Stereoselective Single Step Cyclization to Give Belt-Functionalized Pillar[6]arenes

Area of Science:

• Supramolecular Chemistry
• Organic Synthesis
• Materials Science

Background:

• Pillar[6]arenes are macrocyclic compounds with a unique pillar shape.
• Functionalization of pillararene belts can alter their structure and properties.

Purpose of the Study:

• To synthesize novel belt-substituted pillar[6]arenes.
• To investigate the stereoselectivity and structural characteristics of these new macrocycles.
• To compare the conformational stability with unsubstituted pillararenes.

Main Methods:

• Cyclization reactions of secondary benzylic alcohols.
• Stereoselective synthesis and isolation of macrocyclic products.
• X-ray crystallography for structural determination.
• Density functional theory (DFT) calculations for conformational analysis.

Main Results:

• Two pillar[6]arene macrocycles with methyl substituents at each belt position were synthesized.
• The synthesis yielded exclusively the *rtctct* isomer with alternating methyl group orientations.
• X-ray structures revealed an unexpected capsule-like conformation.
• DFT calculations showed a higher energy barrier for the conventional pillar conformation.

Conclusions:

• Belt functionalization significantly impacts pillararene macrocycle conformation.
• The synthesized macrocycles possess a unique capsule-like structure, deviating from the typical pillar shape.
• Conformational rigidity is increased in these belt-substituted pillararenes compared to their unsubstituted counterparts.