A Revised Mechanism of Thermal Decay of Arylnitroso Oxides

Jul 17, 2012 - There are several groups of researchers, who .... Prestige-21 Fourier transform infrared spectrometer. ... group and H-atom at C(5) are...
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A Revised Mechanism of Thermal Decay of Arylnitroso Oxides Ekaterina M. Chainikova,* Rustam L. Safiullin, Leonid V. Spirikhin, and Marat F. Abdullin Institution of the Russian Academy of Sciences Institute of Organic Chemistry, Ufa Scientific Center of the Russian Academy of Sciences, 71 prosp. Oktyabrya, 450054 Ufa, Russian Federation S Supporting Information *

ABSTRACT: The electronic spectra were measured and the unimolecular decay kinetics of the isomeric forms (cis and trans) of 4-methoxyphenylnitroso oxide in acetonitrile, benzene, and hexane was studied using flash photolysis. The cis form absorbed in a shorter wavelength region and was more labile than the trans form. The difference between the reactivity of the two species increased on going from hexane to acetonitrile. The temperature dependences of reaction rate constants were studied for both isomeric forms. The analysis of products of flash photolysis of 4-methoxyphenyl azide in the presence of oxygen allowed for understanding the mechanism of thermal decay of nitroso oxides. It was shown that the trans nitroso oxide is converted into cis nitroso oxide. The latter undergoes an unusual ring cleavage reaction to form 4-methoxy-6-oxohexa-2,4-dienenitrile N-oxide derivative. We conclude that the nitroand nitrosobenzenes, which are the main products of the steady-state photolysis of aromatic azides in the presence of oxygen, are formed by the photochemical transformation of the nitroso oxides.

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previous work14 electronic spectra and kinetic regularities of decay for phenylnitroso oxide, 4-N,N-dimethylamino-, 4-methoxy-, 4-methyl, 4-bromo-, and 4-nitrophenylnitroso oxides were studied by flash photolysis at near room temperatures. The isomeric forms of all mentioned nitroso oxides are consumed by a first-order reaction. The corresponding nitro- and nitrosobenzenes are always present in reaction mixtures obtained by the oxidation of aromatic azides under conditions of stationary photolysis.2,3,13,27,28 They are believed to be end products of nitroso oxides decay. The formation of nitro compounds can be explained by the following reaction:

INTRODUCTION In 1971, Singh and Brinen,1 while studying 1,4-diazobenzene photolysis in glassy matrices, for the first time observed the formation of arylnitroso oxide (ArNOO), which is a product of molecular oxygen addition to triplet aromatic nitrene. There are several groups of researchers, who have studied the properties of these heteroatomcontaining peroxide intermediates using experimental2−18 and theoretical methods4,18−22 over the past 40 years. Several reviews were published in which nitroso oxides are considered as intermediates of photooxidation of aromatic azides23,24 and in the context of chemistry of heteroatom-containing dioxiranes,25 as well as representatives of the class of 1,3-dipolar peroxide species (X+−O−O−).26 The kinetics of 4-amino-, 4-methyl-, and 4-nitrophenylnitroso oxide decay was studied by flash photolysis at room temperature.2,3 It was found that 4-aminophenylnitroso oxide is consumed by the second-order reaction. The kinetic curves of absorption decay of the two others are of nonexponential character. However, they cannot be described by the secondorder reaction either. Using time-resolved infrared spectroscopy in solution at room temperature, Toscano and co-workers12 showed that the decay kinetics of the nonaromatic nitroso oxide PhCH2ONOO is affected by its starting concentration. At a low initial concentration this nitroso oxide decays by a first-order reaction. A second-order decay reaction takes place at a high initial concentration of the species. Owing to the one-and-a-half order of the N−O bond,21 cis− trans isomerism is typical for nitroso oxides:

The quantum chemical calculations show that the activation energy of the nitroso oxide cyclization into dioxaziridine is equal to 160−170 kJ/mol.20,21 Consequently, this transformation is likely impossible under thermal conditions but it is most likely that it can take place upon photoexcitation of nitroso oxide. The generation of species, assigned as dioxaziridines, was detected by UV−vis spectroscopy upon low-temperature photolysis of several nitroso oxides stilbene derivatives.5 These intermediates were shown to undergo a thermal reaction leading to nitro compounds with rate constant of 0.003 s−1 (77 K). In the works of refs 6−8 photooxidation of aryl azides was performed in the presence of the 16O2−18O2 mixture that shows the unimolecular transformation of nitroso oxides to nitrobenzenes, which occurs presumably through dioxaziridine formation. No evidence was shown for dioxaziridines as intermediates of the conversion of Received: February 21, 2012 Revised: July 16, 2012 Published: July 17, 2012

