Nitroso Oxides Research Articles

Para-Substituent Effect on the Decay Kinetics of the Isomeric Forms of Aromatic Nitroso Oxides

para-Substituent Effect on the Decay Kinetics of the Isomeric Forms of Aromatic Nitroso Oxides

Ortho-Cyclization in Asymmetrically Substituted Arylnitroso Oxides.

The mechanism of the photooxidation of a number of asymmetrically substituted phenyl azides in acetonitrile was studied. The key intermediates of this reaction are the corresponding nitroso oxides, the unimolecular consumption of which occurs via the cis form when the terminal oxygen atom of the NOO moiety reacts with the ortho position of the aromatic ring. As a result, it is opened to form a nitrile oxide. In the case of 3-methylphenyl azide, the reaction proceeds via the cis/syn form of nitroso oxide with a regioselectivity of 91%. The methoxy substituent at the para position changes the direction of the ortho-cyclization so that it occurs via the cis/anti form of nitroso oxide independently on the nature of a meta substituent. Nitrile oxides, which are formed as a result of these transformations of nitroso oxides, are stabilized by [3 + 2] cycloaddition with acetonitrile to give 1,2,4-oxadiazoles. The observed regioselectivity of the ortho-cyclization of nitroso oxides was explained using theoretical methods. Its cause consists in the extra-stabilization of the transition state of the reaction of the cis/anti form due to a stereoelectronic effect of the para-methoxy substituent.

Structure‐activity relationship in the case of intramolecular ortho‐cyclization of aromatic nitroso oxides: Inverted steric effect of substituent in the 2‐R‐C6H4NOO transformation

AbstractA systematic theoretical study at the M06L/6‐311+G(d, p) level of theory was carried out to calculate the activation barriers ΔH≠ for the intramolecular ortho‐cyclization of aromatic nitroso oxides 2‐R‐C6H4NOO and to reveal the effect of substituent nature and position in the benzene ring on the nitroso oxides reactivity. A set of 24 substituents with widely differing spatial and electronic properties (inductive, resonant, steric effects of R) was studied. The para‐substituent was shown to have little effect on the ΔH≠ value. The full set of effects of the R substituent contributes to the reactivity of ArNOO for 3‐substituted aromatic nitroso oxides. In the case of 5‐substituted ArNOO the Hammett‐type relationship was obtain to describe inductive and resonant effects of R on the ortho‐cyclization reactivity. The ortho‐cyclization for 2‐substituted nitroso oxides is a nontrivial example of the existence of an “inverted” steric effect, when an increase in substituent size accelerates intramolecular transformation. The substituent in position 6 also exhibits an “inverted” steric effect, but it is noticeably weaker than that for 2‐R‐C6H4NOO.

Kinetics and Mechanism of the Reactions of Aromatic Nitroso Oxides with Tetracyanoethylene

The kinetics of reaction of phenylnitroso oxide and a number of its para-substituted analogues (R = Me2N, MeO, Me, and Br) with tetracyanoethylene in acetonitrile at room temperature was studied using flash photolysis. It was established that nitroso oxides exhibited nucleophilic properties in this reaction, and the rate constants for the trans forms of these species in the above series changed by seven orders of magnitude. In the presence of a strong electron-donating substituent (Me2N or MeO), the cis form, which is usually inert toward organic substrates, also reacted with the olefin along with the trans form. The reaction mechanism was studied using the density functional theory.

Numerical Modeling of Intramolecular Transformations of ortho, meta-Substituted Aromatic Nitroso Oxides

This work is focused on study of ortho, meta-substituted aromatic nitroso oxides. Nitroso oxides have unique properties due to the cis-trans isomerism phenomenon and one- and-a-half N-O bond order in the nitroso oxide fragment. This paper considers the decay reaction of aromatic nitroso oxides. Considering the complexity of aromatic nitroso oxides, conformational transformations development of a new comprehensive approach using experimental and theoretical tools alongside with mathematical methods and informational technologies is particularly relevant. The main objective of the paper is to develop an algorithm for numerical modeling of aromatic ortho, meta-substituted nitroso oxides decay process. We suggested mathematical models for decay reactions of 2,4-dimethoxyphenyl nitroso oxide and 3-trifluoromethyl-4-methoxyphenyl nitroso oxide. The kinetic modeling allowed us not only to get an agreement between quantum-chemical and experimental data, but also to estimate previously unknown rate constants including the constants for the recombination stages.

