Sensitized photolysis of stearic acid azide

Article abstract of DOI:10.1134/S0018143911060087

In an IR-spectroscopic study of the sensitized photolysis of stearic acid azide in a solution of benzene, it was found that the introduction of Michler's ketone into the system decreased the efficiency of isocyanate formation by a factor of about 10. It was hypothesized that the azide photolysis in the presence of Michler's ketone can be explained by the formation of a complex of the acyl azide with the ketone in a triplet excited state, a displacement of electron density from the acyl azide group, and its degradation with the formation of a triplet acylnitrene, which is incapable of rearranging into isocyanate.

Full text of DOI:10.1134/S0018143911060087

ISSN 0018ꢀ1439, High Energy Chemistry, 2011, Vol. 45, No. 6, pp. 521–524. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © A.V. Budruev, A.V. Oleinik, 2011, published in Khimiya Vysokikh Energii, 2011, Vol. 45, No. 6, pp. 559–562.
PHOTOCHEMISTRY
Sensitized Photolysis of Stearic Acid Azide
A. V. Budruev and A. V. Oleinik
Nizhni Novgorod State University, pr. Gagarina 23, Nizhni Novgorod, 603950 Russia
Eꢀmail: budruev@gmail.com
Received March 30, 2011
Abstract—In an IRꢀspectroscopic study of the sensitized photolysis of stearic acid azide in a solution of benꢀ
zene, it was found that the introduction of Michler’s ketone into the system decreased the efficiency of isoꢀ
cyanate formation by a factor of about 10. It was hypothesized that the azide photolysis in the presence of
Michler’s ketone can be explained by the formation of a complex of the acyl azide with the ketone in a triplet
excited state, a displacement of electron density from the acyl azide group, and its degradation with the forꢀ
mation of a triplet acylnitrene, which is incapable of rearranging into isocyanate.
DOI: 10.1134/S0018143911060087
In the study of the photorearrangement of azides
into isocyanates (Curtius rearrangement), three possiꢀ
ble reaction paths should be considered. First, this is a
reaction path without the participation of nitrene; in
this case, the rearrangement occurs simultaneously
There is evidence for the occurrence of each of
these mechanisms. Acyl azides are rearranged into isoꢀ
cyanates both upon photolysis and upon thermolysis
[1–3]. The yield of isocyanates upon thermolysis is
nearly quantitative [4], whereas it is most frequently
no higher than 40% upon photolysis regardless of the
with the degradation of the azide group without the nature of the solvent [5]. The absence of a solvent
effect on the yield of isocyanates can be explained by
the reaction path without the participation of nitrene
[6–8]. However, acylnitrenes and their reaction prodꢀ
ucts were detected along with isocyanates upon the
photolysis of acyl azides [9]; this fact is evidence in
favor of the radical paths of this reaction (Scheme 1).
intermediate formation of nitrene. In the course of a
radical mechanism, an azide decomposes with the forꢀ
mation of acylnitrene; then, the nitrene is rearranged
into isocyanate from a triplet (second reaction path) or
singlet state (third reaction path).
1
*
O
O
hν
R N C O
–N2 (“Curtius”)
R
N3
R
N3
3
Sens *
3
*
1
*
O
O
R'H
R
N3
R
N
O
O
–N2
Intersystem crossing
'
NHR
R
R
R'•
3
*
O
O
R'H
–R'•
•
R
N
R
NH
R'H
–R'•
NH2
R N C O
Scheme 1. Reaction paths in the photolysis of acylazides
521

522
BUDRUEV, OLEINIK
Ratio of the rate constant of isocyanate formation to the rate
constant of azide degradation
in the absorption ranges of the test sensitizers. The
sensitizers were 11 substances with triplet level enerꢀ
gies from 176 to 356 kJ/mol (see the table).
k
/kazide
NCO
E
t, kJ/mol
Sensitizer
[14, 15]
EXPERIMENTAL
FSꢀ7
UFSꢀ1
To study the sensitized photolysis of azide, a benꢀ
zene solution of stearic acid azide (concentration of
5
Anthracene
Benzanthrone
Benzyl
178
0.845
0.939
1.025
0.746
–
–
×
10–2 mol/l) with a sensitizer (azide : sensitizer
193
0.900
–
molar ratios of 1 : 1 and 1 : 2) was irradiated with light
from a DRKꢀ120 mercury quartz lamp through UFSꢀ
1 or FSꢀ7 light filters in an IRꢀspectroscopic cell of
KBr with an optical path length of 0.539 mm at a disꢀ
tance of 28 cm. The photolysis and measurement of
spectra were conducted at room temperature.
