Chlorodifluoroacetyl azide, ClF2CC(O)N3: Preparation, properties, and decomposition

Article abstract of DOI:10.1021/jo3009726

Chlorodifluoroacetyl azide, ClF₂CC(O)N₃, was prepared and characterized by IR (gas, Ar matrix), Raman (liquid), UV-vis (gas), and ¹⁹F, ¹³C NMR spectroscopy. The vibrational spectra were analyzed in terms of a single conformer, gauche-syn, where the Cl-C and the N_α=N_β bonds are gauche and syn to the C=O bond, respectively. The photo and thermal decomposition reactions of the azide were studied with the aid of matrix isolation. In both cases, a new isocyanate species ClF₂CNCO was produced and characterized by matrix IR spectroscopy. The conformational properties and the Curtius rearrangement pathways of this new carbonyl azide were theoretically explored, which suggest the preference of a concerted over stepwise decomposition for the global minimum gauche-syn conformer.

Full text of DOI:10.1021/jo3009726

Article
pubs.acs.org/joc
Chlorodifluoroacetyl Azide, ClF CC(O)N : Preparation, Properties,
2
3
and Decomposition
†
§
†,‡
§
§
Luis A. Ramos, Xiaoqing Zeng, Sonia E. Ulic, Helmut Beckers, Helge Willner,
,
†,∥
́
and Carlos O. Della Vedova*
†CEQUINOR (UNLP-CONICET), Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 47
esq. 115, 1900 La Plata, Argentina
‡
Departamento de Ciencias Bas
́
icas, Universidad Nacional de Lujan
Fachbereich C - Anorganische Chemie, Bergische Universitat Wuppertal, 42097 Wuppertal, Germany
Laboratorio de Servicios a la Industria y al Sistema Científico (UNLP-CIC-CONICET), Buenos Aires, Argentina
S Supporting Information
́ ́
, Rutas 5 y 7 (6700), Lujan, Argentina
§
̈
∥
*
ABSTRACT: Chlorodifluoroacetyl azide, ClF CC(O)N , was prepared
2
3
and characterized by IR (gas, Ar matrix), Raman (liquid), UV−vis (gas),
19
13
and F, C NMR spectroscopy. The vibrational spectra were analyzed in
terms of a single conformer, gauche-syn, where the Cl−C and the N N
α
β
bonds are gauche and syn to the CO bond, respectively. The photo and
thermal decomposition reactions of the azide were studied with the aid of
matrix isolation. In both cases, a new isocyanate species ClF CNCO was
2
produced and characterized by matrix IR spectroscopy. The conforma-
tional properties and the Curtius rearrangement pathways of this new
carbonyl azide were theoretically explored, which suggest the preference
of a concerted over stepwise decomposition for the global minimum
gauche-syn conformer.
INTRODUCTION
RC(O)N, by extruding molecular nitrogen upon photolysis or
pyrolysis, the nitrene may easily isomerize to the correspond-
■
1
Covalent azides are well-known for their potential shock,
thermal, and photolytic sensitivity, and consequently, they have
been classified as explosives or high energy materials.
Compared to the rather sensitive inorganic azides, organic
azides are relatively easier to handle and some of them have
ing isocyanate, RNCO through Curtius rearrangement.
However, study of the intermediates involved in the
10
decomposition of carbonyl azides continues to be of interest.
In view of the rich chemistry of carbonyl azides, we report
herein the first synthesis of chlorodifluoroacetyl azide, ClF CC-
2
been widely used as synthetic reagents in organic and biological
(
O)N . The conformational properties of this compound were
1
3
chemistry. Carbonyl azides (RC(O)N ) are versatile pre-
3
investigated using vibrational (IR, Raman) spectroscopy as
guided by quantum chemical calculations. Its photochemical
and thermal decompositions to a new isocyanate compound
cursors for producing isocyanates (RNCO) through the well-
known Curtius rearrangement. Their synthesis, reactivity, and
physical properties have been extensively studied both
theoretically and experimentally.
Perhaps the best known carbonyl azide is benzoyl azide,
2
ClF CNCO were studied using matrix isolation IR spectrosco-
2
py. The involved Curtius rearrangement mechanism was
theoretically explored using DFT calculations.
3
PhC(O)N , which can be easily prepared and safely handed.
