Intrinsic gas-phase acidity and electrophilicity of model heterocations and carbocations relative to pyridine: Adduct formation versus α- or β-(proton transfer) elimination

Ion/molecule reactions with pyridine (Py) of several gaseous silylium ions (Me3Si+, Cl3Si+, Br3Si+, and bridgehead 3,5,7-trimethyl-1,3,5,7-tetrasilaadamant-1-yl cation), carbocations (tBu+, iPr+, Et+, (MeO)2CH+, CCl3+, CF3+, Ph+, PhCH2+, and C7H7+, and 3,5-dimethyl-1-adamantyl cation), and the dicoordinated boron cation B(OMe)2+ were investigated. The silylium cations, including the unprecedented bridgehead 3,5,7-trimethyl-1,3,5,7-tetrasilaadamantyl gaseous ion, reacted to form mainly the adduct [R3Si–Py]+ via electrophilic attack. Alkyl cations (tBu+, iPr+, and Et+) reacted to give mainly [PyH]+ via β-elimination. CCl3+ forms both [Py–CCl3]+ and [PyH]+, whereas [PyH]+ was the dominant product for the reactions of Ph+, p-Cl–C6H4+, p-H2N–C6H4+, PhCH2+, and the tropylium ion (Tr+). The 3,5-dimethyl-1-adamantyl cation was found to be nearly unreactive towards pyridine. The (MeO)2B+ ion gave a set of three major products: the adduct [(MeO)2B–Py]+, [PyH]+, and [PyMe]+. Reactions involving isotopically labeled pyridine-d5 and CD3CD2+ were used to investigate the source of protons leading to [PyH]+ in reactions of protic and aprotic cations. Great variations of intrinsic reactivity were observed for the cations investigated, even within the same class. These changes were also found difficult to predict, as they were not always in accord with those predicted based on calculated (DFT) reaction thermochemistries.

Graphical abstractIon/molecule reactions of gaseous silylium ions (Me3Si+, Cl3Si+, Br3Si+, and bridgehead 3,5,7-trimethyl-1,3,5,7-tetrasilaadamant-1-yl cation), carbocations (tBu+, iPr+, Et+, (MeO)2CH+, CCl3+, CF3+, Ph+, PhCH2+, C7H7+, and 3,5-dimethyl-1-adamantyl cation), and the dicoordinated boron cation B(OMe)2+ with pyridine were investigated. Three competitive channels: (a) electrophilic addition that forms stable adducts, (b) direct β-elimination that leads to proton transfer, or (c) α-elimination that leads to the transfer of a substituent directly attached to the charge center were identified.Figure optionsDownload full-size imageDownload as PowerPoint slide