20.12 : 自由基反应性：亲核自由基

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.