Solid State Structures and Solution Behavior of Titanium(IV) Octahydrobinaphtholate Complexes. Examination of Nonlinear Behavior in the Asymmetric Addition of Ethyl Groups to Benzaldehyde

In several asymmetric reactions, H 8 -BINOL-based catalysts (H 8 -BINOL = 5,5‘,6,6‘,7,7‘,8,8‘-octahydro-1,1‘-bi-2-naphthol) exhibit higher levels of enantioselectivity than analogous catalysts based on BINOL. A comparison of structures of titanium complexes prepared from H 8 -BINOL and BINOL was, therefore, undertaken. Reaction of ( rac )-H 8 -BINOL with 1 equiv of titanium tetraisopropoxide resulted in formation of the dimer ( meso )-[(H 8 -BINOLate)Ti(O- i -Pr) 2 ] 2 [( meso )- 6 ], which was characterized crystallographically. In a similar fashion, use of ( rac )-BINOL led to formation of the dimer ( meso )-[(BINOLate)Ti(O- i -Pr) 2 ] 2 [( meso )- 7 ]. The torsional angles between the aryl rings of the H 8 -BINOLate and BINOLate ligands in these complexes were 63.2(5)° and 55.7(4)°, respectively. The larger torsional angle of the H 8 -BINOLate ligand results in an increase in the bite angle of the ligand by just over 2°. Upon dissolving dimers ( meso )- 6 and ( meso )- 7, equilibria between the homo- and heterochiral dimers were observed. Reaction of H 8 -BINOL with an excess of titanium tetraisopropoxide provided crystals of the dinuclear complex [(H 8 -BINOLate)Ti(O- i -Pr) 2 ]·Ti(O- i -Pr) 4 ( 8 ). Likewise, reaction of 2 equiv of Ti(OCy) 4 (Cy = cyclohexyl) with H 8 -BINOL furnished [(H 8 -BINOLate)Ti(OCy) 2 ]·Ti(OCy) 4 ( 9 ). These compounds were characterized by X-ray crystallography and compared to the known [(BINOLate)Ti(O- i -Pr) 2 ]·Ti(O- i -Pr) 4, a proposed intermediate in the asymmetric addition of alkyl groups to aldehydes. The solution behavior of 8 and 9 was studied by NMR spectroscopy, revealing that both complexes in solution are in equilibrium with dimers and free titanium tetraisopropoxide. Nonlinear studies with catalytic H 8 -BINOL and an excess of titanium tetraisopropoxide in the asymmetric addition of ethyl groups to benzaldehyde showed no nonlinearity, suggesting that the equilibrium strongly favors formation of dinuclear 8 under the conditions of the asymmetric addition.