Theoretical investigations of β-trimethylsilyl-substituted iminium ions

Calculated reaction energies, NBO analyses, and natural charge distributions show that a trimethylsilyl group at the β carbon atom stabilizes the positive charge of an iminium cation more effectively than a tert-butyl group in the same position. Different from the classical β-silicon effect, where theoretical studies suggest that the positive charge is stabilized hyperconjugatively by interaction of the Si–C σ-bond with a formally empty 2p-orbital of suitable symmetry at the carbonium carbon, the stabilization in the case of iminium ions occurs by interaction of the Si–C σ-bond with the π∗ orbital of the CN double bond. Stabilization of the silyl-substituted derivatives by hyperconjugation might be enhanced by interaction of the positive charge with a neighboring induced dipole.