C-H oxidation in fluorenyl benzoates does not proceed through a stepwise pathway: revisiting asynchronous
Scott C Coste1, Anna C Brezny2, Brian Koronkiewicz1
1Department of Chemistry, Yale University New Haven CT 06520-8107 USA james.mayer@yale.edu.
Chemical Science
|November 8, 2021
Summary
This study refutes a proposed stepwise mechanism for C-H bond oxidation in fluorenyl benzoates. Experimental evidence supports a multi-site concerted proton-electron transfer (MS-CPET) mechanism, not a crossover pathway.
Area of Science:
- Organic Chemistry
- Physical Chemistry
- Reaction Mechanisms
Background:
- 2-Fluorenyl benzoates undergo C-H bond oxidation via proton-electron transfer.
- Unusual rate-driving force relationships were observed, initially suggesting a multi-site concerted proton-electron transfer (MS-CPET) mechanism.
- An alternative stepwise mechanism involving intramolecular proton transfer (PT) was recently proposed.
Purpose of the Study:
- To experimentally evaluate the proposed alternative stepwise mechanism for C-H oxidation in 2-fluorenyl benzoates.
- To determine the validity of the crossover from concerted proton-electron transfer (CPET) to a stepwise PT mechanism.
- To provide definitive evidence for the reaction mechanism governing C-H oxidation in this compound class.
Main Methods:
- Experimental determination of the fluorenyl 9-C-H acidity (pKa).
- Hydrogen-deuterium (H/D) exchange experiments to assess proton transfer rates.
- Kinetic modeling using COPASI to analyze reaction pathways.
Main Results:
- The proposed stepwise mechanism involving initial intramolecular proton transfer is untenable.
- Experimental data, including pKa and H/D exchange rates, contradict the stepwise pathway.
- The multi-site concerted proton-electron transfer (MS-CPET) mechanism remains the most plausible explanation.
Conclusions:
- The crossover from CPET to a stepwise mechanism is definitively ruled out for 2-fluorenyl benzoates.
- The findings reinforce the understanding of MS-CPET mechanisms in C-H oxidation.
- Implications for asynchronous MS-CPET and Marcus theory are discussed.
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