Larger Substituents on Amide Cavitands Induce Bigger Cavities

Area of Science:

• Supramolecular Chemistry
• Organic Synthesis
• Crystal Engineering

Background:

• Cavitands are macrocyclic hosts capable of molecular recognition and encapsulation.
• The design of cavitands allows for tuning of cavity size and properties through substituent modification.
• Understanding structure-property relationships is crucial for developing novel host molecules.

Purpose of the Study:

• To synthesize novel quinoxaline-based cavitands with pendant amide groups of varying steric bulk.
• To investigate the influence of substituent size on cavitand conformation and cavity dimensions.
• To explore the guest encapsulation capabilities of these tailored cavitands.

Main Methods:

• Synthesis of quinoxaline cavitands with ethyl (Et), isopropyl (iPr), and tert-butyl (tBu) amide substituents.
• Structural characterization using X-ray crystallography and Nuclear Magnetic Resonance (NMR) spectroscopy.
• Analysis of molecular conformation and intramolecular hydrogen bonding interactions.

Main Results:

• Vase conformation observed for Et and iPr amide cavitands; kite conformation for the bulky tBu derivative at room temperature.
• X-ray structures confirmed intramolecular hydrogen bonding dictates conformation and cavity size based on functional group bulkiness.
• Proton NMR (1H NMR) demonstrated the Et cavitand's ability to encapsulate an adamantane guest with slow exchange kinetics.

Conclusions:

• The steric bulk of pendant amide groups significantly controls the conformational preferences and cavity dimensions of quinoxaline cavitands.
• Intramolecular hydrogen bonding plays a key role in stabilizing specific conformations and influencing host-guest interactions.
• The synthesized Et cavitand exhibits selective guest encapsulation, highlighting its potential in molecular recognition applications.