In Search of the Weak, Six‐Membered Intramolecular Hydrogen Bond in the Solution and Solid States of Guanidinobenzimidazole

Abstract

The chemical literature presents evidence for the nonexistence of the intramolecular hydrogen bond in neutral 2‐guanidinobenzimidazole, a result that defies chemical intuition. In the current study, analyses of substituted 2‐guanidinobenzimidazoles by dynamic 1H NMR, IR, and X‐ray diffraction unveiled the contribution of the intramolecular hydrogen bond to the overall structure and conformational equilibria. The presence of the intramolecular hydrogen bond in this work and its absence in previous studies of the unsubstituted parent compound is reconciled by the fact that intramolecular hydrogen bonds between the imidazole moieties and guanidino NH2 protons were weak. The intramolecular hydrogen bonds were more apparent in derivatives with guanidino NHR. The behavior of the latter indicated competition between and coexistence of inter‐ and intramolecular hydrogen bonding. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

Resonance-Assisted Hydrogen Bonds: A Critical Examination. Structure and Stability of the Enols of β-Diketones and β-Enaminones

“…[1][2][3][4][5][6][7][8][9][10] These results are summarized in several review articles on RAHB. 11-13 One of the intrinsic difficulties in dealing with IMHBs is the impossibility of evaluating the bond energy in a precise way (although some interesting models have been proposed recently 14 ), and therefore no direct energetic information on RAHB effects can be achieved.…”

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

“…In almost all cases, formation of a hydrogen bonds shifts the 1 H‐NMR signal downfield 36. Hydrogen bonds and the concurrent fast proton exchange contribute to widening of the resonance signal 37 . 1 H‐NMR allows a differentiation between inter‐and intramolecular hydrogen bond, since the chemical shift of NH signal is in the first case dependent on amine concentration, while the NH signals of species with intramolecular hydrogen bonds are not affected by variation in the amine concentration, c (amine) 38…”

Resonance‐Assisted Hydrogen Bonding as a Driving Force in Synthesis and a Synthon in the Design of Materials

“…[61][62][63][229][230][231][232][233] Compared with b-enaminones or b-enaminoimines, the nitrogen atom at the 3-position alters the electronic structures considerably so that conjugation extends over all five atoms. [61][62][63][229][230][231][232][233] Compared with b-enaminones or b-enaminoimines, the nitrogen atom at the 3-position alters the electronic structures considerably so that conjugation extends over all five atoms.…”

“…He has authored one book,o ver 600 research publications, and 33 patents, edited four books, and presented over 100 invited lectures at internationalc onferences.H is work has received over 14 000 citations (h-index = 56;W eb of Science). [6] The most well-established cases of RAHB( Scheme3)i nclude intramolecular Hbonds in b-diketones (I), [24,[40][41][42] b-enaminones (II), [24,[40][41][42]46] Schiff bases (III), [47][48][49][50] b-enaminoimines (IV), [51][52][53][54] hydrazones (V and VI), [55][56][57] formazans (VII), [58][59][60] 1,3,5-triazapentadienes (VIII), [61][62][63] orthosubstituted aromatic compounds (IX-XII), [64][65][66] oximes (XIII-XVI), [29,[67][68][69] mercaptans (or thiols) (XVII-XIX), [70][71][72] thioketones (XX-XXII), [73][74][75] selenones or tellurones (XXIII and XXIV), [76][77][78][79][80]…”