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 9 0% yield and 85-88% enantioselectivity. The axial ligand, 4-(3-phenylp ropyl)pyridine N-oxide (P3NO), increases the rate of epoxidation witho ut affecting enantioselectivity and also stabilizes the catalyst. Thes e two effects afford a reduction in catalyst loading to <1%. The turno ver-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 I 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 a s oxidant, as shown by the dependence of the rate on the NaOH concentr ation. P3NO assists the transport of HOCl to the organic layer as demo nstrated by titration studies and by measuring the rates of oxidation of a redox indicator, diphenylbenzidine. On the other hand, stirring s peed studies indicate that, in the absence of the ligand, oxidation oc curs 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.