Role of neighbouring imidazole groups in the solvolysis of hydrazonyl chlorides

Abstract

Cyclization of the N-imidazolidin-2-ylidenehydrazonyl chlorides (7; R = H) to the 6,7-dihydro-3-aryl-5H-imidazo[2,1-c]-s-triazoles (12) in 4 : 1 dioxan–water at 25° proceeds via rate-determining ionization (azocarbonium ion formation) over the pH range 3–11. The neighbouring group participates after the rate-determining step. The dimethylated model hydrazonyl chlorides (7; R = Me) which cannot cyclize to (12) show similar kinetic behaviour; in this case water reacts with the azocarbonium ion intermediate to give hydrazides (11; R = Me). Evidence is also presented that (7; R = H) exists as the 2,3-diazabuta-1,3-diene tautomer (7) rather than as the hydrazone (4).