Synthesis of 1,5-benzodiazepine from 1,2-phenylenediamine and acetone in the presence of a layered titanosilicate modified with HNO₃

1,5-Benzodiazepine and its derivatives represent the important class of compounds with a wide range of biological applications. One of the ways for the synthesis of 1,5-benzodiazepine is cyclocondensation of 1,2-phenylenediamine (I) with ketones. Here we demonstrated synthesis of 1,5-benzodiazepine from 1,2-phenylenediamine and acetone in the presence of a layered titanosilicate AM-4. The crystal structure of AM-4 is built from octahedra of TiO6 and tetrahedra of SiO4 that form layers perpendicular to [001]. We studied the effect of acid activation with 0.0625-0.25M nitric acid on its physicochemical and catalytic properties. Materials were characterized by X-ray diffraction, elemental analysis and N2-adsorption/desorption analyses. According to elemental analysis, modification of AM-4 with HNO3 leads to the leaching of "crosslinking" sodium cations from the Si and Ti layers. The Na/Si ratio decreases from 1.28 to 0.02 with an increase in acid concentration to 0.25 mol/l. X-ray diffraction data point a decrease in the lattice parameters (d200) from 6.0 to 7.5⁰ (20) after activation of AM-4 with an acid. Moreover, the specific surface and porosity of the system increase with increasing acid concentration. It was found that activation of AM-4 with HNO3 leads to the formation of Brnnsted acidity due to the leaching of interlayer Na cations by acid. Surface acidity was determined as zero point of charge of the surface (pHPZC) by the method of mass titration in aqueous solution. Surface acidity was found to raise with increasing HNO3 concentration from 11.5 (AM-4) to 4.5 (0.25M AM-4). According to Infrared spectroscopy using CDCl3 as probe molecules, strength of basic sites decreases of with increasing HNO3 concentration from 1019 kJ/mol to 797 kJ/mol (0.25MAM-4). The catalytic behaviour of AM-4 activated with HNO3 was investigated in the cyclocondensation of (I) with acetone at the acetone/(I) molar ratio of 2.5 in a metha-nol solution at 50 °C. It was found that the reaction rate and yield of 1,5-benzodiazepine depend on HNO3 concentration. The increasing HNO3 concentration led to a rise in reaction rate and yield of 1,5-benzodiazepine, simultaneously. The maximal yield of 1,5-benzodiazepine (75.1%) was observed in the presence of 0.25M AM-4 with a high surface acidity (pHPZC = 4.5). Recycling experiments point that yield of 1,5-benzodiazepine does not change during at least three catalytic cycles for 0.25M AM-4. Activity of 0.25M AM-4 was higher in compared with zeolites H-ZSM-5 (Si/Al = 28) and ff-Zeolite (Si/Al = 30). At the same time activity of montmorillonite activated with 0.25M HNO3 than that of 0.25M AM-4 was higher in spite similar surface acidity (pHpzc 4.8 (0.25M Mt) and 4.5 (0.25M AM-4)). This phenomenon can be related to the difference in textural properties which affect the accessibility of reactants.

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