Influence of 1,3-dichloroacetone on the regularities of decay of arylnitroso oxides

Article abstract of DOI:10.1007/s11172-009-0341-7

The decay kinetics of isomeric forms of a series of arylnitroso oxides in the presence of 1,3-dichloroacetone in acetonitrile was studied by the flash photolysis technique. The regularities observed are explained by the complex formation between the molecules of nitroso oxide and additive.

Full text of DOI:10.1007/s11172-009-0341-7

Russian Chemical Bulletin, International Edition, Vol. 58, No. 12, pp. 2437—2442, December, 2009
2437
Influence of 1,3ꢀdichloroacetone on the regularities of decay
of arylnitroso oxides
E. M. Chainikova and R. L. Safiullin
Institute of Organic Chemistry, Ufa Research Center of the Russian Academy of Sciences,
71 prosp. Oktyabrya, 450054 Ufa, Russian Federation.
Fax: +7 (347) 235 6066. Eꢀmail: kinetic@anrb.ru
The decay kinetics of isomeric forms of a series of arylnitroso oxides in the presence of
1,3ꢀdichloroacetone in acetonitrile was studied by the flash photolysis technique. The regulariꢀ
ties observed are explained by the complex formation between the molecules of nitroso oxide
and additive.
Key words: kinetics, flash photolysis, nitroso oxides, aryl azides, 1,3ꢀdichloroacetone.
Compounds 2a,c (see Ref. 8) and 2b,d,e (see Ref. 9) were
synthesized according to described procedures. Azides 2a,c
were distilled in vacuo, azide 2b was recrystallized from hexane,
and compounds 2d,e were recrystallized from ethanol.
4ꢀMethoxynitrosobenzene10 and 4ꢀmethoxynitrobenzene11 were
synthesized using published procedures.
The reaction of triplet aromatic nitrenes with oxygen
affords nitroso oxides (ArNOO),1 viz., labile species existꢀ
ing as geometric (cisꢀ and transꢀ) isomers.
transꢀIsomer
cisꢀIsomer
Isomers were detected by the matrix isolation methꢀ
ods2—4 and timeꢀresolved spectroscopy.5,6 In the absence
of oxidizable substrates, the isomeric forms of nitroso
oxides decay according to the firstꢀorder kinetic law. The
rate constants and activation parameters of this reaction
in various solvents have been measured previously.6
The present work is devoted to the study of the influꢀ
ence of 1,3ꢀdichloroacetone (DCA) on the regularities
of consumption of the isomeric forms of nitroso oxides
1a—e in acetonitrile. The decay kinetics of the isomers
at different DCA concentrations was studied by the flash
photolysis technique. The composition of the products
of ArNOO transformations in the presence of DCA was
studied by HPLC using nitroso oxide 1b as an example.
R = H (a), OMe (b),
Me (c), Br (d), Cl (e)
Kinetic studies were carried out on a flash photolysis setup
of the known design.12,13 An IFP 5000ꢀ2 lamp served as a
photolytic source, the maximum energy of a pulse was 400 J at
U = 5 kV, C = 32 μF, and ~90% light energy are irradiated within
50 μs. The reactor was a quartz cell with the optical path length
l = 10 cm and internal diameter ~1 cm. Flash photolysis of a
solution of aryl azide saturated with air oxygen in acetonitrile
was carried out with the filtered light (UFSꢀ2 light filter,
transmission range λ = 270—380 nm). The initial concentration
of aryl azides was 2.5•10–4 mol L–1. The DCA concentration
was varied in the range (7.5—625)•10–4 mol L–1. The error of
determination of the rate constants did not exceed 10%.
The transformation products of nitroso oxide 1b in the
presence of DCA were studied under the conditions of steadyꢀ
state photooxidation of azide 2b. A solution (10 mL) of azide 2b
in acetonitrile (1•10–3 mol L–1) was placed in a temperatureꢀ
maintained cylindrical quartz reactor. Photolysis was carried
out with the light with λ = 270—380 nm (light filter UFSꢀ2,
mercury lamp DRTꢀ400) at 293 K and permanent oxygen
bubbling in the presence of DCA (1.5•10–3 or 0.02 mol L–1) or
without it. The distance from the reactor to the light source was
~20 cm. During the process, sampling was carried out at an
interval of 5 min, and the overall reaction time was 20 min. The
reaction mixtures were analyzed by HPLC on an Acmeꢀ9000
R = H (a), OMe (b),
Me (c), Br (d), Cl (e)
Experimental
Acetonitrile was purified using a known procedure,7 and
DCA was sublimed for purification. Azides 2a—e were used.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2359—2364, December, 2009.
