A flash photolysis and theoretical study of the reaction of arylnitroso oxides with phosphorus(III) compounds

Article abstract of DOI:10.1134/S0018143910040053

The kinetic features of the reaction of aromatic nitroso oxides (ArNOO) with tri-n-butyl phosphite and substituted phosphines were studied by flash photolysis. It was shown that the trans-isomers of nitroso oxides enter into the reaction. The mechanism of the reaction was studied by theoretical methods, and the inertness of the cis-form of ArNOO was explained.

Full text of DOI:10.1134/S0018143910040053

ISSN 0018ꢀ1439, High Energy Chemistry, 2010, Vol. 44, No. 4, pp. 284–289. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © V.S. Khursan, O.A. Kovaleva, E.M. Chainikova, M.R. Talipov, R.L. Safiullin, 2010, published in Khimiya Vysokikh Energii, 2010, Vol. 44, No. 4, pp. 313–
318.
PHOTOCHEMISTRY
A Flash Photolysis and Theoretical Study of the Reaction
of Arylnitroso Oxides with Phosphorus(III) Compounds
V. S. Khursan, O. A. Kovaleva, E. M. Chainikova, M. R. Talipov, and R. L. Safiullin
Institute of Organic Chemistry, Ufa Scientific Center, Russian Academy of Sciences,
pr. Oktyabrya 71, Ufa, 450054 Russia
eꢀmail: kinetic@anrb.ru
Received January 14, 2010
Abstract—The kinetic features of the reaction of aromatic nitroso oxides (ArNOO) with triꢀnꢀbutyl phosꢀ
phite and substituted phosphines were studied by flash photolysis. It was shown that the transꢀisomers of
nitroso oxides enter into the reaction. The mechanism of the reaction was studied by theoretical methods,
and the inertness of the cisꢀform of ArNOO was explained.
DOI: 10.1134/S0018143910040053
Nitroso oxides (ArNOO) are the products of the the reaction at room temperature in acetonitrile are on
reaction of triplet aromatic nitrenes with oxygen. They the order of 104–106 l mol–1 s–1.
are shortꢀlived species produced as intermediates in
In this work, we complemented the available
the photooxidation of aromatic azides [1] or deoxyꢀ
experimental material and investigated the kinetics of
genation of nitrosobenzenes with trivalent phosphorus
the reaction of nitroso oxides Ia–Id with triꢀ
nꢀbutyl
compounds in the presence of oxygen [2, 3]. The oneꢀ
andꢀhalf order of the N–O bond in their molecules [4]
provides that nitroso oxides can exist in the cisꢀ and
transꢀisomeric forms.
phosphite ( ꢀPhO)3P and nitroso oxides Ib and Ic
n
with the substituted phosphines tris(4ꢀmethoxypheꢀ
nyl)phosphine (4ꢀCH3OꢀC6H4)3P), tris(4ꢀmethꢀ
ylphenyl)phosphine (4ꢀCH3ꢀC6H4)3P), and tris(4ꢀ
fluorophenyl)phosphine (4ꢀFꢀC6H4)3P).
NOO
N
Ia: R = H
R
O
O
Ib: R = CH3O
Ic: R = CH3
cisꢀform
R
Id: R = Br
N
R
O
O
Using the reactions of nitroso oxides Ib and Ic with
triꢀ ꢀbutyl phosphate as an example, we investigated
transꢀform
n
the effect of the medium on the rate constant of the
reaction at room temperature. The main objective of
the study was to explain the cause of the inertness of
the cisꢀisomers of nitroso oxides in the given reaction.
To solve this task, we employed quantumꢀchemical
methods.
The isomers do not undergo conversion into one
another and differ in spectral properties and reactivity,
thus being identifiable in lowꢀtemperature matrices [5, 6]
and, by means of flash photolysis, in solutions [7].
