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(OiPr)2 (1a‐3a) and (L1‐L3)TiCl2(thf) (1b‐3b). 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 3b 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 3a was remarkably active with pivalaldehyde as substrate, yielding the cyanosilylated product with 99% conversion and 61% ee after only 30 min at ‑60°C.