A Revised Mechanism of Thermal Decay of Arylnitroso Oxides

Abstract

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.

The chemistry of nitroso oxides

“…This is due to the fact that the DFT methods are known to overestimate the stability of the singlet diradical states. Indeed, the study of the kinetics and products of transformations of arylnitroso oxides leads to the conclusion that the rate-limiting step of the irreversible consumption of trans-ArNOO is the conformational transition to the cis-form, activation energy of which is 64-74 kJ mol −1 depending on the structure nitroso oxide and the nature of the used solvent 72,73 . A low rate of a conformational transition explains the experimental fact, according to which cisand trans-ArNOO behave as kinetically independent species with substantially different chemical nature and reactivity (see below).…”

“…It was interesting to find out what happens to the nitroso oxides when they are consumed in the dark. Most recently, this was done by means of a very simple experiment 73 . The solution of 4-methoxyphenyl azide (53) (5 × 10 −4 mol L −1 ) in oxygen-saturated acetonitrile was irradiated by one light pulse duration of 50 μs.…”

“…The solution of 4-methoxyphenyl azide (53) (5 × 10 −4 mol L −1 ) in oxygen-saturated acetonitrile was irradiated by one light pulse duration of 50 μs. The lifetime of 4-methoxyphenylnitroso oxide (54) was determined to be more than 0.1 s 73,98 , which is three orders of magnitude longer than the pulse time. Thus, under these conditions, nitroso oxide 54 is formed during the pulse time and is then consumed in the dark.…”

“…Therefore, the reaction of the consumption of 54b is isomerization to 54a. Thus, the complete sequence of the transformations, which takes place during the photooxidation of azide 53, is shown in Scheme 15 73 .…”

“…Indeed, the reaction of 4-methoxynitrosobenzene with triphenylphosphine in oxygen-saturated acetonitrile also leads to the formation of the nitrile oxide 56 73 .…”