4-N,N-Dimethylaminophenyl azide photooxidation: effect of conditions on the reaction pathway. Ring contraction of benzene to cyclopentadiene due to a strongly electron-donating substituent

Article abstract of DOI:10.1016/j.tetlet.2015.06.042

Abstract Depending on the reaction conditions employed, the photooxidation of 4-N,N-dimethylaminophenyl azide led to the formation of 4-N,N-dimethylaminonitrosobenzene and 4-N,N-dimethylaminonitrobenzene or (5Z)-2-(dimethylamino)-5-(hydroxyimino)cyclopenta-1,3-diene-1-carbaldehyde.

Full text of DOI:10.1016/j.tetlet.2015.06.042

Tetrahedron Letters 56 (2015) 4661–4665
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
4-N,N-Dimethylaminophenyl azide photooxidation: effect
of conditions on the reaction pathway. Ring contraction of benzene
to cyclopentadiene due to a strongly electron-donating substituent
Ekaterina Chainikova a, , Sergey Khursan , Alexander Lobov , Alexey Erastov , Leonard Khalilov ,
⇑
a
a
a
b
b
a
Ekaterina Mescheryakova , Rustam Safiullin
a
Ufa Institute of Chemistry of the Russian Academy of Sciences, 71 prosp. Oktyabrya, 450054 Ufa, Russian Federation
Institute of Petrochemistry and Catalysis of the Russian Academy of Sciences, 141 prosp. Oktyabrya, 450075 Ufa, Russian Federation
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Depending on the reaction conditions employed, the photooxidation of 4-N,N-dimethylaminophenyl
azide led to the formation of 4-N,N-dimethylaminonitrosobenzene and 4-N,N-dimethylaminonitroben-
zene or (5Z)-2-(dimethylamino)-5-(hydroxyimino)cyclopenta-1,3-diene-1-carbaldehyde.
Ó 2015 Elsevier Ltd. All rights reserved.
Received 6 April 2015
Revised 9 June 2015
Accepted 15 June 2015
Available online 20 June 2015
Keywords:
Photooxidation of aryl azides
Reaction mechanism
Arylnitroso oxides
Ring contraction
Nitrile oxides
O
N
O
N
O
O
O
O
Chemical processes proceeding via the participation of triplet
aromatic nitrenes (photolysis of aromatic azides, deoxygenation
of nitrosobenzenes) in the presence of oxygen lead to the forma-
tion of arylnitroso oxides (ArNOO) which are labile species, of
which further transformations determine the composition of the
final reaction products.1 A remarkable feature of nitroso oxides is
their cis-trans isomerism, owing to the NAO bond order of ꢀ1.5
N
R
R
R
Scheme 1. Mechanism of the unimolecular transformation of the cis form of
arylnitroso oxides.
2
in the nitroso oxide moiety. Isomeric forms of most nitroso oxides
3
studied are consumed by first-order reactions, which in the case
2
-azidophenazines.4 Benzisoxazole and pyranoisoxazole contain-
ing heterocycles were formed in the presence of oxygen during
of the trans isomer is isomerization to the cis isomer.
Consumption of the cis isomer is a rather unusual process where
an aromatic ring-opening reaction occurs to form a diene nitrile
oxide (Scheme 1).
3
c,5
6
photolysis of 4-allyl-
and 4-vinyloxyphenyl azide, respectively.
Finally, 6-azidoquinoline photooxidation led to the formation of a
5
substituted indolizine.
The nitrile oxide formed from the reaction of 4-methoxyphenyl-
nitroso oxide (Scheme 1, R = 4-MeO), was sufficiently stable to be
isolated and identified after the photooxidation of 4-methoxyphe-
Analysis of the reaction mixtures obtained upon thawing the
glassy matrices containing 4-aminophenylnitroso oxide showed
that the final products of this species were 4-aminonitro- and 4-
3
b
nyl azide. When the molecule possesses a suitable reaction
center, the nitrile oxide undergoes further intramolecular
rearrangements to form stable heterocyclic products. Thus,
quinoxalines have been formed during the photooxidation of
7
aminonitrosobenzene. In addition, using flash photolysis, it was
found that this nitroso oxide was consumed in a second-order
reaction. On the basis of these facts, it was suggested that the
7
recombination of 4-aminophenylnitroso oxide leads to the forma-
tion of an intermediate peroxide dimer which decays into two
molecules of nitrobenzene or two molecules of nitrosobenzene
⇑
Corresponding author. Tel.: +7 347 235 5496; fax: +7 347 235 6066.
