The photochemical and thermal decomposition of azidoacetylene in the gas phase, solid matrix, and solutions

Article abstract of DOI:10.1002/ejoc.201402153

Decomposition of the extremely explosive and unstable parent compound of 1-azidoalkynes, HCCN₃ (azidoacetylene), was studied in the gas phase, solid argon matrix, and solutions. In the gas phase, this azide decomposes quickly at room temperature with a half-life time (t_1/2) of 20 min at an initial pressure (p₀) of 0.8 mbar. The decay (p₀ = 1.0 mbar) is significantly increased in an atmosphere of O₂ with t _1/2 of 3 min, in which HC(O)CN was identified as the trapping product of the cyanocarbene intermediate HCCN. Trapping products of this carbene by solvent molecules (CH₂Cl₂ and CHCl₃) were also found during decomposition of the azide in solution, whereas the reaction with a solution of bromine to form dibromoacetonitrile is interpreted as taking place by nucleophilic attack of the alkyne itself. The intermediary formation of triplet HCCN by flash vacuum pyrolysis and photolysis (255 nm) of the azide in the gas phase and in solid argon matrices, respectively, was confirmed by IR spectroscopy and mutual photo-interconversion of HCCN with isomeric cyclo-C(H)CN (azirinylidene) and HCNC by selective irradiations at 16 K. Although azidoacetylene is highly explosive, it can be irradiated in an argon matrix to generate cyanocarbene. The same species is also formed and analyzed after flash vacuum pyrolysis under relatively mild conditions. Decomposition of the azide in solution is combined with trapping reactions. Copyright

Full text of DOI:10.1002/ejoc.201402153

Job/Unit: O42153
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FULL PAPER
DOI: 10.1002/ejoc.201402153
The Photochemical and Thermal Decomposition of Azidoacetylene in the Gas
Phase, Solid Matrix, and Solutions
Xiaoqing Zeng,*[a] Helmut Beckers,[b] Jennifer Seifert,[c] and Klaus Banert*[c]
Keywords: Azides / Alkynes / Carbenes / Flash pyrolysis / Matrix isolation / Gas-phase reactions / Photolysis
Decomposition of the extremely explosive and unstable par-
ent compound of 1-azidoalkynes, HCCN3 (azidoacetylene),
was studied in the gas phase, solid argon matrix, and solu-
tions. In the gas phase, this azide decomposes quickly at
room temperature with a half-life time (t1/2) of 20 min at an
initial pressure (p0) of 0.8 mbar. The decay (p0 = 1.0 mbar) is
significantly increased in an atmosphere of O2 with t1/2 of
3 min, in which HC(O)CN was identified as the trapping
product of the cyanocarbene intermediate HCCN. Trapping
products of this carbene by solvent molecules (CH2Cl2 and
CHCl3) were also found during decomposition of the azide
in solution, whereas the reaction with a solution of bromine
to form dibromoacetonitrile is interpreted as taking place by
nucleophilic attack of the alkyne itself. The intermediary for-
mation of triplet HCCN by flash vacuum pyrolysis and pho-
tolysis (255 nm) of the azide in the gas phase and in solid
argon matrices, respectively, was confirmed by IR spec-
troscopy and mutual photo-interconversion of HCCN with
isomeric cyclo-C(H)CN (azirinylidene) and HCNC by selec-
tive irradiations at 16 K.
