Ortho-Cyclization in Asymmetrically Substituted Arylnitroso Oxides

The mechanism of the photooxidation of a number of asymmetrically substituted phenyl azides in acetonitrile was studied. The key intermediates of this reaction are the corresponding nitroso oxides, the unimolecular consumption of which occurs via the cis form when the terminal oxygen atom of the NOO moiety reacts with the ortho position of the aromatic ring. As a result, it is opened to form a nitrile oxide. In the case of 3-methylphenyl azide, the reaction proceeds via the cis/syn form of nitroso oxide with a regioselectivity of 91%. The methoxy substituent at the para position changes the direction of the ortho-cyclization so that it occurs via the cis/anti form of nitroso oxide independently on the nature of a meta substituent. Nitrile oxides, which are formed as a result of these transformations of nitroso oxides, are stabilized by [3 + 2] cycloaddition with acetonitrile to give 1,2,4-oxadiazoles. The observed regioselectivity of the ortho-cyclization of nitroso oxides was explained using theoretical methods. Its cause consists in the extra-stabilization of the transition state of the reaction of the cis/anti form due to a stereoelectronic effect of the para-methoxy substituent.