Acylarylnitrosamines. Part VIII. 15N-Labelling experiments and their relevance to the mechanisms of formation of benzyne from benzenediazonium acetate and of the benzenediazonium ion from hydroxyazo-compounds
Experiments using 15N-labelled N-nitrosoacetanilide and related compounds have shown that the complex reaction leading to benzyne and/or phenyl radicals does not involve the reversible extrusion of nitrogen. The contrasting observation that nitrogen exchange did occur in the formation of benzenediazonium ions by the in situ nitrosation of 15N-labelled acetanilide is attributed to the generation of unlabelled benzenediazonium ions by the reaction of phenyl radicals with the nitrosating agent. The reactions of nitrosating reagents such as p-chlorobenzoyl nitrite with 1-phenylazo-2-naphthol and related hydroxyazo-compounds to give benzenediazonium ions have been investigated, and it is suggested that nitrosation of the hydrazone from of the hydroxyazo-compound followed by rearrangement and cleavage provides the primary source of the diazonium ions but that these are again regenerated in the system from phenyl radical sand the nitrosating reagent. In accord with this, azobenzene and 4-phenylazo-1-methoxynaphthalene, which cannot exist as hydrazones do not so react, in contrast to phenylazotetrachlorocyclopentadiene, which does exist in the hydrazone form. Reactions of p-chlorobenzoyl nitrite or phentyl nitrite with 1-phenylazo-2-naphthol in benzene at 50° lead to biphenyl, via phenyl radicals, in 30 and 41% yield (82% based on azo-compound consumed).