ABSTRACT
The protonation of the carboxamide nitrogen atom is an essential part of inâ vivo and inâ vitro processes (cis-trans isomerization, amides hydrolysis etc). This phenomenon is well studied in geometrically strongly distorted amides, although there is little data concerning the protonation of undistorted amides. In the latter case, the participation of amide nitrogen in hydrogen bonding (which can be regarded as the incipient state of a proton transfer process) is less well-studied. Thus, it would be a worthy goal to investigate the enthalpy of this interaction. We prepared and investigated a set of peri-substituted naphthalenes containing the protonated dimethylamino group next to the amide nitrogen atom ("amide proton sponges"), which could serve as models for the study of an intramolecular hydrogen bond with the amide nitrogen atom. X-Ray analysis, NMR spectra, basicity values as well as quantum chemical calculations revealed the existence of a hydrogen bond with the amide nitrogen, that should be attributed to the borderline between moderate and weak intramolecular hydrogen bonds (2-7â kcal â mol-1 ).
ABSTRACT
A possibility of non-conventional two-step protonation of 1,8-bis(dimethylamino)naphthalene (proton sponge) is discussed. Unlike the generally accepted mechanism, involving relatively slow direct penetration of a proton into the cleft between the peri-NMe2 groups, it consists of the rapid addition of a proton to the out-inverted NMe2 group with the subsequent slower rotational transfer of the proton into the inter-nitrogen space to produce a stable chelated cation. The following approaches were employed during the work: (1) competitive hydrogen bond formation in a specially designed alcohol in which the OH group might chelate either the proton sponge 1-NMe2 group or another basic center (N,N-dimethylaniline) of known basicity; (2) measuring the basicity of naphtho[1,8-b,c]diazabicyclo[3.3.3]undecane considered to be a close analogue of the proton sponge capable exclusively of out-protonation; (3) X-ray, spectral and DFT studies of structure, energy and stereodynamics of compounds obtained, including their conformers. For the first time, the pKa value of an organic base with a perfectly flat nitrogen atom is reported. The final conclusion is made that both pathways of proton sponge protonation, traditional and non-conventional, contribute in parallel with a still undefined ratio. The estimated out-basicity of the proton sponge is at least 5.5 orders of magnitude lower than the directly measured in-basicity.
ABSTRACT
Monoprotonated N,N,N'-trimethyl-1,8-diaminonaphthalene demonstrates fast exchange of H(in) and H(out) protons, in which a counterion (BF4(-) and Br(-) were tested) participates. The process can be frozen below 185 K revealing a tremendous magnetic separation (up to Δδ = 11.6 ppm) of these otherwise equal NH protons with the enzyme-like proton transfer and a â¼7 kcal mol(-1) energetic barrier.
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.
