Arylhydrazine Trapping of Benzynes: Mechanistic Insights and a Route to Azoarenes

Arylhydrazines
(ArNαHNβH2) are ambident
nucleophiles. We describe here their reactivity
with benzynes generated in situ by thermal cyclization of several
multiynes. Products arising from attack of both the alpha- and beta-nitrogen
atoms are observed. These competitive modes of reaction were explored
by DFT calculations. Substituent effects on the site-selectivity for
several substituted phenylhydrazines were explored. Interestingly,
the hydrazo products from beta-attack (ArNHNHAr′) can be oxidized,
sometimes in situ by oxygen alone, to give structurally complex, unsymmetrical
azoarenes (ArNNAr′). Toluenesulfonohydrazide and benzohydrazide
analogues were each demonstrated to undergo similar transformations,
including oxidation to the corresponding benzyne-trapped azo compounds.