Redox isomerization of aromatic nitroso oxides

Full text of DOI:10.1134/S001250081201003X

ISSN 0012ꢀ5008, Doklady Chemistry, 2012, Vol. 442, Part 1, pp. 12–14. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © E.M. Chainikova, R.L. Safiullin, L.V. Spirikhin, 2012, published in Doklady Akademii Nauk, 2012, Vol. 442, No. 2, pp. 200–202.
CHEMISTRY
Redox Isomerization of Aromatic Nitroso Oxides
E. M. Chainikova, R. L. Safiullin, and L. V. Spirikhin
Presented by Academician M.S. Yunusov May 26, 2011
Received June 9, 2011
DOI: 10.1134/S001250081201003X
The reaction of triplet aromatic nitrenes with slowly decomposes when exposed to light in an acetoꢀ
molecular oxygen leads to nitroso oxides ArNOO, nitrile solution at ambient temperature but stable for
labile species that are intermediates of photooxidation several weeks upon storage in a freezing chamber.
of aromatic azides [1] and deoxygenation of aromatic
The UV spectrum of the isolated compound shows
nitroso compounds with trivalent phosphorus comꢀ
a maximum at 300 nm (Fig. 2). The IR spectrum
pounds (phosphines and phosphites) in the presence
exhibits an intense band in the region of stretching
of oxygen [2]. In the course of the photooxidation of
vibrations of triple bonds (2295 cm–1). The 1H NMR
aromatic azides, nitroso oxides are believed to produce
spectrum of compound
X in CD3CN displays four
corresponding nitroꢀ and nitrosobenzenes [3, 4].
doublets with the following chemical shifts (
, Hz): 9.90 (1H, = 7.3), 7.42 (1H, = 11.3),
5.95 (1H, = 11.3), 5.60 (1H, = 7.3), and a singlet at
δ, ppm,
We have revealed in this study that nitroso oxides
are consumed under conditions of thermal activation
and undergo an unusual isomerization reaction. It was
shown previously by flash photolysis [5] that the conꢀ
sumption of ArNOO follows a firstꢀorder kinetics. The
reaction proceeds thermally because, under condiꢀ
tions of flash photolysis, exposure time is several
orders of magnitude shorter than the lifetime of
nitroso oxides (~0.005 to 10 s [5]). This work deals
with the elucidation of the nature of this reaction.
4ꢀMethoxyphenylnitroso oxide was studied as an
example.
J
J
J
J
J
3.82 ppm (3H). The 13C NMR spectrum shows signals
at 189.92, 167.59, 134.80, 121.57, 108.26, 105.29, and
57.44 ppm. From the analysis of spectral data, we drew
a conclusion that compound
hexaꢀ2,4ꢀdienylnitrile oxide (
Scheme 1.
X is 4ꢀmethoxyꢀ6ꢀoxoꢀ
1) formed according to
–O
+N
1
O
NOO
N
O
O
2
3
6
5
We analyzed a reaction mixture obtained on expoꢀ
sure of an acetonitrile solution of 4ꢀmethoxyphenyl
4
7
azide (5 × 10–4 M) to one 50ꢀ
s light pulse (270–
μ
OCH3
OCH3
OCH3
Scheme 1.
1
380 nm) with an energy of 400 kJ in the presence of
oxygen using reversedꢀphase HPLC (mobile phase is
acetonitrile or acetonitrile–water (70 : 30)) (Fig. 1).
The main reaction products were found to be comꢀ
The terminal oxygen atom of the nitroso oxide
group interacts with the orthoꢀposition of the benzene
ring to give a fiveꢀmembered ring. The ring undergoes
opening with the cleavage of C–C and O–O bonds so
that to the aromatic ring transforms into a conjugated
diene with the aldehyde and nitrile oxide functional
groups at the termini of the molecule. Such a transforꢀ
mation may be considered a redox isomerization of
nitroso oxides.
pound
X and 4,4'ꢀdimethoxyazobenzene (identified
by comparison with an authentic sample).
Dimethoxyazobenzene results from the recombinaꢀ
tion of triplet nitrenes or their reaction with the initial
azide. The comparison with authentic samples showed
that compound
These compounds were absent in the reaction mixture.
We managed to obtain ~3 mg of compound , which
X is neither nitroꢀ nor nitrosobenzene.
X
1
Signal assignment in the H NMR spectrum is as
follows (
δ
, ppm): 9.90 (Cꢀ6), 7.42 (Cꢀ3), 5.95 (Cꢀ2),
5.60 (Cꢀ5), 3.82 (Cꢀ7). 13C NMR (
δ
, ppm): 189.92
Institute of Organic Chemistry, Ufa Research Center,
Russian Academy of Sciences, pr. Oktyabrya 71,
Ufa, 450054 Bashkortostan, Russia
(Cꢀ6), 167.59 (Cꢀ4), 134.80 (Cꢀ3), 121.57 (Cꢀ1),
108.26 (Cꢀ2), 105.29 (Cꢀ5), 57.44 (Cꢀ7). On the basis
12

