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(sp(3))-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 (KOBu)-Bu-t the Zn(II)-catalyst [(ZnLCl2)-L-II-Cl-a] (1 a) (L-a=2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline) undergoes one-electron reduction to the active catalyst [Zn-II(L-a)Cl-center dot-(2)] [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(sp(3))-H activation, addition, and cyclization steps.