3
Its decomposition reactions have been studied for several
4
RESULTS AND DISSCUSSION
decades. Following the early attempts to obtain N C(O)C-
■
3
5
6
(
O)N , metastable simple carbonyl azides CH C(O)N and
CF C(O)N3 were merely generated as in situ reagents, and
General Properties. Solid chlorodifluoroacetyl azide,
3
3
3
7
ClF CC(O)N , melts at −64 °C to a colorless liquid. For
2
3
3
safety reasons, the vapor pressure of the liquid was determined
only at −1 °C (24.2 mbar).
their spectroscopic and structural properties remain unknown
still. Recently, the extremely explosive carbonyl diazide,
N C(O)N , was isolated as a neat substance and structurally
The 19F NMR spectrum of the liquid shows one singlet at δ
3
3
8
=
−65.7 ppm attributed to the fluorine atoms of the CClF
characterized. The elusive parent compound HC(O)N has
2
3
13
group; the C NMR spectrum exhibits two triplet signals
been only very recently prepared in solution and spectroscopi-
cally characterized; however, no isolation could be achieved due
2
centered at δ = 166.1 ppm ( J
= 34.4 Hz) and δ = 117.2
(C−F)
9
to its facile decomposition at ambient conditions. One of the
most interesting reactions of carbonyl azides, RC(O)N , is the
Received: May 11, 2012
3
generation of the highly reactive carbonyl nitrene intermediate,
©
XXXX American Chemical Society
A
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The Journal of Organic Chemistry
Article
ppm (1J(C−F) = 299.3 Hz), corresponding to the carbon atoms
distributions of the four conformers at room temperature are
listed in Table 1.
of the CO and CClF groups, respectively (Figure S1,
2
Supporting Information). The chemical shifts are close to those
For the lowest energy gauche-syn form, the C(l)−C(2) bond
is gauche (ϕ(Cl−C(1)−C(2)−O) = 102°) to the CO double
bond, while the N(1)N(2) bond is syn (ϕ(O−C(2)−N(1)−
N(2) = 0°) to the same group. This gauche-syn conformer was
predicted by all methods to be the most abundant form in the
gas phase with an estimated contribution of 64% based on the
CBS-QB3 calculated Gibbs free energy difference (ΔG) at
room temperature. The higher energy syn-syn form is predicted
to make a contribution of 36%, while the other two have
negligible contributions. The predicted distribution of con-
formers varies somewhat between calculation models, but all
three predict gauche-syn and syn-syn to be present in observable
quantities.
1
1,12
reported for similar compounds.
The UV−vis spectrum of gaseous ClF CC(O)N (Figure S2,
2
3
Supporting Information) shows two absorption bands with λ
max
=
228 and 192 nm similar to those of other carbonyl azides.
There features are assigned to the n → π* and π → π*
transitions, respectively.
8
Calculated Conformers and Relative Energies. The
conformational properties of ClF CC(O)N were explored by
2
3
locating the minimum energy structures on the potential energy
curves around the C(1)−C(2) and C(2)−N(1) bonds by using
the B3LYP/6-311+G(d) method (Figure 1). Four low energy
An interesting result is the rather shallow rotational energy
−1
barrier (<1 kcal mol , B3LYP/6-311+G(d)) around the
C(1)−C(2) bond in ClF CC(O)N (Figure 1), which should
2
3
allow an easy rotational interconversion of the higher energy
syn-syn conformer to the gauche-syn form. This feature, which is
consistent with the measured vibrational spectra, will be
discussed in connection with the matrix experiments (see
below). In contrast, ClF CC(O)N has much larger rotational
2
3
−1
energy barrier (∼5 kcal mol , B3LYP/6-311+G(d)) for the
conversion from the higher energy anti conformer to the syn
around the C(2)−N(1) bond, and the energy difference ΔE°
−1
between them is more than 4 kcal mol . This barrier (Figure
) and large Gibbs free energy differences (Table 1) to the
1
most stable gauche-syn conformer suggest neither the gauche-
anti nor syn-anti form is likely to be detected at room
temperature, a conclusion also consistent with the experimental
results discussed below.
Figure 1. Calculated potential energy curves for the internal rotation
around the C1−C2 and C2−N1 bonds of ClF CC(O)N by using the
B3LYP/6-311+G(d) method. The relax scan of the potential energy
surface was performed by rotating the dihedral angle, either Cl−C1−
C2−O or O−C2−N1−N2, in steps of 30° while optimizing the rest of
the molecule.
