Mechanistic Study of the Jacobsen Asymmetric Epoxidation of Indene

The asymmetric epoxidation of indene using aqueous NaOCl, catalyzed by Jacobsen's chiral manganese salen complex, provides indene oxide in 90% yield and 85-88% enantioselectivity. The axial ligand, 4-(3-phenylpropyl)pyridine N-oxide (P₃NO), increases the rate of epoxidation without affecting enantioselectivity and also stabilizes the catalyst. These two effects afford a reduction in catalyst loading to <1%. The turnover-limiting step in the catalytic cycle has been determined to be the oxidation of the manganese catalyst, based on reaction orders of 0 in indene and 1 in catalyst and also based on the dependence of the rate on the hypochlorite concentration. In the presence of the ligand P₃NO, this rate-limiting oxidation occurs in the organic phase with HOCl as oxidant, as shown by the dependence of the rate on the NaOH concentration. P₃NO assists the transport of HOCl to the organic layer as demonstrated by titration studies and by measuring the rates of oxidation of a redox indicator, diphenylbenzidine. On the other hand, stirring speed studies indicate that, in the absence of the ligand, oxidation occurs at the interface. Thus, the axial ligand plays at least two roles in the epoxidation of indene: it stabilizes the catalyst, presumably by ligation, and it increases the epoxidation reaction rate by drawing the active oxidant, HOCl, into the organic layer.