Synthesis of 1,5-benzodiazepine from 1,2-phenylenediamine and acetone in the presence of catalytic systems based on montmorillonite: effect of the surface acidity

The effect of the surface acidity of catalytic systems based on montmorillonite (Mt) on the formation of 1,5-benzodiazepine from 1,2-phenylenediamine and acetone was investigated. Two strategies were used for designing the acid sites in the framework of a Mt. The Bronsted type catalysts were prepared by clay modification, changing the nature of the treating acid (0.25 M solutions of HNO3, H2SO4, HOAc and HCl) or the concentration of the solution (0.125–3.0 M HNO3). The Lewis type catalysts were prepared by the pillaring method using bulky Al-hydroxypolycations (OH−/Al3+ = 2.4 mol/mol) and mixed Al, Fe-polycations (OH−/(Al3+ + Fe3+) = 2.4 mol/mol, Al3+/Fe3+ = 12/1 mol/mol), followed by calcination at 500 °C in air. The acidity of the catalysts, i.e., the type and amount of acid sites, was found to play a critical role in the cyclocondensation and dictated the performance of the catalysts. Acid-activated Mt materials gave high yield of 1,5-benzodiazepine in a shorter reaction time compared to Al- and Fe, Al-pillared Mts.