Gas-phase heteroaromatic substitution. 8. Electrophilic attack of ethyl cation on pyrrole, N-methylpyrrole, furan, and thiophene

Article abstract of DOI:10.1021/ja00164a029

Ethyl cation, obtained in the dilute gas state, together with CH₅⁺, from the γ-radiolysis of methane, has been allowed to react with pyrrole, N-methylpyrrole, furan, and thiophene, in the pressure range 50-760 Torr and in the presence of variable concentrations of a gaseous base (NMe₃). The mechanism of the substitution and of the subsequent isomerization of the relevant ionic intermediates is discussed, and the intrinsic positional selectivity of the C₂H₅⁺ ions is evaluated. Gas-phase C₂H₅⁺ ion attack on pyrroles is characterized by a significant positional selectivity toward those substrate positions with the highest net negative charge (N:α:β = 13%:10%:77% for pyrrole; α:β = 19%:81% for N-methylpyrrole). Interaction of C₂H₅⁺ with the furan center having the maximum value of the negative charge, i.e. the O atom, favors occurrence of α-substitution (α:β = 57%:43%). Thiophene displays no significant positional discrimination (α:β = 54%:46%). Gas-phase attack of C₂H₅⁺ on simple five-membered heteroaromatics is mainly governed by electrostatic interactions established within the encounter pair. This characterizes gaseous C₂H₅⁺ as a very ''hard'' electrophile, rather than a borderline acid, as expected on the grounds of the alkyl cation ''hardness'' scale. This deviation is explained in terms of the bridged geometry for C₂H₅⁺ and its effect on the LUMO energy level of the ion.