Vibrational Spectra. IR spectra of ClF CC(O)N (gas
2
3
2
3
phase and Ar matrix) and the Raman spectrum of the liquid
were recorded; the results are shown in Figure 3. The observed
band positions are listed in Table 2 and compared with the
calculated frequencies for the two lowest energy conformers.
According to the theoretical calculations (Table 1), the syn-
syn conformer of ClF CC(O)N3 is expected to be in
conformers, gauche-syn, gauche-anti, syn-syn and syn-anti,
depicted in Figure 2, were found on the potential energy
surface. Full structural optimizations for these molecules were
performed using DFT (B3LYP) and ab initio (MP2) methods
with 6-311+G(d) and 6-311+G(3df) basis sets, and the CBS-
QB3 method was also applied. The calculated relative energies,
standard Gibbs free energy differences, and the Boltzmann
2
equilibrium with the global minimum gauche-syn form.
However, given the bandwidths of observed IR and Raman
spectra and the calculated frequency differences (Δν) for these
two rotamers, no conclusive evidence for the presence of a
second rotamer could be obtained.
Matrix-isolation IR spectroscopy is a well-known technique
13
for studying conformational equilibria. The IR fundamentals
of Ar-matrix-isolated ClF CC(O)N3 (1:2000) show weak
2
satellite bands within a few wavenumbers, which are attributed
to matrix effects rather than the presence of the syn-syn
conformer, as evidenced by the disparity between the
experimental and calculated Δν in Table 2. The matrix IR
spectrum indicates that the lowest energy gauche-syn conformer
is the only form present in the matrix. However, the very low
−1
energy barrier (<1 kcal mol , Figure 1) for the syn-syn →
gauche-syn interconversion makes it likely that the syn-syn form,
if present in the gas phase, may isomerize to the lowest energy
gauche-syn conformer during the deposition process. The
conformational composition of the gas-phase remains un-
changed during the cooling process only if the barrier exceeds 3
−1
14
kcal mol , a value which is substantially higher than that
calculated for the syn-syn → gauche-syn interconversion of
ClF CC(O)N (Figure 1).
Figure 2. Optimized structures with atomic labeling and relative
energies (kcal mol ) for the minimum energy conformations of
−1
ClF CC(O)N at the B3LYP/6-311+G(3df) level of theory.
2
3
2
3
B
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Article
−
1
−1
Table 1. Calculated Relative Total Energies (ΔE, kcal mol ), Gibbs Free Energies (ΔG, kcal mol ) for the Conformers of
a
ClF CC(O)N and Their Predicted Population in the Gas Phase at 298 K
2
3
gauche-syn
syn-syn
gauche-anti
syn-anti
method
ΔE°
ΔG°
%
ΔE°
ΔG°
%
ΔE°
ΔG°
%
ΔE°
ΔG°
%
B3LYP/6-311+G(d)
B3LYP/6-311+G(3df)
CBS-QB3
0.00
0.00
0.00
0.00
0.00
0.00
74
84
64
1.20
1.15
0.93
0.21
0.57
26
16
36
4.22
3.78
3.45
4.58
4.17
3.84
0
0
0
4.92
4.47
3.94
5.17
4.77
4.26
0
0
0
−0.08
a
Degeneracies of the forms are taken into account.
with the values calculated for the gauche-syn conformer at 1164,
−1
1
064, and 934 cm , respectively.
The strongest band in the Raman spectrum of the liquid is at
31 cm− and is identified as the characteristic CF deformation
1
6
2
mode (coupled with ν(C−Cl) vibration). This frequency is
1
1
close to the same mode observed in ClF CC(O)SH at 639
2
−1
cm . The N deformation in ClF CC(O)N is attributed to
the medium-strong scattering at 518 cm , and it is close to that
in FC(O)N at 565 cm . In addition, what is substantially
3
2
3
−1
−1
16
3
the ν(C−Cl) fundamental of ClF CC(O)N is attributed to the
2
3
−
1
intense Raman band at 432 cm , which is without an obvious
3
5
37
IR counterpart. The calculated Cl/ Cl isotopic shift is less
than 1 cm− (B3LYP/6-311+G(3df)) and consequently could
not be observed. The Raman bands at wavenumbers below 400
1
−
1
cm
correspond mainly to deformation and torsional
vibrations, as described in Table 2.
