Steric Effects of pH Switchable, Substituted (2-pyridinium)urea Organocatalysts: a Solution and Solid Phase Study

Area of Science:

• Organocatalysis
• Hydrogen Bonding
• Organic Chemistry

Background:

• Hydrogen bonding organocatalysis is a growing field.
• Research often focuses on catalyst activity and selectivity.
• Fundamental catalyst design strategies require further exploration.

Purpose of the Study:

• To systematically investigate the impact of steric hindrance on pH-switchable urea organocatalysts.
• To explore how intramolecular hydrogen bonds influence catalyst conformation and performance.
• To understand the relationship between catalyst structure and substrate binding entropy.

Main Methods:

• Systematic increase of steric hindrance at the active site of urea organocatalysts.
• Utilizing protonated pyridines to form strong intramolecular hydrogen bonds.
• Employing single crystal X-ray diffraction to analyze structural changes.
• Conducting kinetic experiments using a benchmark reaction to assess catalytic activity.

Main Results:

• Increased steric hindrance was successfully implemented in pH-switchable urea organocatalysts.
• Strong intramolecular hydrogen bonds from protonated pyridines to oxygen were shown to stabilize the active conformation.
• A reduction in the entropic penalty associated with substrate binding was observed.
• Kinetic experiments demonstrated the effect of steric hindrance on reaction rates.

Conclusions:

• Systematic steric modification is a viable strategy for designing advanced organocatalysts.
• Stabilizing the active conformation through intramolecular hydrogen bonding enhances catalytic efficiency.
• Understanding and controlling entropic factors is crucial for optimizing organocatalyst performance.