Gas-Phase Heteroaromatic Substitution. 2. Electrophilic Methylation of Pyrrole and N-Methylpyrrole by CH3XCH3+ (X = F or Cl) Ions

Article abstract of DOI:10.1021/ja00389a034

The gas-phase methylation of pyrrole (1) and N-methylpyrrole (2) by CH3XCH3⁺ (X = F or Cl) ions, from the γ radiolysis of CH3X, has been investigated at pressure ranging from 50 to 760 torr, in the presebce of a thermal radical scavenger (O2) and variable concentrations of an added base (NMe3).Both the reactivity of the selected pyrroles relative to benzene, used as the refernce substrate in competition experiments, and the isomeric distribution of their methylated derivatives depende markedly on the total pressure of the system and the concentration of NMe3.The apparent k_P/k_B ratios increase from 0.2 (1)-0.3 (2), in neat CH3F at 50 torr, to over 0.4 (1)-1.0 (2), at 760 torr containing 10 torr of NMe3.Concurently, the isomeric distribution of the methylated products changes from β:α:N = 80percent:13percent:7percent (from 1) and β:α = 65percent:35percent (from 2) to β:α:N = 50percent:15percent:35percent and β:α = 70percent:30percent.These results are consistent with a methylation mechanism involving kinetically predominant CH3FCH3⁺ attack on the β-carbons of the pyrrolic substrate 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 isomerization that appears of intramolecular nature are discussed in the light of the recent theoretical predictions on heteroaromatic reactivity and compared with the available data of related methylation reactions, occuring both in the gaseous and condensed phase.