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 chir al C-2-symmetric titanium complexes. When combined with tetrahydrosalen lig ands 2a and 2b, titanium tetraisopropoxide liberated 2 equiv of isopropyl a lcohol and generated the (tetrahydrosalen)Ti(O-i-Pr)(2) complexes 3a and 3b . These complexes were shown to be C-2-symmetric by H-1 and C-13{H-1} NMR s pectrometry and X-ray crystallography. X-ray structures of 3a and 3b indica te that the bonding of the tetrahydrosalen ligand to titanium is different than the bonding of salen ligands to titanium. Whereas salen ligands usuall y bind to titanium in a planar arrangement, the tetrahydrosalen is bonded w ith the phenoxide oxygens mutually trans. When bound in this fashion, the n itrogens of the tetrahydrosalen ligand and the titanium become stereogenic centers. The use of titanium complexes of high enantiopurity in the generat ion of tetrahydrosalen titanium adducts resulted in a maximum diastereosele ctivity of 2:1. The diastereoselectivity obtained using chiral titanium alk oxide complexes was greater than the diastereoselectivity observed when a t etrahydrosalen ligand derived from (SS)-trans-diaminocyclohexane was employ ed.