Gas-phase heteroaromatic substitution. Part 4. Electrophilic attack of tert-butyl cation on pyrrole, N-methylpyrrole, furan, and thiophene

The kinetics were studied for the substitution reaction of Me3C+, obtained in the gas phase from γ-radiolysis of Me4C, with pyrrole (I), N-methylpyrrole (II), furan (III) and thiophene (IV) at 50-760 Torr in the presence of variable Me3N concns. The reactivity of simple heteroarom. compds. relative to PhMe in competition expts. depended markedly upon the compn. of the gaseous reaction environment. The apparent ks/k1 ratios decrease slightly with increasing pressure. At atm. pressure, these ratios increase with the Me3N concn., leveling off to values of 1.0, 2.2, 2.6 and 0.6 for I-IV, resp., at the highest base concn. (PMe3N = 10 Torr). Under the same conditions, predominant β substitution occurs in I and II, whereas α-attack is favored with III and IV. The gaseous electrophile appears rather unselective between the n- and π-type centers of III and IV, and displays very limited substitution on the heteroatom of I and II. A mechanism based upon reversible addn. of the Me3C+ to the heteroarom. nucleus, involving preliminary formation of an electrostatic adduct, accounts for the formation of the tert-butylated products and their isomeric compn. Gas-phase tert-Bu alkylation of simple 5-membered heteroarom. compds. is regulated by electrostatic interactions established within the encounter pair.