E-mail address: kinetic@anrb.ru (E. Chainikova).
7
and oxygen (Scheme 2).
http://dx.doi.org/10.1016/j.tetlet.2015.06.042
040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
0

4
662
E. Chainikova et al. / Tetrahedron Letters 56 (2015) 4661–4665
N3
NOO
NOO
Results and discussion
Based on the information presented, it can be assumed that
changes to the reaction conditions such as the nitroso oxide struc-
ture, reaction medium, and temperature can change the mecha-
nism of the decay reaction of these species from unimolecular to
bimolecular and vice versa. In the present work we have studied
the effect of various factors on the product composition obtained
during the photooxidation of 4-N,N-dimethylaminophenyl azide
N
N
OMe
Me
Me
Me
Me
1
2a
2b
Figure 1. Starting azide (1), corresponding nitroso oxide (2a), and 4-
methoxyphenylnitroso oxide (2b)
(
1, Fig. 1), making it possible to draw certain conclusions about
N3
OH
NO
NO2
the influence of reaction conditions on the mechanism of transfor-
mations of 4-N,N-dimethylaminophenylnitroso oxide (2a, Fig. 1). It
was found that competition existed between the bimolecular and
unimolecular channels for the consumption of this nitroso oxide.
The predominance of one or the other channel depended on the
reaction conditions. Instead of the corresponding nitrile oxide
5N
O
hν, O2
1
4
6
+
+
2
3
N
N
Me
7
N
N
Me
Me
Me
Me Me
Me
Me
8
1
3
4
5
(
2
Scheme 1, R = Me N), the substituted cyclopentadiene, which is
Scheme 3. Products resulting from the photooxidation of 4-N,N-dimethy-
laminophenyl azide (1).
formed as a result of further transformations due to the presence
of the strongly electron-donating substituent, turned out to be
the final product of the unimolecular reaction of 2a. The unimolec-
ular transformations of nitroso oxide 2a into the end product have
been studied using theoretical methods in comparison with the
similar transformations of 4-methoxyphenylnitroso oxide
products 4 and 5 decreases. Thus, if the assumption of a competi-
tion between the bimolecular and unimolecular reactions of the
nitroso oxide is true, then, on the basis of the data in Table 1, it
can be concluded that a polar solvent promotes the formation of
the products of bimolecular reactions of this species, and the
products of a unimolecular transformation are predominant in a
non-polar solvent. Thus, we were able to find optimal conditions
(2b, Fig. 1), permitting an explanation of why in the case of 2b
3b
the reaction stops at the stage of nitrile oxide formation.
The photolysis of azide
1 was carried out in oxygen-
saturated solutions using light of wavelength range 270–380 nm.
The main products of the reaction were (5Z)-2-(dimethylamino)-
9
for the synthesis of cyclopentadiene 3; a non-polar solvent, high
5
-(hydroxyimino)cyclopenta-1,3-diene-1-carbaldehyde (3), 4-N,
temperature, low azide concentration, and a low irradiation inten-
sity (steady-state photolysis).
N-dimethylaminonitrosobenzene (4), and 4-N,N-dimethylaminoni-
trobenzene (5) (Scheme 3).8
Simultaneous formation of nitroso 4 and nitro compound 5
could easily be explained by the mechanism shown in Scheme 2,
whereas the mechanism for the formation of cyclopentadiene 3
from nitroso oxide 2a was not obvious. To clarify a sequence of ele-
mentary stages required for the transformation of 2a into 3, the
reaction was theoretically modeled using the composite
Table 1 shows the yields of the products 3–5 depending on the
details of the experiment. The change of conditions exerted an
influence on the yield of product 3 and the total yield of products
4
and 5. It can be seen that the yield of cyclopentadiene 3 decreases
sharply in acetonitrile in comparison with hexane, and the total
yield of nitroso 4 and nitro compound 5 increases (entry 5 vs
1
0
G3MP2B3 method. To explain the differences in the product
1
2). With increasing concentration of the initial azide 1, the total
yield of 4 and 5 increases and the yield of 3 decreases steadily
entries 4–8). A similar effect was observed with increasing the
compositions of the photooxidation of azide 1 and 4-metho-
3
b
xyphenyl azide we performed a similar investigation into the
unimolecular transformations of nitroso oxide 2b. A solvent effect
was described within the framework of the IEFPCM model11 for
structures optimized at the B3LYP/6-31G(d) level of theory,12
which is a method for optimization and a frequency calculation in
the composite G3MP2B3 method. The energy diagram of the pro-
cess is shown in Figure 2 and the structures of the transition state
10a and cyclopentadiene 3 in Figure 3.
