Regioselectivity and stereoelectronic effects in the reactions of the dinitroaniline herbicides trifluralin and benefin with nucleophiles

Abstract

The reactions of two members of the dinitroaniline class of herbicides, N,N-di-n-propyl-2,6-dinitro-4- (trifluoromethyl)aniline (trifluralin; 1) and N-ethyl-N-n-butyl-2,6-dinitro-4-(trifluoromethyl)aniline (benefin; 2), along with their analogue, N-phenyl-2,6-dinitro-4-(trifluoromethyl)aniline (3), with the nucleophiles, OD- and SO32- , have been investigated using 400 MHz 1H NMR spectroscopy. The reactions of both 1 and 2 with OD- result in formation of Meisenheimer anionic σ -complexes according to a K3T1 (kinetic preference for formation of the C-3 adduct with thermodynamic preference for formation of the C-1 adduct) reaction sequence while the reaction of 3 with OD- and that of 1 with SO32- follow a K3T3 (kinetic and thermodynamic preference for formation of the C-3 adduct) sequence. There was no observation of the C-1 adducts of 1 and 2 with OD-, but the products of SNAr displacement at C-1 were observed as the final thermodynamic products. Geometry optimization calculations support our hypothesis of n –> σ * stabilization of the C-1 adduct leading to SNAr displacement. In the reaction of 3 with OD-, initial N-deprotonation to form the anion, 3a, is followed by σ -complex formation. The final thermodynamic product observed in this system is 3,5-dinitro-4-(N-phenylamino)benzoic acid formed through hydrolysis of the trifluoromethyl group on the anion, 3a. Aryl H-D exchange has been found for the systems of 1 and 2 with OD-, but not for the SO32- system and neither for the reaction of 3 with OD-. Since dimethylpicramide showed significantly slower H-D exchange under identical conditions, it is argued that this discrepancy has as origin the ability of the amino N lone electron pair to interact with the π -system of the ring. With both 1 and 2 the larger size of the amino alkyl chains prevent the amino N lone pair from aligning with the π -system of the ring, thus hindering electron density donation to the electron-deficient ring carbons.Key words: trifluralin, benefin, herbicides, Meisenheimer complexes, stereoelectronic effects.