Preparation and reactivities of chiral manganese(III) and copper(II) complexes of binaphthyl Schiff bases

Abstract

A series of chiral Schiff bases, 2,2′-bis(3-R 1-5-R 2-2-hydroxybenzylideneamino)-1,1′-binaphthyl H 2L (R 2 = Cl, R 1 = Cl, Me, Et or NO 2; R 2 = Me, R 1 = Bu t; R 2 = NO 2, R 1 = H, Me, Et, Pr i, Bu t or Cl), and their complexes [CuL 1] 1 and [Mn 2L 1 2(OMe) 2] 2 (R 1 = R 2 = Cl) have been prepared. The crystal structure of the racemic form of 1 has been determined. Complex 1 is an active catalyst for the oxidation of alkenes by tertbutyl hydroperoxide. On the contrary, 2 is inert towards alkene epoxidation by PhIO. However, upon mixing Mn(O 2CMe) 3·xH 2O and H 2L in acetonitrile a green solution was obtained which could effect asymmetric epoxidation of alkenes by PhIO. The effects of the steric and electronic effects of the R 1 and R 2 substituents, temperature, and the addition of donors like N-methyl- and 2-methyl-imidazole and pyridine N-oxide on the catalytic activity of the Mn III + (S)-H 2L systems towards alkene epoxidation have been investigated. When R 1 = Et and R 2 = NO 2 the best enantiomeric excesses of 58 and 43% were found for epoxidation of cis-β-methylstyrene to (1S,2R)-cis-β-methylstyrene oxide and 4-chlorostyrene to 4-(S)-chlorostyrene oxide respectively.