(
irradiation intensity when the experiments were conducted under
flash photolysis conditions (entry 2 vs 11). Increasing the temper-
ature resulted in a decrease in the yield of nitroso- and nitroben-
zene and an increase in the yield of cyclopentadiene 3 (entries 1,
2
, 5 and 9, 12, 13). Additionally, the ratio of 4 and 5 was dependent
on the solvent polarity and in hexane this ratio was greater than in
acetonitrile (entries 5 and 12).
Product 3 was formed via the sequence of unimolecular trans-
formations 2a ? 3, for which all intermediate stable structures
and transition states were localized (Fig. 2). During the nitrile
oxide cyclization, we found a major difference between the reac-
tivities of 9a and 9b. The activation Gibbs energy in the case of
These results indicated that cyclopentadiene 3 was the product
of unimolecular transformations of nitroso oxide 2a, while the
nitroso and nitro compounds were formed via bimolecular reac-
tions of this species (Scheme 2).7 Increasing the irradiation inten-
sity and/or initial concentration of the starting azide led to an
increase in the concentration of the intermediate species, including
nitroso oxide, that promote the recombination processes and are
reflected in an increasing total yield of products 4 and 5. Clearly,
the unimolecular transformations are more energy-consuming
than the recombination reactions; therefore, with increasing tem-
perature the yield of product 3 increases and the total yield of
À1
the dimethylamino derivative was determined as 88.4 kJ mol
providing an acceptable rate of reaction for 9a ? 11 under the
experimental conditions. However, in the case of the methoxy
–
derivative, the
D
G
value for the similar transformation was much
À1
higher—121.4 kJ mol . This result explained the relative stability
of 9b, which permitted the isolation of this nitrile oxide and its
3
b
characterization by spectroscopic methods.
reorganization ability of 9a seems to come from the strong
mesomeric effect of the Me N substituent, promoting increasing
nucleophilicity of the reaction center in the transition state 10a,
The further
2
2
ArNO2
that is, the carbon atom at the
(Scheme 4).
a-position to the carbonyl group
O
O
O
O
2
ArNOO
Ar
N
N Ar
Indeed, the Atoms in Molecules (AIM) analysis of the electron
density distribution in 9a and 9b testifies to its growth on the
CACAO fragment of 9a by 0.042 a.e. in comparison with 9b, which
2
ArNO + O2
Scheme 2. Mechanism of the bimolecular reaction of arylnitroso oxides.

E. Chainikova et al. / Tetrahedron Letters 56 (2015) 4661–4665
4663
Table 1
Effect of experimental conditions on the yields of products of photooxidation of azide 1
5a
5b
5c
Entry
[1]
0
 10 (M)
Solvent
T (°C)
D
[1] Â 10 (M)
[Product] Â 10 (M)
3
4
5
1d
2.2
2.3
5.6
1.1
5.3
11.1
20.7
101
5.4
2.3
2.0
5.4
5.4
2.2
2.0
C
C
C
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
H
H
À10
2
2.0 (91)
1.0 (43)
5.2 (93)
1.0 (91)
4.8 (90)
9.7 (87)
19.9 (96)
25.6 (25)
4.9 (91)
1.5 (65)
1.2 (60)
2.8 (52)
2.8 (52)
0.9 (41)
1.5 (75)
0.11 (6)
1.1 (55)
0.33 (33)
2.0 (38)
0.060 (6)
1.0 (21)
3.2 (33)
7.2 (36)
19.4 (76)
0.29 (6)
0.28 (19)
0.65 (54)
0.85 (30)
0.54 (19)
0.37 (41)
0.41 (27)
0.24 (12)
0.038 (4)
0.16 (3)
0.012 (1)
0.097 (2)
0.27 (3)
6
6
6
6
6
6
6
6
6
6
6
14
14
14
14
14
14
14
14
14
14
14
d
2
0.40 (40)
2.2 (42)
0.93 (93)
3.1 (64)
4.2 (43)
4.3 (22)
d
3
14
23
23
23
23
21
45
12
0
24
70
23
60
d
4
d
5
d
6
d
7
0.94 (5)
d
8
—
2.56 (11)
0.045 (1)
0.052 (3)
0.11 (9)
0.81 (29)
0.32 (11)
0.22 (24)
0.34 (23)
d
9
4.5 (92)
0.68 (45)
—
0.084 (3)
1.4 (50)
—
1
1
1
1
1
1
0e
e
1
d
2
MeCN
MeCN
MeCN
MeCN
d
3
e
4
e
5
0.36 (24)
a
b
c
Initial concentration of the starting azide.