1066ꢀ5285/09/5812ꢀ2437 © 2009 Springer Science+Business Media, Inc.

2438
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 12, December, 2009
Chainikova and Safiullin
liquid chromatograph (Young Lin Instrument) equipped with a
twoꢀwave UV—Vis detector; column Exsil Silica, mobile phase
hexane—propanꢀ2ꢀol (90 : 10 vol.%), flow rate of the mobile
phase 2 mL min–1, ambient temperature of the column.
A0/A0,max
1.0
0.8
0.6
0.4
0.2
Results and Discussion
2
Kinetic regularities of the decay of arylnitroso oxides in
the presence of DCA. Flash photolysis of solutions of aryl
azides 2a—e in the presence of oxygen generates cisꢀ and
transꢀisomers of the corresponding nitroso oxides. The
kinetic curves of the absorbance decrease for nitroso
oxides 1a—e, detected at the wavelengths at which the
both isomers absorb the light, consist of two components
and are well described by the fiveꢀparameter biexponential
equation6
1
380
400
420
440
460
480
λ/nm
Fig. 2. Optical absorption spectra of the cisꢀform of nitroso
oxide 1b in acetonitrile at 295 2 K: [DCA] = 0 (1) and
1.25•10–3 mol L–1 (2).
cis
trans
t
A = A∞ + A0cise–k t + A0transe–k
,
where A0cis, A0trans, kcis, and ktrans are the initial absorbꢀ
ances and rate constants for the decay of cisꢀ and transꢀ
isomers, respectively; A∞ is the final absorbance caused
by the absorption of the reaction products.
The regularities observed can be explained assuming
that complex formation occurs between the components
of the system under study. Let us assume that the following
reactions occur under our experimental conditions:
The influence of DCA on the kinetic regularities
of isomer decay appears at a certain concentration of the
additive in the system. The dependence of the apparent
rate constant for the decay of the cisꢀ and transꢀforms of
nitroso oxide 1b (kapp) on the DCA concentration is shown
in Fig. 1: the linear increase in the kapp value begins after
some [DCA] value is achieved. Similar regularities were
observed for other nitroso oxides except for 1d, for which
the apparent rate constants for isomer decay increase
linearly in the whole range of additive concentrations
((1.25—6.25)•10–3 mol L–1). The concentration at which
DCA begins to affect the decay kinetics of the nitroso
oxides is ~1.5•10–3, ~2.5•10–3, and ~2.5•10–2 mol L–1
for 1c, 1b,e, and 1a, respectively.
ArNOO + (ClCH2)2C=O
ArNOO...(ClCH2)2C=O,
Products.
(1)
(2)
ArNOO...(ClCH2)2C=O + (ClCH2)2C=O
A complex between the nitroso oxide and additive is
formed at the first stage. Then the complex reacts with the
DCA molecule to form the final products. The DCA conꢀ
centration, at which kapp begins to increase (see Fig. 1),
corresponds to the conditions where the main fraction
of ArNOO is bound in a complex with DCA. The further
increase in [DCA] results in a linear increase in the
apparent rate constant for nitroso oxide decay.
The complex formation of arylnitroso oxide with DCA
is indicated by the broadening of the longꢀwavelength
absorption band and the shift of its maximum in the elecꢀ
tronic spectrum of the cisꢀisomer of nitroso oxide 1b in
the presence of DCA (Fig. 2). It should be mentioned
that no changes in the spectral characteristics in the presꢀ
ence of DCA were observed for the transꢀisomer.
kapp•10–2/s–1
6.0
In the region of high DCA concentrations, where
almost all nitroso oxide exists in the bound state, we moniꢀ
tored the decay kinetics of the complex, and the decay
rate of ArNOO was determined by the rate of reaction (2)
4.0
2
2.0
–d[ArNOO]/dt = –d[ArNOO...(ClCH2)2C=O]/dt =
= k2[ArNOO...(ClCH2)2C=O][(ClCH2)2C=O].