Photolysis and Pyrolysis. The UV spectrum of gaseous
ClF CC(O)N shows two intense absorptions at 192 and 228
2
3
nm (Figure S2, Supporting Information), which invite the
performance of irradiation studies. The IR difference spectrum
showing the spectral changes after λ > 225 nm photolysis of Ar-
matrix isolated ClCF C(O)N is given in Figure 4 (lower
2
3
trace). After 10 min of UV−vis broad band photolysis, virtually
complete depletion of the IR bands of ClF CC(O)N was
2
3
observed, together with the simultaneous appearance of a few
new bands. The assignments are collected in Table 3, while a
complete list of all the new bands which develop on photolysis
is given in Table S1 in the Supporting Information.
Among the new IR bands, the most intense one occurs at
−1
2
282.3 cm , which is assigned to the antisymmetric stretch of
the NCO group (ν (NCO)) of the expected Curtius-
as
rearrangement product ClF CNCO after nitrogen loss from
Figure 3. Upper trace: IR spectrum of ClF CC(O)N isolated in solid
2
2
3
−1
the azide precursor. The symmetric stretch (ν (NCO)) appears
Ar at 15 K (resolution: 0.25 cm ). Middle trace: IR spectrum of
s
−
1
−1
gaseous ClF CC(O)N at 298 K (resolution: 2 cm ). Lower trace:
at 1463.0 cm as a much weaker band. The other weak
absorptions observed at 1150.5, 1116.0, and 1001.7 cm can
2
3
−1
Raman spectrum of liquid ClF CC(O)N at 298 K (resolution: 2
2
3
−
1
cm ).
be assigned to the vibrational modes of the CClF group. The
2
assignments are supported by the calculation of the low energy
gauche conformer of ClF CNCO (Table 3). No clear IR bands
were found for the possible nitrene intermediate. The exclusive
All 21 fundamental vibrational modes of the ClF CC(O)N3
2
2
molecule are expected to be active in both the IR and Raman
spectra. The antisymmetric and symmetric stretching modes of
formation of isocyanate ClF CNCO from the photolytic
2
−1
decomposition of ClF CC(O)N3 is consistent with the
the azide group (N ) were observed at 2164 and 1279 cm ,
2
3
previous observations that the carbonyl nitrene intermediates
are photosensitive to UV−visible irradiations. For example, the
recently observed triplet nitrene FC(O)N evolves rapidly to
respectively, as strong and medium-strong bands in the IR
−1
spectrum of the gas. The IR band at 1759 cm and Raman
band at 1740 cm− are assigned to the ν(CO) fundamental
1
10b
−1
FNCO under UV radiation.
in keeping the calculated frequency (1801 cm , gauche-syn)
Photolysis of matrix-isolated ClF CC(O)N3 was also
and also by comparison with the data reported for similar
2
−1
15
molecules, such as F CC(O)NCO (1795 and 1785 cm )
performed using an ArF excimer laser (193 nm). Similarly,
3
−1
8
and N C(O)N (1756 and 1721 cm ). The intense IR band
the decomposition product ClF CNCO was found. Some
2
3
3
−1
at 1195 cm is assigned to the C(1)−C(2) stretching mode on
additional very weak IR bands were observed after the laser
photolysis, which could be associated with the elusive nitrene
intermediate; however, no positive conclusion could be drawn
due to the rather low abundance of the carrier in the matrix.
11
the basis of the IR frequencies reported for similar molecules.
