Electron charge redistribution following electrophilic attack on heterocycles: nitrogen as a charge transducer

It is shown that an electrophilic attack on the ring nitrogen in pyrrole or tryptamine produces an electron charge redistribution that is qualitatively different from the redistribution caused by an attack on the oxygen in furan. The electrophilic attacks are represented theoretically as interactions with a positive point charge and calculated by an ab-initio LCAO-SCF method with gaussian basis sets. Results show that an attacked nitrogen responds to the perturbation by moving electronic charge to the adjacent carbons whereas oxygen retains most of the charge polarized by the interaction. The nitrogen also acts as a charge transducer in other systems that are structurally very different. As a consequence of the charge redistribution, the comparative susceptibility of various sites in the heterocyclic molecules to an electrophilic attack may also depend on the response of the molecule to a prior attack on the heteroatom. The results indicate the need for dynamical reactivity considerations which reflect the variability in the molecular response to an incipient attack and the possibility that enhanced reactivity can be induced at certain sites by this response.