Structure and Basicity of 2-Guanidiobenzimidazoles

Versatile behavior of 2-guanidinobenzimidazole nitrogen atoms toward protonation, coordination and methylation

“…These facts and the observation that C-4(C-7) and C-8(C-9) present averaged sharp signals indicate that the first protonation site of 1a occurs at N-3 at any acidic concentrations ( Figure 6). This conclusion was reached in spite of the fact that semiempirical calculations suggested that the first protonation site in water was at N-10 [2] . Under stronger acidic conditions [1b in 12.1 M solution of sulfuric acid (98%)], the equilibration of the imidazolic N-H protons with the medium is very slow [17] C NMR spectrum of 1b shows a stronger shielding effect (20 ppm) at C-2, which has been explained by the electronic effect of positively charged substituents on an aromatic ring [18], indicating protonation at N-10.…”

“…A careful NMR study allowed us to establish that the open structure without an intramolecular hydrogen bond is the more populated one in DMSO or DMF. The imidazolic N-3 is the preferred basic site in solution for protonation, methylation, and coordination and not N-10 as was suggested from semiempirical calculations [2]. Under strongly acidic conditions, diprotonation occurs at N-3 and N-10.…”

“…Its structure and dynamical behavior have been studied in solution by 1 H [2], 13 C [2,3], and 15 N [4] NMR spectroscopy, and the equivalent conformers I and II were suggested as the principal contributors to the molecular structure of 1a in solution. These are in equilibrium and stabilized by intramolecular hydrogen bondings [2]. In the solid state, the X-ray diffraction structure [5] shows intramolecular hydrogen bonding giving rise to a six-membered ring ( Figure 1).…”

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

“…Examining X-ray crystal structures [19Ϫ22] and performing calculations [12,23] led to the notion that S(6) in 2-guanidinobenzimidazole 1 should hold all atoms coplanar. Contradic-parent compound is reconciled by the fact that intramolecular hydrogen bonds between the imidazole moieties and guanidino NH 2 protons were weak.…”

“…The chemical literature supports the N10-dehydrotautomer of 1 drawn in Figure 1 versus N12-or N13-dehydrotautomers. [11,12] The intramolecular hydrogen bond, S(6), Etter's notation, [13] in neutral 1 should be weak because of the non-optimal relative positions of the hydrogen-bond donor guanidine NH and the imidazole hydrogen-bond acceptor.…”