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 sigma-bonding abilities of the nitrogen donors, as related to their pK(a) (BH+) values, were not a reliable factor for determining their co-catalytic activities, unless there were no pi-interactions and/or no significant involvement of steric hindrance. On the other hand, the pi-donating ability of the nitrogen donors was found to be of importance in defining their role as co-catalysts. The strong pi-donor N-H imidazoles displayed cocatalytic activities greater than strong sigma-donor amines and weak pi-donor pyridines in the presence of manganese(III)tetraphenylporphyrin acetate (MnTPP(OAc)). In the case of hindered man ganese(III)tetramesityl porphyrin 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 sigma* orbitals of the pentafluorophenyl groups in MnTPFPP(OAc) complex and C-H U 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. (C) 2007 Elsevier B.V. All rights reserved.