Transfer Hydrogenation and Catalysis

Introduction to Transfer Hydrogenation

  • Discusses a form of hydrogenation utilizing sacrificial hydrogen donors rather than molecular hydrogen (H₂).
  • Examples of sacrificial hydrogen donors:
    • Propan-2-ol (isopropanol)
    • Formic acid

Mechanism of Sacrificial Hydrogen Donors

  • Propane-2-ol: Converts to acetone, releasing 2 hydrogen atoms.
  • Formic Acid: Converts to carbon dioxide (CO₂), releasing 2 hydrogen atoms.
  • Note: Both reactions reach equilibrium, affecting choice of donor based on reaction conditions.

Importance of Choice of Donor

  • Propan-2-ol less preferred in unfavorable equilibria.
  • Formic acid suggested for more efficient reactions due to:
    • Mild reaction conditions
    • No need for sophisticated equipment
    • Adaptation for fine chemical synthesis instead of bulk synthesis.

Application in Fine Chemistry

  • Especially relevant for synthesis of chiral compounds.
  • Importance of chirality in pharmaceuticals; racemic mixtures not acceptable.
  • Process involves adding hydrogen to:
    • Oxygen on carbonyls
    • Nitrogen in amines.

The Mechanism of Transfer Hydrogenation

  • Often utilizes the NPV mechanism.
  • Basic process:
    • Deprotonation of alcohol to bind with metal catalyst (often aluminum).
    • Formation of a transition state leading to the transformation of ketones to alcohols without regenerating the alcohol used in the reaction.
    • Mention of enantiomer generation without chirality in catalysts.

Chirality Generation Issues

  • Historical anecdote about false claims of chirality induced by magnetic fields.
  • Emphasizes that true chirality results from catalysts having inherent chirality, not external factors.

Cooperative Catalysis Concept

  • Use of two active sites for activation, unlike conventional hydrogenation which relies on one metal site.
  • Mechanism involves:
    • Metal abstracting a hydrogen (H⁻)
    • Nitrogen abstracting a proton (H⁺).
  • Leads to the creation of a hydrogen-rich catalyst that efficiently gives back hydrogens to substrates in a concerted manner.

Mechanism Summary

  • Mechanistically simple mechanism of moving from hydrogen abstraction to transferring to substrate:
    • Uses outer-sphere mechanism without direct substrate contact.

Notable Example: Asymmetric Transfer Hydrogenation

  • Viral catalytic mechanism involving coordinated interaction leading to chiral alcohols.
  • Mechanism involves a palladium or ruthenium catalyst and aspects of the transition state.
  • Emphasizes the ability to generate high enantiomeric excess (>95%) using chiral catalysts.

Additional Concepts: Tandem Catalysis

  • Introducing two catalysts for two sequential transformations.
  • Each catalyst must perform its task without interference:
    • Example: Metal 1 conducts one transformation, followed by Metal 2 conducting another.
  • Can be done with tethered or separate entities, influencing catalytic efficiency.

Dynamic Kinetic Resolution

  • Focus on kinetic resolution involving racemic substrates.
  • Key distinction:
    • Kinetic resolution converts one enantiomer faster than the other, limiting yield to 50%.
    • Dynamic kinetic resolution leverages existing equilibrium between enantiomers to drive full conversion.
  • Example involving Shiva's catalyst to demonstrate this concept.

Implementation Example

  • The case study illustrates using unsymmetrical alcohol and enzyme together with the catalyst:
    • Resulted in high conversion (>99%) with good isolated yields (92%).
    • Notable efficiency attributed to synchronizing the catalyst with the enzyme's functionality, optimizing process throughput.

Conclusion

  • The lecture concludes with re-emphasizing the goal of simplifying chemical processes and enhancing efficiency, particularly regarding chiral product synthesis in pharmaceuticals.
  • Notes that these mechanisms are presented as effective methods to achieve desired outcomes in fine chemical synthesis.