Theoretical study of free-radical migrations

1,2-Fluorine Radical Rearrangements: Isomerization Events in Perfluorinated Radicals

“…In the TS, the migrating fluorine is perpendicular to the plane of the carbon atoms, residing at the midpoint between the two atoms. Early computational investigations into free-radical migrations also predicted similar bridged intermediates for various atomic rearrangements such as chlorine and bromine . During the F atom shift, the C–C bond length shrinks from a typical C–C single bond (1.54 Å) to 1.37 Å, indicating that the C–C has more double bond character.…”

“…The least investigated of the rearrangements, 1,2-fluorine atom shifts in radicals, have since been hypothesized or directly measured in a diverse consortium of chemical systems, both in the gaseous and the condensed phases. − For example, 1,2-fluorine shifts were observed in the condensed phase via NMR for the tertiary fluorine adjacent to a tertiary perfluorocarbanion . Furthermore, F atom rearrangements in polyfluorinated cyclohexadienyl radicals generated by photochemical decomposition or heating of perfluoro- p -xylene and pentafluorobenzoyl peroxide was observed via electron paramagnetic resonance (EPR). ,, An early theoretical study by Fossey and Nedelec found the activation barriers for 1,2-F and 1,2-H shifts to be 107 and 88 kcal/mol respectively, indicating that these processes would not readily occur . Surprisingly, further studies on the 1,2-migration of hydrogen and fluorine in • CF 2 –CHFT and • CF 2 –CFHT radicals, where T is a tritium atom, found that 1,2-F atom rearrangements had lower barriers than 1,2-H atom shifts .…”

Addition reaction of 3,3,3-trifluoropropene to tetrahydrofuran. Telomeric growth via free-radical rearrangements

“…Very little quenching of radical (ii) occurs. Instead a 1,Shydrogen shift leads to the more stable radical (iii), which couples with 6) from the y-radiation-induced reaction of diethyl ether with fluorotrichloroethylene, but tetrahydrofuran gave only the monoadduct (7) under the same conditions (Scheme 3). Dedek and Fikar made a similar observation with addition reactions of chlorotrifluoroethylene and diethyl ether and proposed that the diadducts were a result of an intramolecular 13-H shift.'…”

Radicals by Design

“…This effect has been explained by the decrease in the energy of the SOMO upon protonation, which is a major predictor for the ease of migration. [45] This protonation mechanism has important implications in rationalising the catalysis mediated by free-radical enzymes, such as diol dehydratase, ethanolamine ammonia lyase and ribonucleotide reductase and provides a mechanism by which reactions may be directed. Full proton transfers are quite rare, except under strongly acidic conditions and with an appropriately basic substrate.…”