Interplay between the diamine structure and absolute helicity in Ni–salen metallofoldamers

Abstract

The nature of internal chiral diamines can greatly influence the ratio of helical diastereomers for Ni–salen based metallofoldamers. The diastereomer ratio is small for metallofoldamers derived from (1R, 2R)-cyclohexanediamine, (11R, 12R)-9,10-dihydro-9,10-ethanoanthracene-11,12-diamine, or (1R, 2R)-cyclopentanediamine. By contrast, the foldamer from (1S, 2S)-1,2-diphenylethylenediamine provides a relatively large bias (6 ∶ 1) for the P-helical diastereomer as evidenced by NMR studies, chiroptical data, and X-ray studies. A model is proposed to explain the origin of the helical bias. These findings underscore the need to consider helical diastereomers in models for asymmetric induction in metal–salen catalyzed reactions.