Mechanism of Hemin-Catalyzed Alkene Epoxidation. The Effect of Catalyst on the Regiochemistry of Epoxidation
Traylor, T.G., Nakano, T., Danlap, B.E., Traylor, P.S., and David Dolphin
J. Am. Chem. Soc.
Epoxidation of norbornene by pentafluoroiodosylbenzene or iodosylxylene under catalysis by iron(III) tetraarylporphyrin chlorides affords substantial amts. of endo-2,3-epoxynorbornane in addition to the usual exo isomer. The fraction of the endo isomer increases with increasing electronegative substitution on the aryl groups of the catalyst, reaching as high as one endo to three exo products. Since no other addn. to norbornene gives substantial endo product, whereas abstraction by norbornyl radical does, it is concluded that this epoxidation proceeds through initial electron transfer from norbornene to produce a radical cation. Subsequent collapse of the radical cation with the geminate porphyrin iron(IV) oxide affords a carbocation which either closes to an epoxide or undergoes rearrangement.