Isomers were detected for 4-amino-2−4 and 4-nitrophenylnitroso oxides9 in matrices by UV- and IR-spectroscopy. In our © 2012 American Chemical Society

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nitroso oxide to nitro compounds upon 4-amino-4 and 4-nitrophenyl azides’9 photolysis in argon4 and xenon9 matrices in the presence of oxygen. Benzyl nitrate is the product of transformations of benzylO-nitroso oxide.12 On the basis of the fact that the decay kinetics of this nitroso oxide is affected by its starting concentration, the authors12 state that the bimolecular reaction leads to a sixmembered-ring diperoxide, [1,2,4,5]-tetraoxadiazinane, and that the monomolecular reaction is an intramolecular cyclization to dioxaziridine:

work. Unfortunately, Albini’s article has gone unnoticed by researchers involved in the study of nitroso oxides, and until now, the corresponding nitro- and nitrosobenzenes were considered to be the main products of transformations of these species.24

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EXPERIMENTAL SECTION HPLC grade acetonitrile (Panreac) and hexane (Cryochrom) were used without further purification. Benzene was purified as described in ref 30, 4-methoxyphenyl azide,31 4-methoxynitrosobenzene,32 4-methoxynitrobenzene33 and 4,4′-dimethoxyazobenzene34 were synthesized using the described methods. Triphenylphosphine was recrystallized from ethanol. NMR spectra were registered on a Bruker AM-300 MHz spectrometer in deuterated acetonitrile (Acros Organics, degree of deuteration, 99.95%) using tetramethylsilane (TMS) as internal standard. FTIR spectra (on KBr glass) were recorded on a Shimadzu Prestige-21 Fourier transform infrared spectrometer. UV-vis spectra were registered on a Shimadzu UV-365 spectrometer. Atmospheric pressure chemical ionization (APCI) mass spectra were obtained on a HPLC mass-spectrometer LCMS2010EV (Shimadzu) (direct syringe sample inlet, sample solution was in acetonitrile, mobile phase was acetonitrile/water (70:30)) in positive and negative ions mode at the corona discharge needle ionizing electrode potential of 4.5 kV and −3.5 kV, respectively. The mobile phase flow rate was 1 mL min−1. The temperature and voltage of the interface capillary was 250 °C and 5 to −5 V, respectively. The nebulizer gas (nitrogen) flow rate was 2.5 L min−1. The high-frequency lenses (Q-array) voltage was 5 to −5 V. The chromatographic column was Luna 5C18(2), 150 × 4.6 mm (Phenomenex). Kinetic Experiments. A flash photolysis system of known design15 was used for the kinetic experiments. The photolytic source was an IFP 5000-2 lamp; maximum pulse energy was 400 J at U = 5 kV and C = 32 μF; ∼90% light energy was emitted in 50 μs. The reactor was a quartz cell with optical path length l = 10 cm, an inner diameter of ∼1 cm and volume of ∼8 mL. The flash photolysis of 2.5 × 10−4 M 4-methoxyphenyl azide solutions saturated with oxygen was performed with filtered light (UFS-2 light filter; transmittance range, λ = 270−380 nm). To investigate the products of the reaction, photolysis of 4-methoxyphenyl azide solution (1000 mL, 5 × 10−4 M) saturated with oxygen was performed using a flash photolysis system. The solution was irradiated by one light pulse of maximum intensity. Then the obtained reaction mixture was concentrated to about 0.5 mL and divided using a reversed-phase HPLC on an Acme-9000 (Young Lin Instrument) liquid chromatograph equipped with a two-wave UV−vis detector. The column is a Wakosil II 5C18RS 4.6 × 250 mm (SGE), the mobile phase is acetonitrile, the flow rate of the mobile phase is 1 mL min−1, the column is operated at room temperature. Detection wavelengths are 230 and 300 nm (240 and 340 nm for analysis of 4,4′-dimethoxyazobenzene). (2Z,4E)-4-Methoxy-6-oxohexa-dienenitrile oxide (4) was isolated in an amount of ∼3 mg. IR spectrum (KBr), ν/cm−1: 2853 (C(O)−H); 2295 (CN); 1653 (CO). 1H NMR spectrum (CD3CN, δ, ppm, J, Hz): 9.90 (d, 1 H, H(6), J = 7.3); 7.42 (d, 1 H, H(3), J = 11.3); 5.95 (d, 1 H, H(2), J = 11.3); 5.60 (d, 1 H, H(5), J = 7.3); 3.82 (s, 3 H, OMe). 13C NMR spectrum (CD3CN, δ, ppm): 189.92 (C(6)); 167.59 (C(4)); 134.80 (C(3)); 128.91 (C(1)); 108.26 (C(2)); 105.29 (C(5)); 57.44 (C(7)). On the basis of the spin−spin coupling constants

