Manganese Porphyrin-Catalyzed C–H Iodination and Bromination with Alkyl Halides through Biomimetic Oxidation

Abstract

Heme oxygenases catalyze C–H oxidation primarily via the oxygen rebound mechanism to form hydroxylated products. While hydroxylation is the principal pathway in iron-containing oxygenases and corresponding biomimetic systems, manganese porphyrin analogues can mediate alternative reaction pathways. The oxygen rebound step is much slower in manganese porphyrin systems, permitting radical escape and thereby enabling interception by various nucleophiles. Based on these findings, chlorination, bromination, fluorination, azidation, and isocyanation of unactivated C–H bonds have been accomplished via bioinspired oxidation reactions when related anions were introduced (such as F^–, Cl^–, Br^–, N₃^– and NCO^–). However, the iodination of hydrocarbons in biomimetic systems remains unreported. The susceptibility of I^– to oxidation by common oxidants (e.g., PhIO) to form I₂ precludes its use as a direct halogen source in biomimetic C–H functionalization. Recently, it has been found that carbon radicals generated by hydrogen atom abstraction (HAA) with a Mn(IV)-oxo porphyrin can escape the solvent cage and react with halohydrocarbon solvents (e.g., CH₂Cl₂) to form chlorinated products. Inspired by this finding, we have achieved manganese porphyrin-catalyzed C–H iodination using CHI₃ as the halogen source. Furthermore, when the halogen source was replaced with CBr₄, the bromination of unactivated C–H bonds was also accomplished. In all cases, only the halogenated products were detected, exhibiting satisfactory yields. Mechanistic studies indicate that the active intermediate, Mn(V)-oxo porphyrin complex (or Mn(IV)-oxo porphyrin complex), generates carbon radicals by abstracting hydrogen atoms from hydrocarbon substrates. These radicals undergo cage escape and ultimately react with halogen sources to form halogenated products. This strategy offers a potential route for synthesizing iodinated hydrocarbons from simple hydrocarbons under mild conditions.

Keywords

Biomimetic catalysis / Heme manganese porphyrin / Unactivated C–H functionalization / Hydrogen atom abstraction / Iodination / Bromination / Alkyl halides / C–H halogenation

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