The standard reagent used for detecting cysteine sulfenic acids is dimedone (5,5-dimethyl-1,3-cyclohexanedione) and derivatives labeled with biotin, 4 fluorophores, 4b alkynes, or azides. While dimedone probes are reported to be selective for the sulfenic acid oxidation state of cysteine, they often react slowly. To increase the reactivity of sulfenic acid probes, a number of structural variations of dimedone and carbon-centered nucleophiles have been explored, 7a, 8 but there remains a need for probes that react both rapidly and selectively with cysteine sulfenic acid.