Iprobenfos

The Uses of Iprobenfos

Iprobenfos is a systemic fungicide which provides effective protective and curative control of leaf and ear blast (Pyricularia oryzae), stem rot (Helminthosporium spp.) and sheath blight (Rhizoctonia solani) in rice.

Definition

ChEBI: An organic thiophosphate that is the S-benzyl O,O-diisopropyl ester of phosphorothioic acid. Used as a rice fungicide to control leaf and ear blast, stem rot and sheath blight.

Metabolic pathway

Iprobenfos undergoes extensive degradation and metabolism. Primary metabolic pathways include isomerisation to the thionate (P=S) ester, cleavage of the P-S-benzyl and P-O-isopropyl moieties, and transesterification. Benzyl mercaptan, upon cleavage, undergoes additional oxidation reactions to yield the alcohol, carboxylic acid, disulfide and sulfonic acid. The formation of the oxon from the isomerisation product (P=S ester) was minor and was observed only under UV light irradiation. The primary metabolic pathways of iprobenfos are presented in Scheme 1.

Degradation

Iprobenfos (1) is stable to hydrolytic degradation (<50 °C). The DTN value of iprobenfos in aqueous solution was approximately 301-324 days(PM).
Iprobenfos degraded rapidly when deposited as a thin film and exposed to UV light (DT₅₀ ca. 10 min). Two major degradation reactions were observed during the initial phase of the irradiation. The first reaction involved the isomerisation of iprobenfos to O,O-diisopropyl O- benzyl phosphorothioate (2) followed by the oxidative desulfuration of compound 2 to yield the corresponding oxon analogue 3 (O,O-diisopropyl O-benzyl phosphate). The second reaction involved the hydrolytic cleavage of the S-C linkage of iprobenfos to yield O,O-diisopropyl hydrogen phosphorothioate (4) and the cleavage of the P-O-benzyl linkage of compound 3 to yield O,O-diisopropyl hydrogen phosphate (5). Other major photolytic degradation reactions include the cleavage of the parent P-S linkage to O,O-diisopropyl phosphonate (6). Benzyl alcohol (7) and benzyl mercaptan (8) were also detected. Further oxidation of compound 7 yielded benzoic acid (9) and the oxidation of compound 8 yielded benzylsulfonic acid (10), dibenzyl disulfide (11) and sulfuric acid as terminal products. Other minor products detected included transesterification products [O,O,S-triisopropyl phosphorothioate (12), O,O,O-triisopropyl phosphate (13)] and benzyl isopropyl sulfide (14).