[1]rotaxanes and pretzelanes: synthesis, chirality, and absolute configuration [In Process Citation]

Area of Science:

• Supramolecular Chemistry
• Organic Synthesis
• Stereochemistry

Background:

• Rotaxanes and catenanes are mechanically interlocked molecules with unique topological properties.
• Chiroptical properties of supramolecular structures are crucial for applications in chiral recognition and sensing.
• Controlling the bridge length in rotaxanes and pretzelanes is key to tuning their molecular architecture and function.

Purpose of the Study:

• To synthesize aliphatically bridged [1](n)rotaxanes and (n)pretzelanes.
• To investigate the dependency of their chiroptical properties on the length (n) of the bridge.
• To introduce a new specification for the absolute configuration of supramolecules and nomenclature additions.

Main Methods:

• Intramolecular dialkylation of cycloenantiomeric bis(sulphonamide)[2]rotaxanes and bis(sulphonamide)[2]catenanes using homologous bifunctional oligomethylene reagents and 1,omega-dibromoalkanes.
• Characterization using 1H NMR spectroscopy to observe structural changes with decreasing bridge length.
• Enantiomeric resolution of racemates using chiral High-Performance Liquid Chromatography (HPLC) columns.
• Analysis of circular dichroism (CD) spectra to determine chiroptical properties.

Main Results:

• Successful synthesis of aliphatically bridged [1](n)rotaxanes and (n)pretzelanes in preparative yields.
• Demonstrated successful covalent connection of axle and wheel in [2]rotaxanes with bridges as short as three methylene groups ([1](3)rotaxane).
• Observed decreasing yields for (n)pretzelanes with decreasing bridge length, with the shortest isolated being (6)pretzelane.
• Structural changes in [1](n)rotaxanes correlated with increasing high-field shifts in 1H NMR spectra.
• Enantiomeric resolution achieved for seven [1](n)rotaxane and two (n)pretzelane series.
• Strong dependency of circular dichrograms on bridge length, with unexpected deviations for the shortest bridges.

Conclusions:

• Aliphatically bridged rotaxanes and pretzelanes can be synthesized, with bridge length significantly influencing their properties.
• The study establishes a method for creating rotaxanes with very short bridges and introduces a new nomenclature system for supramolecular structures.
• Chiroptical properties are highly sensitive to bridge length, offering potential for designing sophisticated molecular architectures.