On the mechanism for the photooxidation of aromatic azides containing a secondary N–H bond: A sequence of intramolecular transformations with the formation of heterocyclic oximes

During the photooxidation of aromatic azides containing a secondary N–H bond at the para-position, a sequence of intramolecular transformations of nitroso oxides led to the formation of heterocyclic oximes along with the corresponding nitroso and nitro compounds.

Interplay of Conformational and Chemical Transformations of Ortho-Substituted Aromatic Nitroso Oxides: Experimental and Theoretical Study.

The mechanism of the photooxidation of aromatic azides containing a substituent at one of the ortho positions (2,4-dimethoxyphenyl azide (1a) and 2-methyl-4-[(2E)-1-methylbut-2-en-1-yl]phenyl azide (1b)) was studied in acetonitrile. The electronic spectra and the kinetic regularities of the consumption of corresponding nitroso oxides, which are the reaction intermediates, were investigated by flash photolysis. Owing to the one-and-a-half order of the C-N and N-O bonds and asymmetric molecule structure these nitroso oxides exist as four conformers (cis/syn, cis/anti, trans/syn, and trans/anti). The conformers differ in the spectral properties and in the reactivity in various irreversible transformations. The only product, (2Z,4E)-4-methoxy-6-oxohepta-2,4-dienenitrile oxide (7a), was observed during photooxidation of 1a, whereas transformations of the nitroso oxide isomers derived from 1b led to a set of stable products: the cis/anti isomer was transformed into (3,4,7-trimethyl-3a,4-dihydro-2,1-benzisoxazol-5(3H)-ylidene)ethanal (10), the trans isomers recombined forming the corresponding nitro and nitroso compounds, and the most reactive cis/syn isomer was transformed into ortho-nitrosobenzyl alcohol 11. The last was oxidized slowly to the corresponding benzaldehyde 12. Interaction of 11 and 12 led to the formation of (Z)-1,2-bis(2-formyl-4-((2E)-1-methylbut-2-en-1-yl)phenyl)diazene-1-oxide (13). The DFT simulation and kinetic modeling of the nitroso oxide transformations as well as the product analysis allowed revealing the fine details of the mechanism of decay for these species.

Conformational Transformations in Aromatic Nitroso Oxides.

A systematic theoretical study on conformational transformations of monosubstituted (ortho- and para-) aromatic nitroso oxides R-C6H4NOO was performed. The existence of two rotation axes enables two types of conformational transitions in substituted arylnitroso oxides: trans/cis (rotation around the N-O bond) and syn/anti (rotation around the C-N bond, which is important in ortho isomers). The complete set of conformers was localized for R-C6H4NOO using four selected density functional (M06-L, mPWPW91, OLYP, and HCTH) and augmented polarization basis set of triple splitting. It was found that the activation enthalpy of the trans-cis conformational transition is nearly insensitive to the nature of R and ranges within 58-60 kJ/mol for para isomers. The ortho substituent has an insignificant effect on ΔH(≠)trans→cis: it increases this value by ∼5 kJ/mol in syn isomers and decreases it by ∼3 kJ/mol in anti isomers. On the contrary, the syn-anti conformational barrier is considerably affected by the substituent R; an increase in the electron-withdrawing properties of R decreases ΔH(≠)syn→anti. The activation enthalpies grow with increasing polarity of the solvent, as it was found using IEFPCM calculation. The values of relaxation time for all conformational equilibria were calculated and compared with known lifetimes of aromatic nitroso oxides. Our results suggest that syn/anti transitions occur fast enough in the scale of the experimental lifetime. However, trans/cis transformations proceed more slowly. And under certain conditions discussed in the paper, the rate of this conformational transition limits that of irreversible decay of nitroso oxide.

