Homolytic and Heterolytic Scission of Organic Hydroperoxides by (meso-Tetraphenylporphinato)iron(III) and Its Relation to Olefin Epoxidation

Article abstract of DOI:10.1021/ja00199a022

Reaction of 10-hydroperoxyoctadeca-8,12-dienoic acid (10-OOH-18:2) with chloro(meso-tetraphenylporphinato)iron(III) (Fe(3+)-TPP) in CH2Cl2 produces two products identified as 10-oxodec-8-enoic acid (10-oxo-10:1) and 10-oxooctadeca-8,12-dienoic acid (10-oxo-18:2).These compounds are derived from an alkoxyl radical intermediate generated by homolytic cleavage of 10-OOH-18:2.Only a trace of the heterolytic cleavage product 10-hydroxyoctadeca-8,12-dienoic acid (10-OH-18:2) is observed.Inclusion of imidazole alters the product profile so that 10-OH-18:2 is the majorproduct (62percent).Reaction of 10-OOH-18:2 or t-BuOOH (0.05 M) with Fe(3+)-TPP (0.001 M) and cis-stilbene (5 M) produces trans-stilbene oxide as the only oxidation product (55percent based on hydroperoxide).Oxidation in the presence of imidazole (0.3 M) produces approximately equal amounts of cis-stilbene oxide (22percent) and trans-stilbene oxide (23percent).Butylated hydroxytoluene inhibits formation of both epoxides from cis-stilbene whereas p-methoxyanisole selectively inhibits formation of cis-stilbene oxide.The data indicate that two separate oxidizing agents are produced in the reaction of hydroperoxides with Fe(3+)-TPP.In the absence of imidazole, Fe(3+)-TPP cleaves 10-OOH-18:2 and t-BuOOH homolytically to alkoxyl radicals and Fe(4+)=O.TPP.Both species oxidize the hydroperoxide to peroxyl radicals that epoxidize cis-stilbene nonstereospecifically.In the presence of imidazole, Fe(3+)-TPP also cleaves the hydroperoxides heterolytically to alcohol and Fe(4+)=O.TPP^.+, which epoxidizes cis-stilbene stereospecifically.The results reconcile apparently conflicting observations in the literature and suggest that the pathway of hydroperoxidecleavage by heme complexes and heme proteins is a sensitive function of the environment of the heme.