1,8-Bis(dimethylamino)naphthalene

1,8-Bis(dimethylamino)naphthalene is a chemical compound that was first prepared in 1968 by Roger Alder FRS at the University of Bristol. It is often referred by the trade name "Proton Sponge", a trademark of Aldrich Chemical Co., of Milwaukee, Wisconsin, USA.^[1][2] This compound is a diamine in which the two dimethylamino groups are attached on the same side or peri position of a naphthalene system. Proton-sponge has several very interesting properties; one is its very high basicity; another is its spectroscopic properties.

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High basicity

With a pK_a of 12.34 for its conjugate acid in aqueous solution, 1,8-Bis(dimethylamino)naphthalene is one of the strongest amine bases known, although it only absorbs protons slowly—hence the trade name. The high basicity is attributed to the relief of strain upon protonation and/or the strong interaction between the nitrogen lone pairs.^[1] However, the molecule is sterically hindered, making it a weak nucleophile. Because of this combination of properties, it has been used in organic synthesis as a highly selective non-nucleophilic base.

Spectroscopic properties

The spectroscopic properties of Proton-sponge are very interesting for researchers of molecular chemistry and have been researched for a long time. Proton-sponge emits a double fluorescence in various solutions due to the mixture of two ground-state species.^[2]

Trimethylated Proton-sponge

Proton-sponge is methylated by using dimethyl sulfate. The pK_a of trimethylated Proton-sponge is 6.43 in aqueous solution.

Proton sponges

Second generation proton sponges are known with even higher basicity. 1,8-Bis(hexamethyltriaminophosphazenyl)naphthalene or HMPN ^[3] is prepared from 1,8-Bis(dimethylamino)naphthalene by reaction with Tris(dimethylamino)phosphine. HMPN has a pK_BH+ of 29.9 in acetonitrile which is 12 orders of magnitude higher than Proton Sponge.

References

1. ^ ^{a b} F. Gerson, E.Haselbach, G. Plattner (December 1971). "Radical anion of 1,8-bis(dimethylamino)naphthalene ("proton sponge")". Chem. Phys. Lett. 12 (2): 316–319.
2. ^ ^{a b} A. Szemik-Hojniak, W.Rettig, I. Deperasinska (2001-08-03). "The forbidden emission of protonated proton sponge". Chem. Phys. Lett. 343 (3): 404–412. doi:10.1016/S0009-2614(01)00690-X.
3. ^ Volker Raab, Ekaterina Gauchenova, Alexei Merkoulov, Klaus Harms, Jörg Sundermeyer, Borislav Kovačević, and Zvonimir B. Maksić (2005). "1,8-Bis(hexamethyltriaminophosphazenyl)naphthalene, HMPN: A Superbasic Bisphosphazene "Proton Sponge"". J. Am. Chem. Soc. 127 (45): 15738–15743. doi:10.1021/ja052647v.