1
Thus, the rate constants of the reaction of the comꢀ
plexes of the isomeric forms of nitroso oxides with DCA
(k2) were determined from the slope ratios of the linear
plots presented in Fig. 1. The rate constant values are
listed in Table 1. The reactivity of the complexes of the
0.2 0.4
0.6 0.8
1.0
[DCA]•102/mol L–1
Fig. 1. Apparent rate constants for the decay of the transꢀ (1) and
cisꢀisomers (2) of nitroso oxide 1b (kapp) vs DCA concentration
(acetonitrile, 295 2 K, detection wavelength 450 nm).

Effect of dichloroacetone on nitroso oxides
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 12, December, 2009 2439
acetone. Second, we have shown in several works6,13,16,17
that only the transꢀform of nitroso oxides is reactive in the
reaction with the substrates (olefins, triphenylphosphine).
The reactivity of the cisꢀform in the reaction with DCA
is even several times higher than that of the transꢀform
(see Table 1).
Table 1. Decay rate constants of isomeric forms of
nitroso oxides 1a—e (k2) in the presence of DCA
(acetonitrile, 295 2 K)*
Isomer
k2•10–2/L mol–1 s–1
1a
1b
1c
1d
1e
Photooxidation products of 4ꢀmethoxyphenyl azide (2b)
in the presence of DCA. The photooxidation of aromatic
azides includes the following main stages (Scheme 1).2,18
cis
trans
0.7
0.2
460
120
40
7.9
5.0
1.7
4.3
1.8
* The error of determination of the rate constants did
not exceed 10%.
Scheme 1
logk2/L mol–1 s–1
MeO
4.0
Me
3.0
2.0
Br
Cl
2
H
1
Resin
Singlet nitrenes are reversibly isomerized with ring
expansion through the corresponding benzazirines to
dehydroazepines or undergo intersystem crossing to form
triplet nitrenes, which yield nitroso oxides in the presence
of oxygen. At high azide concentrations, its reaction with
dehydroazepine produces resins.18 If the system contains
a triplet sensibilizer, triplet nitrene can be obtained
directly omitting the stage of formation of singlet nitrene
and avoiding related secondary reactions.
To understand the mechanism of influence of DCA
on the kinetic regularities of nitroso oxide decay, we
studied the composition of the photooxidation products
of azide 2b in the presence and in the absence of the
additive by HPLC. The photolysis of acetonitrile soluꢀ
tions of 2b (1•10–3 mol L–1) was carried out at 293 K with
permanent oxygen bubbling. Three experiments were
carried out at a DCA concentration of 0, 5•10–3, and
0.02 mol L–1. In all cases, the azide conversion was <50%.
In all the three cases, the qualitative composition of
the reaction mixture was the same: the products were the
corresponding nitrosoꢀ and nitrobenzenes, which were
determined by the retention times compared with the
authentic samples (Table 2). The low yield of the prodꢀ
ucts based on decomposed azide in the absence of DCA is
due, most likely, to the fact that side reactions of resin
formation occur at rather high azide concentration taken
by us (see Scheme 1). No new compounds were observed
in the studied system in the presence of DCA; however,
the quantitative composition of the reaction mixture
changed substantially. The fraction of azide, which
–0.3
–0.2
–0.1
0.0
0.1
0.2
σ
Fig. 3. Rate constants for the reaction of the complexes of the
isomeric forms of nitroso oxides 4ꢀXC6H4NOO (X = H, Me,
MeO, Br, Cl) with DCA (k2) vs electronic properties of the
substituent X in the coordinates of the Hammett equation for
the transꢀ (1) and cisꢀisomers (2) (acetonitrile, 295 2 K).
cisꢀisomers is higher than that of the transꢀisomers and
depends strongly on the nature of the substituent in the
aromatic ring of the nitroso oxide, and both the electronꢀ
releasing and electronꢀwithdrawing substituents increase
the reaction rate constant (see Table 1). The dependence
of logk2 on the σ constants of substituents in the nitroso
oxide molecule in the Hammett scale14 has a Vꢀlike charꢀ
acter (Fig. 3). A similar shape of the plots indicates the
change in the mechanism of formation of the transition
state due to different effects of the electronꢀreleasing and
electronꢀwithdrawing substituents in the nitroso oxide
molecule on the reaction course (nitroso oxide manifests
biphilic properties). Remind that we consider the comꢀ
plex of nitroso oxide with the DCA molecule.
The regularities obtained seem rather unexpected
for two reasons. First, we have earlier15 studied the
decay kinetics of nitroso oxides in acetone used as a
solvent, and for nitroso oxide 1a we obtained the rate
constant of the same order as that in acetonitrile, while
for 4ꢀN,Nꢀdimethylaminophenylnitroso oxide the conꢀ
stant turned out to be even lower than that in acetonitrile.
