Synthesis, NMR spectroscopic characterization, quantum investigations on [3+2] cycloaddition reaction of nitrile oxide, and anti-corrosion on mild steel in 1M HCl of two novel 1,5-benzodiazepines

Abstract

This study explores the synthesis and reactivity of benzodiazepine derivatives through [3 + 2] cycloaddition and condensation reactions. Benzodiazepine 1, featuring two dipolarophilic centers (C = C and C = N), was synthesized via condensation and subsequently a reaction with nitrile oxides to yield oxadiazole-benzodiazepines and pyrazole-benzodiazepine compounds. Structural characterization was performed using ¹H and ¹³C NMR. The reaction mechanisms were analyzed using density functional theory (DFT) at the B3LYP/6–311G(d,p) level, supported by molecular electrostatic potential (MEP) and Fukui function analyses, which confirmed regioselective behavior under thermodynamic (condensation) and kinetic (cycloaddition) controls. The corrosion inhibition efficiency of synthesized compounds 1,5-benzodiazepine 1 (Inh1) and Oxadiazole-benzodiazepines 9 (Inh2) was evaluated using potentiodynamic polarization (PP) and electrochemical impedance spectroscopy (EIS). The results demonstrated that both inhibitors acted as mixed inhibitors, forming protective films on the steel surface. The maximum inhibition efficiencies reached by PP are 97 % for Inh1 and 92 % for Inh2 at an optimal concentration of 10^-³M. EIS data, modeled using an equivalent electrical circuit with two-stage time constants, confirmed a two-step adsorption process. Overall, this study highlights the successful integration of experimental and theoretical approaches, providing valuable insights into the synthesis and reactivity of heterocyclic compounds while demonstrating their practical applications in corrosion inhibition.