Probing substituent effects in aryl-aryl interactions using stereoselective diels-alder cycloadditions

Abstract

Stereoselective Diels−Alder cycloadditions that probe substituent effects in aryl−aryl sandwich complexes were studied experimentally and theoretically. Computations on model systems demonstrate that the stereoselectivity in these reactions is mediated by differential π-stacking interactions in competing transition states. This allows relative stacking free energies of substituted and unsubstituted sandwich complexes to be derived from measured product distributions. In contrast to gas-phase computations, dispersion effects do not appear to play a significant role in the substituent effects, in accord with previous experiments. The experimental π-stacking free energies are shown to correlate well with Hammett σ[subscript m] constants (r = 0.96). These substituent constants primarily provide a measure of the inductive electron-donating and -withdrawing character of the substituents, not donation into or out of the benzene π-system. The present experimental results are most readily explained using a recently proposed model of substituent effects in the benzene sandwich dimer in which the π-system of the substituted benzene is relatively unimportant and substituent effects arise from direct through-space interactions. Specifically, these results are the first experiments to clearly show that OMe enhances these π-stacking interactions, despite being a π-electron donor. This is in conflict with popular models in which substituent effects in aryl−aryl interactions are modulated by polarization of the aryl π-system.