Inducing Propeller Chirality in Triaryl Boranes with Chiral Amines

Area of Science:

• Organic Chemistry
• Stereochemistry
• Supramolecular Chemistry

Background:

• Stabilizing chiral propeller conformations in triaryl compounds is challenging due to low racemization barriers.
• Previous work induced chiral conformational preferences in triaryl boranes using point-chiral alkylether chains and ammonia.
• A four-point interaction involving Lewis-adduct formation and hydrogen bonds was proposed for stabilization.

Purpose of the Study:

• To investigate if three strong interactions are sufficient to induce preferential propeller handedness in triaryl boranes.
• To explore the role of amine steric demand in the induction and stabilization of chiral conformations.
• To compare computational predictions with experimental VCD spectroscopic data.

Main Methods:

• Synthesis of triaryl borane derivatives with chiral alkylether chains.
• Formation of adducts with chiral primary amines.
• Density Functional Theory (DFT) calculations to predict conformational preferences.
• Vibrational Circular Dichroism (VCD) spectroscopy to experimentally determine propeller handedness.

Main Results:

• Three strong interactions were shown to be sufficient for inducing preferential propeller handedness.
• VCD spectroscopy revealed a measurable excess of one propeller handedness, even when DFT calculations predicted no preference.
• The steric bulk of the amine was found to be crucial for inducing chirality and preventing rotation.

Conclusions:

• Preferential propeller handedness can be reliably induced in triaryl boranes using a combination of Lewis-adduct formation and hydrogen bonding.
• VCD spectroscopy is a powerful tool for characterizing subtle chiral preferences in molecular conformations.
• Amine structure significantly influences the stereochemical outcome and conformational stability of triaryl borane adducts.