Journal of the American Chemical Society p. 7091 - 7098 (1982)
Update date:2022-08-10
Topics:
Angelini, Giancarlo
Lilla, Gaetano
Speranza, Maurizio
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 (kS/kB = 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.
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