Precatalyst separation paradigms: alkane functionalization in water utilizing in situ formed [Fe2O(η1-H2O)(η1-OAc)(TPA)2]3+, embedded in surface-derivatized silica, as an MMO model, and fluorous biphasic catalysis for alkane, alkene, and alcohol oxidation chemistry

Abstract

Two precatalyst separation paradigms will be reviewed. The first involves the biomimetic, methane monoxygenase enzyme (MMO) precatalyst, [Fe2O(η1-H2O)(η1-OAc)(TPA)2]3+ (TPA=tris[(2-pyridyl)methyl]amine), 1 , formed in situ at pH 4.2 from [Fe2O(μ-OAc)(TPA)2]3+, 2 , which was embedded in an amorphous silicate surface modified by a combination of hydrophilic polyethylene oxide and hydrophobic polypropylene oxide, for ease of separation from the products formed. The resulting catalytic assembly was found to be a biomimetic model for the MMO active site within a hydrophobic macroenvironment, allowing alkane functionalization with t- butyl hydroperoxide (TBHP)/O2 in an aqueous reaction medium (pH 4.2). For example, cyclohexane was oxidized to a mixture of cyclohexanone, cyclohexanol, and cyclohexyl-t -butyl peroxide, in a ratio of ~3:1:2. The balance between polyethylene oxide and polypropylene oxide, tethered on the silica surface, was crucial for maximizing the catalytic activity. Moreover, the mechanism for the silica-based catalytic assembly was found to occur via the Haber–Weiss process. The second precatalyst separation paradigm, the use of the fluorous solvents, which is predicated on solubilizing the precatalyst in a fluorocarbon solution, allows the functionalization of alkanes and alkenes, while selective oxidation of alcohols to aldehydes was also possible; both precatalyst and product are in separate solvent phases. A discussion concerning both separation of precatalyst from product approaches will be presented.