Isotopic Fractionation as a Mechanistic Probe in Light-Driven C-H Bond Exchange Reactions

Area of Science:

• Photochemistry
• Chemical Kinetics
• Isotope Effects

Background:

• Traditional thermal hydrogen isotope exchange (HIE) relies on common transition states.
• Light-driven HIE reactions can involve complex, non-degenerate reaction pathways.
• Mechanistic elucidation and optimization of these reactions are challenging.

Purpose of the Study:

• To develop a diagnostic framework for understanding photoredox-based HIE mechanisms.
• To utilize hydrogen/deuterium (H/D) fractionation for mechanistic insights.
• To differentiate between degenerate and non-degenerate reaction pathways in HIE.

Main Methods:

• Employing a diagnostic framework based on H/D fractionation.
• Comparing deuterium incorporation in substrate C-H bonds to solvent H/D ratios.
• Applying classical treatments of equilibrium isotope effects.

Main Results:

• The H/D fractionation method effectively distinguishes between degenerate and non-degenerate HIE mechanisms.
• Fractionation extent is sensitive to the specific bond-forming and bond-breaking steps.
• Results from model systems align with thermochemical predictions.
• The method was successfully applied to HIE reactions with ambiguous thermodynamic mechanisms.

Conclusions:

• The H/D fractionation framework provides a robust tool for mechanistic studies of photoredox HIE.
• This approach simplifies the analysis of complex light-driven exchange reactions.
• The method aids in optimizing reactions where traditional thermodynamic analysis is insufficient.