Protonation of Azines and Purines as a Model for the Electrophilic Aromatic Substitution – Rationalization by Triadic Formula

First gas-phase carbon proton affinities of eleven azines and purines (pyrrole, pyrazole, imidazole, pyridine, pyridazine, pyrimidine, pyrazine, bicyclic purine, pyridine–N–oxide, 2-aminopyrimidine and uracil) were calculated by a composite G3B3 methodology and used to probe their susceptibility to undergo electrophilic aromatic substitution (EAS), taking benzene as a reference molecule. The results revealed excellent agreement with the available experimental data and were interpreted using the triadic approach. We found out that pyrroles, which are more reactive towards EAS reaction than benzene, are stronger carbon bases than the latter compound, whereas pyridines exhibit lower carbon basicity, being at the same time less reactive toward substitution by electrophiles than benzene. In all of the investigated molecules the frontier orbital describing the corresponding π–electron density at the carbon atom to be protonated is HOMO as calculated by the HF/G3large//B3LYP/6–31G(d) level of theory. Our results are in a disagreement with the work by D’Auria (M. D’Auria, Tetrahedron Lett. 2005, 46, 6333–6336; Lett. Org. Chem. 2005, 2, 659–661), who at B3LYP/6–311+G(d,p) level found out that in some of systems investigated here the HOMO orbital is not of π–symmetry, which was used to rationalize the lower reactivity of these systems towards EAS. It turned out that energies of HOMO orbitals alone correlate very poorly with carbon proton affinities, unlike the difference in proton affinities between the most basic carbon atom and thermodynamically the most favourable site of protonation, which performs much better. Triadic analysis demonstrated the importance of considering a complete picture of the protonation process and all three terms appearing in the triadic scheme individually when discussing trends in basicity/nucleophilicity of closely related molecules.