1,8-Bis(dimethylethyleneguanidino)naphthalene: Tailoring the Basicity of Bisguanidine “Proton Sponges” by Experiment and Theory

1,8-Bis(dimethylethyleneguanidino)naphthalene (DMEGN), the second example of a peralkyl
guanidine “proton sponge” based on the 1,8-naphthalene backbone, was prepared and fully
characterized. The crystal structure analysis of monoprotonated DMEGN reveals an unsymmetrical
intramolecular hydrogen bridge. A decrease in the basicity with respect to the noncyclic parent
1,8-bis(tetramethylguanidino)naphthalene was found. Nevertheless, a new proton sponge provides
a new crossbar in the ladder of highly basic neutral organic compounds. A detailed theoretical
study of DMEGN and related cyclic guanidines explains this surprising experimental result.
Homodesmotic reactions reveal that the intramolecular hydrogen bond contributes effectively 10
kcal/mol to proton affinity of DMEGN.