Intramolecular radical additions to pyridines

Intramolecular 6-exo/endo-trig and 5-exo-trig cyclisations of aryl radical intermediates to the alpha-, beta- and gamma-carbons of pyridine have been shown to be facile processes at neutral pH. The tether conjoining the radical donor to the pyridine plays an important role in determining the outcome of the reaction. When a Z-alkene is used as a tether, ortho-cyclisation proceeds in good yield. A more complex course is followed when a saturated two carbon tether is employed, leading to products derived from hydrogen atom abstraction, ipso-cyclisation and ortho-cyclisation pathways. All attempts to effect 5-exo/endo-trig cyclisations failed. Tributyltin hydride, tris(trimethylsilyl) silane, tris(trimethylsilyl) germane and, in part, samarium(II) iodide can each be employed as mediators of the reaction.