Comparative computational studies for nucleophilic aromatic substitution of dinitro-substituted benzannulated heterocycles with 1H-1,2,3-triazole

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 center dot center dot center dot 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.