Non‐Heme Iron Complexes for Stereoselective Oxidation: Tuning of the Selectivity in Dihydroxylation Using Different Solvents

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Abstract
A new class of functional models for non‐heme iron‐based dioxygenases, including [(N3Py‐Me)Fe(CH3CN)2](ClO4)2 and [(N3Py‐Bn)Fe(CH3CN)2](ClO4)2 {N3Py‐Me = [di(2‐pyridyl)methyl]methyl(2‐pyridyl)methylamine; N3Py‐Bn = [di(2‐pyridyl)methyl]benzyl(2‐pyridyl)methylamine}, is presented here. NMR, UV and X‐ray analyses revealed that six‐coordinate low‐spin FeII complexes with the pyridine N‐atoms and the tertiary amine functionality of the ligand bound to Fe are formed. The two remaining coordination sites located cis to each other are occupied by labile CH3CN groups that are easily exchanged by other ligands. We demonstrate that the reactivity and stereoselectivity of the complexes investigated depend on the choice of the solvent. The complexes have been examined as catalysts for the oxidation of both alkanes and olefins in CH3CN. In this solvent alkanes are oxidized to alcohols and ketones and olefins to the corresponding cis‐epoxides and cis‐diols. In acetone as solvent a different reactivity pattern was found, with, as the most striking example, the trans‐dihydroxylation of cis‐olefins. 18O‐labeling studies in CH3CN establish incorporation of 18O from H218O2 and H218O in both the epoxide and the diol implicating an HO‐FeV=18O active intermediate originating from an H218O‐FeIIIOOH species. These results are in full agreement with mechanistic schemes derived for other dioxygenase model systems. Based on labeling studies in acetone an additional oxidation mechanism is proposed for this solvent, in which the solvent acetone is involved. This is the first example of a catalyst that can give cis‐ or trans‐dihydroxylation products, just by changing the solvent. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)