Simultaneous formation of nitro- and nitrosobenzenes upon photooxidation of aryl azides was explained by the authors2,3 in terms of the bimolecular decay mechanism of nitroso oxides, which they detected by flash photolysis for 4-aminophenylnitroso oxide:

Other nitroso oxides are consumed by a first-order law.14 Here we should note that the reaction of ArNOO decay occurring under conditions of flash photolysis is thermal because the time of the light pulse is by several orders of magnitude shorter than the lifetime of these species (∼0.005−10 s).14 In this work, we studied electronic spectra and the kinetics of decay of cis and trans isomers of 4-methoxyphenylnitroso oxide by flash photolysis in acetonitrile, benzene, and hexane. Analysis of reaction mixtures obtained during the photooxidation of 4-methoxyphenyl azide under conditions similar to the kinetic experiments shows that dark transformations of the isomeric forms of this nitroso oxide lead to an unexpected product which was not observed upon photooxidation of aryl azides earlier. On the basis of these data, we proposed the mechanism of monomolecular decay of arylnitroso oxides under thermal conditions. During the preparation of this article, we learned that in 1987, Albini et al. had published the work,29 which is devoted to the study of the photolysis products of phenazines 1 in benzene solutions. In the presence of oxygen, product 3 was found in the reaction mixture. To explain the formation of this compound, authors proposed Scheme 1. Scheme 1

Nitrile oxide 2, which was suggested by Albini as an intermediate, is similar to the product obtained by us in the present 8143

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between the protons at C(2) and C(3) (11.3 Hz) in 1H NMR spectrum, we can conclude that the double bond between these carbon atoms has a cis configuration. The compound 4 is formed by cleavage of the benzene ring (see Scheme 2), so the location of

In the short-wavelength region, kinetic curves of absorption decay consist of two components (Figure 1, curve 2), that is, they correspond to decay of two intermediates different in reactivity. These kinetic curves were described by a five-parameter biexponential equation:

Scheme 2

I

A = A∞ + A 0Ie−k t + A 0IIe−k

II

t

where AI0, AII0 , kI, kII are initial optical densities and rate constants of unimolecular decay of the former and latter intermediate, respectively; A∞ is the final absorbance of the solution due to reaction products. The absorption spectra of the two species were constructed from the calculated initial optical densities in acetonitrile, benzene, and hexane (Figure 2). The absorption maximum of substituents with respect to the double bonds must be maintained as in the parent molecule. Therefore, the methoxy group and H-atom at C(5) are also cis. The reaction of 4-methoxynitrosobenzene with triphenylphosphine was performed as follows. Acetonitrile solution of 4methoxynitrosobenzene (1.1 × 10−3 M) was put into the reactor (10 mL). To saturate solution with oxygen, O2 was bubbled through it for 5 min. Then triphenylphosphine was added to the solution in small portions (∼5 × 10−6 M) every 5 h. The reaction was carried out at oxygen bubbling for 26 h. The obtained reaction mixture was analyzed using HPLC.

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RESULTS AND DISCUSSION Kinetics Decay and the Electronic Spectra of Isomeric Forms of 4-Methoxyphenylnitroso Oxide. We observed the formation of short-lived species absorbing light in the wavelength region 390−500 nm upon flash photolysis of 4-methoxyphenyl azide solutions in the presence of oxygen. Kinetic curves of the decay of these intermediates absorption measured at λ ≥ 490 nm in acetonitrile and benzene and at λ ≥ 450 nm in hexane were described by a first-order equation (Figure 1, curve 1): A − A∞ = (A 0 − A∞)e−kt

Figure 2. Electronic absorption spectra of cis (a) and trans forms (b) of 4-methoxyphenylnitroso oxide in acetonitrile (1), benzene (2), and hexane (3). T = 295 K.

