Probing substituent effects in aryl-aryl interactions using stereoselective Diels-Alder cycloadditions

J Am Chem Soc. 2010 Mar 17;132(10):3304-11. doi: 10.1021/ja903653j.

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 pi-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 pi-stacking free energies are shown to correlate well with Hammett sigma(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 pi-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 pi-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 pi-stacking interactions, despite being a pi-electron donor. This is in conflict with popular models in which substituent effects in aryl-aryl interactions are modulated by polarization of the aryl pi-system.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Alkynes / chemistry
  • Anthracenes / chemistry
  • Benzene Derivatives / chemistry*
  • Cyclization
  • Models, Molecular
  • Molecular Conformation
  • Stereoisomerism
  • Thermodynamics

Substances

  • Alkynes
  • Anthracenes
  • Benzene Derivatives
  • acetylenedicarboxylic acid dimethyl ester