Alkene Epoxidations Catalyzed by Iron(III), Manganese(III), and Chromium(III) Porphyrins. Effects of Metal and Porphyrin Substituents on Selectivity and Regiochemistry of Epoxidation

Article abstract of DOI:10.1021/ja00201a026

The products of epoxidation of norbornene, cyclohexene, and adamantylideneadamantane with pentafluoroiodobenzene using as catalysts variously substituted tetraphenylporphyrin complexes of chloroiron(III), chloromanganese(III), and chlorochromium(III) have been determined.All catalysts afforded the epoxide from adamantylideneadamantane, suggesting that the metallacycle intermediate, impossible in this case, is not required for epoxidation.The ratios of exo- to endo-epoxynorbornanes obtained from norbornene varied from about 1E3 for electropositively substituted Cr(III) complexes gradually through the Mn(III) and Fe(III) series to a value of about 6 for electronegatively substituted iron(III) porphyrins.Mechanisms ranging from limiting electrophilic addition to limiting electron transfer are proposed to account for these changes.The electronegatively substituted iron porphyrins show a greater tendency toward epoxidation rather than allylic oxidation and show more selectivity for cis-alkenes.