−1
The three IR bands at 1165, 1064, and 949 cm are attributed
to the stretching modes of the CClF group by comparison
2
C
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−1
Table 2. Experimental and Calculated Frequencies (cm ) and Assignments of the Fundamental Vibrational Modes of
ClF CC(O)N3
2
d
experimental
bc
calculated
a
c
e
mode
IR (gas)
IR (Ar matrix)
Raman (liquid)
gauche-syn
syn-syn
assignment
ν (N )
ν1
2164 s
1759 s
2164.4 (51)
1752.0 (52)
1280.8 (30)
1193.4 (100)
1160.6 (48)
1060.6 (35)
943.3 (30)
847.1 (21)
714.5 (8)
2167/2160 (72)
1740 (53)
1282 (8)
2295 (448)[113]
1801 (257)[18]
1304 (439)[<1]
1235 (138)[4]
1164 (168)[1]
1063 (248) [13]
934 (134) [4]
851 (115) [6]
722 (55) [2]
625 (33) [8]
586 (<1) [1]
534 (6) [5]
2295 (437) [106]
1804 (201)[13]
1296 (504)[<1]
1205 (74)[6]
1110 (186)[2]
1094 (104) [10]
922 (254) [6]
850 (237) [3]
756 (18) [<1]
662 (3) [9]
as
3
ν2
ν(CO)
ν3
1279 ms
1195 s
ν (N )
s
3
ν4
1185 (4)
ν(C−C)
ν (CF )
ν5
1165 ms
1064 m
949 m
1151 (7)
as
2
ν6
1059/1050 (30)
945 (10)
ν (CF ), ν(C−N)
s 2
ν(CCl), ν (CCN)
ν7
s
ν8
849 m
849 (42)
ν(CCl) + δ(CNN)
γ(CC(O)N)
ν9
721 mw
625 mw
718 (7)
ν10
ν11
ν12
ν13
ν14
ν15
ν16
ν17
ν18
ν19
ν20
ν21
627.8 (5)
631 (100)
δ(CF ), ν(C−Cl)
2
591 (4) [<1]
533 (3) [5]
δoop(N3)
518 w
518 (42)
498 (7)
δ(N3)
500 (4) [1]
380 (1) [5]
ρ(CF2)
432 (67)
360 (32)
327 (15)
249 (37)
209 (28)
421 (<1) [5]
356 (1) [1]
439 (<1) [2]
416 (<1) [1]
264 (2) [3]
ν(C−Cl), δ(CF2)
ρ(CO), δ(CF2)
δ twist(CF2)
δwag(CF2)
323 (<1) [1]
245 (3) [2]
282 (<1) [<1]
225 (5) [2]
199 (1) [1]
γ(O−C−N-N)
ρ(CClF2)
133 (<1) [4]
84 (<1) [4]
121 (<1) [4]
89 (<1) [3]
τ((O)C−N3)
40 (<1) [1]
13 (2) [<1]
τ(CClF −C(O))
2
a
b
c
Relative band intensities: s, strong; ms, medium strong; mw, medium weak; w, weak. Only the most populated Ar-matrix sites are given. Relative
d
−1
intensities based on the integration of band areas are given in parentheses. B3LYP/6-311+G(3df) calculated frequencies (cm ), IR intensities in
−
1
4
−1
e
parentheses (km mol ), and Raman intensities (Å amu ) in square brackets. ν, δ, ρ and τ represent stretching, deformation, rocking and torsion
modes, respectively.
Flash vacuum pyrolysis (FVP) of argon-diluted ClF CC(O)-
2
N at 280 °C was performed by passing the mixture through a
3
hot nozzle which was heated by a 10 mm platinum wire before
deposition onto the matrix. The decomposition products were
quenched as matrix, the IR spectrum of which is shown in
Figure 4 (upper trace). The decomposition of the azide was
indicated by the appearance of a number of new IR bands.
Some of these coincide with the bands previously assigned to
the aforementioned ClF CNCO.
2
The IR spectrum of the matrix-isolated pyrolysis products
−1
includes strong bands at 1941.4, 1913.4, 1237, and 965.4 cm .
These match, respectively, the ν(CO), 2ν (CF ), ν (CF ), and
s
2
as
2
17
ν (CF ) vibrational modes of OCF . On the assumption of a
s
2
2
unimolecular decomposition process and taking into account
the high matrix dilution (1:2000), the formation of OCF2
requires the simultaneous evolution of ClCN following the
elimination of N from ClF CC(O)N . In fact, weak but
2
2
3
−1
distinct IR bands observed at 2208.6 and 717.7 cm can be
attributed to the ν(CN) and ν(ClC) fundamentals of ClCN.
These frequencies are in good agreement with the values
−1
reported for an authentic sample (2208.6 and 717.8 cm ) in
18
the Ar matrix.
Calculated Curtius Rearrangement. The potential
energy surface (PES) for the Curtius rearrangement of the
global minimum conformer of ClF CC(O)N (gauche-syn) was
calculated using the B3LYP/6-311+G(3df) calculations. Two
distinct pathways from the lowest energy gauche-syn conformer
2
3
Figure 4. Upper trace: IR spectrum of matrix-isolated pyrolysis (280
C) products of ClF CC(O)N ; the bands of the azide are marked by
°
2
3
asterisks. Lower trace: IR difference spectrum recorded before and
after 10 min of UV−vis irradiation (λ > 225 nm) of Ar-matrix isolated
ClF CC(O)N . The IR bands of newly formed species point upward
to ClF CNCO were found. The results are depicted in Figure
2
5
. The energetically more favorable path is a concerted way of
2
3
while those of depleted ClF CC(O)N point downward.
