Methyl Radical Addition Reactions to C=X Double Bonds

Radical additions to olefins and related π-systems are essential tools in the modern synthetic toolbox for forging C–C and C–X bonds. We report a systematic DFT study (ZORA-(U)OLYP/TZ2P) on methyl radical (H₃C•) additions to H₂C=X substrates, where X spans tetrel, pnictogen, and chalcogen elements across Periods 2–5. Our analysis reveals clear periodic trends: addition barriers at carbon decrease and those at X increase from tetrels to chalcogens; reaction energies become less favorable across a period but more favorable down a group. Regioselectivity favors addition at X except for X = CH₂, NH, and O, where carbon attack dominates—consistent with experimental data. These trends arise from a balance of orbital interactions and Pauli repulsion, with the latter emerging as a key, yet underappreciated, factor governing radical reactivity and regioselectivity in π-systems.