Concentration of consumed azide, conversions in % are given in parentheses.
Yields in % based on consumed azide were measured by HPLC using calibration graphs9 and are given in parentheses.
Steady-state photolysis.
d
e
Flash photolysis—irradiation of the solution by a light pulse of high power.
O
o
-1
O
ΔG , kJ mol
N
N
N
O
N
OH
O
O
C
O
H
7
7.5
O
X
X
7
3.5
X
X
8
10
12
14
a X = Me N
2
N
O
N
OH
b X = MeO
0.0
0
.0
H
-
21.3
-
36.2
-41.5
X
O
O
X
O
O
-56.3
N
X
-54.9
N
H
O
11
13
-
77.2
O
-
140.4
X
3
-
165.6
6
-176.3
O
O
N
O
N
O
-
216.0
O
-2 21 .9
C
N
O
X
-253.7
-
280.0
X
X
2
7
9
2
Figure 2. Energy diagram (in terms of Gibbs free energy) of the transformation of nitroso oxide 2a (X = Me N) into cyclopentadiene 3. Gibbs energy levels are indicated by the
solid lines; the results of calculations of the first stages of the transformation of nitroso oxide 2b (X = MeO) are provided for comparison (in italic). Calculation conditions:
G3MP2B3, 298 K, solvent is n-hexane (IEFPCM, B3LYP/6-31G(d)).
O
O
N
N
C
O
C
O
N
N
Me
Me
Me
Me
Scheme 4. Resonance structures of the transition state 10a.
was consistent with Scheme 4. In transition states 10a and 10b, the
CACO bond distances were 1.475 and 1.492 Å, respectively, and the
formally double neighboring C@C bond in 10a was substantially
elongated in comparison with 10b (1.433 vs 1.421 Å). It can be
concluded that a zwitterionic resonance structure’s contribution
to the transition state 10a wave function is large, which promotes
an intramolecular cycloaddition of the nitrile oxide through an
electrophilic mechanism with CAC bond formation. The driving
force of this process is the presence of the strongly electron-
donating substituent in the molecule of nitrile oxide 9a. Thus,
1
0a
3
Figure 3. Transition state structure for the cyclization of nitrile oxide 9a and the
equilibrium structure of the product of the transformation of nitroso oxide 2a—
cyclopentadiene 3. Full optimization at the B3LYP/6-31G(d) level of theory. The
only imaginary frequency is observed (
0a. Bond distances are in angstroms.
À1
m
imag = 428i cm ) for the transition state
1

4
664
E. Chainikova et al. / Tetrahedron Letters 56 (2015) 4661–4665
O
O
N
trans-2a
3N
N
N3
N
Me
Me
NO
N
NO2
N
hν/ isca
MeCN
+
low temperature
high irradiation
intensity
N
N
Me
Me
Me
Me
1
Me
Me
Me
Me
hexane
high temperature
low irradiation
intensity
O
O
O
O
O
N
N
O
4
5
N
C
cis-2a
N
N
Me
Me
Me
Me
Me
Me
9
a
7a
N
O
N
OH
H
O
N
O
N
Me
Me
Me
Me
1
1
3
a
Intersystem crossing
Scheme 5. Mechanism of photooxidation of 4-N,N-dimethylaminophenyl azide.
the result of the transformation sequences, which take place in the
system under study, is the ring contraction of benzene to a
cyclopentadiene. The 2-(dimethylamino)-5-nitroso-cyclopenta-2,
Supplementary data
Supplementary data (experimental details, characterization
1
13
4
-diene-carbaldehyde (11) formed is stabilized by proton transfer
data of compounds and copies of H NMR and C NMR spectra,
ORTEP diagram, and the HPLC chromatogram) associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1016/j.tetlet.2015.06.042.
