Comparison of Thiyl, Alkoxyl, and Alkyl Radical Addition to Double Bonds: The Unusual Contrasting Behavior of Sulfur and Oxygen Radical Chemistry

High-level ab initio calculations have been used to compare prototypical thiyl, alkoxyl, and alkyl radical addition reactions. Thiyl radical addition to the sulfur center of thioketones is exothermic and rapid, occurring with negative enthalpic barriers and only weakly positive Gibbs free energy barriers. In stark contrast, alkoxyl radical addition to the oxygen center of ketones is highly endothermic and occurs with very high reaction barriers, though these are also suppressed. On the basis of analysis of the corresponding alkyl radical additions to these substrates and the corresponding reactions of these heteroatom radicals with alkenes, it suggested that addition reactions involving thiyl radicals have low intrinsic barriers because their unpaired electrons are better able to undergo stabilizing resonance interactions with the π* orbitals of the substrate in the transition state.I.D. gratefully acknowledges the Scientific and Technological Research Council of Turkey (TUBITAK) under 2219 grant and Naomi Haworth for her valuable support. M.L.C gratefully acknowledges generous allocations of supercomputing time on the National Facility of the Australian National Computational Infrastructure and financial support from the Australian Research Council Centre of Excellence for Electromaterials Science