Terpenols as substituents for the diastereoselective formation of enantiomerically pure triple lithium-bridged helicate type-coordination compounds

Abstract

The terpenols L(−)-borneol, (1S2S3S5R)-3-pinanol, (−)-menthol, and (−)-myrtenol are easily available chiral alcohols for the preparation of enantiomerically pure catechol esters 1–4-H₂. Those ligands are used for the hierarchical assembly of triple lithium-bridged dinuclear titanium(IV) triscatecholate helicates Li[Li₃(1–4)₆Ti₂]. In solution, the dimeric species are in a solvent dependent equilibrium with the monomer Li₂[(1–4)₃Ti]. The equilibrium is studied by ¹H NMR. CD spectroscopy indicates that the configuration at the complex units of the enantiomerically pure dimeric α-chiral derivatives Li[Li₃(1–3)₆Ti₂] is opposite to the configuration of the monomers Li₂[(1–3)₃Ti]. For the γ-chiral complex Li₂[(4)₃Ti] only a de of 25% is observed and in this case no interpretation of the mechanism of stereocontrol is possible.