losing N with simultaneous formation of ClF CNCO, which
2
2
2
3
−1
corresponds to a calculated barrier of 31.9 kcal mol (TS1). In
contrast, the higher energy route comprises a first conforma-
D
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The Journal of Organic Chemistry
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−1
Table 3. Observed Band Positions (cm ), Relative Intensities, and Assignments for the IR Bands of the Ar-Matrix Isolated
Pyrolysis (280 °C) and Photolysis (λ > 225 nm, 10 min) Products of ClF CC(O)N at 15 K
2
3
IR (Ar matrix)
pyrolysis
ν (cm−1)a
photolysis
assignment
mode
ν (cm−1
)
a
species
ref
b
2
2
1
1
1
1
1
1
280.7 (100)
2282.3 (100)
ClF CNCO
νas(NCO)
ν(CN)
2359 (1139)
2
c
208.6 (2)
ClCN
OCF2
OCF2
2208.6
d
941.4/1935.7 (56)
913.4/1906.8 (41)
460.3 (7)
ν(CO)
1941
d
2(ν (CF ))
1913
s
2
b
1463.0 (14)
ClF CNCO
νs(NCO)
ν (CF )
1501 (172)
2
d
237.7/1231.2 (67)
146.8 (11)
OCF2
1239
as
2
b
1150.5 (19)
ClF CNCO
ν (CClF )
1156 (337)
2
as
2
b
118.9 (<1)
1116.0 (<1)
ClF CNCO
ν (CClF )
1101 (297)
2
s
2
b
9
9
7
98.0 (26)
1001.7/995.7 (39)
ClF CNCO
ν (CClF )
965 (412)
2
as
2
d
65.4/960.6 (15)
69.0/759.3 (8)
OCF2
OCF2
ν (CF )
966
s
2
d
δ(FCO)
δ CF2)
764
b
7
6
5
07.1 (1)
67.5 (1)
95.1 (1)
ClF CNCO
720 (38)
2
b
ClF CNCO
δ(NCO)
δ(NCO)
670 (16)
2
b
ClF CNCO
613 (24)
2
a
b
−1
Relative intensities based on the integration of band areas are given in parentheses. Calculated vibrational frequencies (cm ) and absolute
−1
c
d
intensities (km mol ) in parentheses at the B3LYP/6-311+(3df) level of theory. Reference 18. Reference 17.
approach the α-nitrogen atom of the azide group, as a result a
three-membered ring is formed and the N −N bond is
α
β
elongated (TS1 in ClF CC(O)N ). However, in the anti
2
3
conformer, such approach is hindered by the steric repulsion
between the substituent and the azide group (ClF C in
2
ClF CC(O)N ). As a compensation, the stabilizing intra-
2
3
molecular N ···O interaction in singlet carbonyl nitrenes may
α
facilitate the formation of an oxazirene-like intermediate by
reducing the O−C−N angle (∠OCN = 89.9° in ClF CC-
α
α
2
(
O)N). Nonetheless, the rather small energy gap between
−1
singlet and triplet ClF CC(O)N (0.8 kcal mol , CBS-QB3)
2
indicates the possible occurrence of rapid intersystem crossing
(
(
ISC) between the two electronic states. Such a low energy gap
ca. 4.0 kcal mol ) has been experimentally inferred for a
−1
Figure 5. Calculated potential energy surface for stepwise and
concerted Curtius rearrangement of gauche-syn ClF CC(O)N at the
closely related carbonyl nitrene F CC(O)N, for which the
calculated ΔE was found to be dependent on the calculation
method used.
3
2
3
S−T
B3LYP/6-311+G(3df) level of theory. The relative energies are given
19
−1
in kcal mol and selective bond lengths (italics) given in Å.
CONCLUSIONS
■
tional conversion of the syn azide to anti with an activation
−1
The new high energy chlorodifluoroacetyl azide (ClF CC(O)-
N ) has been synthesized and characterized. Quantum chemical
barrier of 9.4 kcal mol (TS2); then the latter decomposes
into the nitrene intermediate with a significant barrier of 31.1
kcal mol− (TS3), eventually the singlet nitrene rearranges to
2
3
1
calculations predict four minimum energy conformers for the
azide and the coexistence of the gauche-syn and syn-syn forms in
the gas phase at room temperature. In practice, however, only
the lowest energy gauche-syn rotamer was observed when the
vapor was trapped in a solid Ar matrix. It is assumed that the
very small rotational barrier (<1 kcal mol ) calculated at the
B3LYP/6-311+G(d) level of approximation for the syn-syn →
gauche-syn conformational conversion causes this change to
occur spontaneously during quenching of the diluted sample
onto the cold matrix support.
