A computational study of multicomponent orbital interactions during the cyclization of silyl, germyl, and stannyl radicals onto C-N and C-O multiple bonds.

BHandHLYP/6-311G** and BHandHLYP/DZP computations of the potential surface of Si-, Ge-, and Sn-radical cyclizations onto the imine double bond reveal that these reactions proceed through simultaneous SOMO --> pi*, LP(N) --> SOMO, and LP(N) --> sigma* interactions. Such multicomponent orbital interactions are responsible for the regioselectivity in these radical cyclizations, where the nucleophilic radical unexpectedly attacks the more electron-rich end of the pi system. Less nucleophilic heteroatoms, for example, the nitrogen atom in nitriles or the oxygen atom in carbonyl compounds, show reduced LP interactions with the radical center in the respective transition states, so that these reactions predominantly occur in the "classical" fashion and with the expected regioselectivities of nucleophilic radicals through SOMO --> pi* interactions. This supports the hypothesis that Si-, Ge- and, to a lesser extent, Sn-radicals are ambiphilic in nature and that the unpaired electron is not necessarily the most reactive site in a radical but can act as an observer of a nucleophilic attack at the radical center.