Iron-catalyzed halogenation of alkanes: Modeling of nonheme halogenases by experiment and DFT calculations

Abstract

When the dichloroiron(II) complex of the tetradentate bispidine ligand L = 3, 7-dimethyl-9-oxo-2, 4-bis(2pyridyl)-3, 7-diazabicyclo[3.3.1]nonane1, 5-dicarboxylate methyl ester is oxidized with H2O2, tBuOOH, or iodosylbenzene, the high-valent Fe=O complex efficiently oxidizes and halogenates cyclohexane. Kinetic D isotope effects and the preference for the abstraction of tertiary over secondary carbon-bound hydrogen atoms (quantified in the halogenation of adamantane) indicate that C-H activation is the rate-determining step. The efficiencies (yields in stoichiometric and turnover numbers in catalytic reactions), product ratios (alcohol vs. bromo- vs. chloroalkane), and kinetic isotope effects depend on the oxidant. These results suggest different pathways with different oxidants, and these may include iron(IV)- and iron(V)-oxo complexes as well as oxygen-based radicals. © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.