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.