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The mechanistic pathways for nucleophilic aromatic substitution of the nitro groups of 5,7-dinitro-3H-quinazolin-4-one and 1,3-dinitro-10H-dibenzo[b,f][1,4]oxazepin-11-one with 1H-1,2,3-triazole have been studied in both gas and solvent phase employing DFT/B3LYP calculations using 6-31G(d,p) basis set. The energetic parameters, i.e., free energy profiles along the reaction route and H-bond energy, charge analysis, and structural parameters have supported mechanistic pathway involving one-step concerted mechanism via formation of transition state during the reaction course. Despite substantial steric hindrance at adjacent position to fusion point, the regioselective attack of 1H-1,2,3-triazole has been observed at peri-position of benzannulated heterocycles. This regioselectivity is credited to intramolecular hydrogen bond C–H···O = C which stabilizes peri-transition state formation rather than para-transition state. The hydrogen bond acceptor–donor interactions have been studied using AIM method. The DFT and AIM analysis-derived results are compared for different sized dinitro-substituted benzannulated heterocycles and these studies have confirmed the contribution of substantial steric hindrance at peri-position with increased size of heterocycle. As benzannulated heterocycles are quite significant from the perspective of medicinal and natural product chemistry, this work will fulfill the need to find an efficient synthetic method for benzannulated heterocycles. [ABSTRACT FROM AUTHOR]

The nucleophilic aromatic substitution reactions of 4,6-dinitroisoindoline-1,3-dione with 1,2,3-triazole isomers, i.e., 1H-1,2,3-triazole and 2H-1,2,3-triazole, have been investigated theoretically using DFT/B3LYP calculations employing 6-31G(d,p) basis set in gas phase as well as in solvent phase. The computational studies have supported the formation of transition states via one-step concerted mechanism for the nucleophilic aromatic displacement of nitro groups with 1,2,3-triazoles rather than intermediate formation via two-step addition-elimination mechanism. The amination of 4,6-dinitroisoindoline-1,3-dione with 1,2,3-triazole isomers have revealed regioselectivity of peri-attack despite sterically favorable para-attack. The regioselectivity is attributed to the stabilization of the transition state through intra-molecular hydrogen bond C-H···O=C. This work presents the role of C-H bond as an effective hydrogen bond donor. The molecular interactions through hydrogen bonding have been investigated using AIM method. This work shall result in the synthesis of new regioselective 1,2,3-triazole-derived phthalimide compounds possessing potential biological candidature. [ABSTRACT FROM AUTHOR]

Quaternary ammonium salt obtained from the Menshutkin reaction between DABCO and benzyl chloride has been used in the synthesis of a novel Bronsted acidic ionic liquid (IL), namely 1-benzyl-4-(sulfobutyl)-diaza-bicyclo-octane hydrogen sulfate. The reaction of DABCO with benzyl chloride is a crucial step in the synthesis of this IL. Density functional theory calculations at B3LYP/6-31G(d,p) level have been employed to investigate the mechanism of Menshutkin reaction by calculating the energy barriers through possible transition states i.e., five-membered ring transition state and S2 transition state in gas phase and in diethyl ether as a solvent. It was found that while DABCO reacts with benzyl chloride through the well-known S2 transition state mechanism, the corresponding reaction with chlorodiphenylmethane can proceed through both S2 and five-membered ring transition state mechanism. However, S2 transition state mechanism is still the strongly preferred one out of the two possible mechanisms. The electronic structure analysis shows that solvent effects and enhanced resonance stabilization may play a decisive role in guiding the reaction pathway. [ABSTRACT FROM AUTHOR]