The IR spectra of solutions were measured on a
Shimadzu IRPrestigeꢀ21 spectrophotometer (4000–
400 cm–1).
222
Naphthalene
Anthraquinone
Biphenyl
253
–
261
0.703
–
274
0.958
0.134
–
Michler’s ketone
DMPA
275
0.083
0.775
0.692
–
278 [16]
287
Stearic acid azide was synthesized by the diazotizaꢀ
tion of stearic acid hydrazide.
Benzophenone
Acetophenone
Acetone
1.027
0.973
0.788
0.849
Synthesis of stearic acid azide. Dioxane (80 ml) was
added to 3 g of stearic acid hydrazide, and the mixture
was heated in a water bath until the complete dissoluꢀ
tion of the sediment. Then, 25 ml of HCl (conc) was
added to the solution. The solution with the precipiꢀ
tated hydrazide hydrochloride was cooled in an ice
310
335
353
0.689
0.673
–
bath to 0–5 С, and a solution of 5 g of NaNO2 in 20 ml
°
of water was gradually added with stirring. After the
addition of the solution, the mixture was stirred for
20 min. The precipitate was filtered off, reprecipitated
from dioxane with water, and dried in a vacuum.
The rearrangement of acyl azides into isocyanates
is a convenient method for the preparation of carꢀ
bamic acids, urethanes, ureas, and primary amines.
The photochemical method of the synthesis of isocyꢀ
anates can make it possible to use acyl azides as phoꢀ
tochemical modifiers for the surfaces of polymers,
including biopolymers. However, the photochemical
method of the synthesis of isocyanates is ineffective
because of a low yield of this reaction (as compared
with that of thermolysis) [10].
RESULTS AND DISCUSSION
Changes in the concentrations of the azide and its
photolysis products were monitored by measuring the
absorption of analytical absorption bands in the IR
spectra of the solutions before and in the course of
irradiation. The absorption band of the asymmetric
stretching vibration of the azide group (νas N3) at
On the other hand, the control of the photochemiꢀ
cal generation of acylnitrenes, which are capable of
inserting into ordinary and multiple bonds, can also
allow one to use them as photoaffinity labels. Thereꢀ
fore, the study of the mechanism of photolysis of acyl
azides for controlling the conditions of formation of
both isocyanates and acylnitrenes is of considerable
current interest.
1
2134.7 cm⎯ was chosen as the analytical absorption
band of the azide. An absorption band due to the
asymmetric stretching vibration of the isocyanate
1
group (νas NCO) at 2257.2 cm⎯ was chosen for heptaꢀ
decyl isocyanate, whereas the stretching vibration of
the amide carbonyl group (
used for amides.
ν
CO) at 1646.25 cm⎯1 was
Sensitized photolysis will make it possible to conꢀ
vert acyl azide into a triplet excited state and to obtain
triplet nitrene or isocyanate escaping a singlet excited
state and, thus, to exclude or confirm a singlet reaction
path of the formation of isocyanate, to define the reacꢀ
tion scheme of photolysis and, possibly, to choose a
method for controlling this reaction.
The photolysis of azide solutions both in the presꢀ
ence and in the absence of sensitizers decreased the
absorbance of absorption bands due to the asymmetric
stretching vibrations of the azide group and the
stretching vibrations of the carbonyl group of acyl
azide (1720 cm–1). Simultaneously, absorption bands
due to the asymmetric stretching vibrations of the isoꢀ
cyanate group and several weak absorption bands in
the range of 1650–1615 cm–1, which were attributed
The aim of this work was to study the effect of
energy transfer from the triplet states of sensitizers on
the yields of acyl azide photolysis products.
