One-handed single helicates of dinickel(II) benzenehexapyrrole-α,ω-diimine with an amine chiral source

Abstract

Benzenehexapyrrole-α,ω-dialdehyde, composed of a pair of formyltripyrrole units with a 1,3-phenylene linker, was metallated to give dinuclear single-stranded helicates. X-ray studies of the bis-nickel(II) complex showed a helical C2 form with a pair of helical-metal coordination planes of a 3N+O donor set. The terminal aldehyde was readily converted into the imine by optically active amines, whereby helix-sense bias was induced. Bis-nickel(II) and bis-palladium(II) complexes of the benzenehexapyrrole-α,ω-diimines were studied to show that an enantiomer pair of the helical C2 form are interchanged by slow flipping of each coordination plane and fast rotation around the C(benzene)C(pyrrole) bond. The helical screw in the bis-nickel(II) complexes was biased to one side in more than 95 % diastereoselectivity, which was achieved by using a variety of optically active amines, such as (R)-1-cyclohexylethylamine, (S)-1- phenylethylamine, L-Phe(OEt) (Phe=phenylalanine), and (R)-valinol. The nickel complexes showed much better diastereoselectivity than the corresponding palladium complexes.

Keywords: chirality; helical structures; metalation; oligopyrroles; porphyrinoids.