the more reactive intermediate is 410 nm in hexane and 430 nm in acetonitrile or benzene (Figure 2a), and the less reactive has maxima 425, 450, or 460 in acetonitrile, benzene, and hexane, respectively (Figure 2b). According to theoretical calculations,21 cis isomers of nitroso oxides absorb light in a shorter wavelength range than trans isomers. Gritsan et al.4 drew the same conclusion not only on the basis of quantum chemical calculations, but also from IR spectra of cis and trans isomers of 4-aminophenylnitroso oxide registered in an argon matrix. As a consequence, a more labile intermediate

Figure 1. Kinetic curves of absorbance decay of 4-methoxyphenylnitroso oxide in benzene solution measured at wavelengths of 500 nm (1) and 440 nm (2) (T = 295 K). Solid lines correspond to theoretical description.

where A0 and A∞ are optical densities of the solution immediately after excitation and at the end of the reaction, respectively. 8144

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Table 1. Rate Constants (T = 295 K) and Activation Parameters of Decay of Isomeric Forms of 4-Methoxyphenylnitroso Oxide cis

trans

solvent

kcis (s−1)

log k0cis

Ea (kJ mol−1)

ktrans (s−1)

log k0trans

Ea (kJ mol−1)

C6H14 C6H6 CH3CN

4.7 ± 0.6 6.9 ± 0.1 11.3 ± 0.1

11.5 ± 0.2 12.4 ± 0.1 11.7 ± 0.1

61 ± 1 65.5 ± 0.5 60.5 ± 0.3

1.00 ± 0.05 0.56 ± 0.04 0.37 ± 0.01

11.8 ± 0.1 12.2 ± 0.1 11.4 ± 0.2

66.9 ± 0.7 70.4 ± 0.6 67.2 ± 0.9

Figure 3. Chromatogram of concentrated reaction mixture obtained by flash photolysis of acetonitrile solution of 4-methoxyphenyl azide (5 × 10−4 M) saturated with oxygen. The mobile phase is acetonitrile, detection wavelength is 300 nm: tR = 3.17 min (X), tR = 3.87 min (4-methoxyphenylazide), tR = 4.60 min (4,4′-dimethoxyazobenzene).

isomer decays by 5−7 kJ mol−1 higher than that for the cis isomer. Products of Flash Photolysis of 4-Methoxyphenyl Azide in the Presence of Oxygen. A reversed-phase HPLC was used for analysis of reaction mixtures obtained by flash photolysis of acetonitrile solutions of 4-methoxyphenyl azide in the presence of oxygen (see Experimental Section). Its solution saturated with oxygen was irradiated by one light pulse. Nitroso oxide generated in this way was consumed in the dark, while the products of its transformations did not undergo secondary photolysis. As can be seen from the chromatogram of the obtained reaction mixture (Figure 3), a compound X and 4,4′dimethoxyazobenzene (identified by comparison with a simple standard) are the main products. 4,4′-Dimethoxyazobenzene is formed upon recombination of triplet nitrenes and/or during their reaction with the parent azide. Comparison with standard samples showed that X is neither 4-nitro- nor 4-nitrosomethoxybenzene which are expected in the reaction according to the existing concept on the mechanism of aromatic azide photooxidation. By isolating the corresponding fraction from the reaction mixture, we obtained approximately 3 mg of compound X which slowly decomposes when stored in acetonitrile at environmental conditions but is stable for several weeks in the freezing chamber. The compound was stable enough to obtain its spectral characteristics. The UV spectrum of the isolated substance in

formed upon flash photolysis of 4-methoxyphenyl azide solutions in the presence of oxygen is likely the cis form of the corresponding nitroso oxide, while the trans form is less labile. The dramatic influence of the solvent nature on the absorption maximum of the isomers is worth noting (Figure 2). For cis-4methoxyphenylnitroso oxide the λmax is red-shifted by 20 nm in benzene and acetonitrile as compared to hexane, and for trans-4methoxyphenylnitroso oxide this shift is 25 nm in benzene and 35 nm in acetonitrile. This fact can be explained by the more effective solvation of electron-excited state than grounded isomeric forms of nitroso oxide in benzene and acetonitrile. Rate constant values of the unimolecular decay of isomers at 295 K are listed in Table 1. As can be seen from this data, the nature of the solvent has the opposite effect on the stability of the isomeric forms of 4-methoxyphenylnitroso oxide: the magnitude of the rate constant of the consumption of cis form increases and that of trans form decreases on going from hexane to benzene and acetonitrile. As a result, the ratios of the cis/trans rate constants are ∼5 in hexane, ∼12 in benzene, and ∼30 in acetonitrile. Temperature dependences of isomer decay rate constants were studied in the range 279−333 K. These dependences are described well by the Arrhenius law. Table 1 lists the values of activation energy and pre-exponential factors of the rate constants of the decay of cis and trans isomers in acetonitrile, benzene, and hexane. In all solvents, activation energy of trans 8145

