Highly efficient carbon catalysts for the green synthesis of 1,5-benzodiazepines: Experimental and theoretical study

Abstract

A family of sustainable carbon catalysts with different chemical surface, active and selective for the synthesis of benzodiazepine 1 (BDZ), from o-phenylendiamine (OPD) 2 and acetone 3, under mild conditions, is reported. Catalysts were prepared by acids treatments with H2SO4 or H3PO4 of an activated carbon doped with ZnO at 3% wt (N3Zn) and, subsequently, submitted to additional thermal treatment in air. Simultaneously to the ZnO leaching, surface C-SO3 groups were generated by treatment of N3Zn with H2SO4 (N3Zn-S) whereas treatment with H3PO4 led to C-PO3 functions (N3Zn-P sample). The thermal treatment partially removes the C-SO3 groups (N3Zn-S-C sample) while C-PO3 functions were partially oxidized to C-OPO3 groups (N3Zn-P-C). Our results suggest that these chemical surface modifications of the catalysts are key on catalytic performance, pointed out the importance of the nature and distribution of acid sites at the surface. Remarkably, investigated carbon catalysts (N3Zn-S samples) were more active than the NS catalyst obtained by direct treatment of AC with H2SO4, this last mainly functionalized with C-OSO3 groups. Although the catalysts resulting of the H3PO4 treatment showed both a similar activity, some differences on selectivity to BDZ 1 were observed, attributed to certain specificity of P-functions at the surface depending on the acid strength of active sites and the reaction conditions. These results were supported by DFT calculations.