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 (HC) additions to HC═X substrates, where X is varied across elements from the tetrel (group 14: C, Si, Ge), pnictogen (group 15: N, P, As), and chalcogen (group 16: O, S, Se) groups. 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 = CH, 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.