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Table 2. The Results of Analysis of Composition of Different Reaction Mixturesa [products] × 104 (M)

[initial reactants] × 104 (M) run no. 1 2 3 4

b

[ArN3] (Δ[ArN3])

[4] 0.16

[ArNO] c

[Ph3P]

[4]

[ArNNAr]

0.19 [40%]d 0.05 [15%]d 0.1

0.12 [25%]d 0.07

1.0

5.0 (0.47) 5.0 (0.33) 11

[Ph3PO]

[ArNO]

0.16

1.0 0.27e

0.08 [24%]d 0.27

0.08 [24%]d

a

Note: Acetonitrile was used as solvent, T = 295 K. bConcentration of consumed azide. cConcentration of 4 has been determined by yield of phosphine oxide. Reaction time is 19 h. dThe yields per consumed azide. eTotal concentration of triphenylphosphine. Reaction time is 26 h.

Scheme 3

from the addition of Ph3P excess to a solution of compound 4 (Table 2, run 1). Compound 4 was completely consumed over the reaction time to give a new compound slightly more polar than 4 judging from the retention time in the HPLC chromatogram. We have not identified this compound, perhaps it is the corresponding nitrile. The extinction coefficient at absorption maximum of compound 4 in acetonitrile (λ = 300 nm) was determined to be ε ≈ 2 × 104 M−1 cm−1. The intermediate nitroso oxide yield over one light pulse per consumed azide was determined from the yield of compound 4 (Table 2, run 2), which is ∼40%. The yield of azobenzene is ∼25%. Taking into account that one molecule of azobenzene results from two molecules of triplet nitrene, the total yield of triplet nitrene per consumed azide is ∼90%. We have demonstrated that the cis and trans isomers of arylnitroso oxides show a different reactivity toward organic substrates.15,16 Only the trans isomers react with triphenylphosphine, whereas the cis isomers undergo monomolecular consumption.16 Flash photolysis of 4-methoxyphenyl azide in the presence of oxygen and Ph3P (1 × 10−4 M) led to a considerable decrease in the concentration of nitrile oxide 4 (Table 2, run 3 and 2) and to the emergence of triphenylphosphine oxide and 4-methoxynitrosobenzene in the reaction mixture, which resulted from the reaction of trans nitroso oxide and Ph3P. The rate constant for the reaction of trans-4-methoxyphenylnitroso oxide with triphenylphosphine is 5.5 × 105 M−1 s−1.16 Hence, trans nitroso oxide reacts under these conditions with phosphine, while nitrile oxide 4 results only from cis nitroso oxide. The yield of nitrile oxide 4, equal to the yield of cis nitroso oxide, was 15%, the yield of Ph3PO, equal to the yield of trans nitroso oxide and nitrosobenzene, was 24%. A total yield is ∼40%, which coincides with the total yield of nitroso oxide obtained in run 2 (Table 2). Consequently, nitrile oxide 4 is a product of the transformation of both isomers. However, the oxygen atom of the cis isomer of nitroso oxide can only react at the ortho position of the benzene ring. So, monomolecular reaction of trans nitroso oxide decay is isomerization to cis form. The decay of cis form is also isomerization but with aromatic ring cleavage, the first stage of which is interaction of the terminal oxygen atom of nitroso oxide

