REGIOSELECTIVITY AND STEREOELECTRONIC EFFECTS IN THE REACTIONS OF THEDINITROANILINE HERBICIDES TRIFLURALIN AND BENEFIN WITH NUCLEOPHILES

The reactions of two members of the dinitroaniline class of herbicides , di-n-propyl-2,6-dinitro-4-(trifluoromethyl)aniline (trifluralin; 1) and l-N-n-butyl-2,6-dinitro-4-(trifluoromethyl)aniline (benefin; 2), a long with their analogue, N-phenyl-2,6-dinitro-4-(trifluoromethyl)anil ine (3), with the nucleophiles, OD- and SO32-, have been investigated using 400 MHz H-1 NMR spectroscopy. The reactions of both 1 and 2 with OD- result in formation of Meisenheimer anionic sigma-complexes accor ding to a K3T1 (kinetic preference for formation of the C-3 adduct wit h thermodynamic preference for formation of the C-1 adduct) reaction s equence while the reaction of 3 with OD- and that of 1 with SO32- foll ow 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 obse rved as the final thermodynamic products. Geometry optimization calcul ations support our hypothesis of n --> sigma stabilization of the C-1 adduct leading to SNAr displacement. In the reaction of 3 with OD-, i nitial N-deprotonation to form the anion, 3a, is followed by sigma-com plex formation. The final thermodynamic product observed in this syste m is 3,5 -dinitro-4-(N-phenylamino)benzoic acid formed through hydroly sis of the trifluoromethyl group on the anion, 3a. Aryl H-D exchange h as been found for the systems of 1 and 2 with OD-, but not for the SO3 2- system and neither for the reaction of 3 with OD-. Since dimethylpi cramide showed significantly slower H-D exchange under identical condi tions, it is argued that this discrepancy has as origin the ability of the amino N lone electron pair to interact with the pi-system of the ring. With both 1 and 2 the larger size of the amino alkyl chains prev ent the amino N lone pair from aligning with the pi-system of the ring , thus hindering electron density donation to the electron-deficient r ing carbons.