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

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 (C[dbnd]N 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.