Mechanistic Insight Into Sulfides Oxidation Catalyzed By Iron(Ⅲ)Porphyrin

Iron(Ⅲ) porphyrin, as a biomimetic model of cytochrome P450, can activate hydrogen peroxide for addition of a single oxygen atom into a wide variety of substrates, including hydrocarbon, amine, sulfides and many more, under mild conditions. However, intermediate that is responsible for sulfoxidation remains ill defined, and controversy always exits in this point. In the present work, sulfides oxidation by H2O2catalyzed by meso-tetraphenylporphin-atoiron (Ⅲ) chloride was investigated at room temperature. The results showed that protonic solvents can facilitate this oxidation and the catalyst’s life-span was closely related with temperature variations. Axial ligand imidazole remarkably enhanced the conversion of sulfides in aprotic solvents, but accelerated decomposition of catalyst in protic solvents. Iron(Ⅲ) porphyrin, substituted on the phenyl groups with a chlorine atom, displayed higher efficiency than no substituted. The conversions calculated from the GC data are100,99.81,99.78and77.05wt%for di-n-butyl sulfide, thioanisole, diphenyl sulfide and dibenzothiophene respectively after a typical procedure, and maximal slectivity for sulfoxides reached94%.Iron(Ⅲ) porphyrin chloride easily dissociated into free ion in polar solvents, but it is difficult for iron(Ⅲ) porphyrin hydroxide. The Ultraviolet-Visible absorption demonstrated that solvent, sulfides, sulfoxides and additional ligands all can coordinated with the ferric center. They competed with each other, and strong ligand often leads to significant changes in peak shape. In addition to alkalinity of axial ligand, coordination strength may be related with steric hindrance, and decreases in the order:Im>DMSO>MPSO≥MeOH>DBS≈EtOH.TPPFeCl rapidly decomposed when H2O2was added in EtOH according to decay of catalyst absorption curves without delay, but an accumulation of iron(Ⅳ) oxo porphyrin cation radical (CpdⅠ) was observed when mCPBA was added. Ionic species (m/z=684) detected by API-ES in these two processes was assigned to CpdⅠ. So evidence was given that catalyst’s decomposition did not result from CpdⅠ, but caused by hydroperoxoferric porphyrin (Cpd0). Comparative experiment with PhIO further confirmed this assumption. On the basis of online UV test, intermediates capture by API-ES, comparative experiments and product distribution,"Two oxidants" mechanism was proposed for iron(Ⅲ) porphyrin catalyzed sulfides oxidation by H2O2:selective sulfoxidation of sulfides was controled by Cpd0, then sulfoxides were oxidized by CpdⅠ. Cpd0can be changed into CpdⅠ when sulfoxides ran vast majority of substrates. Trendency towards this transformation would be increased when porphyrin ligand substituted with electron-deficient groups (such as meso-tetrakis(pentafluorophenyl) porphinatoiron(III) chloride. TPFPPFeCl), and exhibited higher yield and selectivity for sulfoxides and sulfone. Moreover, oxygen isotopic exchange and cyclohexene catalytic oxidation were also performed to prove proposed mechanism.