Chain processes in the reduction of aromatic nitroso compounds by triphenylphosphine in the presence of oxygen

Article abstract of DOI:10.1007/s11172-009-0116-1

The reduction of aromatic nitroso compounds by triphenylphosphine in the presence of oxygen is accompanied by the chain reaction proceeding due to the interaction of trans-isomers of nitroso oxides formed in this system with triphenylphosphine followed by the regeneration of the initial nitrosobenzene.

Full text of DOI:10.1007/s11172-009-0116-1

926
Russian Chemical Bulletin, International Edition, Vol. 58, No. 5, pp. 926—928, May, 2009
Chain processes in the reduction of aromatic nitroso compounds
by triphenylphosphine in the presence of oxygen
E. M. Chainikova and R. L. Safiullin
Institute of Organic Chemistry, Ufa Research Center of the Russian Academy of Sciences,
7
1 prosp. Oktyabrya, 450054 Ufa, Russian Federation.
Fax: +7 (347) 235 6066. Eꢀmail: kinetic@anrb.ru
The reduction of aromatic nitroso compounds by triphenylphosphine in the presence of
oxygen is accompanied by the chain reaction proceeding due to the interaction of transꢀisomers
of nitroso oxides formed in this system with triphenylphosphine followed by the regeneration
of the initial nitrosobenzene.
Key words: nitroso oxides, nitrosobenzenes, triphenylphosphine, chain reaction.
The primary products of photolysis of aromatic azides are
mations it is necessary to study the products formed.
Steadyꢀstate photolysis of transformations of aryl azides
in the presence of oxygen is not quite adequate for this
purpose, because in this method the occurrence of photoꢀ
chemical reactions of the nitroso oxides themselves
and products of their transformations cannot be excluded.
Thus, a nonꢀphotochemical method of generation of
these particles is required for the study of the mechanism
of dark reaction of ArNOO and a correct comparison
of the obtained results with the results of kinetic investiꢀ
gations.
1
singlet nitrenes, which are further isomerized with cycle
extension or undergo intersystem crossing (Scheme 1).
Triplet nitrenes recombine to give azobenzenes and
2
also react with nitroso compounds and oxygen (see
Scheme 1). The latter reaction affords labile species, viz.,
3
nitroso oxides ArNOO. The spectral properties of these
intermediates and kinetic regularities of their consumpꢀ
tion have been studied earlier4—7 using lowꢀtemperature
matrix isolation and flash photolysis techniques. Flash
photolysis with timeꢀresolved spectroscopic detection
makes it possible to monitor the kinetics of reactions
of species under the thermal activation conditions. For
understanding the mechanism of nitroso oxide transforꢀ
8
It is known that the deoxygenation of nitrosobenzenes
by trivalent phosphorus compounds (phosphines and phosꢀ
phites) generates the corresponding nitrenes (Scheme 2).
Scheme 1
Resin
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 906—908, May, 2009.
1
066ꢀ5285/09/5805ꢀ0926 © 2009 Springer Science+Business Media, Inc.

Reduction of aromatic nitroso compounds
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 5, May, 2009
927
Table 1. Results of analysis of the composition of the reaction mixtures and the concentration of oxygen absorbed by the Ph P—ArNO
3
a
system (Ar = 4ꢀRC H )
6
4
Entry
R
[ArNO]0
3•10–4
[Ph P]
Δ[ArNO] Δ[Ph P] = [Ph PO] [ArNO2] [ArN=N(O)Ar]
Δ[O2]
3
0
3
3
mol L–1
–3
3•10–4
–4
–3
1
2
3
H
H
H
H
H
H
MeO
MeO
1•10
5•10
4.4•10
3.2•10
—
1•10
1•10
2.1•10
4.9•10
6.5•10
—
3•10
—
8•10
6•10
—
—
6•10
—
6•10
4•10
—
–
3
–3
–3
–5
–5
–3
2.4•10
2.1•10
—
1.1•10
1.8•10
—
b
–3
–3
–3
4•10
1•10
2•10
9.6•10
–
–
3
3
–3
–4
–3
–5
–5
–5
–5
4
5
6
7
8
1•10
1•10
8.3•10
–3
–4
–3
9•10
—
c
–3
–3
–3
–3
–3
–3
–3
–3
–3
–3
–3
–3
–4
2.4•10
2.4•10
2.5•10
4.9•10
4.9•10
9.1•10
2.4•10
1.6•10
2.4•10
3.5•10
—
—
1.8•10
3.2•10
d
–3
–3
—
—
d
—
a
O ] = 7.7•10 mol L–1 (with saturation of acetonitrile with oxygen).9
The reaction mixture was not analyzed.
The experiment was carried out under an argon atmosphere.
A white unidentified precipitate is formed during the reaction.
–3
[
2
b
c
d
Scheme 2
triphenylphosphine (Δ[Ph P]) and formed triphenylꢀ
3
phosphine oxide are always higher than that of the conꢀ
sumed nitroso compound (Δ[ArNO]) (see Table 1). This
is explained by the additional channel of Ph P oxidation
3
If the reaction is carried out in the presence of oxygen,
through its interaction with nitroso oxide due to which
nitrosobenzene is regenerated. The blank experiment
shows that triphenylphosphine is not oxidized by oxygen
and produces no triphenylphosphine oxide during the
times comparable with the reaction time. Thus, the folꢀ
lowing sequence of steps is observed in the system studied
it can serve as the desired method for synthesis of nitroso
oxides. In the present work, we studied this system using
the reaction of nitrosobenzene and 4ꢀmethoxynitrosoꢀ
benzene with triphenylphosphine in acetonitrile as an
example. The analysis of the products formed showed
that the Ph P was oxidized via the chain mechanism. The
3
(
Scheme 3).
presence of chains is due to the fact that the nitroso oxide
formed reacts with triphenylphosphine to regenerate the
starting nitroso compound.
Scheme 3
Results and Discussion
In the earlier* studies performed by flash photolysis
we found that the transꢀform of aryl nitroso oxides is
highly reactive toward triphenylphosphine, whereas the
cisꢀform does not react with this substrate. Nitroso oxides
were generated from the corresponding azides. The rate
constants of the reactions of transꢀisomers of phenyl and
6
4
ꢀmethoxyphenyl nitroso oxides with Ph P are 1.9•10
3
5
–1 –1
and 5.5•10 L mol s , respectively. Based on this and
taking into account that the product of the reaction of
nitroso oxide with triphenylphosphine gives the correꢀ
Since the transꢀform of nitroso oxides is reactive
toward triphenylphosphine, reaction (4), which causes
the chain character of the process, is added to the known
reactions occurred during the generation of nitrenes in
the presence of oxygen.
The yield of nitroso oxides was determined by the
oxygen concentration, which is absorbed during the reacꢀ
sponding nitrosobenzene along with Ph PO, it is reasonꢀ
able to assume the chain mechanism of oxidation in the
3
ArNO—Ph P—O system.
3
2
The major reaction products of triphenylphosphine
with nitrosobenzene in the presence of oxygen are triꢀ
phenylphosphine oxide and nitroꢀ and azoxybenzenes
tion time (Δ[O ]) (see Table 1, entries 2, 3, 7, and 8). In
2
(
Table 1). Note that the concentrations of the consumed
the Ph P—PhNO system, this yield based on the conꢀ
3
sumed nitrosobenzene (Δ[O ]/Δ[ArNO]) was 0.52 (see
*
The results will be published in the journal “Kinetics and
2
Table 1, entry 2), and in the Ph P—4ꢀMeOC H NO
Catalysis.”
3
6
4