The test material was stearic acid azide with an to the carbonyl groups of the resulting amides,
absorption maximum at 225 nm, which is transparent appeared. That is, under the described conditions, the
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SENSITIZED PHOTOLYSIS OF STEARIC ACID AZIDE
523
azide decomposed with the formation of isocyanate
and primary or second amides as in the direct photolꢀ
ysis of the azide.
We did not detect qualitative changes in the IR
spectra of irradiated solutions upon changing a sensiꢀ
tizer. Only the rates of formation and the ratios
between of reaction products changed.
Changes in the absorbance of analytical absorption
bands in the azide photolysis were measured to conꢀ
versions of about 10%. The kinetics of the azide phoꢀ
tolysis and the formation of isocyanate and amides
were described by a straight line with a correlation
coefficient of about 0.98.
The efficiency of the formation of isocyanate and
amides with different sensitizers was judged from the
ratios of the rate constants of formation of the photolꢀ
ysis products to the rate constant of azide decomposiꢀ
tion.
k
/k
NCO N
3
1.0
0.8
0.6
0.4
0.2
0
1
2
165
215
265
315
T, kJ/mol
E
Dependence of the ratios of the isocyanate formation to
the .azide degradation rate constants upon the triplet state
energies of sensitizers during irradiation through (
and ( ) UFSꢀ1 light filters.
1) FSꢀ7
The table summarizes the experimental data. The
figure shows the dependence of the ratio between rate
constants on the triplet state energy of the sensitizers.
2
In the photolysis of solutions through a UFSꢀ1
light filter, which passes light in the range of 250–
400 nm, the ratio of the rate constant of isocyanate
formation to the rate constant of azide degradation
was 0.74 0.10 for the majority of the test sensitizers.
However, in the photolysis in the presence of
Michler’s ketone (ET = 275 kJ/mol), the ratio between
the constants was 0.08 0.10, that is, lower by an order
of magnitude.
Upon the replacement of the light filter by FSꢀ7,
which is transparent in the range of 290–450 nm, the
average ratio of the rate constants of isocyanate formaꢀ
tion to the rate constants of azide degradation was
0.91 0.10 for all of the sensitizers other than
overlaps with the absorption of the carbonyl groups of
the sensitizers; this fact made the analysis of IRꢀspecꢀ
troscopic data difficult to perform.
Among a number of absorption bands in the region
of 1650–1615 cm–1, we recognized a band at
1646.25 cm–1, which appeared in all of the test sysꢀ
tems, and attributed it to the stretching vibration of the
carbonyl group of amide.
The found ratios of the rate constants of amide forꢀ
mation to the rate constants of azide photolysis in the
photolysis with both UFSꢀ1 and FSꢀ7 light filters difꢀ
fered only slightly from each other for all of the test
sensitizers. The ratio between the rate constants
increased by a factor of about 2 for sensitizers with an
energy difference between the ground and triplet
Michler’s ketone, for which this ratio was 0.13 0.10
.
After excluding Michler’s ketone from the considꢀ excited states in the range of 222–302 kJ/mol. Howꢀ
eration, the average rate constants of azide degradaꢀ ever, with consideration for a small change in the
tion were 1.8
×
10–3 and 8.7 10–4 for UFSꢀ1 and FSꢀ absorbance of the chosen band, the above data did not
×
7 light filters, respectively, and they did not depend on allow us to reliably state that the sensitizers transfer
the presence of a sensitizer in solution. That is, with energy to the azide molecule and lead to an increase in
the use of the UFSꢀ1 light filter, the azide decomposed the efficiency of amide formation.
more rapidly than with FSꢀ7 by a factor of 2; this fact
It is unlikely that the decrease in the efficiency of
can be explained by a shift of the transmission cutoff of
isocyanate formation in the azide photolysis sensitized
light filters from 250 to 290 nm and, as a result, a
by Michler’s ketone can be explained only by energy
decrease in the portion of radiation directly absorbed
transfer from the ketone in a triplet excited state to the
by the azide.
