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 FAÀ Ti complexes revealed a distorted octahedral geometry around the metallic centre provided by the FA ligand in merfashion, 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 À 60 °C.
Origin | Publication funded by an institution |
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