Semidione

Semidiones are radical anions analogous to semiquinones, obtained from the one-electron reduction of non-quinone conjugated dicarbonyls.^[1]

The simplest possible semidiones are derived from 1,2-dicarbonyls and have structure R−C(−O⁻)=C(−O^•)−R' ↔ R−C(−O^•)=C(−O⁻)−R', making them the second member of a homologous series starting with ketyl radicals and continuing with semitriones.^[1] They are often transient intermediates, appearing in reactions such as the final reduction step of the acyloin condensation.^[2]

Benzil semidione (Ph−C(−O⁻)=C(−O^•)−Ph ↔ Ph−C(−O^•)=C(−O⁻)−Ph), synthesized by Auguste Laurent in 1836, is believed to have been the first radical ion ever characterized.^{[3]: 425 [4]}

Semidehydroascorbate is a relatively stable semitrione produced by hydrogen abstraction from ascorbate (Vitamin C).^[5]