Synthesis, crystal structure, and herbicidal activity of pyrimidinyl benzylamine analogues containing a phosphonyl group

A series of pyrimidinyl benzylamine analogues containing a phosphonyl group (2) was synthesized via the Mannich‐type reactions of 2‐(4,6‐dimethoxypyrimidin‐2‐yloxy)benzoaldehyde 1, aromatic amines, and dialkyl phosphites or triphenyl phosphite in the presence of Mg(ClO4)2. Their structures were characterized by spectroscopic data (infrared, 1H NMR, 31P NMR, and mass spectrometry) and elemental analyses, and compound 2b was further determined by X‐ray diffraction crystallography. The results of preliminary bioassays (in vitro) showed that most of title compounds 2 exhibited higher herbicidal activities against dicotyledonous weeds (Brassica campestris L.) than monocotyledonous weeds (Echinochloa crus‐galli). Further bioassays (in vivo) indicated that some of 2 displayed as good herbicidal activity against amaranth pigweed (Amaranthus retroflexus) as the commercially available herbicide, Bispyribac‐sodium, in both preemergence and postemergence treatments at the dose of 1.5 kg/ha.