Syntheses and insecticidal activities of novel 2-fluorophenyl-5-aryl/cyclopropyl-1,3,4-oxadiazoles

Two 2-fluorophenyl-5-substitued cyclopropyl-1,3,4-oxadiazoles (IIIa–b) with cis/trans isomers were synthesized from the corresponding hydrazide (Ia) and aroyl chlorides by two-step reactions. Six asymmetrical 2-(2,4-dichloro-5-fluorophenyl)-5-aryl-1,3,4-oxadiazoles (IIIc–h) were obtained by one step reaction of 2,4-dichloro-5-fluorobenzoic acid hydrazide (Ic) with the corresponding aromatic carboxylic acid in the presence of phosphorus oxychloride under reflux. However, the similar reaction of 4-fluorophenoxyacetic acid hydrazide (Ib) with 2,4-dichloro-5-fluorobenzoic acid failed to yield the desired asymmetrical 2-(2,4-dichloro-5-fluorophenyl)-5-(fluorophenoxymethyl)-1,3,4-oxadiazole, and the resulting product was characterized as symmetrical 2,5-bis(2,4-dichloro-5-fluorophenyl)-1,3,4-oxadiazole (DCFPO). A possible reaction mechanism is suggested. Insecticidal activities of these nine new compounds against armyworms (Pseudaletia separata Walker) were determined.