(NMR and IR) evidence for the parent molecule, azido-
acetylene (HCϵC–N3), was reported.[6] It was found that
azidoacetylene decomposed in dichloromethane at –30 °C
with a half-life of about 17 h. Similar to the recent isolation
of the unstable parent carbonyl azide, HC(O)N3,[7] the use
of highly soluble tributyl(hexadecyl)phosphonium azide
(QN3) as an N3-introducing reagent together with high-
boiling solvent propylene carbonate allows the isolation of
this kinetically unstable azide as a neat substance at low
temperatures.[8] However, this azide was found to be ex-
tremely sensitive, and explosions were always encountered
during the phase transition of the azide even at liquid-nitro-
gen temperature (77 K). Nevertheless, a full set of its vi-
brational spectra (IR and Raman) was obtained and ana-
lyzed with the aid of high-level ab initio calculations.[8]
Decomposition of organic azides has attracted much at-
tention with respect to both experiment and theory, and the
intermediates formed during their decay are of particular
interest.[9] Generally, organic azides (RN3) can eliminate
molecular N2 upon either photolysis or pyrolysis and form
short-lived nitrene intermediates (RN) that may readily
undergo rearrangement or inter- and intramolecular bond-
insertion reactions. However, ab initio calculations have
shown that bent cyanocarbene (RC–CϵN) instead of a
nitrene intermediate should be formed from the decomposi-
tion of 1-azidoalkynes, RCϵC–N3.[10] Interestingly, for
none of the compounds under quantum chemical investiga-
tion was a local minimum corresponding to a nitrene found
in the geometry optimizations. For the parent cyanocarb-
ene, HCCN, the preferred state is the triplet state, with an
energy difference of about 7 kJmol–1 to the singlet state
calculated at the MP2/cc-pVTZ level of theory.[10] Indeed,
Introduction
Covalent azides are versatile building blocks in organic
synthesis.[1] The high energy content of this class of com-
pounds has stimulated numerous studies on their prepara-
tion, structure, reactivity, and applications for more than a
century.[2] Still, a number of simple covalent azides have
only recently been isolated as neat substances and structur-
ally characterized, although some have been known for
quite some time to exist in solution.[3] The obstacles to their
isolation and characterization is partially due to the lack of
suitable synthetic methods but most importantly because of
their extreme sensitivity to shock and heat as a neat sub-
stance.
The experimental pursuit of synthetically useful 1-azido-
alkynes dates back to 1910 with the work of Forster and
Newman;[4] however, like all the other early attempts, only
unwanted products were obtained, and no conclusive evi-
dence for the formation of 1-azidoalkynes was found.[5]
This class of azides remained elusive for more than 100
years until, very recently, unambiguous spectroscopic
[a] College of Chemistry, Chemical Engineering and Materials
Science, Soochow University,
215123 Suzhou, China
E-mail: xqzeng@suda.edu.cn
http://chemistry.suda.edu.cn/
[b] FB C – Anorganische Chemie, Bergische Universität
Wuppertal,
Gaussstrasse 20, 42119 Wuppertal, Germany
[c] Technische Universität Chemnitz, Organische Chemie,
Strasse der Nationen 62, 09111 Chemnitz, Germany
E-mail: klaus.banert@chemie.tu-chemnitz.de
http://www.tu-chemnitz.de/chemie/org/index.html.en
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201402153.
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X. Zeng, H. Beckers, J. Seifert, K. Banert
FULL PAPER
formation of cyanocarbenes RC–CϵN has been chemically
inferred from their trapping products obtained during the
thermal decomposition of the corresponding azides in solu-
tion.[6,11] As theoretically predicted,[10] the thermal stability
of 1-azidoalkynes is not enhanced by typical donor or ac-
ceptor substituent groups, and – remarkably – it was found
that the energy barrier for the N2 separation of the parent
compound HCCN3 is the highest [90.4 kJmol–1, CCSD(T)/
cc-pVTZ]. This barrier is very close to that of HC(O)N3
(93.3 kJmol–1, B3LYP/def2-QZVPP),[7b] for which the
underlying decomposition mechanism has been extensively
explored from both aspects of experiment and theory.[7]
Nevertheless, owing to the kinetic instability of all 1-
azidoalkynes, knowledge of their properties is rather lim-
ited. Thus, an experimental study on the decomposition of
the parent molecule HCCN3 as a neat substance is desir-
able. Furthermore, as a possible method of producing the
highly interesting interstellar species HCCN in the gas
phase,[12] the decomposition of HCCN3 is of general inter-
est. In this work, the thermal decomposition of HCCN3
was studied in the gas phase and in solutions, and the
photodecomposition was studied in a solid argon matrix;
in all cases, the formation of HCCN was unambiguously
confirmed.