(11 ? 13), and the conformational transition (13 ? 3) to form an
intramolecular hydrogen bond completes the reaction (Fig. 2).
Therefore, the oxime group in compound 3 has only the Z-configu-
8
ration which was established by single-crystal X-ray analysis and
NMR spectroscopy (See ESI).
References and notes
A decrease in the yield of cyclopentadiene 3 in acetonitrile was
due to a change in the mechanism of nitroso oxide 2a consumption
and this was confirmed by the theoretical calculations. In
1
.
(a) Gritsan, N. P.; Pritchina, E. A. Russ. Chem. Rev. 1992, 61, 500–516; (b) Gritsan,
N. P. Russ. Chem. Rev. 2007, 76, 1139–1160; (c) Ishiguro, K.; Sawaki, Y. Bull.
Chem. Soc. Jpn. 2000, 73, 535–552; (d) Sawwan, N.; Greer, A. Chem. Rev. 2007,
107, 3247–3285; (e) Chainikova, E. M.; Khursan, S. L.; Safiullin, R. L. In The
Chemistry of Peroxides; Greer A.; Liebman J. F., Ed.; John Wiley & Sons Ltd:
Chichester, UK, 2014; Vol. 3, pp 357–420.
–
À1
acetonitrile the
DG (6a) value increases by 5.2 kJ mol over the
hydrocarbon solvent which reduces the competitiveness of the
unimolecular decay reaction of nitroso oxide 2a with respect to
the recombination processes. As a result, nitroso 4 and nitro com-
pound 5 were formed as the main products under photooxidation
of azide 1 in acetonitrile at room temperature (Table 1, entry 12).
Thus, we have studied the effect of various factors on the out-
come of the photooxidation of azide 1. The product composition
of this reaction was determined by transformations of its interme-
diate, nitroso oxide 2a. The nature of these transformations, in
turn, depends on solvent polarity, temperature, irradiation inten-
sity, and/or concentration of the starting azide. The unimolecular
reaction to form cyclopentadiene 3 predominates in non-polar
solvent, at high temperatures, low irradiation intensity, and low
concentration of azide 1 (Scheme 5). On the contrary, the bimolec-
ular reaction of nitroso oxide 2a leading to the formation of
nitroso- 4 and nitrobenzene 5 is favored in polar solvent, at low
temperatures, high irradiation intensity, and high concentration
of azide 1 (Scheme 5).
2.
Talipov, M. R.; Ryzhkov, A. B.; Khursan, S. L.; Safiullin, R. L. J. Struct. Chem. 2006,
47, 1051–1057.
3. (a) Chainikova, E. M.; Khursan, S. L.; Safiullin, R. L. Kinet. Catal. 2006, 47, 549–
54; (b) Chainikova, E. M.; Safiullin, R. L.; Spirikhin, L. V.; Abdullin, M. F. J. Phys.
5
Chem. A 2012, 116, 8142–8147; (c) Chainikova, E. M.; Pankratyev, E. Yu.;
Teregulova, A. N.; Gataullin, R. R.; Safiullin, R. L. J. Phys. Chem. A 2013, 117,
2728–2737.
Albini, A.; Betinetti, G.; Minoli, G. J. Org. Chem. 1987, 52, 1245–1251.
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4
5
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2140–2142.
