GAS-PHASE HETEROAROMATIC SUBSTITUTION .7. METHYLATION OF 5-MEMBERED HETEROAROMATIC RINGS BY FREE METHYL CATIONS

The results of a study of the reactions of a powerful alkylating electrophile, the CT3 +ion from the β-decay of CT4, with gaseous pyrrole, IV-methylpyrrole, furan, and thiophene are reported. In all the systems investigated, two major categories of labeled products are recovered: the methylated and the tritiated substrates. The nature and the yields of the methylated products, as well as their isomeric composition, suggest that the gas-phase alkylation reaction of simple heteroaromatics by the CT3 +cation is regulated by factors related to electron mixing between the HOMO of the substrate and the LUMO of the electrophile in the transition state, a behavior that ranks CT3 +as a “soft” Lewis acid. For“harder” alkylating species, such as CH3FCH3 +, the orienting properties of the heteroaromatic substrates are mainly determined by electrostatic interactions established within the encounter pair. Furan and thiophene give, although for different reasons, similar product distributions with both “hard” and “soft” alkylating cations. © 1990, American Chemical Society. All rights reserved.