2,2′-Bipyrrolidine versus 1,2-Diaminocyclohexane as Chiral Cores for Helically Wrapping Diamine−Diolate Ligands

The synthesis of chiral tetradentate dianionic diamine-diolate ligands assembled around either N,N'-dimethyl-trans-1,2-diaminocyclohexane or 2,2'-bipyrrolidine is described. These ligands wrap in a fac-fac helical mode around octahedral titanium and zirconium centers giving chiral-at-metal complexes. Diaminocyclohexane was found to be a poor chiral motif for diastereoselective helical wrapping, and all complexes of this family were obtained as mixtures of stereoisomers. In contrast, bipyrrolidine was found to be a perfect chiral motif for helical wrapping, and the corresponding diamine-diolate complexes were obtained as (enantiomerically pure) single diastereomers.