Figure 1. Decrease of the IR bands of gaseous HCCN3 (p0
=
3.8 mbar) in the spectral range of 2260–2020 cm–1 within 20 min;
for the assignments of the bands, see ref.[8]
292 K. A deviation from the assumed first-order decay is
clear for the higher initial pressure of HCCN3 (Figure 2)
and is probably caused by some surface-catalyzed decom-
position. However, owing to the experience of several ex-
plosions that occurred during the phase transitions of the
azide upon evaporation and condensation, only a limited
set of kinetic data was eventually obtained. Nevertheless,
an activation barrier of about 88 kJmol–1 was roughly esti-
mated for the separation of N2 from HCCN3 at an initial
pressure of 0.8 mbar, and this value is close to that obtained
by ab initio calculations of 90.4 kJmol–1, at the CCSD(T)/
cc-pVTZ level.[10]
Results and Discussion
Decomposition of Gaseous HCCN3
Azidoacetylene has a melting point of –69.5 °C, the li-
quid is quite volatile and can be transferred in vacuo by
very slow evaporation at low temperatures without notice-
able decomposition. Even gas chromatography at ambient
temperature is possible, and GC–MS (EI) led to signals
with m/z (%) = 39 (50) [M – 28]+ and 28 (100) for azido-
acetylene. However, rapid decay of the azide in the gas
phase occurred in the IR cell at room temperature as indi-
cated by the simultaneous decrease of all the IR bands of
the azide. The decomposition rates were estimated based on
the intensity changes of the two strong bands at 2196 (ν4 +
ν5) and 2092 cm–1 (ν3) with time (Figure 1). No clear IR
bands of any decomposition products were observed in the
spectrum. This is probably because of the low vapor pres-
sure of the expected product trans-1,2-dicyanoethene
[H(CN)C=C(CN)H] from a secondary dimerization of the
initially formed carbene intermediate HCCN. In fact, this
dimer was identified in the Raman spectrum of a solid azide
sample kept at 77 K.[8]
Figure 2. Time-dependent decrease of IR intensities ln(I/I0) of the
band at 2092 cm–1 of gaseous HCCN3 (initial pressure, p0) at room
temperature (298 K). The approximate first-order reaction rates
of the three curves are (8.43Ϯ0.09)ϫ10–4 s–1 (squares),
(1.69Ϯ0.04)ϫ10–3 s–1 (cicles), and (6.14Ϯ0.06)ϫ10–3 s–1 (tri-
angles), respectively.
Assuming a first-order kinetics for the thermal decay, at
least at low partial pressures of HCCN3 (Figure 2), a rate
constant (k) of (1.69Ϯ0.04)ϫ10–3 s–1 was obtained at
Thermal decomposition of HCCN3 (1.0 mbar) in the
298 K and an initial pressure (p0) of 3.8 mbar of the azide. presence of O2 (20 mbar) was also monitored by gas-phase
The corresponding half-life time (t1/2) is about 10 min, IR spectroscopy. Interestingly, all the IR bands of the azide
which is significantly shorter than that of HC(O)N3 (k = vanished completely within 20 min, and new bands oc-
8.80ϫ10–5 s–1, t1/2 = 132 min).[7a] Interestingly, the rate is curred at 2892, 2232, 1716, 1383, and 914 cm–1 (Figure 3).
reduced to (8.43Ϯ0.09)ϫ10–4 s–1 when the initial pressure Their band positions and contours strongly suggest the for-
of the azide is lowered to 0.8 mbar at 298 K. The rate is mation of metastable formyl cyanide, HC(O)CN.[13]
further lowered to (4.04Ϯ0.03)ϫ10–4 s–1 (p0 = 0.8 mbar) at plausible explanation for the formation of HC(O)CN is that
A
2
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Decomposition of Azidoacetylene
the initially produced triplet carbene intermediate HCCN condensed onto the cold matrix support at 16 K. The IR
was immediately oxidized by O2 through oxygen transfer, spectrum of the matrix revealed that decomposition started
which has been frequently observed for other carbene spe- at about 235 °C, as indicated by the appearance of new IR
cies in O2-doped noble gas matrices upon irradiation.[14] bands at 3228.9 and 1734.2 cm–1, which can be assigned to
The formation of trace amounts of HCN was also identified triplet HCCN by comparison with those previously ob-
by comparison with the IR spectrum of an authentic HCN served in solid Ar matrices at 3229.4 (ν1, CH stretch) and
sample, and it is likely produced by the decomposition of 1734.9 cm–1 (ν2, CCN asymmetric stretch).[17] In addition,
HC(O)CN.[13b] The decay of HCCN3 in the presence of formation of HCN was ascertained by the occurrence of
excess O2 still follows a first-order kinetics (Figure 2), two IR bands at 3711.0 (ν1, CH stretch) and 720.7 cm–1 (ν3,
but the rate constant is significantly increased to bend). The yield of HCCN was improved when the pyroly-
(6.14Ϯ0.06)ϫ10–3 s–1, which corresponds to a half-life sis temperature was increased to 275 °C (see the Supporting
time of less than 3 min.
