On the Mechanism of Oxygenation of Olefins by Tetraphenyl-porphinatoiron(III)-Peroxide Systems(Organic,Chemical)

The mode of oxygenation by weso-tetraphenylporphinatoiron(III)chloride (Fe(III)TPPCl)-peroxide systems was characterized by using cis-stitbene (1) and choiesteryl acetate (3) as substrates. The stereoselectivity of epoxidation of 1 by the Fe(III)TPPCHerr-butyl hydroperoxide ('BuOOH) system depended on the concentration of Fe(III)TPPCl; the reaction was honstereoselective at 0.1 mM Fe(III)TPPCl but stereoselective at 10 m_M Fe(III)TPPCl. In the reaction of 3 with the same system, both allylic oxidation and epoxidation proceeded. However, the epoxidation was β-selective independently of the concentration of Fe(III)TPPCl. Thus, 'BuOOH is assumed to decompose by a radical chain reaction mechanism to give tert-butylperoxyl radical('BuOO・) and tert-butyloxyl radical('BuO・) at 0.1 mw Fe(III)TPPCl. At 10mM Fe(III)TPPCl, on the other hand, the O-O bond in the Fe(III)TPPCl-'BuOOH complex may cleave in both a homolytic manner(quasi-Fenton mechanism) and a heterolytic one(ferryl ion mechanism). The former leads to 'BuO・ as the active oxygen species responsible for the allylic oxidation of 3 and the latter to ferryl ion(Fe-O・) as that responsible for the stereoselective epoxidation of 1 and β-epoxidation of 3. In the reaction with the Fe(III)TPPCl-m-chloroperbenzoic acid(MCPBA) system at -30℃ in the presence of γ-collidine, β-selective epoxidation of 3 proceeded predominantly while 1 was epoxidized stereoselectively. The stereoselectivities of these epoxidations closely resemble those with the Fe(III)TPPCl-iodosylben7.ene system, which is known to give the ferryl ion. These results suggest that the O-O bond in the Fe(III)TPPCl-MCPBA complex cleaves mainly in the heterolytic manner(ferryl ion mechanism).