly po-
larized in both conformers (see Table 2). However, it is considerably
more polarized in conformer II, since the positive charge in the C7 is
larger in this form and the charge in O9 is also more negative in
conformer II than in I. These results are in agreement with the
2000 1800 1600 1400 1200 1000
800
600
0.6
0.4
0.2
0.0
MCPOC
Ar matrix (as-deposited)
2000 1800 1600 1400 1200 1000
800
600
0.6
0.3
0.0
-0.3
-0.6
-0.9
MACBP
Irradiated Ar matrix
n)
calculated relative infrared intensities of the nCeO bands: intense
bands are predicted for this mode in both conformers, but the in-
tensity in conformer II (595.2 km molꢁ1) is almost twice that in
conformer I (324.0 km molꢁ1). This means that the CeO ester bond
in II is also more sensitive to the polarizability of the media than the
same bond in conformer I, being additionally polarized in greater
extent in more polarizable media. Consequently, the relative in-
2000 1800 1600 1400 1200
0
600
600
0
25
50
75
frared intensities of the nCeO stretching modes in conformer II
MBCAC
B3LYP/6-311++G(d,p)
compared to form I can be expected to grow in the order: gas
phase<argon matrix<xenon matrix, as observed experimentally.
Assignment of a few other less intense bands to a single con-
former was also attempted and is presented in Table 3, but they
must be considered as tentative, e.g., bands ascribed uniquely to the
2000 1800 1600 1400 1200 1000
Wavenumber / cm-1
800
less stable conformer (II) at 1461 (
stretching vibration ( Ph5, as defined in Table S2) mixed with
(C1eC14)], 906 [ (NNN)] in argon, which in
(CeCl)] and 659 cmꢁ1
xenon appear at 1455/1453, 1045, 906, and 660/659 cmꢁ1
d
CH3 as0), 1047 [phenyl ring
Fig. 3. Top: Simulated IR spectrum of CBMK (most stable conformer); Middle top: IR
spectrum of MCPOC in argon matrix (as deposited; 10 K); Middle bottom: IR differ-
l >235 nm irradiated matrix during
90 min minus >235 nm irradiated matrix during 10 min; before subtraction, re-
d
n
n
[d
ence spectrum of irradiated Ar matrix of MACBP (
,
l
sidual bands due MACBP were subtracted from the spectra of the irradiated matrix);
Bottom: simulated IR spectrum of MBCAC (most stable conformer). In the simulated
spectra, bands were represented by Lorenztian functions centered at the DFT(B3LYP)/
6-311þþG(d,p) calculated wavenumbers (scaled by 0.978) and with fwhm (full
width at half maximum) equal to 2 cmꢁ1 (in case of MBCAC, the spectrum was
multiplied by ꢁ1).
respectively.
3.3. Photochemistry of matrix-isolated MACBP
Upon in situ broadband UV irradiation (l >235 nm) of matrix-
isolated monomeric MACBP, a significant decrease in the intensity
of the bands of the compound was observed, while new bands due
to photoproducts emerged. These changes were already clearly
visible after 10 min of irradiation, whereas more than 90% of the
original compound was consumed after 450 min of irradiation of
both argon and xenon matrices (see Experimental section for
methodology used in the matrix isolation experiments). Analysis of
the kinetic profiles of the bands appearing upon photolysis pro-
vided essentially two distinct patterns: (a) a set of bands starting to
grow in the early stages of irradiation and then decreasing of in-
tensity, and (b) the remaining bands starting to noticeably increase
of intensity later on and in a continuous way. This behavior is
shown in Fig. 3, which presents the infrared difference spectrum of
the irradiated argon matrix of MACBP obtained subtracting the
spectrum after 10 min of irradiation from that collected after
90 min of irradiation (before subtraction, residual bands due
MACBP were subtracted from the spectra of the irradiated matrix).
Bands pointing down in this spectrum correspond to the initially
formed species, while those pointing up are due to photoproducts
appearing at later stages of irradiation. Results obtained upon ir-
radiation of the xenon matrix were qualitatively identical (see
Fig. S3 in the Supplementary data).
azirine. On the other hand, the bands corresponding to the species
detected later on are continuously growing with time of irradiation
and fit well with those of methyl 4-chloro-5-phenyl-1,3-oxazole-2-
carboxylate (6, MCPOC) and ketenimine 7 (CBMK) (see Scheme 2).
