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

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.

Enantioselective Additions of Aryltitanium Tris(isopropoxide) to Ketones: Structure of [(i‐PrO)2Ti{μ‐(S)‐BINOLate}(μ‐O‐i‐Pr)TiPh(O‐i‐Pr)2], Study of Mechanistic and Stereochemical Insights

“…Even though the solid state structure and elemental analysis confirmed a dititanium species for (S)-4, the 8 ] structure has been reported recently. [21] are well known in addition to the structure of (S)-4. [21] are well known in addition to the structure of (S)-4.…”

Mechanistic Investigation of Chiral Phosphoric Acid Catalyzed Asymmetric Baeyer–Villiger Reaction of 3‐Substituted Cyclobutanones with H2O2 as the Oxidant

“…[66] For the exploration of the reaction pathway shown in Scheme 5 and the following reactivity calculations, (R)-H 8 -BINOL-derived phosphoric acid (1 c) was used as the model catalyst, in which the initial conformation of the binaphthyl rings was kept the same as that in the H 8 -BINOL/Ti complex, which has been characterized by X-ray crystallography. [67] Models were constructed and fully optimized at the B3LYP [68] /6-311 + G** level. For each optimized structure, a harmonic vibrational frequency calculation was carried out and thermal corrections were made.…”

Nonlinear Effects in Asymmetric Catalysis

“…Subsequent studies using H 8 -binol revealed similar NLEs. [42] rac-H 8 -Binol and one equivalent of Ti(OiPr) 4 resulted in the formation of meso-18, as characterized by single-crystal analysis. The meso dimers were found to be in equilibria with the corresponding homochiral dimers in solution.…”