NMR and semi-empirical study of the tautomerism of�2,2?-bisbenzimidazolyl

Abstract

A dynamic NMR study of the tautomerism of 2,2'-bisbenzimidazolyl in DMSO-d 6 and a mechanistic interpretation of the process, based on a stepwise, single-proton transfer and formation of a zwitterionic intermediate, are presented. This interpretation is substantiated by semi-empirical calculations of the postulated intermediate and transition state, that yield results which are compared with previous studies on related aliphatic systems.

The Influence of Substituents on the Tautomerism of Symmetrically Substituted 2,2'-Bis-benzimidazoles

“…2,3 The tautomeric interconversion barrier for this class of compounds was estimated 1 to be 67 kJ mol -1 .…”

“…For all compounds calculations converged to transition state structures very similar to that previously calculated for the unsubstituted 2,2'-bis-benzimidazole. 1 In cases where two TS structures were possible, we always considered the more stable one. Our theoretical results are summarized in Tables 1 and 2.…”

“…Thus, the introduction of two symmetrical electron-withdrawing Cl substituents in 3, or of two electron-donating methyl substituents in 2 led to energy barriers in DMSO-d 6 / Me 2 CO-d 6 rather close to those of the unsubstituted 2,2'-bis-benzimidazole in DMSO-d 6 (67 kJ mol -1 ). 1 This lack of sensitivity of the energy barriers to the nature of the substituents is in agreement with the postulated mechanism of tautomerization, which involves a two-step process with a zwitterionic intermediate. 1,4,6 Thus, for a transition state with an incipient zwitterionic character, the effects of the two symmetrical substituents X on the benzimidazolium and benzimidazolide moieties should oppose and, at least partially, cancel each other.…”

A twist in the non-slanted H-mers to control π-conjugation in 2-dimensions and optical properties

“…280 K based on 1 H-NMR studies. 36,37 H-mer-1 is expected to exhibit imine-amine tautomerization, whereas tautomerization is not possible in H-mer-2 and H-mer-3 due to the N-methylation. Thus, the conjugation path and absorption spectrum of H-mer-1 may vary depending on temperature due to tautomerization whereas temperature should have a negligible effect on the conjugation path in H-mer-2.…”

Efficient Synthesis and Characterization of Novel Bibenzimidazole Oligomers and Polymers as Potential Conjugated Chelating Ligands

“…1 H NMR spectra of the oligomers ( 1 , 10 , 13 , and 15 ) are shown in Figure . To clarify the NMR spectra by removing the tautomer effect, CF 3 COOD was used as the solvent, as well as forming the trifluoroacetate salts of the oligomers. The crystal structure of dimer 10 shows that the rings are coplanar and the molecule is centrosymmetric.…”