Imidazolidine-4-thiones have been suggested as potential prebiotic organocatalysts for light-driven α-alkylations of aldehydes by bromoacetonitrile. However, imidazolidine-4-thiones react with bromoacetonitrile to giveS-cyanomethylated dihydroimidazoles. Kinetic studies show that enamines derived from these cyclic secondary amines and aldehydes are more nucleophilic than enamines derived from aldehydes and MacMillan organocatalysts.