P-Doped carbon catalyst highly efficient for benzodiazepine synthesis. Tires valorisation

Abstract

Carbon catalysts prepared from pyrolysis of spent tires are found to efficiently catalyse the synthesis of benzodiazepine 1, from o-phenylendiamine 2 and acetone 3, with high conversions and selectivity, under mild reaction conditions, according to Scheme 1.The most acidic catalyst, CPN_H3PO4, obtained by chemical activation with H3PO4, resulted on the most efficient catalyst affording conversion values higher than 90 %, after 4 h of reaction time, and selectively leading to benzodiazepine 1 (90 %). On the other hand, the CPN and CPN_CO2 catalysts reached high conversions of 2 although diminished selectivity to 1, confirming that the carbon matrix is involved in the first steps of the reaction mainly catalyzing the formation of intermediate 4 as the main reaction product.Note that all the investigated catalysts are macroporous materials with pore size distribution large enough to favor diffusion of reactants and products. Therefore, the catalytic performance is mainly governed by the chemical surface, in particular by the presence of acid functions as phosphate groups anchored to the carbon surface or as SiP2O7 supported phase.Finally, considering both experimental and theoretical results, it seems that the most probable catalytic centers comprise phosphate functions in SiP2O7 catalyzing the last cyclization step to 1. Although electrophilicity of carbon acceptor (CN moiety) in the presence of model simulating phosphate groups anchored to the carbon surface was slightly superior, transition structure in the presence of model simulating SiP2O7 showed the smallest free energy barrier.