Kinetics and mechanism of the nitrosobenzene deoxygenation by trivalent phosphorous compounds

The reaction of aryl nitroso compounds with organic phosphines and phosphites in aerated media is a convenient non-photolytic procedure to generate aromatic nitroso oxides. The reaction rate constants and activation parameters of the key (for the proposed method of nitroso oxide generation) reaction of nitrosobenzene with tripenyl phosphite or para-substituted phosphines (4-RC6H4)3P (R = MeO, Me, H, F), as well as that of para-methoxynitrosobenzene with triphenylphosphine in acetonitrile were determined by kinetic spectrophotometry and chemiluminescence. A significant transfer of the electron density to the nitroso compound occurs in the transition state of the reaction as was revealed using the Hammett correlation analysis and DFT calculations in the M06L/6-311+G(d,p) approximation. The introduction of the electron-donor substituent MeO into the para-position of PhNO decreases the reactivity of the nitroso compound by two orders of magnitude. The reactivity of triphenyl phosphite in the oxygen atom transfer reaction is lower by two orders of magnitude compared to that of triphenylphosphine. In the case of the reactions of PhNO with phosphines, the apparent rate constant depends on the oxygen content in the reaction medium.

A Revised Mechanism of Thermal Decay of Arylnitroso Oxides

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 nitro- and 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.

A new intramolecular transformation of aromatic nitroso oxides

The possibility of intramolecular interaction of a nitroso oxide group with an aromatic ring is investigated at the UB3LYP/6-311+G(d,p) and G3MP2B3 levels of theory for a wide series of aromatic nitroso oxides. It is found that this reaction leads to the formation of a dioxazole cycle, its subsequent decay resulting in opening of the benzene ring and formation of nitriloxide and carbonyl functional groups. The activation enthalpy of the transformation of phenylnitroso oxide is 75.1 kJ/mol. It is shown that various sub-stituents at ortho-position (with respect to the nitroso oxide fragment) considerably lower the activation barrier of the investigated transformation, particularly in case of o,p-dimethoxyphenylnitroso oxide ΔH≠ = 43.7 kJ/mol. It is concluded that in the case of polyaromatic nitroso oxides, for which intramolecular cyclization is more typical (ΔH≠ ∼ 50 kJ/mol), a factor favoring the attack on the ortho-carbon atom is the stabilization of the product’s diene group due to its inclusion in the polyaromatic system. It is established that sum of these effects leads to a low activation barrier for the transformation of nitroso oxide that forms during the photooxidizing of 2-azido-1-methoxyphenazine, ΔH≠ = 19 kJ/mol. It is proposed that due to the low activation energy of some nitroso oxides, their intramolecular cyclization may be the primary channel of their unimolecular decay.

Redox isomerization of aromatic nitroso oxides

Redox isomerization of aromatic nitroso oxides

The reaction of nitroso oxides with olefins: Concerted or nonconcerted addition?

The mechanism of the interaction of nitroso oxides (RNOO) with olefins was studied at MCQDPT2/6-311+G(3df, 2p)//CASSCF(10; 9)/6-311G(d) level of theory. The following reaction channels were considered: (1) (3 + 2)-cycloaddition and nonconcerted biradical addition of nitroso oxide (2) through the terminal oxygen atom and (3) through the nitrogen atom to the C=C multiple bond. It was shown for the cases of (A) cis/trans-HNOO + C2H4, (B) cis/trans-HNOO + C2F4, (C) cis/trans-PhNOO + C2H4, and (D) cis/trans-PhNOO + C2H3CH3 model systems that the typical reaction of nitroso oxides with alkenes was cycloaddition. For olefins with a decreased electron density at the multiple bond, as in system B, a substantial contribution of the one-center mechanism with the formation of biradical intermediates is possible.

ChemInform Abstract: Structure and Reactivity of Nitroso Oxides.

The presence of nitroso oxide intermediates has been studied in the photooxidation of phenyl azide. The hydroxylation of benzenes occurred and was accelerated by electron-donating groups. Sulfides were oxidized to sulfoxides, but the oxygenation and deoxygenation of sulfoxides proceeded concurrently; these reactions were facilitated by electron-donating substituents. Trapping of the intermediates by added nitrosobenzenes was most efficient, approaching 80% O transfer. The relative reactivity order (styrenes, PhN=O>>Ph 2 S>Ph 2 SO>PhH), substituent effects, and the stereochemistry of hydroxylation of C-H bonds characterized the electrophilic radical nature of nitroso oxides. An 18 O-tracer study revealed that a unimolecular isomerization of nitroso oxides to nitro compounds proceeds competitively with the O transfers; The reactivity of XOO species is discussed on the basis of electronegativity of X

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

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.