In Fig. 4, the 1300e1150 cmꢁ1 region of the spectrum of the as-
deposited argon matrix of MACBP and of the spectra obtained for
the irradiated matrix at two different times of irradiation is pre-
sented. This spectral range is crowded and contains strong over-
lapping absorptions of the reactant as well as of photoproducts, so
that in the difference spectra shown in Fig. 3 a reliable analysis
cannot be done. However, the data shown in Fig. 4 points to the
same conclusions withdrawn from the general analysis of the
spectral data presented in Fig. 3.
These observations can be rationalized considering that the
oxazole derivative is a photoproduct of 2H-azirine 2 resulting from
ring expansion via nitrile ylide intermediate 5. On the other hand,
the experimental results also indicate that ketenimine 7 must be
formed from the starting 3-azido-acrylophenone 1. In fact, the
conversion of the 2H-azirine into ketenimine 7 would require the
generation of vinyl nitrene 3, which should also lead to its cycli-
zation to the corresponding isoxazole. No formation of the iso-
xazole derivative was observed upon photolysis, which supports
the conclusion40 that the vinyl nitrene species is not involved in the
generation of ketenimine 7. With all probability this species is
obtained from the starting azide via a Curtius type concerted
rearrangement. Thus, the formation of 2H-azirine 2 and keteni-
mime 7 are competitive processes, being the former a kinetically
It can be seen in both Fig. 3 and Fig. S3 that the spectrum of the
initially predominant photoproduct fits well that theoretically
predicted for the postulated azirine, methyl 2-benzoyl-2-chloro-
2H-azirine-3-carboxylate (2, MBCAC). As already mentioned, these
bands first start to grow and then considerably decrease for longer
time of irradiation, indicating subsequent reactions of the 2H-

N
N3
MeO2C
Cl
Cl
COPh
Cl
hν
N
CO2Me
(λ > 235 nm)
MeO2C
COPh
Ph
O
2 (MBCAC)
1
(MACBP)
5
hν
O
Cl
Cl
C
Ph
N
N
N
MeO2C
COPh
Ph
CO2Me
O
MeO2C
Cl
7 (CBMK)
6 (MCPOC)
3
O
Ph
N
MeO2C
Cl
4
N
N
Cl
N
Cl
Curtius type rearrangement
- N2
N
C
MeO2C
COPh
MeO2C
COPh
1
7
Scheme 2.
1300
1250
1200
1150
-1
Wavenumber/cm
Fig. 4. From top to bottom: IR spectra (1300e1150 cmꢁ1 region) of MBCAC (calculated; most stable conformer), MACBP (experimental, 10 K:
compound for 10 min, as-deposited Ar matrix, and >235 nm irradiated Ar matrix of the compound for 90 min), MCPOC (as-deposited Ar matrix; 10 K), and CBMK (calculated; most
stable conformer). In the simulated spectra, bands were represented by Lorentzian functions centered at the DFT(B3LYP)/6-311þþG(d,p) calculated wavenumbers (scaled by 0.978)
and with fwhm (full width at half maximum) equal to 2 cmꢁ1
l >235 nm irradiated Ar matrix of the
l
.

