Asymmetric catalysis by 1,1′-binaphthyl compounds with conformation-defined 3,3′-aryl substituents

Three 1,1′-binaphthyl-based diastereomeric chiral ligands containing conformation-defined 3,3′-aryl substituents are synthesized and characterized. These compounds are isolated from the product mixture of the Suzuki coupling of (S)-3,3′-diiodo-2,2′-bis-methoxymethoxy-[1,1′]binaphthalenyl with 2-methoxy-1-naphthylboronic acid. The absolute structures of these compounds are determined by a single crystal X-ray analysis of the (S,S,S)-diastereomer. Among the three diastereomers, i.e. (S,S,S), (R,S,S) and (R,S,R), the (S,S,S) compound shows the highest catalytic activity as well as the highest enantioselectivity for the diethylzinc addition to aldehydes. At 0°C in toluene solution, 5 mol% of the (S,S,S) isomer catalyzed the reaction of diethylzinc with both aromatic and aliphatic aldehydes with up to 96% ee and 90% yield. The other two isomers show quite a low catalytic activity and enantioselectivity. This demonstrates that the 3,3′-aryl conformation of the 1,1′-binaphthyl ligands plays a very important role in the catalytic process. The study of these diastereomeric compounds provides new insights into the design of chiral 1,1′-binaphthyl-based monomer as well as polymer catalysts for asymmetric catalysis.