Catalytic enantioselective meso-epoxide ring opening reaction with phenolic oxygen nucleophile promoted by gallium heterobimetallic multifunctional complexes

The catalytic enantioselective meso-epoxide ring opening reaction with phen olic oxygen nucleophile (4-methoxyphenol) is described for the first time h erein. This reaction was first found to he promoted by (R)-GaLB (Ga = galli um, L = lithium, B = (R)-BINOL), giving a variety of epoxide opening produc ts 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 res ulted in the decomplexation of GaLB. Application of various known chiral li gands such as 6,6'-bis((triethylsilyl)ethynyl)-BINOL and H-8-BINOL were exa mined, 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 G a-complex was greatly improved. Using 3-10 mol % (R,R)-Ga-Li-linked-BINOL c omplex, a variety of epoxide opening reactions were found to proceed smooth ly, 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 mechan ism of the entitled reaction is also discussed, based on the X-ray structur e of the Ga-complex.