Catalytic Enantioselective meso-Epoxide Ring Opening Reaction with Phenolic Oxygen Nucleophile Promoted by Gallium Heterobimetallic Multifunctional Complexes

Abstract

The catalytic enantioselective meso-epoxide ring opening reaction with phenolic oxygen nucleophile (4-methoxyphenol) is described for the first time herein. This reaction was first found to be promoted by (R)-GaLB (Ga ) gallium, L ) lithium, B ) (R)-BINOL), giving a variety of epoxide opening products in good to high ee (67-93% ee). However, chemical yield was only modest (yield 31-75%), despite the use of more than 20 mol % GaLB. This was due to the undesired ligand exchange between BINOL and 4-methoxyphenol, which resulted in the decomplexation of GaLB. Application of various known chiral ligands such as 6,6′-bis((triethylsilyl)ethynyl)-BINOL and H 8 -BINOL were examined, but satisfactory results were not obtained. To overcome this problem a novel linked-BINOL containing coordinative oxygen atom in the linker has been developed. By linking two BINOL units in GaLB, the stability of the Ga-complex was greatly improved. Using 3-10 mol % (R,R)-Ga-Li-linked-BINOL complex, a variety of epoxide opening reactions were found to proceed smoothly, affording products in analogous ee (66-96% ee) and in much higher yield (yield 67-94%) compared to (R)-GaLB. The structure of the LiCl free Ga-Li-linked-BINOL complex was elucidated by X-ray analysis. This is the first X-ray data for an asymmetric catalyst containing gallium. The possible mechanism of the entitled reaction is also discussed, based on the X-ray structure of the Ga-complex.

The Development of the Asymmetric Morita—Baylis—Hillman Reaction Catalyzed by Chiral Brønsted Acids

“…The use of (R)-6,6'-dibromo-BI-NOL 4 afforded product in low yield and correspondingly low enantioselectivity as well (entry 3). Improvements in enantioselectivity and yield were observed with the use of saturated BINOL derivatives [18] (entries 4 and 5) and (R)-3,3'-dibromo-H 8 -BINOL 5b (entry 5). The use of an additional ligand, 10.5 mol % (R)-3,3'-dibromo-H 8 -BINOL 5b, led to the best enantiose- Table 1.…”

Enantioselective Total Syntheses of Several Bioactive Natural Products Based on the Development of Practical Asymmetric Catalysis

“…Thus, we attempted to develop a new asymmetric catalyst. For preparation of an efficient catalyst, lanthanide and O-linked-BINOL 21, which was previously developed by Shibasaki, et al, [73][74][75][76] were chosen as a Lewis acidic center metal and chiral ligand based on the following properties: (1) the lanthanide (Ln) phenoxide complex is relatively stable against moisture [4][5][6][7][8] and the oxygen in the linker is likely to coordinate to the lanthanide metal in an Ln-O-linked-BINOL complex, stabilizing the Ln complex as a pentadentate ligand 74) and (2) the ionic radius and coordination number of lanthanides are larger than those of aluminum, making a relatively large chiral reaction pocket. First, the asymmetric Michael reaction of dibenzyl malonate (19) to 1 was examined with monometallic (La only) and heterobimetallic (e.g., La-Li, La-Na, La-K) complexes.…”

Facile, mild and convenient preparation and characterization of some novel Schiff base ligands from synthetic diamines and salicylaldehyde

“…These compounds have been used for preparing ion-selective electrodes for determination of cations in solution with potentiometric methods [4]. Cooperative biometallic catalysis has been documented in several recently reported such as asymmetric epoxide ring opening reaction [5][6][7][8][9]. In such systems that metal is complexed with special Schiff bases, one metal is proposed to serve as Lewis acid for epoxide activation and another as counter ion for the nucleophile [10], reaction of some epoxides with elemental iodine and bromine in the presence of catalytic amounts of Schiff base complexes of metal(II) for preparation of haloalcoholes in high yields and regioselectivity [11], mimic peroxidase in the oxidation of phenol hydrogen peroxides in the presence of Schiff base complexes of Mn(III).…”