Design, Synthesis and Biological Evaluation of <i>N</i> -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 <i>N</i>-phenylphthalimide derivatives bearing benzoxazinone moieties were synthesized. Bioassays showed that compounds <b>B2, A5, A30</b>, and <b>A11</b> exhibited excellent inhibitory activity against <i>Nicotiana tabacum</i> PPO (NtPPO) with <i>K</i>_i values of 9.50, 17.8, 11.2, and 11.7 nM, respectively─outperforming flumioxazin (<i>K</i>_i = 52.0 nM). At 9.375 g a.i./ha, <b>B2</b> and <b>A30</b> 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 <b>B2</b> and <b>A30</b> 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 (<b>A30</b>, Δ<i>G</i>_bind = -44.50 kcal/mol; <b>B2</b>, Δ<i>G</i>_bind = -33.03 kcal/mol) with suitable conformational dynamics. Overall, <b>B2</b> and <b>A30</b> are promising PPO inhibitors for agricultural weed control.