Synthesis, theoretical studies, and effect on the photosynthetic electron transport of trifluoromethyl arylamides

Abstract

The photosynthetic apparatus is targeted by various herbicides, including several amides such as diuron and linuron. Considering the need for the discovery of new active ingredients to cope with weed resistance, the synthesis of a series of trifluoromethyl aryl amides is herein described whose inhibitory properties were assessed in vitro on the photosynthetic electron transport chain, and in vivo on the growth of a model cyanobacterial strain. Theoretical studies were also carried out. Starting with 1‐fluoro‐2‐nitro‐4‐(trifluoromethyl) benzene, the preparation of the amides was achieved via a three‐step sequence, namely nucleophilic aromatic substitution, reduction with SnCl2/HCl, and acylation reactions. The measurement of ferricyanide reduction by functionally intact spinach chloroplasts showed that several derivatives are capable of inhibiting the photosynthetic apparatus. The most active amides presented IC50 values close to 1 μmol L−1, and showed the presence of a 4‐bromophenyl group as a common structural feature. The addition of these brominated amides to the culture medium of a model cyanobacterial strain, Synechococcus elongatus PCC 6301, caused various degrees of growth inhibition. Theoretical studies (molecular modeling and quantitative structure–activity relationship) of all amides and their comparison with some known herbicides confirmed these experimental findings and provided more in‐depth information about the possible molecular target of these compounds. Trifluoromethyl amides herein described, which were shown to act at the PSII level, may represent a novel scaffold to be exploited aiming at the development of new active ingredients for weed control.