Methyl Radical Addition Reaction to Substituted C═C Double Bonds

Radical addition reactions have become an indispensable tool for constructing carbon-carbon bonds in organic chemistry. Given the significance of these processes, it is imperative that the underlying rules governing odd electron C-C bond forming processes be better understood. This study explores the fundamental case of methyl radical addition reaction (MRAR) to monosubstituted ethylene derivatives (H₂Cβ═CαHR, with R = H, CHO, and NMe₂) using Density Functional Theory at ZORA-(U)OLYP/TZ2P. We quantitatively analyzed the factors governing regioselectivity and substituent effects using the Activation Strain Model and Energy Decomposition Analysis. Regarding regioselectivity, radical addition@β is universally favored over addition@α due to reduction in the buildup of unfavorable Pauli repulsion (ΔEPauli) and maximization of orbital interactions (ΔEoi) with the β-carbon. Substituents modulate reactivity through electronic effects: NMe₂ (a representative electron-donating group) and CHO (a representative electron-withdrawing group) lower activation barriers with respect to the unsubstituted reactions due to improved orbital interactions. Among the substituents, CHO lowers the activation barrier for radical addition more than NMe₂ due to a change in the orbital interaction mechanism. By linking qualitative theories like Frontier Molecular Orbital Theory with detailed computational insights, this work advances the mechanistic understanding of radical addition reactions and provides a physically sound and intuitive basis for understanding these key transformations.