azide and by a change from a singlet to a triplet reacꢀ
Thus, in the cases under consideration, isocyanate tion path. In this case, sensitizers with close energies
was formed without the participation of sensitizers between the ground and triplet excited states should
only in the direct photolysis of the azide, which cannot behave analogously; however, this was not observed
be completely excluded. The sensitizers introduced experimentally.
into the system did not participate in energy transfer,
It is believed that Michler’s ketone, which transꢀ
and they cannot be called, strictly speaking, sensitizers
ferred to a triplet excited state upon the absorption of
in this case.
a light photon, removed hydrogen from the aliphatic
The sensitizers used are, for the most part, ketones tail of an azide or isocyanate; thus, it became reduced
or other carbonyl compounds. Therefore, the region to 4,4'ꢀ(dimethylamino)diphenylꢀsubstituted methaꢀ
of the absorption of the carbonyl groups of amides nol or substituted pinacol, respectively [11]. The
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524
BUDRUEV, OLEINIK
resulting alcohols interacted with isocyanate to behavior was not observed with other ketones capable
decrease its concentration in the solution. In this case, of forming an alcohol upon photoreduction, for examꢀ
it is impossible to understand why an analogous ple, benzophenone.
−
1
−
−
3
δ
• δ
O
δ
O
O
•
•
Intersystem crossing
hν
+
+
+
+
δ
+
δ
+
δ
δ
δ
δ
N
N
N
N
N
N
unrelaxed triplet
3
Michler’s ketone
Oᮎ
⊕
N
Oᮎ
3
N N N
N
N N
⊕ ᮎ
ᮎ
N
⊕
R
R
3
N
O
O
amides
R
+
–MK
δ
⊕
3
O
N
N
Oᮎ
ᮎ
N
N
N
N N
⊕
R
O
Scheme 2. Hypothetical mechanism of the photolysis of stearic acid azide sensitized by Michler’s ketone.
5. Linke, S., Tisue, G.T., and Lwowski, W., J. Am. Chem.
Soc., 1967, vol. 89, no. 24, p. 6308.
6. Budruev, A.V., Ignatov, S.L., Zelentsov, S.V., and Oleꢀ
It is likely that the azide photolysis in the presence
Michler’s ketone [12, 13] can be explained by its abilꢀ
ity to coordinate stearic acid azide with the formation
of a triplet exciplex (Scheme 2).
inik, A.V., Vestn. Nizhegorodsk. Univ., 1995, p. 32.
7. Tarwade, V., Dmitrenko, O., Bach, R.D., and
Upon the absorption of a light photon, the ketone
transferred to a singlet and then to a triplet excited
state. In the course of the relaxation of the triplet state,
electron density was displaced from the N,Nꢀdimethꢀ
ylamine and phenylene groups to the carbonyl group
with the formation of a quinoneꢀlike structure. This
structure served as an acceptor in the formation of a
complex with the azide to cause the displacement of
electron density from the acyl azide group of stearic
acid azide and its degradation to triplet acylnitrene. It
is likely that the resulting triplet acylnitrene cannot be
rearranged into isocyanate, and it reacted with the forꢀ
mation of primary and secondary amides.
Fox, J.M., J. Org. Chem., 2008, vol. 73, p. 8189.
8. Kubicki, J., Zhang, Y., Wang, J., Luk, H.L., Peng, H.,
Vyas, Sh., and Platz, M.S., J. Am. Chem. Soc., 2009,
vol. 131, p. 4212.
9. Lwowski, W. and Tisue, G.T., J. Am. Chem. Soc., 1965,
vol. 87, no. 17, p. 4022.
10. Mochalova, A.E., Budruev, A.V., Oleinik, A.V., and
Smirnova, L.A., Perspektiv. Mater., 2009, no. 5, p. 61.
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Trans., 1977, vol. 73, p. 1569.
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Chem. A, 2002, vol. 106, no. 25, p. 6084.
13. Levin, P.P. and Kuzmin, V.A., Usp. Khim., 1987,
vol. 56, no. 4, p. 527.
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