Mechanistic Insights into the Enantioselective Epoxidation of Olefins by Bioinspired Manganese Complexes: Role of Carboxylic Acid and Nature of Active Oxidant

Bioinspired manganese and iron complexes bearing nonporphyrinic tetradentate N4 ligands are highly efficient catalysts in asymmetric oxidation reactions by hydrogen peroxide (H₂O₂), in which carboxylic acid is employed as an essential additive to improve product yields and stereo-, regio-, and enantioselectivities. The metal catalysts should possess two cis-binding sites for oxidant (e.g., H₂O₂) and carboxylic acid to generate high-valent metal-oxo species as active oxidants via a "carboxylic acid-assisted" mechanism. In the present study, we have investigated the role(s) of carboxylic acid and the nature of reactive intermediate(s) in the manganese complex-catalyzed enantioselective epoxidation of olefins, by employing non-heme manganese catalysts, such as 1 bearing a tetradentate N4 ligand (L^N4) and 2 bearing a pentadentate N5 ligand (L^N5), and using various oxidants, such as H₂O₂, alkyl hydroperoxides, and iodosylbenzene (PhIO). As expected, 1 possessing two cis-binding sites is an effective catalyst irrespective of the oxidants in the presence of carboxylic acid. In contrast, 2 possessing only one binding site is not an effective catalyst in the reactions of H₂O₂ and alkyl hydroperoxides even in the presence of carboxylic acid. However, unexpectedly, 2 turns out to be an effective catalyst in the asymmetric epoxidation of olefins by PhIO in the presence of carboxylic acid. The latter result indicates that a manganese complex, which cannot bind carboxylic acid as an auxiliary ligand, can afford a high enantioselectivity, probably through a second-sphere coordination interaction between carboxylic acid and the oxo group of a presumed manganese-oxo oxidant. We have also provided indirect evidence supporting that a high-valent Mn(V)-oxo species is the active oxidant in the catalytic asymmetric epoxidation reactions.