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Are pyridinium ylides radicals?
Fan Liao1, Wenhuan Huang1, Biao Chen1
1Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, China. zhxil@mail.ustc.edu.cn yiluo@ustc.edu.cn gzhang@ustc.edu.cn.
Chemical Communications (Cambridge, England)
|August 26, 2020
Summary
Pyridinium ylides, typically nucleophiles, demonstrate unexpected radical character. This discovery, supported by EPR and mass spectrometry, reveals new insights into their chemical behavior and potential radical mechanisms.
Area of Science:
- Organic Chemistry
- Physical Chemistry
- Spectroscopy
Background:
- Pyridinium ylides are conventionally recognized as nucleophiles.
- Their reactivity is typically explained through electron pair interactions.
Purpose of the Study:
- To investigate the potential radical character of N-alkyl-substituted pyridinium ylides.
- To explore the underlying mechanisms responsible for their observed properties.
Main Methods:
- Electron Paramagnetic Resonance (EPR) spectroscopy in solution and solid states.
- Variable-temperature EPR studies.
- High-resolution mass spectrometry (HRMS).
Main Results:
- Pyridinium ylides exhibited significant EPR signals, indicating radical character.
- A correlation was found between lowered π* energy levels and increased EPR activity.
- Variable-temperature EPR revealed a singlet ground state and a thermally activated triplet state.
- HRMS confirmed oligomer formation, supporting a radical mechanism.
Conclusions:
- N-alkyl-substituted pyridinium ylides possess notable radical characteristics.
- Thermally induced electron-transfer processes are likely involved in their behavior.
- The findings challenge traditional views of pyridinium ylide reactivity.
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