Proton-Promoted and Anion-Enhanced Epoxidation of Olefins by Hydrogen Peroxide in the Presence of Nonheme Manganese Catalysts

We report a remarkable Brønsted
acid effect in the epoxidation
of olefins by nonheme manganese catalysts and aqueous hydrogen peroxide.
More specifically, a mononuclear nonheme manganese complex bearing
a tetradentate N4 ligand, MnII(Dbp-MCP)­(OTf)2 (Dbp-MCP = (1R,2R)-N,N′-dimethyl-N,N′-bis­((R)-(3,5-di-tert-butyl-phenyl)-2-pyridinylmethyl)­cyclohexane-1,2-diamine;
OTf– = CF3SO3–), is a highly efficient catalyst in the epoxidation of olefins by
aqueous H2O2 in the presence of H2SO4 (1–3 mol %). The yields of epoxide products
as well as the chemo- and enantioselectivities increase dramatically
in the presence of H2SO4; no formation of epoxides
is observed in the absence of H2SO4. In addition,
the product yields and enantioselectivities are dependent significantly
on the manganese catalysts and Brønsted acids. The catalytic
epoxidation of olefins by other oxidants, such as peracids, alkyl
hydroperoxides, and iodosylbenzene, is also affected by the presence
of H2SO4; product yields and enantioselectivities
are high and similar irrespective of the oxidants in the presence
of H2SO4, suggesting that a common epoxidizing
intermediate is generated in the reactions of [MnII(Dbp-MCP)]2+ and the oxidants. Mechanistic studies, performed with 18O-labeled water (H218O) and cumyl hydroperoxide,
reveal that a high-valent manganese-oxo species is formed as an epoxidizing
intermediate via O–O bond heterolysis of Mn-OOH­(R) species.
The role of H2SO4 is proposed to facilitate
the formation of a high-valent Mn-oxo species and to increase the
oxidizing power and enantioselectivity of the Mn-oxo oxidant in olefin
epoxidation reactions. Density functional theory (DFT) calculations
support experimental results such as the formation of a Mn­(V)-oxo
species as an epoxidizing intermediate.