Reactivity of Arylnitroso Oxide Isomers with Benzoquinones.

The kinetics of the reaction of phenylnitroso oxide, para-CH3-, para-CH3O-, and para-Br-phenylnitroso oxides with benzoquinones in acetonitrile at room temperature was studied by flash photolysis. Both nitroso oxide isomers were involved in the reaction, the rate constants for the cis forms being higher than for the trans isomers for all compounds under study. The reactivity of arylnitroso oxides (ArNOO) with benzoquinones was found to be dual. The electron-donating substituents in the ArNOO molecule increase the reactivity of both conformers with 1,4-benzoquinone, 2-chloro-1,4-benzoquinone, and chloranil. Arylnitroso oxides exhibited electrophilic properties in the reaction with duroquinone, which was determined from the positive slope of the Hammett dependence for the trans isomers of ArNOO (ρ = 0.68 ± 0.08, r = 0.994). The constants ρ for the reaction of the cis isomers with 1,4-benzoquinone (–1.5 ± 0.2, r = 0.990), 2-chloro-1,4-benzoquinone (–2.0 ± 0.2, r = 0.995), and chloranil (–4.8 ± 0.6, r = 0.98) and for the reaction of trans isomers with chloranil (–2.8 ± 0.3, r = 0.990) increased in magnitude with the electron deficiency of the double bond. Along with the negative slope of the Hammett dependence, they suggest a nucleophilic attack of nitroso oxide on benzoquinone. For the reaction of trans-nitroso oxides with 2-chloro-1,4-benzoquinone and 1,4-benzoquinone, the Hammett dependence is nonlinear. [ABSTRACT FROM AUTHOR]