−1
ClF CNCO with a moderate barrier of 11.5 kcal mol (TS4).
2
The higher overall barrier of the stepwise rearrangement route
suggests that the thermal decomposition of the azide will
mostly occur through a concerted way without passing by the
nitrene intermediate. In contrast, photolytic dissociation of the
azide may happen via both ways due to the large excess of
energy input upon irradiation, where the azide in the excited
state may also be involved under the photolysis conditions.
The preference of a concerted decomposition pathway has
also been suggested based on calculations for other carbonyl
−
1
UV photolysis (λ > 225 nm and λ = 193 nm) of
4c
9
azides like CH C(O)N and HC(O)N . As far as we know,
ClF
2
CC(O)N
3
isolated in a solid Ar matrix leads via the loss
CNCO formed through the
3
3
3
no nitrene intermediate but mostly Curtius rearrangement
product has been obtained during the thermal decomposition
of carbonyl azide. For all of these carbonyl azides, the lower
energy syn conformer of the azide prefers a concerted
decomposition due to the steric effect; i.e., the substituent in
the syn conformer (ClF C in ClF CC(O)N ) may easily
of N
2
to a new isocyanate ClF
2
Curtius rearrangement. Pyrolysis of the azide at 280 °C also
produces ClF CNCO together with OCF and ClCN. DFT
2
2
calculations on the Curtius rearrangement of ClF CC(O)N
explain the absence of any nitrene intermediate during the
thermal decomposition of the azide which reacts in a concerted
2
3
2
2
3
E
dx.doi.org/10.1021/jo3009726 | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Article
manner. The photolytic decomposition gives rise to some
unknown weak IR features which might be associated with the
nitrene, further experiments to generate and characterize this
elusive intermediate are currently underway.
scans were added. More details of the matrix apparatus are given
elsewhere.
2
3
(
d) UV Spectroscopy. The UV−vis spectrum of gaseous ClF CC-
3
2
(
O)N was recorded using a glass cell equipped with quartz windows
(100 mm optical path length). Measurements were carried out in the
spectral region from 190 to 700 nm with a sampling interval of 1.0 nm,
−1
EXPERIMENTAL SECTION
Caution! Covalent azides are explosive. Although no explosions were
a scan speed of 200 nm min and a slit width of 2 nm.
e) NMR Spectroscopy. For 13C (100.6 MHz) and 19F (376.5 MHz)
NMR spectra, a pure sample of liquid ClF CC(O)N was flame-sealed
■
(
encountered with ClF CC(O)N during this work, it should be
2
3
2
3
in a thin-walled 4 mm o.d. tube which was placed inside a 5 mm NMR
handled with care in millimolar quantities, and appropriate safety
precautions should be taken, especially when working with the liquid
or solid ClF CC(O)N .
tube. The sample was held at −20 °C, and CDCl was used as an
3
external lock and reference.
2
3
(
f) Theoretical Calculations. Quantum chemical calculations were
Synthesis. Chlorodifluoroacetyl azide, ClF CC(O)N , was synthe-
sized by the reaction of chlorodifluoroacetyl chloride, ClF CC(O)Cl,
2
3
24
performed using a commercial program package. Scans of the
potential energy surface, structure optimizations, and calculations of
vibrational frequencies for ClF CC(O)N were carried out by applying
density functional theory (B3LYP) and complete basis set CBS-QB3
methods. 6-311+G(d) and 6-311+G(3df) basis sets were employed.
2
and sodium azide, NaN . For this purpose, ClF CC(O)Cl (0.3 g, 2.0
3
2
mmol) was distilled into a glass vessel provided with a vacuum valve
2
3
2
5
20
and containing dried NaN (0.5 g, 7.7 mmol). The reaction vessel
3
26
was then placed in a metallic container for protection. The reaction
was complete after 3 h at room temperature. Volatile products were
collected and separated by repeated fractional condensation using
traps held at −70, −95, and −196 °C. The product (0.22 g, 1.4 mmol),
ClF CC(O)N , was retained in the first trap together with a small
ASSOCIATED CONTENT
Supporting Information
Vibrational frequencies and assignments of IR bands observed
■
*
S
2
3
amount of unreacted ClF CC(O)Cl from which it could be freed by
2
for the products trapped in an Ar matrix following pyrolysis
slow vacuum evaporation. The quality of the samples was checked by
19
reference to the IR spectrum of the vapor and to the 19F and 13C NMR
(280 °C) and photolysis (λ > 225 nm) of ClF
2
CC(O)N
;
3
F
and 13C NMR and UV−visible spectra of ClF CC(O)N ; and
2
3
spectra of the liquid. The final yield was around 70% based on
ClF CC(O)Cl. The starting material, ClF CC(O)Cl, was prepared by
calculated energies and atomic coordinates for all optimized
structures at the B3LYP/6-311+G(3df) level of theory. This
material is available free of charge via the Internet at http://
pubs.acs.org.
