Asymmetric catalytic synthesis of polyketones and polycarbonates

Abstract

Two examples are presented for the synthesis of optically active polymers with main-chain chirality from achiral monomers using chiral metal-complexes as catalysts. Asymmetric alternating copolymerization of α-olefins with carbon monoxide provided optically active polyketones when catalyzed by an (R,S)-BINAPHOS-Pd complex. From propene and CO, highly isotactic polyketone with high enantioselectivity (>97% like diad and >95% ee). Spectroscopic and theoretical studies revealed that the olefin insertion is the key step for the enantiofacial selection and that this step takes place at cis to the phosphine part of (R,S)-BINAPHOS. The catalyst is applicable not only to propene/CO but also to styrene/CO, which enabled the first asymmetric terpolymerization of propene/styrene/CO. The catalyst tolerates funational groups such as fluorocarbons and a nitrile group so that they can be incorporated in the side chain. Optically active polycarbonate was also synthesized by the alternating copolymerization of cyclohexene oxide with carbon dioxide via the desymmetrization of the meso-epoxide. Dinuclei zinc species prepared from diethylzinc, ethanol, and α,α′ -diphenylprolinol, was revealed to be the real active species.