That is, nitroso oxides do not react with unsubstituted

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Russ.Chem.Bull., Int.Ed., Vol. 58, No. 12, December, 2009
Chainikova and Safiullin
Table 2. Quantitative composition of the reaction mixture
obtained by the photooxidation of azide 2b in acetonitrile at
However, how is nitrosobenzene formed in this case?
This very difficult question remains yet unanswered. An
analysis of the results obtained in the present work sugꢀ
gests that DCA is involved somehow in the decay of nitroso
oxides, but no new products are formed. The yields of
ArNO and ArNO2 increase nonꢀproportionally in the presꢀ
ence of the additive in the system (see Table 2). In the
case of the parallel formation of these products from
nitroso oxides, one should expect a proportional increase
in their yields in the presence of DCA. The conclusion
that nitroso and nitro compounds are formed in different
transformations of nitroso oxides was drawn25 on the basis
of analysis of the photooxidation products of azidostyrylꢀ
quinolines and their hydrochlorides in ethanol. Perhaps,
nitrosobenzene is not a product of nitroso oxide transforꢀ
mation but is formed in any secondary reaction. Then,
based on the data presented in Table 2, one can assume
that DCA sensitizes the formation of azide in the triplet
state (see Scheme 1). This increases the yield of triplet
nitrene, due to which the yield of nitroso oxide and, hence,
nitrobenzene increases. Since no products of the reaction
of nitroso oxide 1b with DCA were found, the increase in
the decay rates of the both forms of nitroso oxides in the
presence of DCA detected by flash photolysis (see Fig. 1
and Table 1) is explained, probably, by catalysis of nitroꢀ
benzene formation from ArNOO.
Activation parameters of the decay of 4ꢀmethoxyꢀ
phenylnitroso oxide (1b) in the presence of DCA. The temꢀ
perature dependence of the decay rate constant for the
isomers of nitroso oxide 1b in the interval 276—325 K
(ketone concentration 7.5•10–3 mol L–1) was studied by
the flash photolysis technique. The apparent rate constants
for the decay of the isomeric forms obtained at different
temperatures were divided into this concentration and the
k2 values were determined. The dependence of logk2 on
the inverse temperature is linear for the both isomers
(Fig. 4). The following values of the activation parameters
were obtained: logA = 11.1 0.2, Ea = 41 1 kJ mol–1 (for
transꢀ1b) and logA = 9.2 0.1, Ea = 26.0 0.8 kJ mol–1
(for cisꢀ1b). The earlier6 determined activation parameters
for the monomolecular decay of this nitroso oxide had the
following values: logA = 11.4 0.2, Ea = 67.2 0.9 kJ mol–1
(for transꢀ1b) and logA = 11.7 0.1, Ea = 60.5 0.3 kJ mol–1
(for cisꢀ1b). Thus, taking into account the results obꢀ
tained in flash experiments and in experiments on studyꢀ
ing the photooxidation products of azide 2b in the presꢀ
ence of DCA, one can conclude that this ketone accelerꢀ
ates the isomerization of nitroso oxides to nitrobenzene,
decreasing considerably the activation energy of this
transformation for the both conformers. The activation
entropy for the transꢀisomer changes weakly, being at
293 K –34.9 J mol–1 K–1 in the absence of DCA and
–40.6 J mol–1 K–1 in the presence of DCA. In the case
of the cisꢀisomer, the changes are more significant:
–29.2 and –77.0 J mol–1 K–1, respectively.
293 K and different durations of the reaction (t) ([ArN3]0
=
= 1•10–3 mol L–1
)
[DCA]•103
/mol L–1
t
[ArN3]•104 [ArNO]•105 [ArNO2]•105
/min
mol L–1
0
5
10
15
20
5
10
15
20
5
7.6
6.6
6.0
5.3
7.8
7.6
6.4
5.6
9.1
7.8
7.2
5.8
0.9
1.4
2.0
2.8
2.0
2.4
3.0
3.4
2.4
3.0
3.6
4.1
0.6
1.4
2.8
4.2
2.2
4.0
5.8
7.2
3.8
7.6
10.4
15.8
5
20
10
15
20
decayed every moment, somewhat decreased with some
increase in the yield of nitrosobenzene and a considerable
increase in the yield of nitrobenzene (see Table 2).
