Hydrazination of pyridazines and phthalazines

It is shown by ¹⁵N-labelling techniques that hydrazination of pyridazines partly occurs according to a reaction pathway, involving addition of the nucleophile (hydrazine), ring opening and ring closure (S_N(ANRORC)-mechanism). It is also proved that phthalazines undergo hydrazination without ring opening (S_N(AE)-mechanism).