Experimental and theoretical investigations into the manifestation of the γ-effect in 2- and 4-[2-silylethyl)]pyridines and pyridinium ions

• Published in: Journal of organometallic chemistry 2021-11, Vol.953, p.122050, Article 122050
• Main Authors: Karnezis, Asimo, Brydon, Samuel C., Molino, Andrew, Wilson, David J.D., O'Hair, Richard A.J., White, Jonathan M.
• Format: Article
• Language: English
• Subjects: DFT; Homo-hyperconjugation; Percaudal interaction; Pyridinium ions; Silicon; γ-effect
• ISSN: 0022-328X; 1872-8561
• DOI: 10.1016/j.jorganchem.2021.122050

•Group 14 γ-effect.•Silicon pyridinium ions.•Homo-hyperconjugation.•Percaudal interaction. The γ-effect of the group 14 metals is a stabilizing interaction involving the back-lobe of the σC-M (M = Si, Ge, Sn, Pb) interacting with the developing carbocation γ to the tetrel. This percaudal interaction has been observed in solvolysis experiments with significant rate enhancements over unsubstituted systems, and subsequent isolation of three-membered ring products. In this study, the γ-effect is experimentally investigated in the solid phase with X-ray crystallography, the solution phase with Si-C coupling constants, and the gas phase via collision-induced dissociation (CID). Moving from 2- and 4-silylethyl substituted pyridines to the more electron demanding substituted pyridinium ion systems results in systematic shifts in key structural parameters including the 29Si-13C NMR coupling constants consistent with the cyclopropane-like resonance form attributable to the γ-effect. However, these shifts are much less in magnitude than those previously reported for the well documented β-effect. The gas phase CID results also reflect this with the fragmentation pathways for the silylethyl substituted systems not being dominated by the percaudal interaction, compared to previous studies where all major fragments were attributable to the β-effect. Density functional theory (DFT) calculated hyperhomodesmotic equations in combination with natural bond orbital (NBO) theory also supported the experimental trends with the γ-effect providing stabilization in the pyridinium ion systems studied, but to a much smaller extent to the β-effect. [Display omitted]