Olefin cis-dihydroxylation versus epoxidation by non-heme iron catalysts: Two faces of an Fe^III-OOH coin

The oxygenation of carbon-carbon double bonds by iron enzymes generally results in the formation of epoxides, except in the case of the Rieske dioxygenases, where cis-diols are produced. Herein we report a systematic study of olefin oxidations with H₂0₂ catalyzed by a group of non-heme iron complexes, i.e., [Fe^II(BPMEN)CH₃CN)₂]²⁺ (1, BPMEN = N,N′-dimenthyl-N,N′-bis(2-pyridylmethyl)-1,2-diaminoethane) and [Fe^II(TPA)(CH₃CN)₂]²⁺ (4, TPA = tris(2-pyridlymethyl)amine) and their 6- and 5-methyl-substituted derivatives. We demonstrate that olefin epoxidation and cis-dihydroxylation are different facets of the reactivity of a common Fe^III-OOH intermediate, whose spin state can be modulated by the electronic and steric properties of the ligand environment. Highly stereoselective epoxidation is favored by catalysts with no more than one 6-methyl substituent, which give rise to low-spin Fe^III-OOH species (category A). On the other hand, cis-dihydroxylation is favored by catalysts with more than one 6-methyl substituent, which afford high-spin Fe^III-OOH species (category B). For catalysts in category A, both the epoxide and the cis-diol product incorporate ¹⁸O from H₂¹⁸O, results that implicate a cis-H¹⁸O-Fe^v=O species derived from O-O bond heterolysis of a cis-H₂¹⁸O-Fe^III-OOH intermediate. In contrast, catalysts in category B incorporate both oxygen atoms from H₂¹⁸O₂ into the dominant cis-diol product, via a putative Fe^III-η²-OOH species. Thus, a key feature of the catalysts in this family is the availability of two cis labile sites, required for peroxide activation. The olefin epoxidation and cis-dihydroxylation studies described here not only corroborate the mechanistic scheme derived from our earlier studies on alkane hydroxylation by this same family of catalysts (Chen, K.; Que, L, Jr. J. Am. Chem. Soc. 2001, 123, 6327) but also further enhance its credibility. Taken together, these reactions demonstrate the catalytic versatility of these complexes and provide a rationale for Nature's choice of ligand environments in biocatalysts that carry out olefin oxidations.