Co(II) mediated cascade cyclization of unactivated olefin to afford N-alkylacetamided tetrahydropyridazine and dihydropyrazole

Abstract

We demonstrate an efficient and simple protocol to afford structurally varied tetrahydropyridazine and dihydropyrazole scaffolds, via Co(OAc)2. 4H2O salt catalysed cascade cyclization of unactivated alkenes without external coupling partner under nitrogen free condition. In this strategy, N-alkylacetamide group is effortlessly attached to the tetrahydopyridazine skeleton by reacting N-homoallylacetohydrazides and N-allylacetohydrazides with different N-alkylformamide as a solvent cum radical sources through intramolecular 6-endo-trig and 5-endo-trig cyclization respectively. The salient features of this protocol include easy availability of inexpensive cobalt salt as catalyst, one-pot conversion, and good functional group tolerance even in heteroatomic systems to afford in total 31 new compounds in good yields. Numerous control experiments including radical trapping experiments along with Hammett analysis elucidate the mechanistic details of this cascade cyclization reaction.