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-6oxo-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 2k 4 = (8.7 ± 1.0) × 10 8 L/(mol s), or adds molecular oxygen to produce cis−trans isomers of nitroso oxide 2 with a total rate constant k 5 = (1.0 ± 0.1) × 10 6 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 k 6 = 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 k 7 = 12.5 c −1 . The final product, nitrile oxide 3, has intense absorption in the near UV region, λ max = 300 nm and ε max = 2 × 10 4 L/(mol•cm). This made it possible to determine the spectral characteristics of 2 isomers using nonlinear regression analysis. It was found λ max = 421 nm, ε max = (1.23 ± 0.07) × 10 4 L/(mol•cm), and δ = 1640 ± 10 cm −1 (half width at half maximum) for cis-2 and λ max = 462 nm, ε max = (1.57 ± 0.03) × 10 4 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.
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 CC 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
CC 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.
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