Proton transfer in the benzimidazolone and benzimidazolthione tautomerism process catalyzed by polar protic solvents

Abstract

The tautomeric equilibrium of benzimidazolone and benzimidazolthione have been studied by the density functional theory method using the CAM-B3LYP functional together with the 6-311G(d,p) basis set. Two separate mechanisms have been investigated: a direct intramolecular transfer using the polarizable continuum model and an indirect proton transfer assisted by a molecule of solvent (C6H12, H2O, CH3OH, and H2O2). In both cases, the results obtained indicate that ketone and thione are the most stable forms. However, the enhanced height of the activation barrier for the four-center mechanisms describing the tautomerism reaction as a direct intramolecular transfer implicates a relatively disadvantaged process. The participation of a polar protic solvent molecule allows the lowering of the activation energy barrier. Potential energy profiles of keto-enol and thio-enol tautomerism assisted by methanol and water are very different. The former one describes a concerted mechanism but the latter does not because it is associated with asynchronous processes that take place during the thio-enol tautomerism.

Competing Excited‐State Hydrogen and Proton‐Transfer Processes in 6‐Azaindole‐S3,4 and 2,6‐Diazaindole‐S3,4 Clusters (S=H2O, NH3)

“…The ESHT process in 6AI and 26DAI with 0–2 solvent molecule could not be achieved because of larger distance between the N(1)‐H a ⋯and N(6) groups. However, it has been reported previously that three or more polar protic solvents enhance the tautomerization reaction in the excited state via the hydrogen bonded network [58,59] . We have generated a relaxed potential energy surface along the solvent assisted hydrogen atom transfer pathway from the N(1)‐H a group to the N(6), in the excited state S 1 for each system.…”

“…However, it has been reported previously that three or more polar protic solvents enhance the tautomerization reaction in the excited state via the hydrogen bonded network. [58,59] We have generated a relaxed potential energy surface along the solvent assisted hydrogen atom transfer pathway from the N(1)-H a group to the N(6), in the excited state S 1 for each system. Figure 3 represents the PES along the hydrogen atom transfer coordinate R N(1)À Ha from 1H6AI-S 3,4 to 6H6AI-S 3,4 (i. e., R(6AI-S 3,4 )) and 1HDAI-S 3,4 to 6HDAI-S 3,4 , i. e., R(DAI-S 3,4 ), in the S 1 state.…”

Synthesis and evaluation of the oil removal potential of 3-bromo-benzimidazolone polymer grafted silica gel

“… 32 This band was seen to be broad probably due to OH groups formed as a result of tautomerism in the benzimidazolone ring and also as a result of intra-molecular hydrogen bonding. 33 Also worthy of note was the fact that the band for asymmetric N–H stretching seen in the spectrum for 3-bromo-1,2-diaminobenzene, was absent in the 3-bromobenzimidazolone spectrum because the primary NH 2 group (amine) in the former becomes a secondary NH group (amine) in the later. There was also an observed red shift in band for the C O group in urea from 1678 cm −1 to 1629 cm −1 .…”