New Synthetic Methods via Free Radicals. Free-Radical Generation via Photolytic Homolysis of Alkyl-Cobaloxime C-Co Bonds. Efficient Radical Trapping with Useful Functional Groups

Visible-light photolyses of primary and secondary (pyridine)alkylcobalt(III) cobaloximes, R-Co^III(dmgH)₂py (2-7), generate free radicals that can be trapped in good to excellent yield (75% to nearly quantitative) with radical-trapping agents PhSSPh (to produce R-SPh), PhSeSePh (to produce R-SePh), or BrCCl₃(to produce R-Br). Studies with CH₂═CH(CH₂)₄-Co^III(dmgH)₂py (6) demonstrate that a 5-hexenyl intramolecular radical-olefin cyclization can intervene between radical generation and radical trapping. At the dilute concentrations required to suppress premature radical trapping prior to 5-hexenyl cyclization, competing side reactions limit the yields of cyclized, trapped product to about 60%. Studies of n-decyl-Co^III(dmgH)₂py (2) and n-pentyl-Co^III(dmgH)₂py (9) photolyses in the presence of PhSSPh at various concentrations demonstrate that β-hydride elimination (4%) was the major side reaction at high concentrations (18-22 mM) and that at low concentrations (1 mM) β-hydride elimination (17-31%) and as yet uncharacterized radical-cobaloxime ligand reactions (14-26%) were the major side reactions.