Free electrons offer chemists a new way to control reactions that conventional electron transfer rules would normally prevent. For decades, however, a basic rule has restricted which reactions chemists can design. When two molecules compete to receive an electron, the electron ordinarily moves to the molecule that is easier to reduce. Reported in Nature, the strategy overcomes a persistent problem in electron transfer selectivity and may make previously inaccessible coupling reactions possible. “Our calculations reveal how the decisive selectivity emerges after electron transfer has already occurred,” says Paton.