Transformations of 4-R-6-oxohexa-2,4-diene nitrile oxides - Intermediates of photooxidation of para-R-C6H4N3, R = Ph, CH2Ph, OPh

The photooxidation of aromatic azides with the general formula 4-R-C6H4N3 (R = -Ph, -CH2Ph, -OPh) leads to the sequential generation of two reactive intermediates: first, the corresponding aromatic nitroso oxide 4-RC6H4NOO is formed. Its chemical potential is sufficient to destroy the aromatic ring as a result of an intramolecular ortho-cyclization. This reaction gives 4 -aryl -substituted 6-oxohexa-2,4-diene nitrile oxide, which retains a significant part of the chemical potential of its precursor and reacts further via various directions depending on the nature of the substituent R. It was found that in the case of R = -Ph, yet another aromatic system was destroyed as a result of the intramolecular (3 + 2)-cycloaddition of nitrile oxide at the ortho-meta- carbon atoms of the phenyl substituent, which led to the formation of 2Z/E-(5H-naphtho[1,8-cd]isoxazol-5- ylidene)acetaldehyde as a mixture of isomers. In the case of R = -OPh, nitrile oxide was involved in the intermolecular (3 + 2)-cyclization with the C---N triple bond of acetonitrile used in our experiments as solvent. In the case of R = -CH2Ph, both possibilities of (3 + 2)-cyclization were competitively realized, namely, (a) the formation of an isomeric mixture of (2E/Z,4E)-3-benzyl-5-(5-methyl-1,2,4-oxadiazol-3-yl)penta-2,4-dienal, which is the product of the reaction of nitrile oxide with acetonitrile, and (b) accumulation of 2-(E/Z)-4aH-dibenzo[c,d] isoxazol-9(10H)-ylidene)acetaldehyde formed as a result of the intramolecular cyclization of nitrile oxide at the aromatic ring of the substituent R. The presence of the methylene spacer shifts the aromatic C-C bond reacting with the nitrile oxide group: the ipso-ortho-cycloaddition leads to a tricyclic structure with a spiro-carbon atom. The proposed reaction mechanism and the most probable composition of the products were confirmed by DFT calculations in M06L/6-311+G(d,p) + IEFPCM(SD) approximation.