Catalytic Activity of Chloro and Triflate Manganese(II) Complexes in Epoxidation Reactions: Reusable Catalytic Systems for Alkene Epoxidation

摘要

The catalytic oxidation of a series of alkenes by three MnII complexes, MnCl_2[(-)-L]_2 (1), ({MnCl[(-)-L]}_2(μ-Cl)_2) (2), and Mn(CF_3SO_3)_2[(-)-L]_2 (3), having the bidentate N-ligand (-)-pinene[5,6]bipyridine, (-)-L, and using peracetic acid as oxidant in acetonitrile is described. The performance of catalysts 1 and 2 is enhanced by the use of additives (NaHCO_3 and imidazole) and also using a [bmim]/acetonitrile mixture as reaction media. The latter conditions allow recyclability of the catalytic system, keeping good selectivity and conversion values in some cases. The nature of the species formed in acetonitrile after the addition of peracetic acid has been investigated through EPR and UV/Vis spectroscopy. The results point to the formation of high-valent manganese species, among which a di-μ-oxo-bridged Mn_2(III,IV) binuclear species was unambiguously identified for complex 3 through a typical 16-line EPR signature.