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.

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The electronic spectra, kinetic regularities, and the mechanism of decay of the cis and trans isomeric forms of 4-[(2E)-1-methylbut-2-en-1-yl]phenylnitroso oxide (2) were studied by flash photolysis and product analysis. The mechanism of the consumption of this nitroso oxide is the same as the one proposed earlier for 4-methoxyphenylnitroso oxide. The trans-2 isomer is converted into cis-2, which undergoes cyclization to the substituted benzo[d][1,2,3]dioxazole 3. The reopening of the dioxazole ring yields nitrile oxide 4. The final product (3,4-dimethyl-3a,4-dihydro-2,1-benzisoxazol-5(3H)-ylidene)acetaldehyde (5) is formed by the intramolecular [3 + 2]-cycloaddition of the nitrile oxide group of 4 to the allylic double bond. To support the proposed mechanism, the quantum chemical calculations have been employed.