acetonitrile shows a maximum at 300 nm. The IR spectrum exhibits an intense band in the region of stretching vibrations of triple bonds (2295 cm−1). The mass spectrum of negative ions obtained by chemical ionization at atmospheric pressure shows the ion peak [M−H]− with m/z 152 of maximum intensity. The mass spectrum of positive ions displays the peaks with m/z 154 (MH+, 11.7%), 195 ([MH+CH3CN]+, 32.0%), 213 ([MH +CH3CN+H2O]+, 13.7%) and 236 ([MH+2CH3CN]+, 10.5%). These data show that the molecular weight of the isolated product X is 153, which equals the molecular mass of 4-methoxyphenylnitroso oxide (C7H7NO3). Taking into account this fact, we propose that X is formed by the rearrangement of the nitroso oxide molecule. The 1H NMR spectrum of the compound X in CD3CN displays four doublets with the following chemical shifts (δ, ppm, J, Hz): 9.90 (1 H, J = 7.3), 7.42 (1 H, J = 11.3), 5.95 (1 H, J = 11.3), 5.60 (1 H, J = 7.3) and a singlet at 3.82 ppm (3 H). The 13 C NMR spectrum shows signals at 189.92, 167.59, 134.80, 121.57, 108.26, 105.29, and 57.44 ppm. Analyzing spectral data, we concluded that compound X is (2Z,4E)-4-methoxy-6oxohexa-dienenitrile oxide (4) (assignments of signals in the NMR spectra are given in the Experimental Section). Formation of 4 occurs according to Scheme 2. The terminal oxygen atom of nitroso oxide group interacts with the ortho position of the benzene ring to give a five-membered cycle. The cycle undergoes opening with cleavage of C−C and O− O bonds. As a result, the aromatic ring is transformed into conjugated diene with aldehyde and nitrile oxide functional groups at the ends of the molecule. Such a transformation may be considered as a redox isomerization of nitroso oxides. The band in the IR spectrum at 2295 cm−1 corresponds to the stretching vibrations of the CN bond in the nitrile oxide group. Its position agrees well with the position of the corresponding band in the IR spectrum of nitrile oxide stabilized on a polymeric support (2296 cm−1).35 It is known, that nitrile oxides are reduced to nitriles by trivalent phosphorus compounds.36 Compound 4 oxidizes triphenylphosphine to triphenylphosphine oxide. The concentration of 4 was determined from the concentration of Ph3PO resulting 8146

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(7) Ishikawa, S.; Tsuji, S.; Sawaki, Y. J. Am. Chem. Soc. 1991, 113, 4282−4288. (8) Ishikawa, S.; Nojima, T.; Sawaki, Y. J. Chem. Soc., Perkin Trans. 1996, 127−132. (9) Inui, H.; Irisawa, M.; Oishi, S. Chem. Lett. 2005, 34, 478−479. (10) Zelentsov, S. V.; Zelentsova, N. V.; Zhezlov, A. B; Oleinik, A. V. High Energy Chem. 2000, 34, 164−171. (11) Makareeva, E. N.; Lozovskaya, E. L.; Zelentsov, S. V. High Energy Chem. 2001, 35, 177−180. (12) Srinivasan, A.; Kebede, N.; Saavedra, J. E.; Nikolaitchik, A. V.; Brady, D.; Yourd, E.; Davies, K. M.; Keefer, L.; Toscano, J. P. J. Am. Chem. Soc. 2001, 123, 5465−5472. (13) Safiullin, R. L.; Khursan, S. L.; Chainikova, E. M.; Danilov, V. T. Kinet. Catal. 2004, 45, 640−648. (14) Chainikova, E. M.; Khursan, S. L.; Safiullin, R. L. Kinet. Catal. 2006, 47, 549−554. (15) Chainikova, E. M.; Safiullin, R. L.; Faizrakhmanova, I. M.; Galkin, E. G. Kinet. Catal. 2009, 50, 174−175. (16) Chainikova, E. M.; Safiullin, R. L. Kinet. Catal. 2009, 50, 527−529. (17) Laursen, S. L.; Grace, L. E.; DeKock, R. L.; Spronk, S. A. J. Am. Chem. Soc. 1998, 120, 12583−12594. (18) Ling, P.; Boldyrev, A. I.; Simons, J.; Wight, C. A. J. Am. Chem. Soc. 1998, 120, 12327−12333. (19) DeKock, R. L.; McGuire, M. J.; Piecuch, P.; Allen, W. D.; Schafer, H. F.; Kowalski, K.; Kucharski, S. A.; Musial, M.; Bonner, A. R.; Spronk, S. A.; Lawson, D. B.; Laursen, S. L. J. Phys. Chem. A 2004, 108, 2893− 2903. (20) Zelentsov, S. V.; Zelentsova, N. V.; Shchepalov, A. A. High Energy Chem. 2002, 36, 326−332. (21) Talipov, M. R.; Ryzhkov, A. B.; Khursan, S. L.; Safiullin, R. L. J. Struct. Chem. 2006, 47, 1051−1057. (22) Talipov, M. R.; Khursan, S. L.; Safiullin, R. L. J. Phys. Chem. A 2009, 113, 6468−6476. (23) Gritsan, N. P.; Pritchina, E. A. Russ. Chem. Rev. 1992, 61, 500− 515. (24) Gritsan, N. P. Russ. Chem. Rev. 2007, 76, 1139−1160. (25) Sawwan, N.; Greer, A. Chem. Rev. 2007, 107, 3247−3285. (26) Ishiguro, K.; Sawaki, Y. Bull. Soc. Jpn. 2000, 73, 535−552. (27) Abramovitch, R. A.; Challand, S. R. J. Chem. Soc. Chem. Commun. 1972, 964−965. (28) Go, C. L.; Waddel, W. H. J. Org. Chem. 1983, 48, 2897−2900. (29) Albini, A.; Betinetti, G.; Minoli, G. J. Org. Chem. 1987, 52, 1245− 1251. (30) Weisberger, A., Proskauer, E. S., Riddik, J. A., Toops, E. E. Organic Solvents: Physical Properties and Methods of Purification; Interscience: New York, 1955. (31) Organic Synthesis; Rabjohn, R., Ed.; Wiley: New York, 1963; Vol. 4. (32) Houben, J. Die Methoden der Organischen Chemie; Verlag Georg Thieme: Leipzig, 1941. (33) Rarick, M. J.; Brewster, R. Q.; Dains, F. B. J. Am. Chem. Soc. 1933, 55, 1289−1290. (34) Ong, S. Y.; Chan, P. Y.; Zhu, P.; Leung, K. H.; Phillips, D. L. J. Phys. Chem. A 2003, 55, 3858−3865. (35) Faita, G.; Mella, M.; Martoni, A.; Paio, A.; Quadrelli, P.; Seneci, P. Eur. J. Org. Chem. 2002, 1175−1183. (36) Grundmann, C.; Frommeld, H.-D. J. Org. Chem. 1965, 30, 2077− 2078. (37) Chainikova, E. M.; Safiullin, R. L. Russ. Chem. Bull. Int. Ed. 2009, 58, 926−928.

