Abstract.
The tautomeric preferences of the conjugated acids of 2-aminopyrrole derivatives have been examined both in the gas phase and in aqueous solution by using a combination of quantum mechanical, self-consistent reaction field and Monte Carlo–free-energy perturbation methods. The results show that the nature of substituents, the solvent and the presence of cosolute are relevant factors in modulating the relative stability between the tautomeric conjugate acids protonated at the heterocyclic ring and at the exocyclic amino nitrogen. Thus, attachment of electron-withdrawing groups to the ring, solvation in polar solvents, and the presence of negatively charged cosolutes tend to favor protonation at the exocyclic amino nitrogen. Nevertheless, none of these factors alone suffice to change the tautomeric preference for the ring-protonated forms. The results point out that the concerted occurrence of the three factors is necessary to shift the tautomeric preference towards the conjugated species protonated at the exocyclic nitrogen.
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Acknowledgments.
ensp;We thank R. Lavilla for helpful disccusions. We are grateful to J. Tomasi and W. L. Jorgensen for providing the original codes of the MST continuum method and the BOSS program, respectively. We thank the Ministerio de Ciencia y Tecnología for financial support (grants PB98-1222 and PB99-0046) and the Centre de Supercomputació de Catalunya for computational facilities. M.D.R. thanks the National Science Foundation for financial support.
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Fradera, X., De Rosa, M., Orozco, M. et al. Tautomeric conjugate acids of 2-aminopyrroles: effect of substituents, solvation and cosolute. Theor Chem Acc 111, 223–230 (2004). https://doi.org/10.1007/s00214-003-0508-5
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DOI: https://doi.org/10.1007/s00214-003-0508-5