Solution and Gas-Phase Investigations of Trimethylsilylpropyl-Substituted Pyridinium Ions. Manifestation of the Silicon δ Effect

Abstract

Computational studies on the N-methyl-2-trimethyl-M-propylpyridinium ions 15a (M = Si), 15b (M = Ge), 15c (M = Sn), and 15d (M = Pb) and N-methyl-4-trimethyl-M-propylpyridinium ions 16a (M = Si), 16b (M = Ge), 16c (M = Sn), and 16d (M = Pb) provide evidence for a significant through-bond (double hyperconjugative) interaction between the M–CH2 bond and the low-lying π* orbital of the pyridinium ion. The strength of this interaction increases in the order Si < Ge < Sn < Pb, in line with the σ-donor abilities of the C–M bond. The through-bond interaction for M = Si has been studied in solution using 13C and 29Si NMR studies; however, the effect is small. The collision-induced dissociation fragmentation reactions of 15a and 16a are strongly influenced by the through-bond interaction, with the major fragmentation pathway proceeding via extrusion of ethylene to yield the trimethylsilylmethyl-substituted pyridinium ions 1a and 2a.