Information, Figure S1). Further increase of the tempera-
ture to 315 °C caused more decomposition of the azide;
however, the yield of HCCN remained almost unchanged.
When the temperature reached 382 °C, the azide decom-
posed almost completely, and only traces of HCCN were
found among the products, but the yield of HCN clearly
increased (see the Supporting Information, Figure S1).
The formation of HCCN from thermal decomposition of
HCCN3 at relatively low pyrolysis temperatures (Ͻ300 °C)
is now directly confirmed. Despite the poor thermal sta-
bility and explosive nature of the azide precursor, pyrolysis
of HCCN3 can still be used for the production of HCCN
in the gas phase for further spectroscopic studies, because
the azide can be easily distilled off from the cold reaction
mixture (–35 °C) for direct pyrolysis when the high-boiling
propylene carbonate is used as the solvent.[8] According to
the observations noted above, triplet HCCN is thermally
unstable and furnishes HCN at higher temperatures. The
low thermal stability of HCCN also accounts for its absence
among the reported flash pyrolysis products of diazoaceto-
nitrile at 700 °C.[17] At such high temperatures only com-
plete decomposition products HCN and black carbon were
Figure 3. Gas-phase IR spectra of HCCN3 (red line) and of a mix-
ture of HCCN3/O2 (1:20, blue line) after 10 min at room 298 K.
The bands of HC(O)CN are labeled by asterisks.
Flash Vacuum Pyrolysis of HCCN3
Owing to the facile separation of molecular N2, flash identified.
vacuum pyrolysis of covalent azides has been found to be a
powerful approach to some synthetically difficult molecules
in the gas phase, such as cyclo-N2CO[15] and O2SN.[16]
Reactions of HCCN3 in Solution
Cyanomethylidene, HCCN, is an important precursor for
nitrogenated organic solid production in the upper atmo-
We expected the formation of addition products when a
sphere of Titan.[12a] In previous gas-phase studies of this solution of HCCN3 (1) was treated with bromine, because
reactive intermediate, the compound was produced by the reaction of vinyl azide with bromine is known[6,18] to
either laser photolysis of Br2HC–CϵN[12h,12i] or hydrogen quantitatively yield 1-azido-1,2-dibromoethane. However,
abstraction of CH3CN with fluorine atoms.[12j,12k] Owing to exposure of 1 to bromine led to dibromoacetonitrile (3) in-
the rather low yield, its detection requires highly sensitive stead of the corresponding simple addition products
spectroscopic methods such as high-resolution IR and mi- (Scheme 1). Nitrile 3 might be interpreted as a trapping
crowave spectroscopy. Decomposition of diazoacetonitrile product of HCCN, but even when a precooled solution of
[HC(N2)–CN] by irradiation (405 nm) in solid gas matrices bromine in CD2Cl2 was added dropwise to a cold solution
was performed by G. Maier et al. and proved to be another of 1 in CD2Cl2 at –80 °C, dibromoacetonitrile (3) was ob-
1
straightforward procedure to generate HCCN. However, tained in 95% yield (low-temperature H and 13C NMR).
this method cannot be transferred to gas-phase thermolysis Because 1 has a half-life time of about 17 h in CD2Cl2 at
(see below) and is thus limited to photochemical studies.[17] –30 °C,[6] the unimolecular decay of 1 and the generation of
The availability of HCCN3 may provide optional access to HCCN cannot be responsible for the formation of 3 at such
gaseous HCCN through facile N2 separation upon pyroly- a low temperature. Thus, it can be assumed that the ter-
sis.
