Green synthesis of 1,4-benzodiazepines over La2O3 and La(OH)3 catalysts: Possibility of Langmuir-Hinshelwood adsorption

Lanthanum oxide [La<inf>2</inf>O<inf>3</inf>] and lanthanum hydroxide [La(OH)<inf>3</inf>] have been utilized as bases for the three-component reaction of aldehyde, dimedone and o-phenylenediamine for the synthesis of 1,4-benzodiazepine derivatives. The X-ray diffraction pattern shows pure phase formation of both catalysts. The basicity of La<inf>2</inf>O<inf>3</inf> and La(OH)<inf>3</inf> was measured by CO<inf>2</inf>-TPD (temperature programmed desorption) experiment. The measurements indicate the presence of basic sites that are useful for the reaction. Surface area measurements using Brunauer-Emmett-Teller (BET) reveal a low surface area for both the materials. TEM measurements indicate agglomerated particles with heterogeneous size distribution. The multicomponent reaction also proceeds smoothly in aqueous media as well as organic solvents. The heterogeneous catalyst was successfully recycled for 11 iterations without losing the catalytic activity. Several reactions were performed to understand the mechanism of the adsorption and the experiments suggest that the reaction may possibly proceeds by a Langmuir-Hinshelwood adsorption mechanism.