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

Abstract

In several asymmetric reactions, H8-BINOL-based catalysts (H8-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 H8-BINOL and BINOL was, therefore, undertaken. Reaction of (rac)-H8-BINOL with 1 equiv of titanium tetraisopropoxide resulted in formation of the dimer (meso)-[(H8-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 H8-BINOLate and BINOLate ligands in these complexes were 63.2(5)° and 55.7(4)°, respectively. The larger torsional angle of the H8-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 H8-BINOL with an excess of titanium tetraisopropoxide provided crystals of the dinuclear complex [(H8-BINOLate)Ti(O-i-Pr)2]·Ti(O-i-Pr)4 (8). Likewise, reaction of 2 equiv of Ti(OCy)4 (Cy = cyclohexyl) with H8-BINOL furnished [(H8-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 H8-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.