Gas-phase heteroaromatic substitution. 2. Electrophilic methylation of pyrrole and N-methylpyrrole by MeXMe+ (X = fluoro, chloro) ions

The gas-phase methylation of pyrrole (I) and N-methylpyrrole (II) by Me2X+ (X = F, Cl) from the γ radiolysis of MeX is examd. at 50 to 760 torr, in the presence of a thermal radical scavenger (O2) and variable concns. of an added base (NMe3). Both the reactivity of I or II relative to C6H6, used as the ref. substrate in competition expts., and the isomeric distribution of their methylated derivs. depend markedly on the total pressure of the system and the concn. of NMe3. The apparent kpyrrole/kC6H6 ratios increase from 0.2 (I)-0.3 (II), in neat MeF at 50 torr, to over 0.4 (I)-1.0 (II), at 760 torr contg. 10 torr of NMe3. Concurrently, the isomeric distribution of the methylated products changes from β:α:N = 80%:13%:7% (from I) and β:α = 65%:35% (from II) to β:α:N = 50%:15%:35% and β:α = 70%:30%. This is consistent with a methylation mechanism involving kinetically predominant Me2F+ attack on the β-C atoms of the pyrrole rings, and subsequent isomerization of the resulting excited intermediates to the thermodynamically most stable 3-methylpyrrole protonated on the 2-position. The substrate and positional selectivity of the gas-phase methylation and the mechanism of the intramol. isomerization are related to theor. predictions on heteroarom. reactivity and are compared with the available data of related methylation reactions, occurring both in the gaseous and condensed phase.