S. Lopes et al. / Tetrahedron 67 (2011) 7794e7804
7801
Table 4
Vibrational assignments of the bands observed in the UV irradiated matrices of MACBPa
Irradiated 1 (MACBP) Ar matrix
Irradiated 1 (MACBP) Xe matrix
Experimental81 or calculated data
Approximate descriptionb
6 (MCPOC)
n
Ar matrix
n
Xe matrix
n
n
1764/1760/1753
1541/1536/1530
1492
1478/1472
1456
1763/1758/1751
1537
1490/1488
1474
1456
1451/1448
1443
1364/1340
1313/1310/1298
1223/1199
1171/1163
1114
1769/1763/1751/1750
1541/1530
1492/1487
1479
1763/1759/1748
1535/1532/1529
1489
1478/1474
1455
1450/1444
1439/1436
1361/1341
1330e1302
1213e1197
1174/1170/1168/1163
1118/1114/1110/1109
1095/1092/1090
1038/1031/1027
1016/1007/1005
980
n
(C]O)
nOx1
d
d
d
d
d
d
(CeH2)
CH3 as0
CH3 as00
(CeH3)
1460
1450
1447
1453/1452/1451/1450
1442
CH3
(CeH1);
Ox2; (CeCa)
s
1360/1358
1317/1313/1300
1226e1198
1173/1168
1118
1093/1085
1034
1018/1008
977
1360/1358/1346
1332e1300
1215e1202
1174/1172/1168/1166
1117
1096/1094
1041/1033
1020/1016/1008/1006
981
nPh2
n
n
n
0
(CeCIR);
n(CeO);
g
CH3
0
g
CH3
;
n
(CeO)
nOx5
nPh6
nPh5
1089
1034
1015/1006
977
d
Ox1
nPh1
814
813
817/815
815/512
d
(OCO)
778
765
776
762
781/778
765
778/775
762
g
(C]O)
(CeH1)
g
690
691
690
dPh3
688
653
686
651
689/688/687
652
686
650/648
s
s
Ox1
Ox2
2 (MBCAC)
n
Calculated
n
nc
I
1776
1742
1706/1696
1608
1772
1740
1694
1604
1584
1463
1454
1442
1432
1320
1283/1270/1262
1251/1236/1232
1194.
1175
1045
1027/1021
975
1777
1752
1697
1602
1582
1460
1452
1446
1439
1322
1239
1223
1178
1174
1041
1019
971
871
858
815
767
735
702
689
619
188.4
144.7
193.8
42.6
11.0
9.5
11.3
18.2
12.8
29.8
501.6
244.3
38.2
32.8
7.3
11.9
44.9
20.3
70.6
33.0
17.7
4.8
81.1
29.5
51.1
n
n
n
n
n
(C]N) A
(C]O) E
(C]O)
Ph
Ph
1464
1454
1444
1438
1322
1285
1254/1244/1234
1195
1181
1047
1029/1025
990
862
854
d
d
d
d
CH3 as00
CH3 as0
(CeH) Ph
CH3
s
n
n
n
(CeC) A
(CeCPh
(CeO)
)
d
(CeH) Ph
0
g
CH3
n
(CeC(¼O));
d(CeH) Ph
dPh
n(OeCH3)
n(CeN) A
n(CeCl)
854
819
784/771
724
694
d(OCO)
785/773
724
702/694
668/661
g
g
g
AeE
(C]O) E
(CeH) Ph
Ph
(CC]O)
667/660
623
d
d
7 (CBMK)
n
Calculated
nc
n
I
2059
1764
1680
2055
1763
1685
1588
1440
1366
2077
1756
1682
1601
1441
1366
1235
1207
1173
1055
816
875.3
311.6
350.9
19.5
45.7
123.2
56.7
1042.2
304.8
198.1
60.3
n
n
n
n
C¼C]N as
(C]O) E
(C]O)
Ph
1440
1378
dCH3 s
n
n
n
C¼C]N s
1238
1242
(CeCPh)
d
d
(CeO)
0
1187
1067
1184/1178
gCH3
n
(CeC(¼O));
d(CeH) Ph
823
n(CeCl)
718/711
680
715/708
678
709
673
76.9
23.5
g(CeH) Ph
d
Ph
643
643
643
28.7
d
(CC]O); g(C]C]N)
a
Wavenumbers (
n
) in cmꢁ1 calculated intensities (I) in km molꢁ1
,
n, bond stretching,
d, bending,
g
, rocking, s, torsion, s, symmetric, as, asymmetric, Ox, oxazole ring, Ph,
phenyl ring, A, azirine ring; E, ester, IR, inter-rings.
b
Approximate descriptions for MCPOC as in Ref. 83.
Scaled wavenumbers (0.978).
Buried within the profile due to MCPOC bands.
c
d

7802
S. Lopes et al. / Tetrahedron 67 (2011) 7794e7804
more favorable process, since at a shorter time of irradiation 2H-
azirine dominates.
MACBP, the presence of such band could not be confirmed (see
Fig. S5 for the calculated spectrum of nitrile ylide 5).