Nitroso and nitro compounds are typical photooxidaꢀ
tion products of aromatic azides.2,19—21 Their yield and
ratio vary, depending on the reaction conditions. It has
earlier been found2 by flash photolysis that 4ꢀaminoꢀ
phenylnitroso oxide is consumed bimolecularly with the
rate constant close to the diffusion one. It is assumed that
the bimolecular decay of nitroso oxides proceeds through
the intermediate formation of dimeric peroxide, which
decomposes to either two molecules of the nitro comꢀ
pound, or to two molecules of the nitroso compound and
oxygen (Scheme 2).
Scheme 2
The parallel formation of nitrosoꢀ and nitrobenzene
upon the photooxidation of azides can be explained using
this mechanism. However, all nitroso oxides studied6 deꢀ
cay in a monomolecular reaction, for example, in isomerꢀ
ization to nitrobenzene (Scheme 3).
Scheme 3
The occurrence of this reaction was proved by the
method of labeled atoms.22—24

Effect of dichloroacetone on nitroso oxides
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 12, December, 2009 2441
logk2/L mol–1 s–1
kapp•10–2/s–1
4.0
5.0
2
3.0
4.5
2
2.0
1
4.0
3.5
1.0
1
3.0
3.1
3.2
3.3
3.4
3.5
(1/T)•103/K–1
1.0
2.0
3.0
4.0
5.0 [Ph3P]•104/mol L–1
Fig. 4. Temperature dependences of the rate constant for the
reactions of the transꢀ (1) and cisꢀisomers (2) of nitroso oxide 1b
bound in complexes with DCA (k2) (acetonitrile, detection waveꢀ
length 450 nm).
Fig. 5. Apparent rate constants for the decay of the transꢀ (1) and
cisꢀisomers (2) of nitroso oxide 1b (kapp) in the presence of DCA
(2.5•10–3 mol L–1) vs triphenylphosphine concentration (acꢀ
etonitrile, 295 2 K, detection wavelength 450 nm).
Perhaps, an interaction similar to the addition of carꢀ
bonyl oxides to the carbonyl group with the formation
of “secondary” ozonide occurs in the system under
study.26 Then the occurring processes can be presented by
Scheme 4. At first a complex of nitroso oxide with a DCA
molecule is formed and reacts with the second ketone
molecule to form an intermediate product of the type of
“secondary” ozonide. The latter decomposes to nitroꢀ
benzene and ketone.
the decay of the complexes of the isomeric forms of 1b
on the triphenylphosphine concentration was obtained
(Fig. 5). It turned out that at first kapp for transꢀ1b someꢀ
what increases and then remains unchanged with an
increase in the phosphine concentration. A linear depenꢀ
dence of kapp on [Ph3P] is observed for cisꢀ1b, and its
slope ratio gave the rate constant for the reaction of the
cisꢀisomer of 1b bound in a complex with triphenylphosꢀ
phine: 4.5•105 L mol–1 s–1. This value almost coincides
with the earlier17 measured rate constant for the reaction
of the transꢀform of 1b with Ph3P (5.5•105 L mol–1 s–1).
Thus, some inversion of the reactivity of the isomeric
forms toward the oxidized substrate under the action of
DCA was observed. The nature of this phenomenon
remains yet unexplained.
Scheme 4
This work was financially supported by the Division of
Chemistry and Materials Science (Program “Theoretical
and Experimental Investigation of the Chemical Bond
Nature and Mechanisms of the Most Important Chemical
Reactions and Processes”) and the Russian Foundation
for Basic Research (Project No. 09ꢀ03ꢀ00411).
Kinetics of the reaction of the complexes of the isoꢀ
meric forms of 4ꢀmethoxyphenylnitroso oxide (1b) with
triphenylphosphine. We have earlier17 studied the reactꢀ
ivity of the isomeric forms of some arylnitroso oxides
toward triphenylphosphine. Only the transꢀisomers of
nitroso oxides are involved in this reaction, and the rate
constant is about 105—106 L mol–1 s–1. Since the presence
of DCA strongly changes the reactivity of the isomeric
forms in the decay reaction, we decided to check how this
additive would affect the kinetics of the reaction of ArNOO
with triphenylphosphine. The studies were carried out by
the flash photolysis technique for nitroso oxide 1b. Such a
concentration of DCA was used (2.5•10–3 mol L–1) that
the all nitroso oxide would be bound into a complex (see
Fig. 1). The dependence of the apparent rate constant for
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Received May 20, 2009;
in revised form August 21, 2009