Design, Synthesis and Biological Evaluation of N-Phenylphthalimide Protoporphyrinogen IX Oxidase Inhibitor Herbicides

Protoporphyrinogen IX oxidase (PPO) is a key target for herbicide development. To develop novel PPO herbicides, a series of N-phenylphthalimide derivatives bearing benzoxazinone moieties were synthesized. Bioassays showed that compounds B2, A5, A30, and A11 exhibited excellent inhibitory activity against Nicotiana tabacum PPO (NtPPO) with K_i values of 9.50, 17.8, 11.2, and 11.7 nM, respectively─outperforming flumioxazin (K_i = 52.0 nM). At 9.375 g a.i./ha, B2 and A30 achieved over 80% control efficacy against three broadleaf weeds; they also displayed broad-spectrum herbicidal activity at 75 g a.i./ha, with a low injury index toward wheat, rice, and maize even at 150 g a.i./ha. Molecular docking indicated that B2 and A30 formed stable H-bonds with Arg98 (2.7 Å), π-π stacking with Phe392, and sandwich π-alkyl interactions with Leu356/Leu372. Molecular dynamics simulations and Gibbs free energy analysis confirmed their stable binding to NtPPO (A30, ΔG_bind = -44.50 kcal/mol; B2, ΔG_bind = -33.03 kcal/mol) with suitable conformational dynamics. Overall, B2 and A30 are promising PPO inhibitors for agricultural weed control.