Intramolecular hydrogen bonding in 1,8-bis(dimethylamino)naphthalenes containing two to four heteroatomic functions in the ortho- and peri-positions: from CD3 to sulfur and selenium

Abstract

Using 4,5-dibromo- and 2,7-dibromo-4,5-dimethyl-derivatives of 1,8-bis(dimethylamino)naphthalene (naphthalene “proton sponge”) in bromine-lithium exchange and subsequent reactions with appropriate electrophiles, a set of intriguing new derivatives with such functionalities as Br, Me, SMe, and CD₃ in various combinations, including compounds with peri-S–S and Se–Se bridges, were synthesized. The subsequent transformation of neutral polyfunctional proton sponges into monoprotonated cations was investigated both in solution and in crystals to reveal new possible limits of the so-called “buttressing effect”, “clothespin effect” and their combination affecting the length (2.50–2.61 Å), symmetry and dynamics (barrier free or low-barrier proton transfer) of the intramolecular H-bridge. In a series of 2,4,5,7-tetrasubstituted cations with an extremely compressed but still accessible for nucleophilic species hydrogen bond, a weak influence of the counterion (tetrahedral BF₄⁻, linear SCN⁻, spherical Br⁻) on the properties of the [NHN]⁺ bond in spatially overloaded proton sponges was demonstrated. It seems, in such cations a “micro-buttressing effect” is possible, since the replacement of hydrogen by deuterium in ortho-methyl groups results in a new record of compression of the internitrogen distance till 2.498 Å. The reversible redox behavior of the disulfide bridge in the 4,5-S–S-derivative of 1,8-bis(dimethylamino)naphthalene was also described.