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A forbidden rearrangement
Martin Leivers1, Iris Tam, Kevin Groves
1Department of Chemistry, Columbia University, New York, New York 10027, USA.
Barrelene derivatives undergo fragmentation to yield benzene and cyanocyclopropenes. The barrelene anion rearranges via a formally forbidden cyclopropyl ring cleavage, forming an allyl anion.
Area of Science:
- Organic Chemistry
- Reaction Mechanisms
- Physical Organic Chemistry
Background:
- Barrelene derivatives are strained polycyclic hydrocarbons.
- Understanding the fragmentation and rearrangement pathways of strained molecules is crucial in organic chemistry.
Purpose of the Study:
- To investigate the fragmentation products of a barrelene derivative.
- To elucidate the mechanism of the barrelene anion's thermal rearrangement.
- To rationalize a formally Woodward-Hoffmann-forbidden reaction.
Main Methods:
- Chemical reaction analysis of barrelene derivatives.
- Spectroscopic characterization of reaction products.
- Theoretical analysis of reaction pathways.
Main Results:
- Barrelene derivatives fragment to benzene and 1,2,3-tricyanocyclopropene.
- The barrelene anion fragments to benzene and the 1,2,3-tricyanocyclopropenyl anion.
- The major thermal product of the barrelene anion is a rearranged allyl anion, resulting from disrotatory cleavage of the cyclopropyl ring.
Conclusions:
- The thermal rearrangement of the barrelene anion proceeds through a cyclopropyl ring cleavage that is formally forbidden by Woodward-Hoffmann rules.
- Several mechanistic proposals are presented to explain this unexpected rearrangement pathway.
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