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

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.