ALKENE EPOXIDATIONS CATALYZED BY CHEMICALLY ROBUST MN(III) PORPHYRINSAND PROMOTED BY HOCL UNDER AQUEOUS-ORGANIC 2-PHASE CONDITIONS IN THE ABSENCE OF AXIAL LIGAND AND PHASE-TRANSFER CATALYST - REACTION-MECHANISM AND LARGE-SCALE PREPARATIVE APPLICATIONS

Under CH2Cl/2H2O two-phase conditions alkene epoxidations, catalysed b y Mn(III)-porphyrins and promoted by aqueous NaOCl adjusted at pH 10.0 with NaHCO3, proceed in the absence of a phase-transfer catalyst and, for electron-rich olefins, also in the absence of axial ligands. Unde r these conditions, reliable kinetic measurements on cyclooctene epoxi dation can be obtained by using the chemically robust Mn(III)-tetra(2, 6-dichlorophenyl)por-phyrin chloride, as the side reactions associated with the oxidative demoliton of the axial ligand and of the quaternar y onium salts do not occur. The results indicate that reactions follow Michaelis-Menten saturation kinetics which are confirmed by the oxida tion rates of different substrates in competitive epoxidations. The ab sence of strong axial ligands, like pyridine or imidazole bases, highl ights the autocatalytic behaviour of the reactions, probably because t he produced epoxides act as weak axial ligands. Indeed, reactions are noticeably accelerated by initial addition of consistent amounts of ep oxides. Under the conditions reported here electron-rich olefins can b e oxidised on a large scale, the overall catalyst turnovers being in t he range 30,000-100,000 in 20-24 h at 20-degrees-C.