Abstract
The so far poorly understood factors controlling the complete meta‐selectivity observed in the C−H activation reactions of alkylarenes promoted by aluminyl anions have been explored in detail by means of Density Functional Theory calculations. To this end, a combination of state‐of‐the‐art computational methods, namely the activation strain model of reactivity and energy decomposition analysis, has been applied to quantitatively unveil the origin of the selectivity of the transformation as well as the influence of the associated potassium cation. It is found that the selectivity takes place during the initial nucleophilic addition step where the key LP(Al)→π*(C=C) molecular orbital interaction is more stabilizing for the meta‐pathway, which results in a stronger interaction between the reactants along the entire transformation.
Highlights
In contrast to neutral, low-valent Al(I) compounds, which have been extensively studied since the isolation of [AlCp*4] in 1991,[1,2] the chemistry of related anionic species, known as aluminyl anions, is still in its infancy
We first explored the meta-selective CÀ H activation reaction involving toluene and the monomeric aluminyl species 1, described by Aldridge and co-workers,[12] where the bulky tBu groups in A were replaced by methyl groups
The Meisenheimer intermediate INT1 is formed in an endergonic reaction (ΔGR = 21.6 kcal/mol from the separate reactants) via TS1, a saddle point associated with the formation of the new AlÀ C bond (ΔG1⁄46 = 33.3 kcal/mol)
Summary
Low-valent Al(I) compounds, which have been extensively studied since the isolation of [AlCp*4] in 1991,[1,2] the chemistry of related anionic species, known as aluminyl anions, is still in its infancy. The presence of potassium clearly favors the entire reaction pathway from the initial reactant complex RC-K over the process involving the naked anion 1.[35] This is mainly due to the stabilizing interaction of the cation with the π-system of toluene along the entire reaction coordinate and during the key SNAr step.
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