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-R¹-5-R²-2-hydroxybenzylideneamino)-1,1′-binaphthyl H₂L (R²= Cl, R¹= Cl, Me, Et or NO₂; R²= Me, R¹= Bu^t; R²= NO₂, R¹= H, Me, Et, Prⁱ, Bu^t or Cl), and their complexes [CuL¹]1 and [Mn₂L¹₂(OMe)₂]2(R¹= R²= 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 tert-butyl hydroperoxide. On the contrary, 2 is inert towards alkene epoxidation by PhIO. However, upon mixing Mn(O₂CMe)₃·xH₂O and H₂L 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 R¹ and R² 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)-H₂L systems towards alkene epoxidation have been investigated. When R¹= Et and R²= NO₂ 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.