Gas-phase heteroaromatic substitution. Part 6. Alkylation of pyrrole, N-methylpyrrole, furan, and thiophene by isopropyl cation

Abstract

Isopropyl cation, obtained in the dilute gas state from the γ-radiolysis of propane, has been allowed to react with pyrrole (1), N-methylpyrrole (2), furan (3), and thiophene (4), both neat and in competition with benezene, in the pressure range 50–760 Torr, and in the presence of variable concentrations of a gaseous base (NMe₃). Both the reactivity of the selected heteroaromatic compounds and the isomeric distribution of their isopropylated derivatives depend upon the total pressure of the system and the concentration of NMEe₃. The apparent heteroaromatic substrate (S)vs. benzene (B) selectivity k_s/k_B ratios decrease slightly with increasing pressure. At atmospheric pressure, the k_s/k_B ratios for pyrroles are found to increase with NMe₃ concentrations reaching a value of 0.7 (1) and 0.8 (2) at the highest base concentration [P(NMe₃) 10 Torr]. The k_s/k_B values concerning furan and thiophene are found to decrease slightly with the NMe₃ concentration, levelling off to 0.3 (3) and 0.8 (4) at P(NMe₃) 10 Torr. Under the same conditions, predominant β substitution takes place in pyrroles [β:α:N 70:20:10 for (1) and β:α 61:39 for (2)], whereas α-attack is favoured in the case of furan (β:α 29:71). Thiophene (4) displays no significant positional discrimination (β:α 49:51). The mechanism of substitution, and the subsequent isomerization and dealkylation processes, are discussed, and the substrate and positional selectivity of i-C₃H₇⁺ compared with that of related gaseous alkylating reactants. The hypothesis of gas-phase electrophilic alkylation reactions of simple five-membered heteroaromatic compounds regulated essentially by electrostatic interations within the encounter pari is confirmed in the present study.