Influence of the conformation of salen complexes on the stereochemistry of the asymmetric epoxidation of olefins

Compared with Katsuki's or Jacobsen's catalysts (R,R,R,R)-1 or (R,R)-2, the phenomenon of reversed asymmetric induction in the course of the epoxidation of unfunctionalised olefins in the presence of (salen)manganese complex (S,S)-3 is indirectly explained by means of quantumchemical calculations, as well as by H-1 NMR spectra of salen ligand (S,S)-10 and diamagnetic low-spin nickel complex (S,S)-7 and by its X-ray structure analysis. Whereas in (R,R,R,R)-1 and (R,R)-2 the phenyl and cyclohexyl substituents occupy equatorial positions, the origin for the reversed enantioselection caused by manganese catalyst (S,S)-3, is based on an axial position of the acyclic heteroalkyl substituents in the diimine backbone of (S,S)-3 as exemplarily confirmed by model nickel complex (S,S)-7. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005).