4-Azidoquinoline N-oxide: Synthesis and photolysis

Article abstract of DOI:10.1134/S1070428008050163

In photolysis of 4-azidoquinoline N-oxide azoxy compound was isolated as the final reaction product. This result may be ascribed to the dimerization of the intermediate nitrene to azo compound followed by oxidation of the latter with air oxygen. The initially arising nitrene is stabilized by resonance conjugation involving the aromatic system and the N-oxide group. The rate constants of 4-azidoquinoline N-oxide photolysis were measured in various solvents and the values of spin density and bond lengths in the formed nitrene were calculated.

Full text of DOI:10.1134/S1070428008050163

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2008, Vol. 44, No. 5, pp. 728−730. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © K.V. Shaforost, A.V. Ryzhakov, L.L. Rodina, 2008, published in Zhurnal Organicheskoi Khimii, 2008, Vol. 44, No. 5, pp. 736−
738.
4-Azidoquinoline N-Oxide: Synthesis and Photolysis
K. V. Shaforosta, A. V. Ryzhakovb, and L. L. Rodinaa
aSt. Petersburg State University, St. Petersburg, 198504 Russia
e-mail: lrodina@VN6646.spb.edu
bKarelia Scientific Center, Russian Academy of Sciences, Petrozavodsk, Russia
Received June 20, 2007
Abstract—In photolysis of 4-azidoquinoline N-oxide azoxy compound was isolated as the final reaction product.
This result may be ascribed to the dimerization of the intermediate nitrene to azo compound followed by oxidation
of the latter with air oxygen. The initially arising nitrene is stabilized by resonance conjugation involving the
aromatic system and the N-oxide group. The rate constants of 4-azidoquinoline N-oxide photolysis were measured
in various solvents and the values of spin density and bond lengths in the formed nitrene were calculated.
DOI: 10.1134/S1070428008050163
The decomposition of organic azides with liberation
of a nitrogen molecule can be performed by photolysis
or thermolysis. The first way is obviously preferable for
aryl azides for they possess adsorption bands in the long-
wave region of the spectrum thus making possible their
photolysis without affecting the other chromophores
present in the molecule.
In the study of reactions in the dark of quinolines
N-oxides we found that even in the presence of an azide
group in the heteroaromatic framework these compounds
reacted with active dipolarophiles, e.g., with dimethyl
acetylenedicarboxylate, only involving the N-oxide
function [9]. In this connection it was reasonable to
investigate the behavior of this compound under the
photolysis conditions. Just the investigation of photo-
transformations of 4-azidoquinoline N-oxide was the
target of this study.
The photodecomposition of aryl-substituted azides is
known to produce nitrenes whose stabilization may occur
intramolecularly: As a result of conversion into azirine
followed by electrocyclic cycle opening and trans-
formation into azepine [1]. However relatively stable
nitrenes are prone to dimerization even in the presence
of “nitrene traps” [1–3]. It should be also considered that
under the UV irradiation the heteroaromatic N-oxides
are capable of various rearrangements [4, 5]. The ease of
nitrogen ejection from the azide molecule can be strongly
affected by the other functional groups present in the
initial molecule. The unsubstituted quinoline N-oxide is
known to be converted under the action of light along
two routes, yielding 2-quinolone in hydroxy-containing
solvents and in those containing no hydroxy groups,
benzazepine, and a small amount of quinoline formed in
both cases [6, 7].
Initial N-oxide (I) we synthesized in ~80% yield by
a new simple procedure we had developed: 4-Nitro-
quinoline N-oxide was kept in solution with sodium azide
in the dark for 2–3 days at room temperature. Previously
compound I was prepared in 45% yield from the 4-chloro-
quinoline N-oxide and sodium azide at 115–125ºC in
a sealed ampule, and the the 4-chloroquinoline N-oxide
was synthesized from 4-nitroquinoline N-oxide and
acetyl chloride [10].
Azide I was irradiated with UV light of >220 nm
wavelength (through a quartz filter) in various solvents
(concentration 1×10–4–5×10–3 mol l−1). The progress of
the photochemical reaction that proceeded for ~1 min
was followed by registering the UV spectra every 5–
10 s (see the figure).
Yet in the case of aryl azides the photolysis can leave
intact the other chromophore groups. Actually, 4-azido-
pyridine N-oxide was converted by photolysis not into
an azo compound but to benzazirine and afterwards to
dehydroazepine [8].
Practically immediately after the start of irradiation
of the N-oxide azide solution the color changed from
yellow to bright red, and a settling of a dark-red
crystalline precipitate was observed. Its elemental
728

