Interplay of Conformational and Chemical Transformations of Ortho-Substituted Aromatic Nitroso Oxides: Experimental and Theoretical Study

Abstract

The mechanism of the photooxidation of aromatic azides containing a substituent at one of the ortho positions (2,4-dimethoxyphenyl azide (1a) and 2-methyl-4-[(2E)-1-methylbut-2-en-1-yl]phenyl azide (1b)) was studied in acetonitrile. The electronic spectra and the kinetic regularities of the consumption of corresponding nitroso oxides, which are the reaction intermediates, were investigated by flash photolysis. Owing to the one-and-a-half order of the C-N and N-O bonds and asymmetric molecule structure these nitroso oxides exist as four conformers (cis/syn, cis/anti, trans/syn, and trans/anti). The conformers differ in the spectral properties and in the reactivity in various irreversible transformations. The only product, (2Z,4E)-4-methoxy-6-oxohepta-2,4-dienenitrile oxide (7a), was observed during photooxidation of 1a, whereas transformations of the nitroso oxide isomers derived from 1b led to a set of stable products: the cis/anti isomer was transformed into (3,4,7-trimethyl-3a,4-dihydro-2,1-benzisoxazol-5(3H)-ylidene)ethanal (10), the trans isomers recombined forming the corresponding nitro and nitroso compounds, and the most reactive cis/syn isomer was transformed into ortho-nitrosobenzyl alcohol 11. The last was oxidized slowly to the corresponding benzaldehyde 12. Interaction of 11 and 12 led to the formation of (Z)-1,2-bis(2-formyl-4-((2E)-1-methylbut-2-en-1-yl)phenyl)diazene-1-oxide (13). The DFT simulation and kinetic modeling of the nitroso oxide transformations as well as the product analysis allowed revealing the fine details of the mechanism of decay for these species.

Ortho-Cyclization in Asymmetrically Substituted Arylnitroso Oxides

“…Second, it was found that the presence of a substituent at the ortho position facilitates the ortho-cyclization. Thus, under the photooxidation of 2,4-dimethoxyphenyl azide, a nitrile oxide, which was formed as a result of the reaction of the nitroso oxide group with the substituted ortho -carbon atom, was obtained as the sole product . The effect of an ortho substituent was studied in detail using density functional theory calculations .…”

“…Nitrile oxides of common structure 4 are stable enough, and they were isolated and identified during the photooxidation of 4-methoxyphenyl azide and 2,4-dimethoxyphenyl azide . In other cases, a number of carbocyclic and heterocyclic compounds ,− were obtained by the photooxidation of aromatic azides.…”

“…So, 2,4-dimethoxyphenylnitroso oxide has an asymmetrical structure that leads to two possible ways of the ortho-cyclization with the participation of the cis/syn and cis/anti form of this species (Scheme ). It turned out that the reaction proceeds regiospecifically toward the ortho substituent to form nitrile oxide with an ester group …”

“…Nitrile oxides of common structure 4 are stable enough, and they were isolated and identified during the photooxidation of 4-methoxyphenyl azide 4 and 2,4-dimethoxyphenyl azide. 7 In other cases, a number of carbocyclic 8 and heterocyclic compounds 4,9−11 were obtained by the photooxidation of aromatic azides. Their formation was due to further intramolecular transformation of nitrile oxides with participation of the CNO group and a suitable reactive center.…”

“…It turned out that the reaction proceeds regiospecifically toward the ortho substituent to form nitrile oxide with an ester group. 7 On the other hand, the products of the transformation both the cis/syn and cis/anti nitroso oxides were found under the photochemical oxidation of 2-methyl-4-[(2E)-1-methylbut-2en-1-yl]phenyl azide. 7 In this case, the cis/syn form of ArNOO reacted via the oxidation of the methyl substituent (Scheme 3).…”