Synthesis, structure, and catalytic activity of titanium complexes with chiral biaryl Schiff-base ligands

Abstract

A series of chiral organo-titanium complexes have been prepared from the reaction between Ti(OiPr)4 and chiral biaryl Schiff-base ligands 1H2–12H. The steric demand of the ligand plays an important role in the formation of the titanium complexes. For example, treatment of ligand 1H2 with 1 equiv of Ti(OiPr)4 in toluene at room temperature gives, after recrystallization from a toluene solution, the chiral bis-ligated titanium complex (L1)2Ti (14). While under similar reaction conditions, the more bulky ligands 2H2, 4H2, and 6H2 form the mono-ligated titanium complexes (L2)Ti(OiPr)2 (15), (L4)Ti(OiPr)2 (19), and (L6)Ti(OiPr)2 (22), respectively, in good yields. The mono-ligated titanium alkoxides can be converted to bis-ligated complex via ligand redistribution reaction. For one instance, treatment of mono-ligated complex (L2)Ti(OiPr)2 (15) in benzene at 60°C results in the isolation of the bis-ligated complex (L2)2Ti (16) in 92% yield. All titanium complexes have been characterized by various spectroscopic techniques and elemental analyses. The solid-state structures of complexes 14–21, 23, 24 and 29 have further been confirmed by X-ray diffraction analyses. The titanium complexes are active catalysts for the asymmetric hydrophosphonylation of aromatic aldehydes with moderate enantioselectivities.