minal carbon atom of 1 attacks the bromine molecule to
Flash pyrolysis of HCCN3 was performed by passing a form intermediate salt 2. The cationic part of 2 then adds
mixture of HCCN3/argon (estimated ratio of 1:1000) bromide at the terminal carbon atom with simultaneous lib-
through a heated quartz tube (i.d. 1.0 mm, length 30 mm). eration of dinitrogen to yield the final product 3. When a
The resulting decomposition products were immediately mixture of 1 and acetylene was treated at –80 °C with an
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1
excess of bromine, a few minutes later, low-temperature H shows very weak absorption in this region, it was com-
NMR showed that 1 was completely consumed with the pletely consumed after 7 min of irradiation. The IR differ-
formation of 3 (95%), while acetylene was still present. ence spectrum showing the changes after irradiation is de-
Therefore, 1 is an electron-rich alkyne that is more nucleo- picted in Figure 4.
philic and reactive than acetylene.
Figure 4. IR difference spectrum showing the photodestruction
(255 nm, 7 min) of HCCN3 (IR bands point downwards) in solid
argon at 16 K. The IR bands of HCCN (a), cyclo-C(H)CN (b), and
HCNC (c) are indicated.
Scheme 1. Reactions of HCCN3 (1) in solution.
In the absence of reactive partners, azidoacetylene (1)
underwent unimolecular loss of dinitrogen to generate carb-
ene 4, which can lead to C–Cl insertion products in the
presence of chlorinated solvents such as dichloromethane.
Thus, decay of 1 in deuterated dichloromethane at room
temperature gave propionitrile 5, which was identified by
Among the IR bands of the decomposition products, the
two strongest bands of triplet cyanocarbene (HCCN, a) ap-
peared at 3257.7 and 1746.4 cm–1. A close examination of
the spectrum revealed weaker matrix sites at 3229.3 and
1734.7 cm–1, respectively, which are close to those (3228.9
and 1734.2 cm–1) observed in the pyrolysis experiment. The
occurrence of matrix site splittings and large matrix shifts
as observed for these two vibrational modes by changing
the matrix gas from Ar (3229.4 and 1734.9 cm–1) to N2
(3213.3 and 1750.3 cm–1)[17] indicates interactions of this
carbene with surrounding molecules in the solid matrix.
The remaining IR bands (see the Supporting Infor-
mation, Table S1) of the photolysis products can be reason-
ably assigned to azirinylidene [cyclo-C(H)CN, b] and iso-
cyanocarbene (HCNC, c). The mutual photoinduced in-
terconversion and the electronic spectra of the HC2N iso-
mers a–c isolated in argon matrices have already been re-
ported.[17] Thus, selective interconversion between these
three isomers by narrow-band irradiations into their known
electronic absorption bands is a further unambiguous proof
for their formation (Figure 5).
1
the characteristic H NMR (δ = 4.66 ppm) and 13C NMR
spectroscopic data (δ = 44.1 ppm) of the C–H unit. When
1
was prepared in non-deuterated dichloromethane,
warmed to ambient temperature, and recondensed in vacuo,
removal of the solvent by distillation at normal pressure
caused elimination of hydrogen chloride and formation of
α-chloroacrylonitrile (7) in ca. 50% yield. The C–Cl inser-
tion product 8 was found after degradation of 1 in deuter-
ated chloroform. This was confirmed by analysis of signals
1
of the CH moiety in the H NMR (δ = 4.92 ppm) and 13C
NMR spectra (δ = 49.3 ppm), and especially by the signal
1
of the CDCl2 unit [δ(13C) = 69.0 ppm, t(1:1:1), JCD
=
28.4 Hz]. When the decay of 1 was performed in non-deu-
terated chloroform, product 9 was identified by two dou-
3
1
blets (δ = 4.96, 5.86 ppm, J = 4.5 Hz) in the H NMR
spectrum. Compounds 6 and 9 are already known,[19] but
NMR spectroscopic data have not yet been published.
