Achiral tetrahydrosalen ligands for the synthesis of C(2)-symmetric titanium complexes: a structure and diastereoselectivity study.

Achiral tetrahydrosalen ligands have been employed in the synthesis of chiral C(2)-symmetric titanium complexes. When combined with tetrahydrosalen ligands 2a and 2b, titanium tetraisopropoxide liberated 2 equiv of isopropyl alcohol and generated the (tetrahydrosalen)Ti(O-i-Pr)(2) complexes 3a and 3b. These complexes were shown to be C(2)-symmetric by (1)H and (13)C[(1)H] NMR spectrometry and X-ray crystallography. X-ray structures of 3a and 3b indicate that the bonding of the tetrahydrosalen ligand to titanium is different than the bonding of salen ligands to titanium. Whereas salen ligands usually bind to titanium in a planar arrangement, the tetrahydrosalen is bonded with the phenoxide oxygens mutually trans. When bound in this fashion, the nitrogens of the tetrahydrosalen ligand and the titanium become stereogenic centers. The use of titanium complexes of high enantiopurity in the generation of tetrahydrosalen titanium adducts resulted in a maximum diastereoselectivity of 2:1. The diastereoselectivity obtained using chiral titanium alkoxide complexes was greater than the diastereoselectivity observed when a tetrahydrosalen ligand derived from (S,S)-trans-diaminocyclohexane was employed.