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Reduction of benzophenones with SmI2. Post electron transfer processes
1Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel.
The Journal of Organic Chemistry
|February 10, 2009
Summary
This study reveals that substituted benzophenone radical anions form ion pairs and dimers. Protonation of these dimers is significantly faster than monomer protonation, impacting reaction kinetics.
Area of Science:
- Organic Chemistry
- Physical Chemistry
- Photochemistry
Background:
- Benzophenone derivatives are common organic compounds.
- Samarium diiodide (SmI2) is a powerful reducing agent used in organic synthesis.
- Understanding reaction kinetics is crucial for optimizing chemical processes.
Purpose of the Study:
- To investigate the reaction kinetics of substituted benzophenones with SmI2.
- To elucidate the mechanism of electron transfer and subsequent reactions.
- To characterize the species formed, including radical anions and their aggregation states.
Main Methods:
- Stopped-flow spectrometry was employed to monitor the reaction in real-time.
- Kinetic data was analyzed to determine reaction orders with respect to reactants.
- The influence of substrate and SmI2 concentrations on reaction rates was examined.
Main Results:
- Electron transfer between benzophenones and SmI2 is rapid and nearly quantitative.
- Radical anions exist as a mixture of monomeric ion pairs and Streitwieser dimers.
- Dimerization to pinacol is second order in radical anion and shows negative order in SmI2 concentration.
- Protonation of Streitwieser dimers by trifluoroethanol is faster than monomer protonation.
Conclusions:
- The aggregation state of radical anions (monomers vs. dimers) significantly influences reaction pathways.
- Reaction kinetics are dependent on the relative concentrations of substrate and SmI2.
- The findings provide insights into the complex behavior of radical anions in solution.
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