Nitroso oxides are typical oxidants and can react
with deoxygenating agents, such as phosphorus(III)
compounds (R3P). Earlier, we studied the kinetic feaꢀ
tures of the reaction of some arylnitroso oxides with
triphenylphosphine Ph3P [8] and triphenyl phosphite
EXPERIMENTAL
Acetonitrile, benzene, and hexane were purified
(PhO)3P [9]. It was shown that the cisꢀisomers are according to known procedures [10]; triphenyl phosꢀ
unreactive toward these substrates and the transꢀisoꢀ phite was distilled in a vacuum. Azides IIa and IIc were
mers alone enter the reaction. The rate constants of synthesized according to [11] and distilled in a vacꢀ
284

A FLASH PHOTOLYSIS AND THEORETICAL STUDY OF THE REACTION
285
uum. Azides IIb and IId were synthesized according to absorption by both isomer species, twoꢀcomponent
another procedure [12] and recrystallized from hexane absorbance decay curves were recorded, which are
and ethanol, respectively.
described well by the following fiveꢀparameter twoꢀ
exponential equation:
N3
IIa: R = H
I
IIt
A = A∞ + AI0e–k t + A0IIe–k
,
IIb: R = CH3O
IIc: R = CH3
IId: R = Br
R
I
II
where
kI kIIare the initial absorbances and the
,
A0, A0 ,
conversion rate constants of the cisꢀ and transꢀisoꢀ
mers, respectively, and A∞ is the residual absorbance
due to the reaction products.
Kinetic studies were carried out on the flash phoꢀ
tolysis facility detailed in [13, 14]. The light source was
an IFP 5000ꢀ2 lamp having a maximum pulse energy
The conversion rate of the transꢀisomers of nitroso
oxides drastically increases in the presence of R3P, as
compared to that in the absence of the substrate. The
decay rate of the cisꢀisomers is not affected by the presꢀ
ence of the oxidation substrate. From the linear plots
of the pseudoꢀunimolecular conversion rate conꢀ
stants of the transꢀisomers versus the R3P concentraꢀ
tion, values for the reaction rate constant kP(III) were
obtained (Table 1). Table 1 also includes the results
published for triphenylphosphine [8] and triphenyl
phosphite [9].
of 400 J at
U
= 5 kV,
С
= 32
µ
F, a n d ~ 9 0 % o f l i g h t
energy emitted for 50
µ
s. The spectrophotometric unit
of the facility comprised a probing source of continuꢀ
ous radiation, the DKSShꢀ150 xenon lamp with a set
of quartz lenses and diaphragms used to form the
probe beam, an MDRꢀ4 monochromator, an FEUꢀ97
photomultiplier (spectral range of 200–800 nm), and
an S9ꢀ8 storage oscilloscope.
The reactor was a quartz cell with an optical path
length
sis of airꢀsaturated arylazide solutions (2
was carried out with filtered light (UFSꢀ2 filter with a
transmission range of = 280–380 nm). The initial
concentration of arylazides was 2
10–4 mol/l. The
l
of 10 cm and i.d. of ~1 cm. The flash photolyꢀ
×
10–4 mol/l)
The oxidation of phosphorus(III) compounds by
nitroso oxides in the transꢀform is electrophilic in
character. The nature of the substituent on the aroꢀ
matic ring of nitroso oxide affects the reaction rate
λ
×
constant as follows: Br > H > CH3
mett values are 1.5 ( = 0.94) for triphenylphosphine,
1.2 ( = 0.93) for triphenyl phosphite, and 2.4 ( = 0.94)
for triꢀ ꢀbutyl phosphite (the values of are borrowed
>
CH3O. The Hamꢀ
absorbance of ArNOO was monitored at a wavelength
at which both particles absorb light [7]. The concenꢀ
tration of phosphites was varied in the range of (5–
ρ
r
r
r
320)
over (5–500)
×
10–4 mol/l, and that of phosphines was changed
n
σ
×
10–6 mol/l depending on their activity.
from [17]). The reverse order is observed with varying
the substituent on the aromatic ring in the phosphine:
The errors of determination of the rate constants did
not exceed 10%.