Thus, 6[19a] was prepared by addition of chlorine at acrylo-
For the conversion of a Ǟ c, a 266 nm laser was utilized,
and this irradiation caused complete depletion of a and ex-
clusive formation of c in 10 min. Similarly, efficient and se-
lective conversion of c Ǟ b was found by using a far-red
LED (740 nm). Depletion of b was efficiently achieved by
employing a UV 365 nm LED; however, both a and c were
formed, with a being the dominant product. These observa-
1
nitrile. The corresponding H and 13C NMR spectra of 6
are in agreement with those of 5.
Photolysis of Argon-Matrix-Isolated HCCN3
Similar to most covalent azides, the UV/Vis spectrum of tions are consistent with previous studies[17] and calcula-
gaseous HCCN3 reveals two absorptions at λmax = 214 and tions at the CCSD(T)/cc-pVTZ level of theory, which pre-
232 nm (see the Supporting Information, Figure S2). Thus, dict that a is more stable than b and c by 209 and
UV irradiation of argon-matrix-isolated HCCN3 was per- 104 kJmol–1, respectively, and that the barriers for the con-
formed by using a high-pressure mercury lamp in combina- versions of b Ǟ a (22 kJmol–1) and b Ǟ c (18 kJmol–1) are
tion with a 255 nm interference filter. Although the azide similar.[12g] The large activation barrier (Ͼ200 kJmol–1) for
4
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Decomposition of Azidoacetylene
sensitive to variation in pressure, and several explosions were en-
countered during condensation of its vapor. Thus, appropriate
safety precautions (face shields, leather gloves, and protective
leather clothing) are strongly recommended. The risk of explosion
is reduced when the synthesis of HCCN3 is completed by recon-
densing the volatile product in vacuo from the reaction mixture
directly into a strongly cooled but liquid solvent. This procedure,
however, leads to lower yields.
Azidoacetylene, HCCN3, was prepared according to the reported
procedure.[6,8] Its purity was checked by gas-phase IR spectrum or
low-temperature 1H NMR spectroscopy. Oxygen (Ն99.995%, Mes-
ser-Griesheim, Krefeld, Germany) was purified by first liquifying
the gas in a glass container and then taking the most volatile part
for experiment.
Gas-phase IR spectra were recorded with an FTIR spectrometer
(Bruker, Vector 22, resolution of 2 cm–1) in a gas cell (optical path
length 20 cm, Si windows, 0.6 mm thick) without any buffer gas.
The UV/Vis spectrum of a gas sample was recorded in a quartz cell
(10 cm optical path length) with a Perkin–Elmer Lambda EZ210
spectrophotometer. Matrix IR spectra were recorded with an FTIR
spectrometer (IFS 66v/S Bruker, resolution of 0.5 cm–1) in reflec-
tance mode using a transfer optic. A KBr beam splitter and an
MCT detector were used in the region of 4000–550 cm–1. The gas-
eous sample was diluted in argon gas by passing a flow of argon
gas through a U-trap (–110 to –102 °C) containing ca. 3 mg of
the azide. The mixture (azide/argon ≈ 1:1000 estimated) was then
directed (2 mmol/h) through a quartz furnace (i.d. 1.0 mm, length
30 mm), which can be heated over a length of ca. 10 mm by a plati-
num wire (o.d. 0.25 mm, resistance 0.8 Ω at room temperature),
prior to deposition on the cold matrix support (16 K for Ar-matrix
Rh-plated Cu block) in high vacuum. Details of the matrix appara-
tus have been described elsewhere.[22]
Figure 5. IR difference spectra showing the photoinduced intercon-
version of HCCN isomers in solid argon matrix at 16 K: HCNC
(c) Ǟ cyclo-C(H)CN (b) (bottom trace, 740 nm, 10 min), cyclo-
C(H)CN (b) Ǟ HCCN (a) (middle trace, 365 nm, 18 min), and
HCCN (a) Ǟ HCNC (c) (top trace, 266 nm, 10 min).
the conversion from a into c via cyclic b rules out the for-
mation of higher-energy isomers b and c during the pyroly-
sis of HCCN3.
Conclusions
The thermal and photo decomposition of the highly ex-
plosive and unstable parent alkynyl azide, HCCN3, was
studied. In the gas phase, HCCN3 decomposes rapidly at
room temperature featuring a half-life time of Յ20 min.
