Karl-Otto Westphalen

Synthesis and herbicidal activity of phenylpyridines: A new lead

Novel phenylpyridines were synthesized based on molecular modeling oriented design by superposition of herbicidal phenyl-uracils, -indazoles, and diphenylethers. Their preparation follows Suzuki-type coupling conditions of reactive 2-chloro-pyridines with appropriate 2,5-substituted 4-chloro-phenyl boronic acids. Depending on the nature of substituents in the pyridine, and in particular the phenyl group, a large number of derivatives are readily accessible. Besides open-chain phenylpyridines also anellated compounds, such as pyridine-substituted 2H-1,4-benzothiazin-3(4H)-one or 3,4-dihydro-1H-quinolin-2-one, were prepared. Phenylpyridines act by inhibition of protoporphyrinogen-IX-oxidase. Their synthesis will be described in conjunction with structure-activity relationships. Phenylpyridines are highly active against many important broadleaf and grass weed species under pre- and - in particular - post-emergent conditions.

New fluoro intermediates for herbicidal sulfonylureas

New pyrimidine and triazine intermediates for herbicidal sulfonylureas are prepared as follows: 2,4-dichloro-6-methylpyrimidine is converted via a halogenation, halogen exchange and substitution sequence to 2-amino-4-trifluoromethyl-6-trifluoromethoxypyrimidine. New fluoromethyl-triazines are available starting from guanidine, trichloroacetonitrile and difluoroacetic anhydride, or alternatively from thiocarbamoyl guanidine and chlorodifluoroacetic ester ring closure. To obtain new o-fluoroalkyl-benzenesulfonamide precursors, o-chlorobenzaldehyde was fluorinated with sulfur tetrafluoride, or a bromobenzene derivative was subjected to a substitution reaction with sodium pentafluoropropionate. Sulfonylureas derived from trifluoromethylpyrimidines with an m-methylthio substituent are selective post-emergence herbicides in cotton, presumably due to sulfone metabolization. Selectivity in wheat is obtained by combining 4-methoxy-6-trifluoromethylpyrimidine with a lipophilic difluoromethyl-benzenesulfonamide moiety. Also in the difluoromethyl-triazine series, the combination with the difluoromethyl-benzenesulfonamide moiety is a good choice for wheat selectivity. Chlorodifluoromethyl- and trifluoromethyltriazines, however, combine better with an aromatic ester for best activity and selectivity in wheat. Selected compounds are undergoing broad field tests in wheat.