2
2
chlorination of the corresponding acid, ClF CC(O)OH (98%), with
2
21
PCl (>98%). NaN was a commercial reagent.
5
3
Instrumentation and Procedure. (a) General Procedure.
Volatile materials were manipulated in a glass vacuum line equipped
with a capacitance pressure gauge, three U-traps, and valves with PTFE
stems. The vacuum line was connected to an IR cell (optical path
length 200 mm, Si windows 0.5 mm thick) placed in the sample
compartment of a FTIR spectrometer. This arrangement made it
possible to follow the course of the preparative reaction and the
purification processes. The pure compound was stored in flame-sealed
glass ampules under liquid nitrogen in a Dewar vessel. Each ampule
AUTHOR INFORMATION
Corresponding Author
E-mail: carlosdv@quimica.unlp.edu.ar.
■
*
Notes
The authors declare no competing financial interest.
22
could be opened with an ampule key at the vacuum line, an
appropriate amount was withdrawn for the experiments, and the
ampule was then flame-sealed again. The vapor pressure of a sample
was measured in a small vacuum line equipped with a calibrated
capacitance pressure gauge and a small sample reservoir. The melting
point was determined using a small amount of the sample contained in
a 4 mm glass tube immersed in a cold bath in a transparent Dewar
ACKNOWLEDGMENTS
We thank the Deutscher Akademischer Austauschdienst
■
́
Germany (DAAD), Agencia Nacional de Promocion
́
Cientifica
nica (ANPCYT), Consejo Nacional de Investigaciones
Cientificas y Tecnicas (CONICET), Comision de Investiga-
y Tec
́
́
́
́
ciones de la Provincia de Buenos Aires (CIC), Facultad de
Ciencias Exactas, Universidad Nacional de La Plata (UNLP),
−1
vessel; the temperature was increased at a rate of about 1.0 °C min
starting at −90 °C (cold ethanol bath).
and Departamento de Ciencias Bas
Nacional de Lujan for financial support. L.A.R. gratefully
acknowledges the DAAD, UNLP, and Bergische Universitat
́
icas de la Universidad
(
b) Vibrational Spectroscopy. Infrared spectra of gaseous samples
́
−1
were recorded at a resolution of 2 cm in the range from 4000 to 400
−
1
̈
cm , using a glass cell with Si windows and an optical path length of
00 mm. Raman spectra of the neat liquid in a flamed-sealed capillary
Wuppertal. S.E.U. thanks the Deutscher Akademischer
Austauschdienst Germany (DAAD) for an equipment grant
and financial support. C.O.D.V. acknowledges the DAAD,
which generously sponsors the DAAD Regional Program of
Chemistry for Argentina supporting Latin American students to
carry out Ph.D. research in La Plata. X.Z., H.B., and H.W.
acknowledge the support of the Deutsche Forschungsgemein-
schaft.
2
(
−1
3 mm o.d.) at room temperature in the region from 4000 to 100 cm
−
1
at a resolution of 2 cm .
c) Matrix Isolation Experiments. For matrix isolation experiments,
ClF CC(O)N was diluted with argon in the proportions 1:2000 in a 1
(
2
3
L stainless-steel storage container. Small amounts of the mixture were
deposited within 10 min on the cold matrix support (15 K, Rh-plated
Cu block) in a high vacuum. Temperature-dependent experiments
were carried out by passing the gaseous sample-Ar mixtures through a
quartz nozzle (1 mm i.d.), heated over a length of ∼10 mm with a
platinum wire (0.25 mm o.d.) prior to deposition on the matrix
support. The nozzle was held at 110 or 280 °C. Photolysis experiments
were performed using an ArF excimer laser (193 nm) with a total
exposure time of 7 min (2.5 mJ, 5 Hz repetition time) or with a high-
pressure mercury lamp using a cutoff filter giving radiation with λ >
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