Selective aromatic halogenation by a manganese compound I model: a mimic of chloroperoxidase

Abstract

A high-valent manganese(IV)-hydroxo porphyrin π-cation radical complex, [Mn^IV(OH)(Por˙⁺)(OTf)]⁺ (a protonated manganese compound I analogue), was studied in the halogenation of aromatic compounds. By replacing the triflate anion with Cl⁻ or Br⁻, we were able to halogenate toluene with high selectivity for C(sp²)–H bonds over C(sp³)–H bonds, such as chlorination with Cl⁻ or bromination with Br⁻ in the aromatic ring. We have also examined the halogenation of naphthalene and benzene derivatives with [Mn^IV(OH)(Por˙⁺)(X)]⁺ (X = Cl⁻ and Br⁻). In all of these reactions, halogenated products were formed dominantly, and the source of the halogens in the products was found to be halides present in the [Mn^IV(OH)(Por˙⁺)(X)]⁺ complexes. In the absence of halides, naphthalene was found to undergo dimerization. Kinetic isotope effect (KIE) experiments on this reaction showed no isotopic effect in the halogenation reactions. DFT calculations on models with the naphthalene substrate supported a mechanism involving an initial (rate-limiting) electron transfer from the substrate to [Mn^IV(OH)(Por˙⁺)(OTf)]⁺, coupled by the attachment of Cl⁻ to the C₁ position of the naphthalene radical cation. This mechanism was also supported by the Marcus theory of outer-sphere electron transfer. The so-formed [Mn^IV(OH)(Por)(OTf)] (a manganese compound II analogue) underwent a hydrogen atom transfer from the C₁ position of the substrate to form chlorinated naphthalene and [Mn^III(H₂O)(Por)(OTf)]. DFT calculations showed that [Mn^IV(OH)(Por)(OTf)] can also undergo direct OH-transfer to the substrate competitively, leaving open possibilities for side reactions or alternative reactions in a different environment. This study provides a deeper understanding of chloroperoxidase-like reactions.