Radical Reactivity: Nucleophilic Radicals

A nucleophilic radical has an electron-donating group attached to the radical center. This electron-donating substituent increases electron density at the radical site, making it electron-rich and reactive toward electron-deficient species. Nucleophilic radicals readily react with electrophilic alkenes due to favorable orbital interactions.

The reaction is driven by SOMO-LUMO interactions between the high-energy SOMO of the nucleophilic radical and the low-energy LUMO of the electrophilic alkene. These orbital interactions are energetically favorable and form the basis of reactive radical traps. This selectivity allows nucleophilic radicals to preferentially attack electron-deficient alkenes over other reaction pathways.

Addition generates a new radical that is electrophilic in nature. This secondary radical can react with either the alkene or a hydrogen source like tributyltin hydride. Since the electrophilic radical and electrophilic alkene have unfavorable interactions, the radical abstracts hydrogen from tributyltin hydride instead, yielding the final addition product.

Radical traps exploit SOMO-LUMO interactions to direct radical reactivity. By using electrophilic alkenes as traps, nucleophilic radicals are selectively captured through favorable orbital interactions. This prevents unwanted side reactions and ensures the desired radical intermediate reacts with the intended alkene rather than alternative pathways.

Tributyltin hydride serves as a hydrogen source that reacts with the secondary electrophilic radical generated after the nucleophilic alkyl radical adds to the alkene. Since the electrophilic radical cannot favorably interact with the electrophilic alkene, it abstracts hydrogen from the tin hydride, completing the addition and regenerating the tin radical.

The secondary radical generated from addition is electrophilic in nature. Electrophilic radicals do not favorably interact with electrophilic alkenes because both species are electron-deficient. This unfavorable SOMO-LUMO interaction prevents further addition, directing the radical to abstract hydrogen from tributyltin hydride instead.

AIBN is a radical initiator that decomposes thermally to generate free radicals. These initiator radicals abstract a hydrogen atom from tributyltin hydride, producing a nucleophilic alkyl radical. This nucleophilic alkyl radical then reacts with the electrophilic alkene, beginning the cascade of radical transformations that leads to the addition product.