REDOX ISOMERIZATION OF AROMATIC NITROSO OXIDES
4500
13
3500
2500
1500
1
2
500
0
3
0
1
2
3
Time, min
4
5
Fig. 1. Chromatogram of the concentrated reaction mixture obtained by the flash photolysis of a solution of 4ꢀmethoxyphenyl
–4
–1
azide in acetonitrile (5 × 10 mol L ) saturated with oxygen.
Mobile phase is acetonitrile, detection wavelength is 300 nm. Components: (
dimethoxyazobenzene.
1) X , (2) 4ꢀmethoxyphenyl azide, and (3) 4,4'ꢀ
1
cules of triplet nitrene, the total yield of triplet nitrene
per consumed azide is ~90%.
of the spin–spin coupling constants in the H NMR
spectrum between the protons at (Cꢀ2) and (Cꢀ3)
(11.3 Hz), one can draw a conclusion that the double
bond between these carbon atoms has the cis configuꢀ
ration. Calculation with the use of increments [6]
indicates that the C(4)=C(5) bond also has cis configꢀ
uration because, in the case of trans configuration, the
resonances of C(4) and C(5) should appear downfield:
at 171.0 and 107.6 ppm, respectively. The band in the
IR spectrum at 2295 cm–1 corresponds to the stretchꢀ
The oneꢀandꢀaꢀhalf N–O bond in nitroso oxide
molecules [9] causes their existence as cis and trans
isomers.
R
N
R
N
O
O
O
O
cis
trans
ing vibrations of C≡N bond in the nitrile oxide group.
Scheme 2.
Its position agrees well with the position of the correꢀ
sponding band in the IR spectrum of nitrile oxide staꢀ
bilized on a polymeric support (2296 cm–1) [7].
The oxygen atom of only cis isomer of the nitroso
oxide can react at the ortho position of the benzene
Compound
1 oxidizes triphenylphosphine to triꢀ
A
2.5
phenylphosphine oxide. Trivalent phosphorus comꢀ
pounds are known to reduce nitrile oxides into correꢀ
sponding nitriles [8]. The concentration of compound
2.0
1.5
1.0
0.5
0
1
was determined from the concentration of Ph3PO
resulting from the addition of Ph3P to a solution of
compound (table, run 1). Compound was comꢀ
pletely consumed over the reaction time to give a new
compound slightly more polar than judging from the
1
1
1
retention time. It is obvious that this compound is the
corresponding nitrile. The extinction coefficient at
absorption maximum (
acetonitrile was assessed to be
The yield of intermediate nitroso oxide over one light
pulse per consumed azide was determined from the
λ
= 300 nm) of compound
1 in
ε
~ 2 × 104 L mol–1 cm–1.
200
250
300
350
400
450
500
, nm
concentration of compound
1 (table, run 2). It is
λ
~40%, while the yield of azobenzene is ~25%. Taking
into account that azobenzene results from two moleꢀ
Fig. 2. UV spectrum of compound
X in acetonitrile.
DOKLADY CHEMISTRY Vol. 442
Part 1
2012

14
CHAINIKOVA et al.
The results of analysis of the composition of different reaction mixtures
[Initial reactants]
104, mol L–1
×
[Products]
×
104, mol L–1
Run no.
[ArN3]
Δ[ArN3])
[1
]
[ArNO]
[Ph3P]
[
1
]
[ArN=NAr] [Ph3PO]
[ArNO]
(
1
2
3
–
0.16
–
–
–
–
1.0
–
–
–
0.16
–
–
–
5.0 (0.47)
5.0 (0.33)
0.19 [40%] 0.12 [25%]
0.05 [15%] 0.07
–
1.0
0.08 [24%] 0.08 [24%]
Note: Acetonitrile was used as a solvent, 295 K. The concentration of consumed azide is given in parentheses, and the yield of products is preꢀ
sented in square brackets. The concentration of compound 1 was determined from the yield of the phosphine oxide. Reaction time is 19 h.
ring. We show below that compound 1 results from the methods for the first time. Nitroꢀ and nitrosobenzenes
transformation of both isomeric forms. The trans form detected upon a steadyꢀstate photooxidation of aryl
initially undergoes isomerization to the cis isomer azides are most likely to appear from the photolytic
(rate constant at 295 K in acetonitrile kiso = 0.37
0.02 s–1 [5]), which, in turn, transforms into nitrile
oxide (kiso = 11.3 0.2 s–1 [5]).
transformations of the nitroso oxides.
ACKNOWLEDGMENTS
We demonstrated recently that the cis and trans isoꢀ
mers of arylnitroso oxide show different reactivity
both in the monomolecular consumption reaction [5]
and toward organic substrates [10, 11]. Only trans isoꢀ
mer reacts with triphenylphosphine, whereas cis isoꢀ
mer undergoes monomolecular consumption [11].
The flash photolysis of 4ꢀmethoxyphenyl azide in the
presence of Ph3P (1 × 10–4 mol L–1) led to a considerꢀ
This work was supported by the Russian Academy
of Sciences (the program of the Division of Chemistry
and Materials Science “Chemical Reaction Intermeꢀ
diates: Their Detection, Stabilization, and Determiꢀ
nation of Structural Parameters) and the Russian
Foundation for Basic Research (project no. 09–03–
00411).
able decrease in the concentration of nitrile oxide
1
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rate constant for the reaction of transꢀ4ꢀmethoxypheꢀ
nylnitroso oxide with triphenylphosphine is 5.5 ×
105 L mol–1 s–1 [5]. Hence, trans nitroso oxide reacts
under these conditions with phosphine, while nitrile
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,
groups, was isolated and characterized by spectral
DOKLADY CHEMISTRY Vol. 442
Part 1
2012