Zn(II)-Stabilized Radical Ligand Enabled Radical-Type C(sp³)-H Activation-Cascade Cyclization to Imidazopyridines

A Zn(II)-stabilized radical-ligand enabled tandem cyclization via radical-type C(sp³)-H functionalization of N-benzylpyridin-2-amines with terminal alkynes producing straightforward access to a wide variety of imidazo[1,2-a]pyridines in moderate to good yields is reported. In the presence of KO^tBu the Zn(II)-catalyst [Zn^IIL^aCl₂] (1 a) (L^a=2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline) undergoes one-electron reduction to the active catalyst [Zn^II(L^a)^•-Cl₂] [1 a]^- bearing a ligand-centered radical. Upon coordination of N-benzylpyridin-2-amine to [1 a]^-, the radical-ligand abstracts a hydrogen atom from the benzylic position, forming a benzylic radical intermediate which, through radical addition with the alkyne generates a vinyl radical intermediate. Subsequent cyclization via intramolecular nucleophilic attack by the pyridine nitrogen produces imidazo[1,2-a]pyridines. Control experiments and spectroscopic investigation confirm the radical-ligand assisted tandem radical-type C(sp³)-H activation, addition, and cyclization steps.