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