Probing the C-O Bond-Formation Step in Metalloporphyrin-Catalyzed C-H Oxygenation Reactions

The oxygen rebound mechanism, proposed four decades ago, is invoked in a wide range of oxygen- and heteroatom-transfer reactions. In this process, a high-valent metal-oxo species abstracts a hydrogen atom from the substrate to generate a carbon-centered radical, which immediately recombines with the hydroxometal intermediate with very fast rate constants that can be in the nanosecond to picosecond regime. In addition to catalyzing C-O bond formation, we found that manganese porphyrins can also directly catalyze C-H halogenations and pseudohalogenations, including chlorination, bromination, and fluorination as well as C-H azidation. For these cases, we showed that long-lived substrate radicals are involved, indicating that radical rebound may involve a barrier in some cases. In this study, we show that axial ligands significantly affect the oxygen rebound rate. Fluoride, hydroxide, and oxo ligands all slow down the oxygen rebound rate by factors of 10-40-fold. The oxidation of norcarane by a manganese porphyrin coordinated with fluoride or hydroxide leads to the formation of significant amounts of radical rearranged products. cis-Decalin oxidation afforded both cis- and trans-decalol. Xanthene afforded dioxygen trapped products and the radical dimer product, bixanthene, under aerobic and anaerobic conditions, respectively. DFT calculations probing the rebound step show that the rebound barrier increases significantly (by 3.3, 5.4, and 6.0 kcal/mol, respectively) with fluoride, hydroxide, and oxo as axial ligands.