Synthesis of titanium-TADDOLate complexes containing bidentate nitrogen donors and the asymmetric ethylation of benzaldehyde

Reaction of TiCl2(TADDOLate)L2 (L=THF (1) or AcOEt (2)) with a bidentate nitrogen-containing ligand such as 2,2′-bipyridine (bipy) or 1,10-phenanthroline (phen) affords six-coordinate complexes TiCl2(TADDOLate)(L2) (L2=bipy (3) or phen (4)). Complexes 3 and 4 can also be obtained from reaction of 1.3 eq. of TiCl4 with a TADDOL (α,α,α′,α′-tetraphenyl-1,3-dioxolane-4,5-dimethanol) ligand in diethyl ether followed by the addition of the bidentate ligand. The complex 3 reacts further with 2.2 eq. of MeLi in toluene to give the mono-methyl complex TiCl(Me)(TADDOLate)(bipy) (5). Complexes 3 and 4 were subjected to X-ray structural analyses and both structures reveal the same C2 molecular symmetry with the bidentate bipy or phen ligand trans to the TADDOLate ligand. In the asymmetric reaction of diethylzinc addition to benzaldehyde, the monomeric metal complex 3 or 4 alone shows good catalytic reactivities with low to moderate enantioselectivities of 31% or 52% ee, respectively. However, with the addition of excess Ti(O-i-Pr)4, the reaction gives nearly quantitative yields of the desired product with good enantioselectivities up to 88% ee.