Prolinol‐derived phenoxy‐amidine titanium complexes in asymmetric silylcyanation of aldehydes

Abstract Three new tridentate phenoxy‐amidine ligands (FA) have been prepared via the ring‐opening of benzoxazole by 2‐(methylamino)ethanol, L ‐prolinol and L ‐diphenylprolinol. Coordination studies of these FA ligands with Ti have led to the synthesis of the complexes (L1–L3)Ti(O i Pr) 2 ( 1 a – 3 a ) and (L1–L3)TiCl 2 (thf) ( 1 b – 3 b ). The solid state structures of the dichlorido FA−Ti complexes revealed a distorted octahedral geometry around the metallic centre provided by the FA ligand in mer ‐fashion, the two residual chloro ligands in cis arrangement and a THF molecule. The chiral Ti‐FA complexes were shown to promote the asymmetric silylcyanation reaction. With benzaldehyde as substrate, the best catalyst was the dichlorido Ti complex 3 b with a diphenylprolinol in the ligand backbone. It afforded the cyanhydrine with 68 % conversion and 74 % ee after 24 h in DCM at –20 °C. The diisopropoxy Ti complex 3 a was remarkably active with pivalaldehyde as substrate, yielding the cyanosilylated product with 99 % conversion and 61 % ee after only 30 min at −60 °C.