Catalytic Enantioselective <i>m</i><i>eso</i>-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.