Probing substituent effects in aryl-aryl interactions using stereoselective diels-alder cycloadditions
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Swager_Probing substituent.pdf
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Author(s)
Wheeler, Steven E.
McNeil, Anne J.
Muller, Peter
Swager, Timothy Manning
Houk, K. N.
Date Issued
February 2010
Journal
Journal of the American Chemical Society
Publisher
American Chemical Society (ACS)
Citation
Wheeler, Steven E. et al. “Probing Substituent Effects in Aryl−Aryl Interactions Using Stereoselective Diels−Alder Cycloadditions.” Journal of the American Chemical Society 132.10 (2010): 3304–3311.
Version
Author's final manuscript
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.
MIT Department
Massachusetts Institute of Technology. Department of Chemistry
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DOI of Published Version
https://doi.org/10.1021/ja903653j
