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

Area of Science:

• Organometallic Chemistry
• Supramolecular Chemistry
• Physical Organic Chemistry

Background:

• Aryl-aryl interactions are crucial in supramolecular chemistry and organometallic complexes.
• Understanding substituent effects on these interactions is key to designing molecular systems.
• Previous models often focused on pi-system electronic effects.

Purpose of the Study:

• To investigate substituent effects on stereoselective Diels-Alder reactions involving aryl-aryl sandwich complexes.
• To elucidate the role of pi-stacking interactions versus electronic effects in mediating these substituent effects.
• To experimentally derive relative pi-stacking free energies and correlate them with substituent properties.

Main Methods:

• Experimental Diels-Alder cycloadditions to probe stereoselectivity.
• Computational modeling of transition states and pi-stacking interactions.
• Analysis of product distributions to determine free energy differences.
• Correlation of experimental data with Hammett sigma constants.

Main Results:

• Stereoselectivity is mediated by differential pi-stacking interactions in competing transition states.
• Experimental pi-stacking free energies correlate well with Hammett sigma(m) constants (r = 0.96).
• Dispersion effects were found to be insignificant in gas-phase computations, aligning with prior experiments.
• Results indicate OMe enhances pi-stacking, contradicting models relying on pi-system polarization.

Conclusions:

• Substituent effects in aryl-aryl sandwich complexes are primarily governed by through-space interactions, not pi-system electronic donation.
• The findings challenge established models of substituent effects in aryl-aryl interactions.
• This study provides the first experimental evidence that electron-donating groups like OMe can enhance pi-stacking interactions.