CH3O
>
CH3 > H > F. In this case,
ρ
= –1.5 (r = 0.995)
for nitroso oxide Ib and
ρ
= –1.2 (r= 0.995) for nitroso
Quantumꢀchemical calculations were run using the
Gaussian 03 software package [15]. The search for staꢀ
tionary points on the potential energy curves and the
calculation of the thermochemical parameters of the
reactions in question were performed by means of the
density functional theory UB3LYP/6ꢀ31G(d). The
values of the orbital energies were obtained via an
additional single calculation by the MP2/ccꢀpVTZ
method. The results were visualized with the use of the
program available from [16].
oxide Ic. The influence of the solvent nature on the reacꢀ
tion kinetics was studied for the reactions of nitroso
oxides Ib and Ic with (PhO)3P [9] and (nꢀBuO)3P
(Table 1). In the case of triphenyl phosphite, the rate
constants of the reaction in benzene and acetonitrile
are the same accurate to within the experimental
error and are higher by a factor of 2–3 in hexane. A
more profound effect of the nature of the medium
on the characteristics of this interaction was
observed for triꢀnꢀbutyl phosphite. In the given
case, an increase in the reaction rate constant by an
almost an order of magnitude was observed on passꢀ
ing from acetonitrile to hexane.
RESULTS AND DISCUSSION
Experimental Study of the Reactivity of Nitroso Oxides
Toward R3P
Thus, the isomeric state in which the species of
interest occurs has a determining effect on the activity
of arylnitroso oxides in the reaction of oxygen atom
The flash photolysis of the solutions of arylazides
IIa–IId in the presence of oxygen yields the correꢀ
sponding nitroso oxides. In the spectral range of transfer to trivalent phosphorus compounds. While the
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KHURSAN et al.
Table 1. Rate constants for the reaction of the Ar NOO transꢀisomers with phosphorus(III) compounds (kP(III) × 10–5, l mol–1 s–1) at
295 2 K
Nitroso oxide
Substrate
(4ꢀCH OꢀC H ) P
Ib
15.0
Ic
31.0
Ia
Id
3
6 4 3
(4ꢀCH ꢀC H ) P
9.0
20.0
15.0
11.0
3
6 4 3
a
Ph P
5.5
19.0
40.0
0.66
3
(4ꢀFꢀC H ) Pb
5.0
6
4 3
(PhO) Pb
0.14
0.31
0.32
0.83
3
(0.13)c
(0.45)d
0.09
(0.27)c
(0.68)d
0.48
(nꢀBuO) P
3
(0.34)c
(1.2)d
(1.0)c
(2.0)d
a
The rate constants are as measured in [8].
The rate constants are as measured in [9].
The solvent is benzene.
b
c
d
The solvent is hexane.
transꢀform is a very reactive electrophilic oxidant, as
indicated by the values of the reaction rate constants
(Table 1), the cisꢀform is surprisingly inert. In order to
clarify the reasons behind this phenomenon, we perꢀ
formed a theoretical investigation.
QuantumꢀChemical Study of the Nature
of the Primary Step in the Reaction of Arylnitroso
Oxides with R3P
It turned out that nitroso oxides can enter the reacꢀ
tion with phosphines via two routes (scheme 1).
#
A
PR3
ArN
cis/
trtansꢀArNOO + PR3
ArNO + R3PO
O
O
B
#
#
O
O
O
ArN
ArN
R
ArN
R
O
O
O
P
P
P
R
R
R
R
R
R
R
Scheme 1.
Nitroso oxide in the reaction via route A acts as a bonds. The products of both possible routes of the
reaction (A and B) are nitroso compounds and phosꢀ
phine oxide.
biradical, with the transfer of the terminal oxygen
atom of the nitroso oxide group onto the phosphorus
atom occurring in one elementary step. A similar
mechanism was proposed in [18] for the oxidation of
sulfides by nitroso oxides. Route B can be classified as
The experimentally observed inertness of the cis
ꢀ
form of nitroso oxides in the reaction of interest can be
explained in terms of the calculated values of the actiꢀ
vation barrier for routes A and B (Table 2). The data
(3+1)ꢀcycloaddition in which the ꢀelectron group of
π
ArNOO interacts with the lone electron pair of phosꢀ
phorus. This route is typical for the reaction of R3P
with ozone, an isoelectronic analogue of HNOO [19].
