Stable chiral salen Mn(III) complexes with built-in phase-transfer capability for the asymmetric epoxidation of unfunctionalized olefins using NaOCl as an oxidant

• Published in: Journal of catalysis 2012-03, Vol.287, p.170-177
• Main Authors: Luo, Rongchang, Tan, Rong, Peng, Zhigang, Zheng, Weiguo, Kong, Yu, Yin, Donghong
• Format: Article
• Language: English
• Subjects: Aqueous NaOCl; Aqueous solutions; Catalysis; Chemistry; Chiral salen Mn(III) complex; Enantioselective epoxidation; Exact sciences and technology; General and physical chemistry; Ionic liquid; Oxidation; Phase transitions; Phase-transfer catalysis; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2011.12.016

PEG chain-modified ionic liquid functionalized chiral salen Mn(III) complexes with built-in phase-transfer capability were first prepared and proved highly efficient in the asymmetric epoxidation of unfunctionalized olefins using aqueous NaOCl as an oxidant. [Display omitted] ► PEG-IL modified chiral salen Mn(III) complexes were first prepared by covalent linkage. ► The amphipathic PEG chain endowed the catalysts with built-in phase-transfer capability. ► The catalysts could overcome the mass transfer disadvantage in biphasic epoxidation. ► The catalysts were stable and could be separated from the reaction mixture. A series of novel chiral salen Mn(III) complexes with inherent phase-transfer capability were prepared by covalent linkage of the limidazolium IL moieties containing various PEG chains with chiral salen ligand at two sides of 5,5′-position. Technologies of characterization well suggested the presence of polyether chain, the IL linker, and the intact active sites in the complexes. The amphipathic nature of the PEG chain allowed the PEG-based catalysts to undergo built-in phase transfer, which in turn increased the reaction rates in water/organic biphasic systems. Enantioselective epoxidation of styrene, α-methylstyrene, indene, 1,2-dihydronaphthalene, 6-cyano-2,2-dimethylchromene, and 6-nitro-2,2-dimethylchromene catalyzed by the complexes with NaOCl gave >99% conversions within 60min. The enantiomeric excess (ee) for the epoxides was in the range of 68–93%, except for styrene (ee, 35%) and α-methylstyrene (ee, 42%). Furthermore, the PEG-based complexes were stable and could be separated from the reaction mixture by control of the solvent.