title: Proton-Promoted and Anion-Enhanced Epoxidation of Olefins by Hydrogen Peroxide in the Presence of Nonheme Manganese Catalysts.
abstract: 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, Mn(II)(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 [Mn(II)(Dbp-MCP)](2+) and the oxidants. Mechanistic studies, performed with (18)O-labeled water (H2(18)O) 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.
authors.affiliation: State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences , Lanzhou 730000, China.
authors.forename: Chengxia
authors.initials: C
authors.lastname: Miao
authors.affiliation: Department of Chemistry and Nano Science, Ewha Womans University , Seoul 03760, Korea.
authors.forename: Bin
authors.initials: B
authors.lastname: Wang
authors.affiliation: State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences , Lanzhou 730000, China.
authors.forename: Yong
authors.initials: Y
authors.lastname: Wang
authors.affiliation: State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences , Lanzhou 730000, China.
authors.forename: Chungu
authors.initials: C
authors.lastname: Xia
authors.affiliation: Department of Chemistry and Nano Science, Ewha Womans University , Seoul 03760, Korea.
authors.forename: Yong-Min
authors.initials: YM
authors.lastname: Lee
authors.affiliation: Department of Chemistry and Nano Science, Ewha Womans University , Seoul 03760, Korea.
authors.forename: Wonwoo
authors.initials: W
authors.lastname: Nam
authors.affiliation: State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences , Lanzhou 730000, China.
authors.forename: Wei
authors.initials: W
authors.lastname: Sun