Mechanistic insights into C–H activation from radical clock chemistry: oxidation of substituted methylcyclopropanes catalyzed by soluble methane monooxygenase from Methylosinus trichosporium OB3b

Abstract

The soluble methane monooxygenase (MMO) system isolated from Methylosinus trichosporium OB3b catalyzes the adventitious oxidation of alkyl substituted methylcyclopropanes. If the chemical mechanism of C–H activation by MMO involves formation of a radical or carbocation intermediate at the methyl C–H of these ‘radical clock’ substrates, then cyclopropyl ring opened alcohols may appear in the product mixture due to rearrangement of the intermediate. The lifetime of radical intermediates can be determined from known rearrangement rate constants, k r. Rearrangement was observed during the oxidation of 1,1,2,2-tetramethylcyclopropane ( k r=1.7–17.5×10 8 s −1, 30°C) but not for cis- or trans-1,2-dimethylcyclopropane ( k r=1.2–6.4×10 8 s −1, 30°C) or the very fast radical clock, trans-2-phenylmethylcyclopropane ( k r=3.4×10 11 s −1, 30°C). The results show that the occurrence of rearranged products fails to correlate with either the chemical nature of the C–H bond being broken, which is very similar for all of the methylcyclopropanes studied here, or the magnitude of the radical k r value. This study suggests that the steric properties of the substrate play an important role in determining the outcome of the reaction. Substrates with bulky substituents near the C–H bond that is attacked appear to yield intermediates with sufficient lifetimes to rearrange. In contrast, substrates with less steric bulk are postulated to be able to approach the reactive oxygen species in the MMO active site more closely so that intermediates are either rapidly quenched or undergo subsequent interaction with the dinuclear iron cluster of MMO that prevents rearrangement.