2-α-Hydroxyalkyl- and 2,7-Di(α-hydroxyalkyl)-1,8-bis(dimethylamino)naphthalenes:  Stabilization of Nonconventional In/Out Conformers of “Proton Sponges” via N···H−O Intramolecular Hydrogen Bonding. A Remarkable Kind of Tandem Nitrogen Inversion

Abstract

A regular set of 2-(alpha-hydroxymethyl)- and 2,7-di(alpha-hydroxymethyl)-1,8-bis(dimethylamino)naphthalenes has been prepared. Their X-ray, NMR, and IR studies have demonstrated that in tertiary mono-alcohols the orientation of free nitrogen electron pairs in crystals and solution corresponds to nonconventional in/out conformers stabilized by O-H...N intramolecular hydrogen bonding. For tertiary 2,7-dialcohols, the superimposed equilibrating in/out-out/in nitrogen invertomers are observed in solution. Unlike this, primary and secondary mono- and dialcohols commonly exist in the in/in form, which is typical for the parent proton sponge and the majority of its derivatives.

Proton sponges-based (2-naphthyl)pyridylmethanol

“…This compound is only secondary ortho alcohol proton sponge presently known [6,7], whose 1 H NMR spectrum in CDCl 3 has a two-proton OH group signal at δ 5.72 ppm. We should note that this shift is similar to the average chemical shift of OH groups in chelated (δ 6.5-7.7 ppm) [9,10] and nonchelated pyridylcarbinols (δ 3.25-4.45 ppm) [11].…”

“…The energy difference between conformers 1 and 2 calculated for the gas phase is ~4 kcal/mol [5]. We have recently shown that the in/out form can nevertheless be stabilized due to intramolecular hydrogen bonding between the out-inverted NMe 2 group and a suitable proton-donor substituent at C-2 [6,7]. In particular, all tertiary alcohols are found in solution and in crystals exclusively in in/out form 3, while the usual in/in conformation is found for secondary and primary alcohols 4.…”

“…Monobromide 12 and dibromide 13 were taken as the starting compounds for the preparation of alcohols 6-11 as in similar cases [7,8]. Bromides 12 and 13 were converted to the corresponding proton sponge lithium derivatives and then treated with pyridinealdehydes.…”

“…1 H NMR and IR spectroscopy focusing on the OH group bands was used to analyze the conformation of these compounds in solution (when solubility was sufficient). We took into consideration that chelation involving the NMe 2 group shifts the OH signal to 10.5-10.7 ppm [6,7], while chelation involving the 2-aza group shifts this signal to 6.0-7.7 ppm [9,10]. In the absence of chelation due to the formation of aggregated species, within which the proton is rapidly exchanged, the OH group peak in CDCl 3 solution, as a rule, is not seen or is found at δ 3.5-4.5 ppm; the OH group gives a doublet at δ 5.1-5.7 ppm in DMSO-d 6 due to decomposition of the aggregated species [6,7,11].…”

Proton Sponge Phosphanes: Reversibly Chargeable Ligands for ESI‐MS Analysis

“…[18] The methyl groups bound to each nitrogen sterically clash with each other and force the nitrogen lone pairs to point toward each other adopting an "in-in" conformation. [19] This leads to a high degree of lone pair-lone pair repulsion and causes the naphthalene backbone to be twisted. By protonating the neutral molecule a more optimal geometry is attained, steric strain is relieved, and the naphthalene ring returns to planarity.…”