651301-77-8Relevant academic research and scientific papers
Solid state structures and solution behavior of titanium(IV) octahydrobinaphtholate complexes. Examination of nonlinear behavior in the asymmetric addition of ethyl groups to benzaldehyde
Waltz, Karen M.,Carroll, Patrick J.,Walsh, Patrick J.
, p. 127 - 134 (2004)
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.
