Rearrangemements in Piperidine-Derived Nitrogen-Centered Radicals. A Quantum-Chemical Study

[GRAPHICS] Hydrogen migration reactions in piperidine radicals and their protonated counterparts were studied by quantum chemical calculations. G3B3 and G3(MP2)-RAD levels of theory were used as reference procedures in order to evaluate the efficiency of other computational models. In the gas phase, the 1,4-[N <-> C]-H and 1,4-[C-C]-H shifts are the most feasible rearrangements in the piperidine radical cation and neutral piperidine radical, respectively. However, if one explicit water molecule is well placed to facilitate hydrogen migrations, the 1,2-[N <-> C]-H shift becomes the most favorable process in both cases. Three different water-catalyzed [N <-> C]-H shift mechanisms were considered for piperidine radical cation, and only one is found to be operative in the case of neutral piperidine radical. We found that explicit solvation and protonation of piperidine-derived radicals strongly influence the overall mechanism of hydrogen migration reactions.