Gas-phase heteroaromatic substitution. 3. Electrophilic methylation of furan and thiophene by MeXMe+ (X = fluoro, chloro) ions

The gas-phase methylation of furan (I) or thiophene (II) by radiolytically generated Me2X+ (X = F, Cl) is examd. by their competitive reaction with C6H6. The mechanism of the substitution, and of the subsequent isomerization, occurring via intramol. 1,2 Me-group shifts, is related to the reactions regiochem. As for pyrroles, gas-phase Me2F+ methylation of I and II is characterized by a scarce substrate discrimination (relative to C6H6), accompanied by an appreciable positional selectivity toward those substrate positions with the highest neg. net charge. On the contrary, Me2Cl+ confirms its inherent affinity toward n-type nucleophilic centers by attacking preferentially the heteroatom of I and II. Gas-phase attack of Me2X+ on simple 5-membered heteroaroms. 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 theor. predictions.