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

摘要

A series of chiral organo-titanium complexes have been prepared from the reaction between Ti(O~iPr)_4 and chiral biaryl Schiff-base ligands 1H_2-12H. The steric demand of the ligand plays an important role in the formation of the titanium complexes. For example, treatment of ligand 1H_2 with 1 equiv of Ti(O~iPr)_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 2H_2, 4H _2, and 6H_2 form the mono-ligated titanium complexes (L2)Ti(O~iPr)_2 (15), (L4)Ti(O~iPr)_2 (19), and (L6)Ti(O~iPr)_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(O~iPr)_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.