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(a) Pritchina, E. A.; Gritsan, N. P. J. Photochem. Photobiol. A 1988, 43, 165–182;
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(
8. Preparation of cyclopentadiene 3: azide 1 (37 mg, 0.23 mmol) was dissolved in
oxygen saturated acetonitrile (1000 mL) and irradiated with light (wavelength
range 270–380 nm) at room temperature. During the reaction, a brown solid
precipitated. The azide consumption was monitored by reverse-phase HPLC
[analytical column: Wakosil II 5C18 RS 4.6 Â 250 mm (SGE); mobile phase:
2
MeCN/H O = 70/30]. When the conversion of 1 reached 84% (t = 90 min), the
reaction was stopped and the precipitate filtered, dried (14 mg), dissolved in
acetonitrile, and purified by HPLC [column: ReproSil-Pur C18-AQ,
50 Â 8 mm (Dr. Maisch GmbH), mobile phase: MeCN]. After removing the
solvent, brown crystals were obtained. According to single-crystal X-ray
analysis and NMR spectroscopy this compound was cyclopentadiene
11.2 mg, isolated yield is 35% or 29% based on azide consumed or starting
5l,
2
Acknowledgments
3
(
The theoretical investigations were performed and the NMR
and mass spectra were recorded on equipment at the Center for
Collective Use ‘Chemistry’ of the Ufa Institute of Chemistry of the
Russian Academy of Sciences.
1
azide, respectively). Compound 3: mp 133-135 °C; H NMR (500 MHz, CD
TMS): d (ppm) = 3.40 (br s, 6H, CH 3-4 = 5.8 Hz, 1H, H(3)), 6.93 (d,
), 6.79 (d, 3
4-OH = 0.9 Hz, 1H, H(4)), 9.22 (s, 1H; H(6)); 14.40 (br s, 1H, OH).
C NMR (125 MHz, CD CN, TMS): d (ppm) = 44.57 (C(8)), 46.76 (C(7)), 103.62
C(1)), 127.06 (C(3)), 140.42 (C(4)), 157.46 (C(5)), 170.36 (C(2)), 180.04 (C(6))
3
CN,
3
J
3
4-3 = 5.8 Hz, 5
J
J
13
3
(

E. Chainikova et al. / Tetrahedron Letters 56 (2015) 4661–4665
4665
+
(
(
C
the atom numbering is shown in Scheme 3); MS (EI, 70 eV), m/z (%): 166 [M]
ꢀ90% light energy was emitted in 50
l
s. The light from this lamp was passed
+
+
20), 149 [MÀOH] (100), 122 [MÀNMe
2
]
(21); HR-MS (EI) calcd for
[M] : 166.0737. Found: 166.0742. UV–Vis (MeCN): kmax = 300 nm
through a filter with a transmittance range of 270–380 nm. The yields of the
products 3, 4, and 5 were measured by HPLC using calibration graphs. When
carrying out the reaction in hexane the product 3 precipitated. The solvent was
evaporated from the resulting reaction mixture, the residue was dissolved in an
equal acetonitrile volume and the resulting solution was analyzed.
10. (a) Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Rassolov, V.; Pople, J. A. J.
Chem. Phys. 1998, 109, 7764–7776; (b) Curtiss, L. A.; Redfern, P. C.;
Raghavachari, K.; Rassolov, V.; Pople, J. A. J. Chem. Phys. 1999, 110, 4703–4709.
11. Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999–3094.
12. (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652; (b) Ditchfield, R.; Hehre, W.
J.; Pople, J. A. J. Chem. Phys. 1971, 54, 724–728; (c) Hehre, W. J.; Ditchfield, R.;
Pople, J. A. J. Chem. Phys. 1972, 56, 2257–2261; (d) Hariharan, P. C.; Pople, J. A.
Theor. Chim. Acta 1973, 28, 213–222; (e) Hariharan, P. C.; Pople, J. A. Mol. Phys.
1974, 209.
+
H
8 10
N
2
O
2
4
À1
À1
3
À1
À1
(e
= 1.0 Â 10
M
cm
,
380 nm
(e
= 6.3 Â 10
M
cm ). Crystallographic
data (excluding structure factors) for the structure of compound 3 have been
deposited with the Cambridge Crystallographic Data Center as supplementary
publication 973506 CCDC. Copies of the data can be obtained, free of charge, on
application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK, (fax: +44-
(
0)1223-336033 or e-mail: deposit@ccdc.cam.ac.UK.
To select the optimal conditions for preparation of product
photooxidation of azide 1 was conducted in a thermostated quartz reactor
V = 5 mL) under the conditions of the steady-state or flash photolysis. The
steady-state photolysis was performed with a filtered xenon lamp (270–
9
.
3 the
(
3
5
80 nm). In flash photolysis experiments, the photolytic source was an IFP
000-2 lamp; maximum pulse energy was 400 J at U = 5 kV and C = 32 F;
l