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
(LN4) and 2 bearing a pentadentate N5 ligand
(LN5), 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.