Co-catalytic effects of nitrogen donors on the epoxidation of cyclooctene with tetra-n-butylammonium hydrogen monopersulfate in the presence of manganese(III)tetraarylporphyrins: A comparative study

The epoxidation of cyclooctene by tetra-n-butylammonium hydrogen monopersulfate was performed in the presence of four different manganese(III)tetraarylporphyrins as catalysts and a number of nitrogen donors as co-catalysts under a fixed co-catalyst/catalyst ratio (100:1). It was observed that the σ-bonding abilities of the nitrogen donors, as related to their pKa (BH+) values, were not a reliable factor for determining their co-catalytic activities, unless there were no π-interactions and/or no significant involvement of steric hindrance. On the other hand, the π-donating ability of the nitrogen donors was found to be of importance in defining their role as co-catalysts. The strong π-donor N–H imidazoles displayed co-catalytic activities greater than strong σ-donor amines and weak π-donor pyridines in the presence of manganese(III)tetraphenylporphyrin acetate (MnTPP(OAc)). In the case of hindered manganese(III)tetramesitylporphyrin acetate and manganese(III)tetrakis(2,6-dichlorophenyl)porphyrin acetate steric hindrance of the nitrogen donors played a dominant role, and the least hindered imidazole (ImH) was the best co-catalyst. When manganese(III)tetrakis(pentafluorophenyl)porphyrin acetate(MnTPFPP(OAc)) was employed as catalyst then pyridines, in general, demonstrated a higher co-catalytic activities than imidazoles. The same trend was also observed under various co-catalyst/catalyst ratios (1–200), and pyridines were always more effective co-catalysts than imidazoles with MnTPFPP(OAc), whereas for MnTPP(OAc) the order for co-catalytic activities of these nitrogen donors was inverted. Also, in the presence of electron deficient manganese(III)tetrakis(4-nitrophenyl)porphyrin acetate imidazoles acted generally as more effective co-catalysts than pyridines, at co-catalyst/catalyst ratios < 100. It is proposed that the varied attractive C–H⋯F–C interactions between the ortho-C–F σ* orbitals of the pentafluorophenyl groups in MnTPFPP(OAc) complex and C–H σ bonds adjacent to the nitrogen donor sites of pyridines or imidazoles could be responsible for the observed differences. The UV–vis spectra of the catalytic systems containing pyridine and ImH in association with MnTPP(OAc) and MnTPFPP(OAc) were examined for the active oxygen intermediates.

“Hindered” 2,6-Me2Py and 2-MePy demonstrated co-catalytic activities better than imidazoles at all co-catalyst/catalyst ratios (10–200) in the presence of MnTPFPP(OAc), contrasting their behavior with MnTPP(OAc).Figure optionsDownload as PowerPoint slide