Cyclometalations on the imidazo[1,2-a][1,8]naphthyridine framework

Abstract

Cyclometalation on the substituted imidazo[1,2-a][1,8]naphthyridine platform involves either the C₃-aryl or C₄'-aryl ortho carbon and the imidazo nitrogen N₃'. The higher donor strength of the imidazo nitrogen in comparison to that of the naphthyridine nitrogen aids regioselective orthometalation at the C₃/C₄'-aryl ring with Cp*Ir^III (Cp* = η⁵-pentamethylcyclopentadienyl). A longer reaction time led to double cyclometalations at C₃-aryl and imidazo C₅'-H, creating six- and five-membered metallacycles on a single skeleton. Mixed-metal Ir/Sn compounds are accessed by insertion of SnCl₂ into the Ir–Cl bond. Pd(OAc)₂ afforded an acetate-bridged dinuclear ortho-metalated product involving the C₃-aryl unit. Metalation at the imidazo carbon (C₅') was achieved via an oxidative route in the reaction of the bromo derivative with the Pd(0) precursor Pd₂(dba)₃ (dba = dibenzylideneacetone). Regioselective C–H/Br activation on a rigid and planar imidazonaphthyridine platform is described in this work.