title: Olefin cis-dihydroxylation with bio-inspired iron catalysts. evidence for an Fe(II)/Fe(IV) catalytic cycle.
abstract: Iron(II) complexes of a series of N-acylated dipyridin-2-ylmethylamine ligands (R-DPAH) have been investigated as catalysts for the cis-dihydroxylation of olefins to model the action of Rieske dioxygenases that catalyze arene cis-dihydroxylation. The Rieske dioxygenases have a mononuclear iron active site coordinated to a 2-histidine-1-carboxylate facial triad motif. The R-DPAH ligands are designed to provide a facial N,N,O-ligand set that mimics the enzyme active site. The iron(II) complexes of the R-DPAH ligands activate H(2)O(2) to effect the oxidation of olefin substrates into cis-diol products. As much as 90% of the H(2)O(2) oxidant is converted into cis-diol, but a large excess of olefin is required to achieve the high conversion efficiency. Reactivity and mechanistic comparisons with the previously characterized Fe(TPA)/H(2)O(2) catalyst/oxidant combination (TPA = tris(pyridin-2-ylmethyl)amine) lead us to postulate an Fe(II)/Fe(IV) redox cycle for the Fe(R-DPAH) catalysts in which an Fe(IV)(OH)(2) oxidant carries out the cis-hydroxylation of olefins. This hypothesis is supported by three sets of observations: (a) the absence of a lag phase in the conversion of the H(2)O(2) oxidant into a cis-diol product, thereby excluding the prior oxidation of the Fe(II) catalyst to an Fe(III) derivative as established for the Fe(TPA) catalyst; (b) the incorporation of H(2)(18)O into the cis-diol product, thereby requiring O-O bond cleavage to occur prior to cis-diol formation; and (c) the formation of cis-diol as the major product of cyclohexene oxidation, rather than the epoxide or allylic alcohol products more commonly observed in metal-catalyzed oxidations of cyclohexene, implicating an oxidant less prone to oxo transfer or H-atom abstraction.
authors.affiliation: Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota 55455, United States.
authors.forename: Paul D
authors.initials: PD
authors.lastname: Oldenburg
authors.forename: Yan
authors.initials: Y
authors.lastname: Feng
authors.forename: Iweta
authors.initials: I
authors.lastname: Pryjomska-Ray
authors.forename: Daniel
authors.initials: D
authors.lastname: Ness
authors.forename: Lawrence
authors.initials: L
authors.lastname: Que
mesh_terms: D000475:Alkenes / Q000737:chemistry*; D002384:Catalysis; D020134:Catalytic Domain; D006900:Hydroxylation; D007501:Iron / Q000737:chemistry*; D008024:Ligands; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction
chemical_list: D000475:Alkenes; D008024:Ligands; D009942:Organometallic Compounds; D007501:Iron