Gas-Phase Heteroaromatic Substitution. 3. Electrophilic Methylation of Furan and Thiophene by CH3XCH3+ (X = F or Cl) Ions

Article abstract of DOI:10.1021/ja00389a035

A previous radiolytic study on the gas-phase methylation of pyrrole and N-methylpyrrole by CH3XCH3⁺ (X = F or Cl) ions, from the γ radiolysis of CH3X, is extended to furan (3) and thiophene (4).The mechanism of the susbstitution and of the subsequent isomerization occuring via intramolecular 1,2 methyl-group shift is discussed and the substrate and positional selectivity of the selected electrophilic species evaluated.As for pyrroles, gas-phase CH3FCH3⁺ methylation of furan and thiophene is characterized by a scarce substrate discrimination (k_S/k_B = 1.2 (3), 0.8 (4), accompanied by an apprreciable positional selectivity toward those substrate positions with the highest negative net charge (O:α:β = 36percent:35percent:29percent for 3; S:α:β = 19percent:43percent:38percent for 4).On the contrary, CH3ClCH3⁺ confirm its inherent affinity toward n-type nucleophilic centers by attacking preferently the heteroatom of 3 and 4.In light of the previous results concerning CH3XCH3⁺ methylation of pyrroles, it is concluded that gas-phase attack of CH3XCH3⁺ on simple five-membered heteroaromatics is essentially regulated by the electrostatic interaction established within the encounter pair.A close correspondence does exist between this rationalization of the present gas-phase results and recent theoretical predictions.