Flash photolysis study of the reactivity of isomeric forms of arylnitroso oxides toward triphenyl phosphite

The kinetics of the reactions of phenylnitroso oxide and 4-CH3O-, 4-CH3-, and 4-Br-phenylnitroso oxides with triphenyl phosphite was studied in acetonitrile, benzene, and hexane by flash photolysis. The reaction occurs only with the trans-isomers of nitroso oxides. The value of the reaction rate constant increases with an increase in the electron-withdrawing power of substituents on the nitroso oxide aromatic ring, with the rate constant being almost the same in benzene and acetonitrile, and increasing by a factor of 2–3 in hexane. The temperature dependence for the decay rate constants of isomeric nitroso oxides in the presence of triphenyl phosphite was studied in acetonitrile.

The quantum-chemical modeling of the reaction of nitroso oxides with olefins

The density functional theory method was used to localize minima on the singlet potential energy surfaces of the nitroso oxide—ethylene, phenylnitroso oxide—tetramethylethylene, phenylnitroso oxide—hexene-1, and phenylnitroso oxide-styrene systems and on the triplet potential energy surfaces of the nitroso oxide—ethylene and phenylnitroso oxide-hexene-1 systems. The energetically favorable path was found to be that of (3 + 2) cycloaddition with the formation of five-membered rings, which decomposed into nitrone and carbonyl compound with the simultaneous dissociation of the C-C and C-O bonds. The influence of cis-trans isomerism of nitroso oxides on their reactivity with respect to olefins was studied.

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

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.

Reactions of arylnitroso oxides with substituted styrenes: Kinetics and products

The kinetics of the reactions of phenylnitroso oxide and 4-CH3O- and 4-Cl-phenylnitroso oxides with a series of substituted styrenes (4-X-C6H4-CH=CH2; X = H, CH3O, Cl, CN) in acetonitrile at 22 ± 2°C was studied using the flash photolysis technique. It was shown for 4-CH3O-C6H4NOO as an example that only the trans isomers of the nitroso oxides are involved in the reaction. There is a linear correlation between the logarithm of the rate constant and the electronic properties of the substituent in the nitroso oxide aromatic ring on the Hammett scale: ρ = 2.3 ± 0.3 (r = 0.993) for 4-CH3O-styrene ρ = 2.03 ± 0.07 (r = 0.995) for styrene, and ρ = 1.77 ± 0.05 (r = 0.9996) for 4-Cl-styrene. Both the electron-donating and electron-withdrawing substituents in the aromatic ring of styrene increase its reactivity toward a given nitroso oxide. An analysis of the products of phenyl azide photooxidation in the presence of styrene showed that the product of phenylnitroso oxide [3+2]cycloaddition to the double bond of the olefin decomposes into benzalaniline and carbonyl oxide.

Kinetics of reactions between arylnitroso oxides and methyl vinyl ketone

The kinetics of the reactions of phenylnitroso oxide, (4-methylphenyl)nitroso oxide, (4-methoxyphenyl) nitroso oxide, 4-(N, N -dimethylamino)phenylnitroso oxide, (4-chlorophenyl)nitroso oxide, (4-bromophenyl) nitroso oxide, and (4-nitropenyl)nitroso oxide with methyl vinyl ketone in acetonitrile at 295 K was studied. With the use of 4-CH3O-C6H4-NOO as an example, it was found that only the trans isomers of nitroso oxides entered into the reaction. The rate constants of the reactions of the trans isomers of nitroso oxides with methyl vinyl ketone were measured. A linear correlation between the logarithms of reaction rate constants and the electronic properties of substituents in the aromatic rings of nitroso oxides on the Hammett scale was established: ρ = 1.11 ± 0.08; r = 0.990.