Cyclodextrin-Encapsulated NHCs: Increased Selectivity and Reactivity of CO₂ in Amine Formylation

Area of Science:

• Organic chemistry
• Catalysis
• Supramolecular chemistry

Background:

• N-Heterocyclic Carbenes (NHCs) are versatile organocatalysts.
• Cyclodextrins (CDs) are known for their host-guest complexation abilities.
• CO2 utilization in organic synthesis is a growing area of research.

Purpose of the Study:

• To confine NHC organocatalysts within cyclodextrin cavities.
• To investigate the catalytic activity and selectivity of confined NHCs for amine formylation via CO2 hydrosilylation.
• To elucidate the role of cyclodextrin structure on catalytic performance.

Main Methods:

• Encapsulation of NHC organocatalysts within different cyclodextrin cavities (alpha-CD, beta-CD).
• Catalytic reactions for amine formylation using CO2 hydrosilylation.
• Spectroscopic analysis (NMR) and computational studies (DFT) to understand reaction mechanisms and interactions.

Main Results:

• Confined NHC-CD complexes effectively catalyze amine formylation using CO2.
• Significant substrate-selectivity was observed, influenced by the cyclodextrin cavity size.
• The smallest alpha-CD exhibited superior selectivity and reactivity compared to beta-CD.
• NMR and DFT studies revealed hydrogen bonding interactions between complexed CO2 and the alpha-CD cavity, destabilizing the NHC-CO2 adduct and enhancing reactivity.

Conclusions:

• Confinement of NHC organocatalysts in CDs offers a strategy for selective CO2 functionalization.
• Alpha-cyclodextrins enhance both selectivity and reactivity in NHC-catalyzed amine formylation.
• The observed effects are attributed to specific interactions between CO2 and the cyclodextrin cavity.