Tunable asymmetric catalysis through ligand stacking in chiral rigid rods

Area of Science:

• Organic Chemistry
• Catalysis
• Supramolecular Chemistry

Background:

• Chiral ligands are crucial for asymmetric catalysis.
• Achieving high enantioselectivity with catalysts where the chiral center is remote from the metal center remains a challenge.
• Self-assembly offers a novel approach to construct chiral catalytic environments.

Purpose of the Study:

• To investigate the use of chiral benzene-1,3,5-tricarboxamide (BTA) ligands in rhodium-catalyzed asymmetric hydrogenation.
• To explore the role of self-assembly and polymer formation in inducing enantioselectivity.
• To demonstrate the application of the sergeants-and-soldiers principle in catalysis.

Main Methods:

• Synthesis of chiral benzene-1,3,5-tricarboxamide (BTA) ligands with remote chiral side chains.
• Rhodium-catalyzed asymmetric hydrogenation of dimethyl itaconate.
• Control experiments and spectroscopic analysis to elucidate the mechanism.
• Co-polymerization studies involving different BTA monomers.

Main Results:

• Up to 82% enantiomeric excess (ee) was achieved in the asymmetric hydrogenation using chiral BTA ligands.
• Spectroscopic studies confirmed the formation of chiral helical polymers through self-association of BTA monomers.
• Addition of a phosphine-free chiral BTA comonomer increased enantioselectivity to 88% ee.
• An achiral BTA ligand demonstrated a 31% ee, highlighting catalytically relevant chirality transfer between self-assembled units.

Conclusions:

• Chiral BTA ligands can form self-assembled helical polymers that act as effective chiral environments for asymmetric catalysis.
• The enantioselectivity is driven by supramolecular organization rather than direct coordination.
• Mixing different BTA monomers is a simple strategy to enhance catalytic performance.
• This work exemplifies the sergeants-and-soldiers principle in asymmetric catalysis, showcasing efficient chirality transfer in self-assembled systems.