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)degrees and 55.7(4)degrees, 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 2degrees. 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.