Intermediates in the Epoxidation of Alkenes by Cytochrome P-450 Models. 5. Epoxidation of Alkenes Catalyzed by a Sterically Hindered (meso-Tetrakis(2,6-dibromophenyl)porphinato)iron(III) Chloride

Article abstract of DOI:10.1021/ja00199a006

Sterically hindered (meso-tetrakis(2,6-dibromophenyl)porphinato)iron(III) chloride ((Br8TPP)Fe(III)(Cl) was prepared and used as a catalyst for epoxidation of nine structurally different alkenes. (Br8TPP)Fe(III)(Cl) is an extremely efficient catalyst for epoxidation, and nearly quantitative yields of epoxides were obtained in all cases.Computer graphics docking experiments show that alkenes can approach the iron-oxo moiety of the iron(IV)-oxo porphyrin ? cation radical ((^+.Br8TPP)Fe(IV)(O)(X)) from above the oxygen and distal to the iron, but with various angles ofapproach.With the exception of terminal alkenes, the bulky o-bromo substituents prevent interaction of alkenes with the iron or pyrrole nitrogens of the porphyrin ring.With alkyl-substituted terminal alkenes various degrees of porphyrin N-alkylation accompany epoxidation.Selectivity for N-alkylation parallels the selectivity observed with microsomal cytochrome P-450.Arguments are presented that support either a concerted or a radical mechanism for N-alkylation by terminal alkenes.Formation of a metallaoxetane intermediate in the epoxidation reaction via a 2a + 2s cycloaddition is discounted on the basis of (i) docking experiments, which show that all but terminal alkenes are unable to approach the iron(IV) moiety, and (ii) examination of X-ray-based computer graphics constructed metallaoxetane structures formed from (^+.Br8TPP)Fe(IV)(O) + cis-stilbene or 2,3-dimethyl-2-butene regardless of whether the porphyrin ring is flat, ruffled, or saucer shaped.Evidence for rate-determining formation of the carbon radical (Br8TPP)Fe(IV)-O-C-C^., carbocation (Br8TPP)Fe(III)-O-C-C(1+), and the solvent-caged intimate pair/(Br8TPP)Fe(IV)(O)***alkene^.+/ in alkene epoxidation is considered.A mechanism that best correlates the results of epoxidation studies for both (^+.Br8TPP)Fe(IV)(O)(X) and (Br8TPP)Cr(V)(O)(X) involves a rate-determining formation of a charge-transfer complex by partial electron transfer from the alkene to the hypervalent porphyrin metal-oxo species.