group with the ortho position of benzene ring. These are two different transformations, and it is not surprising that the nature of a solvent influences their rates in different ways (Table 1). The deoxygenation of nitrosobenzenes by compounds of trivalent phosphorus in the presence of oxygen is an alternative way of arylnitroso oxides generation under thermal conditions.37 In the case of a slow addition of Ph3P to O2-saturated 4-methoxynitrosobenzene solution (1.1 × 10−3 M) in small portions (∼5 × 10−6 M) (this allows to minimize the interaction of trans nitroso oxide with phosphine), nitrile oxide 4 also becomes the main product of the reaction (Table 2, run 4). At the same time, a small amount of the corresponding nitrile is formed.

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CONCLUSION We present the transformation scheme of nitroso oxides in dark conditions (Scheme 3). Such conditions occur during flash photolysis of aryl azides or deoxygenation of aromatic nitroso compounds by phosphorus(III) derivatives in the presence of oxygen. The reaction of triplet nitrene with oxygen results in cis and trans conformations of nitroso oxide. Decay mechanisms of these species are different. The monomolecular decay reaction of the trans form of nitroso oxide is isomerization to the cis form. The cis form undergoes rearrangement which leads to cleavage of the aromatic ring to give the conjugated diene with nitrile oxide and aldehyde groups at the ends of the molecule. Nitro- and nitrosobenzenes observed under stationary photolysis of aryl azides in the presence of oxygen are most likely generated by photolytic transformations of nitroso oxides.

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ASSOCIATED CONTENT

* Supporting Information S

The UV spectrum of the acetonitrile solution of nitrile oxide 4 and its FTIR spectrum on KBr glass. The 1H and 13C NMR spectra of 4 in CD3CN. The mass spectrum of negative and positive ions of 4 obtained by chemical ionization at atmospheric pressure. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 7 (347) 235 6066. Tel.: 7 (347) 235 5496. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Russian Academy of Sciences (Department of Chemistry and Material Sciences program “Chemical Reaction Intermediates: Their Detection, Stabilization, and Determination of Structural Parameters”).

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REFERENCES

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dx.doi.org/10.1021/jp301699p | J. Phys. Chem. A 2012, 116, 8142−8147