Synthetic approaches to phthalazines: C-N bond cleavage in 2'-benzylbenzohydrazides during reaction with polyphosphoric acid

Abstract

Reaction of 2'-benzylbenzohydrazides with concentrated polyphosphoric acid gave 2,5-diaryl-1,3,4- oxadiazoles and a polybenzyl. Cyclodehydration to phthalazines was not observed. It is inferred from product substitution patterns that CH₂-N bond cleavage occurs with polybenzyls forming from the benzyl moiety and oxadiazoles from the benzohydrazide fragment. Other reported reactions which generate benzyl fragments lead to bibenzyl and are considered to involve trapping of the cleavage products within a solvent cage. The observed production of polybenzyls is therefore interpreted in terms of the formation of free benzyl fragments.

A tetrahydrophthalazine derivative was produced from the addition of diethyl azodicarboxylate to the photodiene of o-methylbenzophenone, but this could not be converted into a phthalazine.