Gas-phase heteroaromatic substitution. 3. Electrophilic methylation of furan and thiophene by CH3XCH3+ (X = fluoro, chloro) ions

Abstract

A previous radiolytic study on the gas-phase methylation of pyrrole and N-methylpyrrole by CH/sub 3/XCH/sub 3//sup +/ (X = F or Cl) ions, from the ..gamma.. radiolysis of CH/sub 3/X, is extended to furan (3) and thiophene (4). The mechanism of the substitution and of the subsequent isomerization occurring via intramolecular 1,2 methyl-group shifts is discussed and the substrate and positional selectivity of the selected electrophilic species evaluated. As for pyrroles, gas-phase CH/sub 3/FCH/sub 3//sup +/ methylation of furan and thiophene is characterized by a scarce substrate discrimination (k/sub S//k/sub B/ = 1.2 (3), 0.8 (4)), accompanied by an appreciable positional selectivity toward those substrate positions with the highest negative net charge (O:..cap alpha..:..beta.. = 36%:35%:29% for 3; S:..cap alpha..:..beta.. = 19%:43%:38% for 4). On the contrary, CH/sub 3/ClCH/sub 3//sup +/ confirms its inherent affinity toward n-type nucleophilic centers by attacking preferentially the heteroatom of 3 and 4. In light of the previous results concerning CH/sub 3/XCH/sub 3//sup +/ methylation of pyrroles, it is concluded that gas-phase attack of CH/sub 3/XCH/sub 3//sup +/ 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 presentmore » gas-phase results and recent theoretical predictions.« less