Different molecular enantiomers can have different chemistry, both in the body and in industrial processes. Controlling the yield of the right one is a time-consuming yet critical step in organic and pharmaceutical synthesis. Shu-Li You and coworkers at the Shanghai Institute of Organic Chemistry have found a surprising and simple shortcut: they can selectively make either enantiomer of some chiral amines just by varying the reaction times (Nat. Chem. 2020, DOI: 10.1038/s41557-020-0489-1). They used an iridium cyclooctadiene compound and a chiral olefin to create a chiral catalyst in solution, a common approach for asymmetric synthesis. After 6 min, S isomers form with 84–99% enantiomeric purity. If the researchers let the reaction go for 10 h, the R isomer forms with 74–99% enantiomeric purity (shown). The catalyst is highly selective for the S isomer and makes the compound quickly, You says. Over time, however, it decomposes and the more stable R isomer forms. The team discovered this while monitoring the reaction every 10 min. You says he was shocked by the results, as no one has reported this effect in asymmetric catalysis before. The team is now studying if the effect occurs with other reactions.