Synthesis, Herbicidal Activity, and Mode of Action of IR 5790

IR 5790, an arylthiadiazolone herbicide structurally related to oxadiargyl and oxadiazon, was synthesized. The herbicidal activity and mode of action of IR 5790 were investigated. This herbicide has broad-spectrum pre-emergence activity against both dicotyledonous and monocotyledonous weeds. The phenotypic responses of susceptible plants, such as interruption of growth and lightdependent development of necrotic areas on the foliage, are consistent with those observed with protoporphyrinogen oxidase-inhibiting herbicides. Tissues exposed to IR 5790 in darkness accumulated protoporphyrin IX, which led to a photodynamic loss of membrane integrity upon exposure to light. Consistent with these physiological symptoms, IR 5790 strongly inhibited protoporphyrinogen oxidase, with an I 50 value of 3 nM. The presence of a sulfur atom did not significantly alter the molecular properties of the thiadiazolone ring, relative to the oxadiazolone ring of oxadiargyl, which explains why IR 5790 has the same mode of action as this herbicide.