Biomimetic oxidation studies. 11. Alkane functionalization in aqueous solution utilizing in situ formed [Fe2O(eta(1)-H2O)(eta(1)-OAc)(TPA)(2)](3+), as an MMO model precatalyst, embedded in surface-derivatized silica and contained in micelles

The biomimetic, methane monooxygenase enzyme (MMO) precatalyst, [Fe2O(eta(1 )-H2O)(eta(1)-OAc)(TPA)(2)](3+) (TPA = tris [(2-pyridyl)methyl]amine), 1, f ormed in situ at pH 4.2 from [Fe2O(mu-OAc)(TPA)(2)](3+), 2, was embedded in an amorphous silicate surface modified by a combination of hydrophilic pol y(ethylene oxide) and hydrophobic poly(propylene oxide). The resulting cata lytic assembly was found to be a biomimetic model for the MMO active site w ithin a hydrophobic macroenvironment, allowing alkane functionalization wit h tert-butyl hydroperoxide (TBHP)/O-2 in an aqueous reaction medium (pH 4.2 ). For example, cyclohexane was oxidized to a mixture of cyclohexanone, cyc lohexanol, and cyclohexyl-tert-butyl peroxide, in a ratio of similar to 3:1 :2. The balance between poly (ethylene oxide) and poly(propylene oxide), te thered on the silica surface, was crucial for maximizing the catalytic acti vity. The silica-based catalytic assembly showed reactivity somewhat higher in comparison to an aqueous micelle system utilizing the surfactant, cetyl trimethylammonium hydrogen sulfate at its critical micelle concentration, i n which functionalization of cyclohexane with TBHP/O-2 in the presence of 1 was also studied at pH 4.2 and was found to provide similar products: cycl ohexanol, cyclohexanone, and cyclohexyl-tert-butyl peroxide, in a ratio of similar to 2:3:1. Moreover, the mechanism for both the silica-based catalyt ic assembly and the aqueous micelle system was found to occur via the Haber -Weiss process, in which redox chemistry between 1 and TBHP provides both t he t-BuO. and t-BuOO. radicals. The t-BuO. radical initiates the C-H functi onalization reaction to form the carbon radical, followed by O-2 trapping, to provide cyclohexyl hydroperoxide, which produces the cyclohexanol and cy clohexanone in the presence of 1, whereas the coupling product emanates fro m t-BuOO. and cyclohexyl radicals. A discussion concerning both approaches for alkane functionalization in water will be presented.