Proceeding via route B, the reaction yields a labile
cyclic intermediate, which readily dissociates involvꢀ
presented in Table 2 indicate that the cisꢀ and trans
isomers must differ in reactivity. Note that the trans
form is, first, substantially more reactive than the cis
form and, second, more prone to follow reaction route
B. The activation energy of this route for trans
ꢀ
ꢀ
ꢀ
ꢀ
ing the synchronous breaking of the P–N and O–O ArNOO does not exceed 9.8 kJ/mol in all of the examꢀ
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A FLASH PHOTOLYSIS AND THEORETICAL STUDY OF THE REACTION
287
Table 2. Activation barriers for the reaction of arylnitroso oxꢀ
ide isomers with triphenylphosphine (UB3LYP/6ꢀ31G(d))
A and B can occur by virtue of the interaction of either
an unoccupied nitroso oxide orbital (Table 4) with
the orbital of the phosphorus loneꢀelectron pair (
–0.290 a.u.) or the occupied orbital of nitrous oxide
(Table 4, a” (NOO)) with the unoccupied orbital
=0.250 a.u.). These interꢀ
ε
=
#
kJ/mol
ΔH298,
4ꢀRꢀC H NOO
6
4
π
d
2
A
B
of the phosphorus atom (
ε
actions for reaction B are illustrated in the figure. An
analysis of the orbital energies of substituted nitroso
oxides shows that electronꢀwithdrawing groups lower
the energies of both occupied and unoccupied orbitals
of the NOO groups (Table 4). Consequently, this effect
enhances interaction only between the higher occuꢀ
pied molecular orbital (HOMO) of Ph3P and lower
unoccupied molecular orbitals (LUMOs) of ArNOO.
Since the enhancement of the HOMO(PPh3)–
LUMO(ArNOO) interaction is accompanied by a
decrease in the activation energy, it can be concluded
that the rate of the reaction is determined by the interꢀ
action of the lone electron pair of the phosphorus atom
(CH ) N
cis
trans
cis
38.3
27.6
39.6
19.9
39.6
27.2
28.9
21.6
–
52.5
7.1
3 2
CH O
43.2
0.8
3
trans
cis
CH
H
36.5
9.8
3
trans
cis
32.4
5.0
trans
cis
Br
27.7
0.2
trans
cis
2.4
with
σ*(OO) (reaction A) or
π*(NOO) (reaction B).
NO
5.9
12.5
0.1
2
Since the
π*(NOO) orbital has a considerably lower
trans
0.6
energy, it is reasonable to assume that it is this orbital
which enters the interaction. That is, the preferred
direction of the reaction is route B.
ined systems. Table 3 lists the activation parameters for
the reaction of triphenyl phosphite with transꢀarylniꢀ
troso oxides in acetonitrile as measured in [9]. The
activation energy is low and slightly depends on the
nature of the substituent in the nitroso oxide molecule.
Based on these results, we may assume that reacꢀ
tions A and B will be accelerated by an increase in the
phosphine HOMO energy. This effect will be observed
for phosphines bearing electronꢀdonating substituꢀ
ents. Indeed, the energy of the frontier orbital
increases somewhat in the case of tris(4ꢀmethylpheꢀ
Reaction B for cisꢀArNOO is impeded by geometꢀ
ric factors: the aromatic ring of nitroso oxide hinders
the approach to the bulky triphenylphosphine moleꢀ
cule. Therefore, reaction B has a higher activation barꢀ
rier required for the deformation of the nitroso oxide
group and route A becomes preferable for cisꢀArNOO.
nyl)phosphine ( HOMO = –0.289 a.u.) as compared to
ε
the unsubstituted triphenylphosphine and substanꢀ
tially decreases in the case of tris(4ꢀfluoropheꢀ
nyl)phosphine ( HOMO = –0.311 a.u.). Experimentally,
ε
this trend is manifested in a decrease in the reaction
rate constant for fluorinated phosphine as compared
to the unsubstituted phosphine (Table 1). Phosphites
are characterized by lowꢀlying occupied orbitals:
Electronꢀwithdrawing substituents in the paraꢀ
position of arylnitroso oxide substantially reduce the
activation barriers to both reactions (Table 2). In the
case of 4ꢀnitrophenylnitroso oxide, the maximal value
of the activation barrier corresponding to reaction B of
the cisꢀform is as low as 12.5 kJ/mol, thereby indicatꢀ
ing that the reaction fairly easily proceeds via both
routes. This fact can be explained in terms of the fronꢀ
ε
= –0.329 a.u. for triphenyl phosphite and
HOMO
ε
HOMO = –0.357 a.u. for triꢀnꢀbutyl phosphite. That is,
phosphite must have a lower reactivity than phosꢀ
phine, which s indeed the case observed experimenꢀ
tierꢀorbital theory. According to this theory, reactions tally (Table 1).
