Synthetic Investigation on the Biomimetic Metal-Catalyzed Sulfoxidations and Photochemical Generation of a Highly Reactive Ruthenium(V)-Oxo Porphyrin

Catalytic oxidation plays a crucial role in current chemical and pharmaceutical industries which is also a leading technology for green chemical processes. In Nature, the ubiquitous cytochrome P450 enzymes can catalyze a wide variety of oxidation reactions with high efficiency and selectivity. Many transition metal catalysts are designed as the biomimetic model of cytochrome P450 enzymes. In this work, series of metalloporphyrins and metallocorroles have been successfully synthesized to investigate and develop catalytic selective oxidation of sulfides to sulfoxides. Manganese(III) porphyrin complexes (2) and manganese(III) corrole complexes (6) with iodobenzene diacetate [PhI(OAc)2] as a mild oxygen source exhibited remarkable catalytic activity toward selective oxidation of sulfides to sulfoxides under mild conditions. Conspicuous is the fact that readily soluble PhI(OAc)2 in the presence of a small amount of water is more efficient than the commonly used PhIO and other oxygen sources under identical conditions. It was found that the reactivity of manganese(III) porphyrin catalysts was greatly affected by axial ligand and the weakly binding chlorate gave the highest catalytic activity in the oxidation of sulfide. Both porphyrin-manganese and corrole-manganese catalysts catalyzed the highly selective oxidation of para-substituted thioanisoles with PhI(OAc)2 in the presence of a small amount of water. Complete conversion and of sulfide and excellent selectivities for sulfoxide were achieved within 120 min. We discovered that photo-disproportionation of a bis-porphyrin-diruthenium(IV)- μ-oxo dimer gave a porphyrin-ruthenium(III) species and a putative poprhyrinruthenium( V)-oxo species that can be detected and studied in real time using laser flash photolysis methods. As determined by its spectral and kinetic behavior, the same oxo transient was also formed by photolysis of a porphyrin-ruthenium(III) N-oxide adduct. Second-order rate constants for reactions with several substrates at 22 °C were determined; representative values of rate constants were kox = 6.6 × 103 M-1 s-1 for diphenylmethanol, kox = 2.5 × 103 M-1 s-1 for styrene, and kox = 1.8 × 103 M-1 s-1 for cyclohexene