One-Handed Single Helicates of Dinickel(II) Benzenehexapyrrole-α,ω-diimine with an Amine Chiral Source

Benzenehexapyrrole-alpha,omega-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 C-2 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-alpha,omega-diimines were studied to show that an enantiomer pair of the helical C-2 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.