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

The soluble methane monooxygenase (MMO) system isolated from Methylosinus t richosporium OB3b catalyzes the adventitious oxidation of alkyl substituted methylcyclopropanes. If the chemical mechanism of C-H activation by MMO in volves 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 rearra ngement rate constants, k(r). Rearrangement was observed during the oxidati on of 1,1,2,2-tetramethylcyclopropane (k(r) = 1.7-17.5 x 10(8) s(-1), 30 de greesC) but not for cis- or trans-1,2-dimethylcyclopropane (k(r)=1.2-6.4 x 10(8) s(-1), 30 degreesC) or the very fast radical clock, trans-2-phenylmet hylcyclopropane (k(r)= 3.4 x 10(11) s(-1), 30 degreesC). The results show t hat the occurrence of rearranged products fails to correlate with either th e chemical nature of the C-H bond being broken, which is very similar for a ll of the methylcyclopropanes studied here, or the magnitude of the radical k(r) value. This study suggests that the steric properties of the substrat e play an important role in determining the outcome of the reaction. Substr ates with bulky substituents near the C-H bond that is attacked appear to y ield intermediates with sufficient lifetimes to rearrange. In contrast, sub strates with less steric bulk are postulated to be able to approach the rea ctive oxygen species in the MMO active site more closely so that intermedia tes are either rapidly quenched or undergo subsequent interaction with the dinuclear iron cluster of MMO that prevents rearrangement. (C) 2000 Elsevie r Science B.V. All rights reserved.