Metal-peroxo versus metal-oxo oxidants in non-heme iron-catalyzed olefin oxidations: computational and experimental studies on the effect of water.

Computational and experimental studies show that Fe(BPMEN)-catalyzed olefin oxidation has two (FeIII-OOH and FeV=O) oxidant species, which act with comparable activation barriers. The presence of water favors formation of an HO-FeV=O oxidant via water-assisted O-OH bond cleavage and leads to both epoxide and cis-diol products. In the absence of water, the oxidant is the FeIII-OOH [or (MeCN)FeIII-OOH], and oxidation mainly leads to epoxide. This conclusion differs from that derived from DFT investigations of iron-porphyrin-catalyzed olefin epoxidation, where the FeIII-OOH pathway is deemed too high in energy to be plausible. The difference between these two systems may lie in the more flexible coordination environment of the non-heme iron complex, which has an available adjacent coordination site that contributes to the activation of the peroxide in both wa and nwa pathways.