Salen-ligands based on a planar-chiral hydroxyferrocene moiety: Synthesis, coordination chemistry and use in asymmetric silylcyanation

Condensation of the O-protected hydroxyferrocene carbaldehyde (Sp)-1 with suitable diamines, followed by liberation of the hydroxyferrocene moiety leads to a new type of ferrocene-based salen ligands (3). While the use of ethylenediamine in the condensation reaction yields the planar-chiral ethylene-bridged ligand [(Sp,Sp)-3a], reaction with the enantiomers of trans-1,2-cyclohexylendiamine gives rise to the corresponding diastereomeric cyclohexylene-bridged systems [(S,S,Sp,Sp)-3b and (R,R,Sp,Sp)-3c], which feature a combination of a planar-chiral ferrocene unit with a centrochiral diamine backbone. Starting with the ferrocene-aldehyde derivative (Rp)-1, the enantiomeric ligand series (3d/e/f) is accessible via the same synthetic route. The (Sp)-series of these newly developed N2O2-type ligands was used for the construction of the corresponding mononuclear bis(isopropoxy)titanium (4a/b/c), methylaluminum (5a/b/c) and chloroaluminum-complexes (6a/b/c), which were isolated in good yields and identified by X-ray diffraction in several cases. The aluminum complexes (5/6) were successfully used in the Lewis-acid catalyzed addition of trimethylsilylcyanide to benzaldehyde, yielding the corresponding cyanohydrins in 45–62% enantiomeric excess.