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Bridgehead carbocations via carbene fragmentation: erasing a 10(10) kinetic preference
Robert A Moss1, Fengmei Zheng, Jean-Marie Fedé
1Department of Chemistry and Chemical Biology, Rutgers, the State University of New Jersey, New Brunswick, New Jersey 08903, USA. moss@rutchem.rutgers.edu
Journal of the American Chemical Society
|May 9, 2002
Summary
The study investigated the fragmentation of bridgehead carbenes, finding that 1-Norbornyloxychlorocarbene (1-NorOCCl) is most readily captured due to its least stable cation. Fragmentation rates correlate inversely with bridgehead carbocation strain.
Area of Science:
- Organic Chemistry
- Photochemistry
- Reaction Kinetics
Background:
- Carbenes are reactive intermediates crucial in organic synthesis.
- Bridgehead carbenes present unique structural and reactivity challenges.
- Understanding carbene fragmentation pathways is key to controlling reaction outcomes.
Purpose of the Study:
- To investigate the photolysis of bridgehead carbenes: 1-Norbornyloxychlorocarbene (1-NorOCCl), 1-bicyclo[2.2.2]octyloxychlorocarbene (1-BcoOCCl), and 1-adamantyloxychlorocarbene (1-AdOCCl).
- To determine the competition between carbene fragmentation and capture by methanol.
- To quantify the absolute rate constants for carbene fragmentation.
Main Methods:
- Generation of carbenes via photolysis of diazirines in dichloroethane (DCE).
- Analysis of reaction products in DCE and methanol/DCE mixtures.
- Laser flash photolysis with UV and time-resolved infrared (TRIR) monitoring.
- Kinetic measurements and activation energy determination.
Main Results:
- Carbene fragmentation leads to bridgehead alkyl chlorides via ion pairs.
- Methanol competes with ion pair collapse, yielding chlorides, alcohols, and methyl ethers.
- Fragmentation rate constants (s⁻¹) in DCE: 1-NorOCCl (3.3 x 10⁴), 1-BcoOCCl (1.5 x 10⁵), 1-AdOCCl (5.9 x 10⁵).
- Fragmentation rates decrease with increasing strain in the resulting carbocation.
- Fragmentation of 1-NorOCCl is significantly faster than acetolysis of 1-NorOTs.
Conclusions:
- Carbene capture ratios follow the order 1-NorOCCl > 1-BcoOCCl > 1-AdOCCl.
- Carbene fragmentation rates are influenced by the stability of the resultant bridgehead carbocation.
- The study provides quantitative kinetic data for bridgehead carbene fragmentation and capture processes.