Chiral bipyrrolidine based iron and manganese complexes [((S,S)-pdp)MII(OTf)2] catalyze the asymmetric epoxidation of various olefins with H2O2 in the presence of carboxylic acid additives with high efficiency (up to 1000 turnover number (TON)) and selectivity (up to 100%), and with good to high enantioselectivity (up to 93% enantiomeric excess (ee)). The enantioselectivity increases with growing steric demand of the acid. On the basis of the electron paramagnetic resonance (EPR) spectroscopy and enantioselectivity studies, the active oxygen-transferring species of the above systems can be identified as structurally similar oxometal(V) species of the type [((S,S)-pdp)MVO(OCOR)]2+ (M = Fe, Mn; R = alkyl).

The key intermediate of a bioinspired iron catalyst for selective hydrocarbon oxidation based on hydrogen peroxide and an iron complex with a tetradentate aminopyridine ligand was trapped by EPR. On the basis of EPR and reactivity data this intermediate is tentatively proposed to be an oxoiron(V) complex.

Herein, we report the EPR spectroscopic study of the bioinspired catalyst systems for selective hydrocarbon oxofunctionalizations based on dinuclear ferric complexes with TPA* and PDP* aminopyridine ligands, hydrogen peroxide, and acetic acid (TPA* = tris(3,5-dimethyl-4-methoxypyridyl-2methyl)amine, PDP* = bis(3,5-dimethyl-4-methoxypyridyl-2methyl)-(S,S)-2,2′-bipyrrolidine). Using very low temperatures, −75 to −85 °C, the extremely unstable and reactive iron−oxygen intermediates, directly reacting with olefins even at −85 °C, have been detected for the first time. Their EPR parameters (g 1 = 2.070−2.071, g 2 = 2.005−2.008, g 3 = 1.956− 1.960) were very similar to those of the known oxoiron(V) complex [(TMC)Fe V O(NC(O)CH 3 )] + (g 1 = 2.053, g 2 = 2.010, g 3 = 1.971, TMC = 1,4,8,4,8,. On the basis of EPR and reactivity data, the detected intermediates were assigned to the Fe V O active oxidizing species of the catalyst systems studied.