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

Abstract

Et cation, obtained in the dil. 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 concns. 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 neg. 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 max. value of the neg. charge, i.e. the O atom, favors occurrence of substitution (α:β = 57%:43%). Thiophene displays no significant positional discrimination (α:β = 54%:46%). Gas-phase attack of C2H5+ on simple five-membered heteroaroms. 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.