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 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.
Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095, USA.
Steven E
SE
Wheeler
Anne J
AJ
McNeil
Peter
P
Müller
Timothy M
TM
Swager
K N
KN
Houk
D000480:Alkynes / Q000737:chemistry; D000873:Anthracenes / Q000737:chemistry; D001555:Benzene Derivatives / Q000737:chemistry*; D003500:Cyclization; D008958:Models, Molecular; D008968:Molecular Conformation; D013237:Stereoisomerism; D013816:Thermodynamics
D000480:Alkynes; D000873:Anthracenes; D001555:Benzene Derivatives; C022918:acetylenedicarboxylic acid dimethyl ester