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

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 constants (r= 0.96). These substituent constants primarily provide a measure of the Inductive electrondonating 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. © 2010 American Chemical Society.