The assignments of the bands due to the photoproducts are
presented in Table 4. It is worth mentioning that the spectrum of
the photoproduced oxazole (MCPOC) in both argon and xenon
matrices is well-known,40,81 so that the identification of this
compound in the irradiated MACBP matrices was doubtless and
band assignment followed that presented before.81 For example,
bands at 1764/1760, 1753, 1541/1536/1530, 1492, 1300, and 1173/
1168 cmꢁ1, in argon, with counterparts at 1763/1758, 1751, 1537,
1490/1488, 1298, and 1171/1163 cmꢁ1, in xenon, respectively, are
intense characteristic bands of MCPOC.81 On the other hand, for
both the azirine and ketenimine species no experimental data for
the matrix-isolated compounds were reported hitherto, so that
their identification and band assignments were based on the
comparison with theoretically predicted spectra obtained at the
DFT(B3LYP)/6-311þþG(d,p) level of theory and on data for other
molecules of the same families.47,50,51,82e91 Moreover, these two
compounds have conformationally flexible molecules and,
according to the structural calculations performed on these species,
several low energy conformers exist in both cases. For the azirine,
the calculations predicted the existence of five different conformers
with relative energies within 10 kJ molꢁ1. In the case of the kete-
nimine, the calculations yielded seven conformers with relative
energies within the same limit. Nevertheless, the calculated in-
frared spectra of all low energy forms predicted theoretically for
each molecule were found to be quite similar, thus facilitating their
experimental identification and band assignments. Fig. 3 (and
Fig. S3, in Supplementary data) shows only the spectra corre-
sponding to the most stable conformer of each molecule, MBCAC
and CBMK (see Fig. S4 for structures of these forms and Fig. S5 for
the calculated spectra of CBMK and the nitrile ylide shown as stick
spectra), while the calculated data for these molecules shown in
Table 4 also belong only to their most stable forms.
4. Conclusions
The 3-azido-acrylophenone MACBP (or methyl (Z)-2-azido-3-
chloro-3-benzoyl-propenoate) has been shown to be a photochem-
ical precursor of the related oxazole, methyl 4-chloro-5-phenyl-1,3-
oxazole-2-carboxylate (MCPOC), in a reaction where the azirine and
nitrile ylide work as intermediates. This reaction is accompanied by
a second one leading to formation of C-chloro-C-benzoyl-N-
methoxycarbonylketenimine (CBMK) via a Curtius type concerted
rearrangement of the starting 3-azido-acrylophenone 1. The alter-
native mechanistic pathway involving the initial formation of the
2H-azirine followed by the CeN bond cleavage giving avinyl nitrene,
which then rearranges to the ketenimine was ruled out. The non-
observation of the isoxazole, the expected vinyl nitrene cyclization
product, points to the concerted nature of ketenimine formation.
MACBP was synthesized, isolated in low temperature argon and
xenon matrices and structurally and vibrationally characterized by
infrared spectroscopy and quantum chemical calculations before
execution of the photochemical studies. Seven different low energy
conformers were found on the DFT(B3LYP)/6-311þþG(d,p) PES,
with the three lower energy forms (which correspond to the con-
formers predicted to have significant populations in the gas phase
equilibrium at the sublimation temperature required to produce
the cryogenic matrices) showing an orientation of the azide group
in a nearly trans orientation. In the matrix isolation experiments,
however, only the two most stable conformers of the compound (I
and II) were observed, in agreement with the predicted easy con-
version of conformer III into conformer II during deposition of the
matrices, due to the very low energy barrier associated with this
conversion (ca. 0.03 kJ molꢁ1).
Among the bands ascribed to MBCAC (see Table 4) the most
intense ones were observed at 1776, 1742, 1706/1696, 1608, 1285,
702/694, and 668/661 cmꢁ1 in argon (1772, 1740, 1694,1604, 1283/
1270/1262, 694, and 667/660 cmꢁ1 in xenon), corresponding to the
5. Experimental section
5.1. Synthesis of MACBP
n
(C]N) stretching of the azirine ring, n(C]O) stretchings in ester
(Z)-3-Azido-3-methoxycarbonyl-2-chloro-acrylophenone
(1)
and benzoyl fragments, the highest frequency stretching mode of
the phenyl group, the stretching vibration of the CeCPh bond, and
the all in phase rocking out-of-plane CeH deformation and a skel-
etal deformational mode of the phenyl group, which were pre-
was prepared using a known synthetic method procedure (Scheme
3).39 The 2-(triphenylphosphoranylidene)acetophenone (9), obtai-
ned by reaction of 2-bromoacetophenone 8 and triphenylphos-
phine, was added to a solution of triethylamine in dry toluene and
treated with the appropriated acid chloride to give the methyl 2,4-
dioxo-4-phenyl-3-triphenylphosphoranylidenebutanoate (10) al-
most in good yield. This ylide reacts with azidotrimethylsilane and
N-chlorosuccinimide in dichloromethane to give the crystalline
dicted to occur at 1777, 1752, 1697, 1602, 1239, 702, and 689 cmꢁ1
,
respectively. The most characteristic band of CBMK was observed at
2059 cmꢁ1 (argon) and 2055 (xenon) cmꢁ1, and is ascribed to the
ketenimine
n
C]C]N anti-symmetric stretching mode.48e51,54,55
O
O
O
O
O
1. PPh3, CH2Cl2
2. NaOH, MeOH
TMSN3, NCS
rt, CH2Cl2
N3
MeO2C
Cl
MeO2C
NEt3
Cl
Ph
Br
MeO2C
Ph
Ph
PPh3
PPh3
COPh
8
10, 86%
1, 40%
9, 88%
Scheme 3. Synthesis of methyl (Z)-3-azido-acrylophenone 1.