The expected triplet cyanocarbene intermediate HCCN was
Photolysis experiments were performed by using a high-pressure
mercury lamp (TQ 150, Heraeus) and conducting the light through
chemically trapped by O2 as HC(O)CN. In solution, trap- water-cooled quartz lenses combined with various cutoff or inter-
ference filters (Schott), Nd3+:YAG laser (266 nm, CryLas,
ping products of HCCN by solvent molecules CH2Cl2 and
FQSS266-Q2, 10 mW), or LEDs of 365 (Qioptiq, ML3–UV365,
CHCl3 were also found during the decomposition of the
3 W) and 740 nm (LedEngin, LZ1-00R305, 5 W).
azide. The formation of HCCN was directly confirmed by
NMR spectra were recorded with a Varian Unity Inova 400 spec-
trometer operating at 400 MHz (1H) or 100 MHz (13C). NMR sig-
nals were referenced to TMS (δ = 0 ppm) or solvent signals and
recalculated relative to TMS. The multiplicities of 13C NMR sig-
nals were determined with the aid of DEPT135 experiments.
IR spectroscopic studies of matrix-isolated products of the
flash pyrolysis and photolysis (255 nm) of HCCN3 in the
gas phase and solid argon matrices, respectively.
About 30 years ago, it was postulated that decomposi-
tion of phenylethynyl azide should lead to the correspond-
ing nitrene.[20] This assumption was based on the erroneous
structure of a trapping product and was not consistent with
quantum chemical calculations.[10] Since then, the correct
structure of this trapping product has been ascertained,
which demonstrates, as several other scavenger reactions
have additionally shown, that cyanocarbenes instead of
nitrenes are generally formed upon decay of ethynyl az-
ides.[6,21] Finally, we have now directly analyzed the genera-
tion of cyanocarbene by the decomposition of the parent
azidoacetylene for the first time.
Gas chromatography was performed with a Hewlett Packard 5890,
Series II using a flame-ionization detector with 30 m quartz capil-
lary column HP-MS 5 (coated with 5% phenylmethyl silicon). Ni-
trogen was used as carrier gas. GC–MS data were recorded with a
Shimadzu GC17A-QP5000 equipped with a 30 m WCOT CP-Sil
8CB-MS capillary column and quadrupole-MS detection (EI); he-
lium was used as carrier gas.
Supporting Information (see footnote on the first page of this arti-
cle): Observed IR spectra of argon-matrix-isolated flash pyrolysis
products of HCCN3, UV absorption spectra of gaseous HCCN3,
1H and 13C NMR spectra of 3, 6, 7, and IR data of HC2N isomers.
Acknowledgments
Experimental Section
Caution! Covalent azides are hazardous and explosive.[1a] HCCN3
was found to be highly volatile and thermally unstable; the neat
substance should be handled in milligram scales (Ͻ20 mg) only.
The azide in both liquid and solid states was found to be extremely
This work was supported by the National Natural Science Founda-
tion of China (21372173) and the Deutsche Forschungsgemein-
schaft (BA 903/12-3). X. Q. Z. highly appreciated a research stay in
the laboratory of Prof. Helge Willner in Wuppertal. We thank
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Job/Unit: O42153
/KAP1
Date: 19-05-14 18:16:14
Pages: 7
X. Zeng, H. Beckers, J. Seifert, K. Banert
FULL PAPER
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Received: February 26, 2014
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Published Online: ᭿
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Eur. J. Org. Chem. 0000, 0–0

Job/Unit: O42153
/KAP1
Date: 19-05-14 18:16:14
Pages: 7
Decomposition of Azidoacetylene
Decomposition of Azides
Although azidoacetylene is highly explos-
ive, it can be irradiated in an argon matrix
to generate cyanocarbene. The same species
is also formed and analyzed after flash vac-
uum pyrolysis under relatively mild con-
ditions. Decomposition of the azide in
solution is combined with trapping reac-
tions.
X. Zeng,* H. Beckers, J. Seifert,
K. Banert* ........................................ 1–7
The Photochemical and Thermal De-
composition of Azidoacetylene in the Gas
Phase, Solid Matrix, and Solutions
Keywords: Azides / Alkynes / Carbenes /
Flash pyrolysis / Matrix isolation / Gas-
phase reactions / Photolysis
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