20.12:
Radical Reactivity: Nucleophilic Radicals
Nucleophilic radicals have an electron-donating group attached to the radical center.
These radicals readily react with electrophilic alkenes.
The reaction between a nucleophilic radical and an electrophilic alkene is favored due to the interactions between the high-energy SOMO of the nucleophilic radical and the low-energy LUMO of the electrophilic alkene.
These interactions form the basis of reactive radical traps and affect the selectivity in radical reactions.
For example, recall the reaction between an alkyl halide and an alkene in the presence of tributyltin hydride and AIBN.
As evident from the reaction mechanism, the nucleophilic alkyl radical reacts with an electrophilic alkene, generating a new radical. This new radical can now react with either the alkene or tributyltin hydride.
Since the radical is electrophilic in nature, its interaction with the electrophilic alkene is not favored. So, the electrophilic radical abstracts hydrogen from tributyltin hydride, yielding the addition product.
20.12:
Radical Reactivity: Nucleophilic Radicals
Radicals adjacent to electron-donating groups are called nucleophilic radicals. These radicals readily react with electrophilic alkenes. The SOMO–LUMO interactions are the driving force for the reaction, where the high-energy SOMO of the electron-rich, nucleophilic radicals interacts with the low-energy LUMO of the electron-deficient, electrophilic alkenes. Such SOMO–LUMO interactions are the basis of reactive radical traps, affecting the selectivity in radical reactions. For instance, consider the reaction between an alkyl halide and an alkene in the presence of tin hydride and AIBN. Initially, a nucleophilic alkyl radical is formed, which reacts with an electrophilic alkene, generating a new radical. This new radical can either react with the alkene or the tin hydride. Since the radical and the alkene are electrophilic, their interactions are not favored. Consequently, the radical reacts with tin hydride, abstracting hydrogen from it and forming the addition product.