928
Russ.Chem.Bull., Int.Ed., Vol. 58, No. 5, May, 2009
Chainikova and Safiullin
system the yields were 1.12 and 1.33 (see Table 1,
entries 7 and 8). The fact that in the case of 4ꢀmethoxyꢀ
nitrosobenzene the amount of oxygen adsorbed is larger
than that of the consumed nitroso compound additionally
indicates that chain processes occur in the system (along
with the formation of triphenylphosphine oxide excessive
with respect to the consumed ArNO).
The oxygen absorption upon the reactions of triphenylꢀ
phosphine with nitrosobenzene and 4ꢀmethoxynitrosobenzene
was studied with a universal manometric differential setup
described earlier. A triphenylphosphine was placed in a
temperatureꢀcontrolled reactor equipped with a magnetic stirrer
and saturated with oxygen. The a solution of ArNO was
introduced, and the amount of O2 adsorbed by the system
was monitored.
1
4
To determine the yield of triplet nitrene, we carried
out the reaction of triphenylphosphine with nitrosoꢀ
benzene in the absence of oxygen (see Table 1, entry 6)
and found that the single product of triplet nitrene transꢀ
formation is azoxybenzene (see Scheme 1). Its yield based
on the starting PhNO is 14.6%. If we take into account
This work was financially supported by the Division
of Chemistry and Materials Science of the Russian
Academy of Sciences (Program “Theoretical and Experiꢀ
mental Investigation of the Chemical Bond Nature and
Mechanisms of the Most Important Chemical Reactions
and Processes”) and the Ministry of Education and
Science of the Russian Federation (Analytical Departꢀ
mental Target Program “Development of Scientific
Potential of the Higher School (2006—2008),” Grant
RNP 2.2.1.1.6332).
only the nitrosobenzene that has reacted with Ph P (see
3
Table 1, [PhNO] – [PhN=N(O)Ph]), then the yield of
0
triplet nitrene is 16.7%. These results agree satisfactorily
1
0
with the published data, according to which the yield of
3
PhN upon the reduction of nitrosobenzene with triethyl
phosphite was 13.4%. Phenylnitrene is mainly consumed
in the singlet state in secondary reactions with resin forꢀ
mation (see Scheme 1), and only small portion undergoes
intersystem crossing. The ratio of yields of nitroso oxide
and triplet nitrene gives the minimum estimate of the
chain length for the reaction of triphenylphosphine with
nitrobenzene in the presence of oxygen. This value is
approximately three units. In the case of 4ꢀmethoxyꢀ
nitrosobenzene, the chain length is sevenꢀ to eight units,
if the yield of triplet nitrene is accepted to be the same as
in the case of nitrosobenzene.
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1
1
The processes in the system studied were monitored analyzing
the qualitative and quantitative composition of reaction mixtures
and measuring the amount of oxygen absorbed by the system.
The reaction products of nitrosobenzene and 4ꢀmethoxyꢀ
nitrosobenzene with triphenylphosphine were studied as follows.
A solution of triphenylphosphine in acetonitrile was saturated
with oxygen for 5 min, then a necessary amount of a solution of
ArNO was added, and the reaction was carried out with
continuous oxygen bubbling. To study the reaction products in
the absence of oxygen, a solution of triphenylphosphine was
purged with argon for 90 min and the preꢀpurged with argon
solution of nitrosobenzene was added. Argon bubbling was
maintained constant until the reaction ceased. The reaction
mixtures were examined by HPLC on an Acmeꢀ9000 liquid
chromatograph (Young Lin Instrument) equipped with a twoꢀ
wave UVꢀVis detector (Exsil Silica column, hexane—isopropyl
alcohol (90 : 10 vol.%) or hexane—diethyl ether (98 : 2 vol.%)
1
1
1
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as mobile phase, flow rate of the mobile phase 2 mL min–1
room temperature of the column).
,
Received May 27, 2008;
in revised form August 12, 2008