Chiral-at-metal tetrahydrosalen complexes of resolved titanium(IV) sec-butoxides: Ligand wrapping and multiple asymmetric catalytic induction

The reaction of the racemic and resolved tetrahydrosalen derivative LH2 (LH2 = N,N’-bis(3,5-dichloro-2-hydroxybenzyl)-trans-1,2-diaminocyclohexane) with the resolved titanium(IV) sec-butoxides Ti(OR-2Bu)4 or Ti(OS-2Bu)4 yielded a series of four compounds, LTi(O2Bu)2 (1–4), which have been characterized by IR, elemental analysis, 1H and 13C NMR and X-ray crystallography. X-ray crystallography revealed the co-crystallization of two pseudo-C2-symmetric products from racemic LH2, whereas a perfect chiral induction of the ligand to the metal occurred when resolved (R,R)-LH2 was used, resulting in a Δ fac–fac wrapping mode of the tetradentate ligand about the titanium center. Ab initio electronic structure calculations (DFT) are in agreement that the lowest energy isomer is that which is experimentally observed. Catalysis screenings show that Ti(OS-2Bu)4, in conjunction with (R,R)-LH2, forms a matched pair that catalyzes the addition of dimethyl zinc to benzaldehyde with higher enantioselectivity than that observed for resolved (R,R)-LH2 with Ti(OR-2Bu)4 or achiral Ti(OiPr)4. Increasing the temperature of the system results in slightly increased enantiomeric excess.