Switching of chiral induction in helical aromatic oligoamides using solid state-solution state equilibrium.

Area of Science:

• Supramolecular Chemistry
• Organic Chemistry
• Chirality Studies

Background:

• Aromatic oligoamides are known to adopt stable helical conformations.
• Controlling the handedness (chirality) of these helical structures is crucial for various applications.
• Understanding the influence of chiral centers on helical equilibrium is an ongoing research area.

Purpose of the Study:

• To investigate the effect of introducing an R asymmetric center into aromatic oligoamides on their helical conformation.
• To determine how this chiral center influences the equilibrium between right-handed and left-handed helices in solution and in the solid state.
• To explore the potential for controlling handedness in helical oligomers.

Main Methods:

• Synthesis of aromatic oligoamides with an R asymmetric center.
• Solution-state analysis using techniques to determine helical preferences (e.g., NMR, Circular Dichroism).
• Crystallization studies to analyze solid-state structures.

Main Results:

• The R asymmetric center significantly shifted the equilibrium towards specific helical forms in solution, creating diastereoisomers (R-P and R-M helices).
• Despite the solution-state chiral induction, the R-P and R-M helices were found to co-crystallize in a 1:1 ratio.
• Chiral induction observed in solution was effectively switched off in the solid state.

Conclusions:

• The introduction of a single chiral center can dramatically influence helical handedness in solution.
• Solid-state packing forces can override solution-state chiral induction, leading to achiral crystalline structures.
• This phenomenon provides a unique mechanism for controlling or switching off handedness in helical oligomers.