Design, synthesis and herbicidal evaluation of novel 4-(1H-pyrazol-1-yl)pyrimidine derivatives.

A series of novel pyrazolylpyrimidine derivatives were designed, synthesised and characterised by IR, (1) H NMR, (13) C NMR, mass spectroscopy and elemental analysis. The herbicidal activities of 30 pyrazolylpyrimidine derivatives were assessed. Nine compounds caused good herbicidal activity for Pennisetum alopecuroides L. Among them, N-ethyl-6-[5-methyl-3-(trifluoromethyl)-1H-pyrazol-1-yl]-pyrimidin-4-amine exhibited the strongest inhibitory activity against the root growth of P. alopecuroides, with an IC50 of 1.90 mg L(-1) . 2-Methyl-4-[5-methyl-3-(trifluoromethyl)-1H-pyrazol-1-yl]-6-(prop-2-yn-1-yloxy)pyrimidine produced the highest inhibition of chlorophyll level in seedlings of P. alopecuroides (IC50  = 3.14 mg L(-1) ). The structure-activity relationship indicated that the alkynyloxy group at the 6-position on the pyrimidine ring played a very important role for bleaching activities. When the alkynyloxy group was replaced by alkoxy, amino, alkylthio and alkylsulfonyl groups, the bleaching activities of the compounds were diminished. However, the compounds substituted by an amino at the 6-position of the pyrimidine ring exhibited excellent inhibition activities against weed root growth.