Inter‐ and intramolecular hydrogen bonds in polyamines: variable‐concentration¹H‐NMR studies

Abstract

Inter-and intramolecular hydrogen bonding play an important role in determining the arrangement, physical properties, and reactivity of a great diversity of structures in chemical and biological systems. Several aromatic nucleophilic substitutions (ANS) in nonpolar aprotic, (non-HBD), solvents recently studied in our laboratory have demonstrated the importance of self-association of amines by hydrogen-bond interactions. In this paper, we describe 1 H-NMR studies carried out at room temperature on bi-and polyfunctionalized amines, namely: N-(3-amino-1-propyl)morpholine (3-APMo), histamine, 2-guanidinobenzimidazole (2-GB), 1,2-diaminoethane (EDA), 3-dimethylamino-l-propylamine (DMPA), and 1-(2-aminoethyl)piperidine (2-AEPip). By 1 H-NMR measurements of amine solutions at variable concentrations we have shown that 3-APMo, histamine and 2-GB are able to form a six-membered ring by intramolecular hydrogen bonding, while EDA, DMPA, and 2-AEPip form dimers by intermolecular hydrogen bonds. Likewise, variable concentration 1 H-NMR studies allowed estimation of the corresponding equilibrium constants for the dimerization. These results are correlated with experimental kinetic results of ANS, confirming hereto the relevance of the ''dimer mechanism'' in reactions involving these amines.

Degradation of dichloromethane by bispidine

“…discussions of the 1 H NMR spectra of N,N ′‐dimethyl‐bispidine and sparteine). Nevertheless, we assume that the pronounced chair–chair conformation with an intramolecular N–H⋯N bridge bonding as it exists in the solid state (see below) persists also in the solution . The NH signal (δ(H) 2.10, w ½ = 16 Hz) is broad at low temperature, which is attributed to relatively slow exchange of the bonding situation of the two NH groups.…”

Aromatic nucleophilic substitution in aprotic solvents using hydrogen‐bonded biological amines. Kinetic studies and quantum chemical calculations

“…In fact, the novel definition for hydrogen bond (H bond) recommended by the IUPAC takes into account the theoretical and experimental knowledge acquired over the last decades . On the other hand, we have recently reported the influence of H bonding in changing mechanisms of aromatic nucleophilic substitutions (ANS) involving amine nucleophiles, when they are carried out in solvents of low permittivity and determined by NMR measurements the degree of H bonding for those amines in each case . In the very well‐known two‐step addition–elimination mechanism for ANS, in the first step, the nucleophile attacks the ipso position corresponding to the leaving group in aromatic ring with electron‐withdrawing groups to yield a zwitterionic σ intermediate that contains an acidic proton; in the second step, the proton is removed by a base, usually the nucleophile itself.…”

Hydrogen-Bond-Assisted, Concentration-Dependent Molecular Dimerization of Ferrocenyl Hydantoins

“…Compound 3 undergoes H-bondassisted dimerization (Scheme 2), as suggested by 1 H NMR experiments in CDCl 3 at different concentrations. 23 Upon dilution from 70 to 4 mM solutions, the N−H signal of 3 shifted upfield from 8.4 to 7.3 ppm (Figure 3), supporting the role of N−H in forming intermolecular H-bonds at high concentrations. At concentrations lower than 70 mM the ferrocenyl and methyl signals broadened and shifted downfield and upfield, respectively, while the Boc signal did not exhibit significant shifts or broadening.…”