Table 3. Activation parameters for the reaction of the transꢀform of nitroso oxides with triphenyl phosphite in acetonitrile*
Nitroso oxide
Parameter
Ib
Ic
Ia
Id
7.3
–113.4
18
6.5
–128.7
12
7.0
–119.2
14.5
7.0
–119.2
12
logA
ΔS2#98
,
J mol–1 K–1
#
E
, kJ/mol
* The data have been obtained in [9].
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288
KHURSAN et al.
Table 4. Energies of some orbitals of the nitroso oxide group (MP2/ccꢀpVTZ//UB3LYP/6ꢀ31G(d)), a.u.
paraꢀRꢀPhꢀNOO
(CH ) N cis
trans
cis
trans
cis
trans
cis
trans
cis
trans
cis
trans
cis
trans
a' n(O )
a''
π
(NOO)
a'' π (NOO)
a''
π
*(NOO)
a' σ*(OO)
t
1
2
ꢀ
–0.397
–0.397
–0.403
–0.398
–0.416
–0.416
–0.385
–0.382
–0.401
–0.398
–0.417
–0.416
–0.425
–0.426
–0.436
–0.438
–0.424
–0.422
–0.466
–0.470
–0.304
–0.291
–0.314
–0.296
–0.330
–0.317
–0.339
–0.319
–0.349
–0.329
–0.350
–0.338
–0.378
–0.358
0.020
0.011
0.016
0.457
0.444
0.454
0.445
0.434
0.409
0.452
0.432
0.445
0.415
0.448
0.445
0.423
0.381
3 2
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
NH
ꢀ
2
–0.001
0.007
CH O
ꢀ
3
0.001
CH
H
ꢀ
–
–
0.422
0.422
–0.001
–0.015
–0.007
–0.020
–0.015
–0.020
–0.047
–0.056
3
ꢀ
–0.430
–0.431
–0.437
Br
ꢀ
–
0.442
NO
ꢀ
–0.461
–0.469
2
Thus, it has been shown in this work that the reacꢀ The evidence for that it is this mechanism indeed is
quite high negative values for the entropy of activation
(Table 3), which should characterize the fourꢀmemꢀ
bered cyclic transition state of the reaction.
tion of nitroso oxides with trivalent phosphorus comꢀ
pounds most likely follows the concerted (3+1)ꢀ
cycloaddition mechanism (Scheme 1, route B) of the
two possible mechanisms, since this route is characterꢀ
The inertness of the cisꢀform of ArNOO in the
ized by considerably lower activation barriers (Table 2). reaction in question is due to steric hindrances, which
H
C
C
H
H
C
C
H
C
H
H
C
H
P
C
C
H
C
H
H
C
C
C
C
H
O
C
H
C
H
C
H
C
C
N
O
C
H
C
H
C
H
C
C
H
H
H
C
H
H
C
C
C
H
H
C
O
H
H
H
C
P
C
C
C
C
H
N
C
O
C
C
C
C
C
H
C
H
H
C
C
H
H
C
H
H
H
Sketch of the orbitals interaction in reaction B.
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A FLASH PHOTOLYSIS AND THEORETICAL STUDY OF THE REACTION
289
5. Pritchina, E.A., Gritsan, N.P., and Bally, T., Phys.
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result in a higher activation barrier as compared to the
transꢀform (Table 2).
The electrophilic nature of oxidation of phosphoꢀ
rus(III) compounds by nitroso oxides is explained by
the fact that the lower unoccupied molecule orbitals of
a nitroso oxide and the higher occupied molecular
orbital of R3P interact during the reaction. That is,
electronꢀwithdrawing substituents in nitroso oxide
molecules and electronꢀdonating substituents in R3P
molecules increase the reaction rate constant.
Kinet. Katal., 2006, vol. 47, no. 4, p. 566.
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ACKNOWLEDGMENTS
This work was supported by the Russian Academy
of Sciences under the program of the Division of
Chemistry and Material Sciences “The theoretical
and experimental study of chemical bonding and
mechanisms of the most important chemical reactions
and processes” and by the Russian Foundation for
Basic Research, project no. 09ꢀ03ꢀ00411.
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