Comprehensive Study of the Mechanism of the "Aromatic Nitroso Oxide-Olefin" Interaction as a Function of the Reagents' Structure.

Using reaction model systems (nitroso oxide ArNOO, Ar = Me2NC6H4 or O2NC6H4; exhaustive set of methyl- and cyano-substituted ethylenes), a detailed study of the reaction mechanism of ArNOO with unsaturated compounds was carried out using the density functional theory (M06L/6311 + G(d,p)). The reaction is preceded by the formation of a reagent complex of stacking type, which is favorable for further transformation. Depending on the structure of alkene, the reaction may proceed via two extreme mechanisms: synchronous (3 + 2)-cycloaddition (the most typical case) or one-center nucleophilic attack of the terminal oxygen atom of ArNOO on the less substituted carbon atom of the double bond. The last direction becomes dominant only under special reaction conditions: ArNOO with a strong electron-donating substituent in the aromatic ring, an unsaturated compound with a significantly depleted electron density on C═C bonds, and a polar solvent. In other cases, a different degree of asynchrony in the (3 + 2)-cycloaddition is possible; however, the main intermediate preceding stable reaction products is 4,5-substituted 3-aryl-1,2,3 dioxazolidine in any event. Both thermodynamic and kinetic arguments suggest the most probable decomposition of dioxazolidine into a nitrone and a carbonyl compound. It has been shown for the first time that the polarization of the C═C bond is a powerful factor regulating the reactivity in the reaction under study. The results of the theoretical study show excellent agreement with known experimental data for a wide variety of reacting systems.

Global Kinetics and Spectral Modeling of p-Methoxyphenyl Azide Photooxidation.

The kinetics of photooxidation of para-methoxyphenyl azide 1 was studied by flash photolysis with spectrophotometric detection of the absorption of active intermediates in an aerated acetonitrile solution at 295 K. The holistic set of experimental data including the consumption of cis-trans isomers of para-methoxyphenyl nitroso oxide 2 and the accumulation of photooxidation products (2Z,4E)-4-methoxy-6-oxo-hexa-2,4-diene-nitrile oxide 3 and bis-p-methoxy-azobenzene 4 monitored via the changes in the optical density of the solution in the wavelength range of 300-500 nm was treated to obtain the most complete information about the system under study. Flash photolysis of 1 results in the formation of the corresponding triplet nitrene, which either recombines to azobenzene 4 with a rate constant 2k4 = (8.7 ± 1.0) × 108 L/(mol s), or adds molecular oxygen to produce cis-trans isomers of nitroso oxide 2 with a total rate constant k5 = (1.0 ± 0.1) × 106 L/(mol s). The latter reaction is the main channel for nitrene consumption: the yield of 2 per consumed nitrene was found to be 90% higher at the cis/trans isomers ratio of 0.94 ± 0.04. The trans-isomer of 2 is chemically inert under the experimental conditions and is consumed in the trans-cis conformational transformation with the rate constant k6 = 0.45 c-1. The cis-isomer is thermodynamically more stable, k-6 = 0.16 c-1, but it is consumed irreversibly in the ortho-cyclization reaction with the rate constant k7 = 12.5 c-1. The final product, nitrile oxide 3, has intense absorption in the near UV region, λmax = 300 nm and εmax = 2 × 104 L/(mol·cm). This made it possible to determine the spectral characteristics of 2 isomers using nonlinear regression analysis. It was found λmax = 421 nm, εmax = (1.23 ± 0.07) × 104 L/(mol·cm), and δ = 1640 ± 10 cm-1 (half width at half maximum) for cis-2 and λmax = 462 nm, εmax = (1.57 ± 0.03) × 104 L/(mol·cm), and δ = 2150 ± 10 cm-1 for trans-2. The obtained spectral parameters are in good agreement with the results of optical spectra modeling for 2 isomers performed in the CIS(D,Full)/aug-cc-PVTZ//CASSCF(14,13)/ma-def2-TZVPP approximation: the calc-exp deviation for the absorption maximum was 0.05 eV (cis) and 0.12 eV (trans), the oscillator strengths were calculated to be 0.59 and 0.65, respectively.

The substituent effects on the [3+2] cycloaddition of nitrile oxides generated by photooxidation of arylazides to acetonitrile.

The DFT approach in M06L/6-311 + G(d,p) approximation was used to study the transformation of unsaturated nitrile oxides (RCNO), which were generated by photooxidation of the corresponding aromatic azide, to oxadiazoles via [3 + 2]cyclization with acetonitrile. It was found that the cycloaddition activation enthalpy was within 60-93 kJ/mol, depending on the structure of the nitrile oxide. A significant mesomeric effect of the substituent and its position in the conjugated molecular system on the activation barrier of the reaction studied was identified. The relationship between the thermodynamic characteristics of [3 + 2] cycloaddition and electron density distribution in RCNO was demonstrated by a representative set of compounds including mono- and disubstituted nitrile oxides.