Solvent steers chiral synthesis

Abstract

Organic chemists who want to make only one of a molecule’s two enantiomers typically use chiral catalysts to steer the synthesis in the right direction. Now, researchers have shown that chiral solvents can act as the sole source of handedness in catalytic reactions that preferentially produce a single enantiomer (ACS Cent. Sci. 2019, DOI: 10.1021/acscentsci.9b00330). The method works by passing the chirality of the solvent, (R)-limonene, to a copolymer, poly(quinoxaline-2,3-diyl), or PQX, which twists into a right-handed helix. Then the helix binds a palladium catalyst, creating a chiral structure that can control the symmetry of a reaction. Michinori Suginome of Kyoto University and colleagues created the catalyst and tested it in the Suzuki-Miyaura reaction, which forms a carbon-carbon bond by coupling an aryl bromide to an aryl boronic acid. It produced the S enantiomer of the coupling product almost exclusively, with an enantiomeric excess of 98% (a 99:1 mixture of