In the present study, among the species predicted to take part in
the azide/oxazole photochemical conversion (Scheme 2), only the
nitrile ylide 5 could not be doubtlessly observed, indicating that for
the system under study this species is a quite unstable in-
termediate, which is promptly converted to the oxazole. The in-
trinsically strong band in infrared due to the anti-symmetric
stretching of the CNþCꢁ moiety of a nitrile ylide appears usually at
target compound 1 after purification by column chromatography
and crystallization.
5.2. Matrix isolation experiments
Matrices were prepared by co-deposition of MACBP vapors
coming out from a specially designed thermoelectrically heatable
mini-furnace, assembled inside the cryostat (APD Cryogenics, model
DE-202A) chamber, and large excess of the matrix gas (argon, N60;
a characteristic frequency of about 1950 cm
infrared spectra of the photolyzed argon and xenon matrices of
.
ꢁ1 48,49,54,55 In both the

S. Lopes et al. / Tetrahedron 67 (2011) 7794e7804
27. Kubota, N. J. Propul. Power 1995, 11, 677.
7803
xenon, N48, both obtained from Air Liquide) onto a CsI substrate
cooled to 10 K (for argon matrices) and 20 K (for xenon matrices). The
IR spectra were recorded with 0.5 cmꢁ1 spectral resolution in
a Mattson (Infinity 60AR Series) Fourier transform infrared spec-
trometer, equipped with a deuterated triglycine sulfate (DTGS) de-
tector and a Ge/KBr beam splitter. Necessary modifications of the
sample compartment of the spectrometer were done in order to
accommodate the cryostat head and allow purging of the instrument
by a stream of dry nitrogen, to remove water vapors and CO2.
Irradiation of the matrices was carried out with unfiltered light
from a 500 W Hg(Xe) lamp (Newport, Oriel Instruments), with
output power set to 200 W, through the outer KBr windows of the
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Acknowledgements
41. Pinho e Melo, T. M. V. D.; Lopes, C. S. J.; d’A Rocha Gonsalves, A. M.; Storr, R. C.
Synthesis 2002, 5, 605.
These studies were partially funded by the Portuguese Science
Foundation (Project No. FCOMP-01-0124-FEDER-007458, cofunded
by QREN-COMPETE-UE), CYTED Program (Iberoamerican Program
for the Development of Science and Technology) [Network
108RT0362]. S.L. and C.M.N. acknowledge FCT for Grants No. SFRH/
BD/29698/2006 and SFRH/BD/28844/2006. A.G.Z. is member of the
Research Career, Conicet (National Research Council, Argentina).
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Supplementary data
ꢀ
R. J. Phys. Chem. A 2006, 110, 8081.
Additional figures with schematic schematic representations of
the four higher energy conformers of MACBP and the low energy
conformers of MBCAC and CBMK, predicted IR spectra for the three
conformers of MBCAC, CBMK and nitrile ylide, and spectral data
showing the observed in xenon matrix photochemistry are pro-
vided as Supplementary data. Also, tables with optimized geome-
tries, IR spectroscopic data of three most stable, experimentally
relevant conformers of MACBP (forms I, II and III), and Cartesian
coordinates and absolute energies for the relevant forms calculated
and discussed. Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.tet.2011.07.084.
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