4-AZIDOQUINOLINE N-OXIDE: SYNTHESIS AND PHOTOLYSIS
729
solution, Dinitial is the optical density of the initial solu-
tion, Dcurrent is the optical density at the given moment.
As seen from Table 1, the solvent character did not
significantly affect the reaction rate.
For the preparative synthesis of azoxy compound III
we carried out the photolysis of N-oxide I in acetone
solution in a quartz cell under the sunlight.
.
.
N3
N
N
O
N
O
Variations in electronic spectrum of a solution of 4-azido-
quinoline N-oxide in acetone (c 1.0×10–4 mol l−1 ) under
irradiation (1, 0 s; 2, 10 s; 3, 15 s; 4, 20 s; 5, spectrum of the
final reaction product).
I
II
O
analysis and spectral characteristics testify to the
structure of azoxy compound III. In the IR spectrum of
the isolated substance recorded from the pellets with KBr
the absorption bands of azide group at 2093 and 1300
cm–1 are lacking, but a new band appears at 1570 cm–1.
This band may be assigned to the stretching vibration of
the N=N bond. In the sprectrum a strong band at 1270
cm–1 is also present characteristic of vibrations of N →
O group of the aromatic N-oxides.
N
N
N
O
N
O
III
Mixture of isomers
The spectral characteristics and rate constants of the
initial compound disappearance are given in Table 1.
The formation of azoxy compound may be
represented by a Scheme assuming the intermediate
formation of nitrene II.
The rate constant of disappearance of the initial azide
was calculated from the variation of the optical density
of its solutions at λmax for various solvents:
To estimate the reactivity of the hypothetical nitrene
II we calculated the values of spin density on each atom
and the bond orders. Method RHF in the basis STO-6G
was used in the calculations. Some calculated data are
given in Table 2.
_
Dinitial Dcurrent
k = (1/t)(1/cinitial)(
),
Dcurrent
where k is the rate constant of the reaction, t is the
irradiation time, cinitial is the concentration of the initial
Table 2. Parameters of nitrene formed from 4-azidoquinoline
N-oxide
Table 1. Characteristics of irradiated solutions of
4-azidoquinoline N-oxide and constants of photolytic
decomposition (c × 6.0 10−3 mol l−1
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SHAFOROST et al.
730
Azides of aromatic compounds commonly have the
chloroform (2 × 5 ml). The extract was dried with
anhydrous sodium sulfate, chloroform was removed in
a vacuum, the residue was recrystallized from acetone.
Yield 0.3 g (81%), mp 140–142ºC (decomp.), light-
sensitive. IR spectrum (1% solution in CHCl3), ν, cm–1:
2093 (asymmetric stretching vibrations of N3), 1292
(N → O), 1318 (symmetric stretching vibrations of N3).
Electronic spectrum (in 96% ethanol), λmax, nm (log ε):
233 (4.31), 258 (3.96), 355(3.88).
spin density on the nitrene nitrogen about 2 [11], but the
introduction of the N-oxide group essentially alters the
situation. In the nitrene formed from 4-azidoquinoline
N-oxide the spin density is distributed between the
oxygen of the N-oxide group and the nitrene nitrogen
atom; this fact apparently results in the nitrene
stabilization.
The latter affects the nitrene reactivity. In particular,
in the photolysis of N-oxide I in the presence of excess
acrylonitrile or butyl vinyl ether only azoxy compound
III was obtained, and no addition products of nitrene
across the multiple bond were detected that could be
ascribed to its low reactivity.
Photolysis of N-oxide I. A solution of 93 mg
(0.5 mmol) of N-oxide I in 3 ml of acetone in a quartz
cell was exposed to the sunlight for 8–10 min. On
completion of the reaction (TLC monitoring, eluent
acetone–hexane, 4:1) and at the end of nitrogen evolution
the formed dark-red crystalline precipitate of azoxy
compound III was filtered off, washed with ethyl ether,
dried in air, and recrystallized from acetone. We obtained
58 mg (70%) of azoxy compound III, mp 278–280ºC.
Found, %: C 64.87; H 3.66; N 17.54. C18H12N4O3.
Calculated, %: C 65.06; H 3.61; N 16.87. The sample
obtained at photolysis of N-oxide I under irradiation with
a mercury lamp was identical to the above described
compound in the melting point and TLC data.
The formation under the experimental conditions of
azoxy compound III is evidently due to the presence of
oxygen both in the solvent and in the atmosphere of the
reaction vessel.
Thus the photolysis of the bifunctional compound,
4-azidoquinoline N-oxide (I) did not involve directly the
N-oxide group. However it played a significant part in
distribution of the spin density in the primarily formed
nitrene that was characterized by reduced activity. The
main route of its transformation was dimerization
followed by oxidation with the oxygen present in the
reaction environment.
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The photolysis of 4-azidoquinoline N-oxide in various
solvents was carried out in a quartz cell under the UV
radiation of a mercury lamp of high pressure Q-139 of a
power 250 W. The spectra in UV and visible regions and
the variation in the intensity of absorption bands of
solutions as a function of irradiation time were measured
on a spectrophotometer Specord M40. IR spectra of the
initial compounds and photolysis products were recorded
on a Specord 75IR instrument.
4-Azidoquinoline N-oxide (I). To a solution of
0.38 g (2 mmol) of 4-nitroquinoline N-oxide in 3 ml of
dioxane was added a solution of 0.14 g (2.2 mmol) of
sodium azide in 1 ml of water and several drops of
ethanol. The reaction mixture was left standing in the
dark at room temperature for 2–3 days (TLC monitoring,
eluent chloroform–ethanol, 10:1). The solvents were
removed in a vacuum, 2 ml of water was added to the
residue, the reaction product was extracted into
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 44 No. 5 2008