Iron Porphyrin Catalyzed Bromination of Unactivated C-H Bonds: Inhibition of Oxygen Rebound by Redox-Inactive Metal Ions

Abstract

Heme-containing oxygenases have been known to catalyze oxidation of unactivated C-H bonds. In most cases, hydroxylated compounds (alcohols) are the predominant products through oxygen rebound pathway. Alternatively, non-hydroxylated products can be obtained under certain conditions when the oxygen rebound pathway is inhibited. However, biomimetic oxidative functionalization reactions catalyzed by synthetic iron porphyrin complexes have yet to be explored, due to the fast oxygen rebound step. In this study, metal bromide LiBr was introduced to the iron-porphyrin catalyzed oxidation of hydrocarbons, such as cycloalkane, straight-chain alkanes and benzyl compounds. In all cases, brominated products were the sole products, indicating that the oxygen rebound pathway was completely inhibited in the presence of LiBr. Mechanistic studies combined with theoretical calculations revealed that the active intermediate iron(IV)-oxo porphyrin π-cation radical species interacted with lithium ion, which significantly inhibited the oxygen rebound pathway. As a result, a carbocation intermediate was formed which was responsible for the formation of brominated products. This carbocation mechanism is reminiscent of the P450 OleT_JE and CYP19A1 enzymatic systems, in which the oxygen rebound is inhibited and desaturated products are obtained. These results demonstrate that the redox-inactive metal ion acting as Lewis-acid is capable to tune the reactivity of high-valent metal-oxo species from oxygen rebound to non-oxygen rebound, providing potential application to produce versatile organic compounds stem from simple hydrocarbons.