Modification of alkoxo ligands of BINOL–Ti ladder: Isolation and X-ray crystallographic analysis

The stable binaphthol–titanium ladder complexes have been successfully prepared by using bulky alkoxo ligands. From the secondary OR ligand (cyclohexyloxo, 2,4-dimethyl-3-pentyloxo or 2-adamantyloxo) and terially OR ligand (tert-butyloxo, 1-adamantyloxo), partial hydrolysis proceeded to give the μ3-oxo titanium complexes. The use of [Ti(BINOLato)(OEt)2]n made it possible to prepare the Ti(BINOLato)(OR)2 complexes using alcohols (ROH) of high boiling point (R = cyclohexyl, 2-adamantyl, 1-adamantyl). X-ray analyses of [(R)-1,1′-bi-2-naphtholato]bis(O-2,4-dimethyl-3-pentyloxo)titanium and [(R)-3,3′-dimethyl-1,1′-bi-2-naphtholato]bis(2-adamantyloxo)titanium showed a good agreement with the estimated ladder complexes. The catalytic activity of BINOL–Ti catalyst analogues, obtained by partial hydrolysis of Ti(BINOLato)(OR)2 with wet MS 4A was studied in asymmetric glyoxylate-ene reaction by two methods. Moderate to good chemical yields and enantioselectivities were obtained.