GAS-PHASE HETEROAROMATIC SUBSTITUTION .8. ELECTROPHILIC ATTACK OF ETHYL CATION ON PYRROLE, N-METHYLPYRROLE, FURAN, AND THIOPHENE

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. © 1990, American Chemical Society. All rights reserved.