Cyclopropyl conjugation and ketyl anions: when do things begin to fall apart?
J M Tanko1, Xiangzhong Li, M'hamed Chahma
1Department of Chemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA. jtanko@vt.edu
Journal of the American Chemical Society
|March 21, 2007
Summary
Electrochemical reduction of cyclopropane derivatives reveals varying ring-opening rates. Some compounds undergo rapid ring opening, while others exhibit slower rates due to resonance stabilization, and one shows a unique rearrangement mechanism.
Area of Science:
- Electrochemistry
- Organic Chemistry
- Reaction Mechanisms
Background:
- Cyclopropane derivatives are known for their strained ring systems.
- Electrochemical reduction offers a pathway to study the reactivity of these strained systems.
- Understanding substituent effects on cyclopropane ring opening is crucial.
Purpose of the Study:
- To investigate the electrochemical reduction of various 1,2-diacylcyclopropane derivatives.
- To determine the kinetics and mechanisms of ring opening following electron transfer.
- To explore substituent effects on the stability and reactivity of radical anions.
Main Methods:
- Electrochemical reduction of 1,2-diacetylcyclopropane, 1-acetyl-2-phenylcyclopropane, 1-acetyl-2-benzoylcyclopropane, and 1,2-dibenzoylcyclopropane.
- Kinetic analysis of radical anion ring opening.
- Computational studies including molecular orbital calculations.
Main Results:
- 1-acetyl-2-phenylcyclopropane radical anion has a very small barrier to ring opening (>10^7 s^-1).
- Benzoyl-substituted cyclopropanes (7 and 8) show slower ring opening (10^5-10^6 s^-1) due to resonance stabilization.
- 1,2-dibenzoylcyclopropane (8) undergoes an unexpected vinylcyclopropane to cyclopentene rearrangement via a radical ion chain mechanism.
- Electrochemical reduction of 1,2-diacetylcyclopropane (5) results are ambiguous, suggesting either simple electron transfer or concerted dissociative electron transfer (DET).
Conclusions:
- Substituent groups significantly influence the electrochemical reduction pathway and ring-opening kinetics of cyclopropane derivatives.
- Resonance stabilization by benzoyl groups lowers the rate of ring opening.
- A novel vinylcyclopropane rearrangement mechanism was observed for 1,2-dibenzoylcyclopropane.
- The reduction mechanism for 1,2-diacetylcyclopropane remains debated, potentially involving concerted dissociative electron transfer.
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