Methyl Radical Addition Reactions to C═X Double Bonds

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Abstract
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 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 = 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.
Funding Information
  • Microsoft Research
  • Nederlandse Organisatie voor Wetenschappelijk Onderzoek

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