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
Authors: Buncel Erwin , Tarkka Richard M. , Dust Julian M. 
Published: May 2006

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 (designated MC-4) are formed with the greatest exothermicity (ΔHc). Exothermicity of complexation increases for both hydroxide and methide adduct formation as the number of nitro groups in the electrophile is increased, from 7 to 9 and to 1, but formation of the methide adducts occurs uniformly with greater exothermicity than that of hydroxide adducts. These results are considered in light of solution calorimetric data that quantify adduct stability in condensed phases. Surprisingly, it is found that regioselectivity inverts for CH3−as compared to OH−and CH3O−in complexation with 2,4,6-trinitroanisole, 2. Thus, while methoxide and hydroxide form adducts at C-1 of TNA with higher exothermicity than at C-3, methide preferentially forms an adduct at C-3 according to the same enthalpy criterion. These results arise from the degree of stereoelectronic stabilization that may be imparted to the respective Meisenheimer complexes formed from ipso attack, that is, the adducts (MC-1) that are geminally disubstituted with electronegative heteroatom groups. For the methoxide MC-1 of TNA, 2, full stereoelectronic stabilization is provided by n–σ* donation from nonbonding electron pairs of the acetal-like methoxyl moieties to suitable C—O acceptor bonds. However, the methide moiety of the comparable MC-1 of TNA cannot partake in such an interaction and, so, with methide, MC-3 formation is preferred over MC-1. Further evidence is provided by consideration of the two energy minima obtained from optimization of the geometry of the oxygen-centred adducts formed by attack of methoxide at C-1 of TNA, 2. In the presence of a point charge that simulates an ion-paired cation, an "M-shaped" conformer is favoured for MC-1, while in the absence of a counterion the "S-shaped" conformer is favoured. Without a complexing counterion M and S conformers are both local minima, while the "S" conformer constitutes the global minimum. The AM1 optimized structure for the "M" conformer compares favourably to published X-ray data. The greater exothermicity of formation of the "S" conformer in the absence of the counterion is indicative of stereoelectronic stabilization of the O-adduct. The geometry is rationalized as a result of minimizing steric repulsion and maximizing the n-σ* stabilization of the C-1 adduct.