AM1 studies on the stabilities of anionic σ-complex regioisomers: thermodynamics of regioselectivity in the reaction of methide, methoxide, and hydroxide anions with electron-deficient aromatics

Heats of formation (ΔHf) for a series of aromatics that are progressively more electron deficient (benzene, 6; nitrobenzene, 7; 4-fluoronitrobenzene, 8; 1,3-dinitrobenzene, 9; 2,4,6-trinitroanisole, 2; and 1,3,5-trinitrobenzene, 1) were determined by semiempirical AM1 calculations. As a probe of the factors that govern the regioselectivity exhibited in the formation of anionic σ-adducts (Meisenheimer complexes), experimental gas-phase ΔHf values for the prototypical oxygen and carbon nucleophiles (hydroxide, methoxide, and methide anions) were used in a thermochemical calculation along with the calculated ΔHf of the electrophiles and the adducts to determine the heats of complexation (ΔHc). The present results show that for the series of nitroaryl electrophiles, 7, 9, and 1, hydroxide and methide anions exhibit the same regioselectivity based on thermodynamics of Meisenheimer complex formation. Specifically, Meisenheimer complexes derived from attack at a position para to at least one nitro group (designa...