Subnanometric Pt clusters supported on MgO-incorporated porous carbon as efficient metal–base bifunctional catalysts for reductive heterocyclization reactions

Abstract

Direct synthesis of 2,1-benzisoxazole and its related N-heterocyclic compounds via a chemoselective reductive heterocyclization route, which involves hydrogenation and subsequent intramolecular cyclodehydration steps, is a viable and attractive approach. However, the efficient transformation of the feedstocks using hydrogen as a hydrogen source at ambient temperature and atmospheric hydrogen pressure remains a great challenge. Herein, we reported a MgO-incorporated porous carbon composite (MgO@C) stabilized subnanometric Pt cluster (Pt/MgO@C) catalyst prepared by annealing a Mg-containing metal–organic framework, Mg-MOF-74, followed by impregnation of a Pt precursor and further reduction treatment strategy. The as-prepared Pt/MgO@C with ultralow Pt loading (0.078 wt%) exhibited superior catalytic performance for synthesizing 2,1-benzisoxazole derivatives by the reductive heterocyclization methodology. Excellent catalytic activity (conversion ≥ 99%), high chemoselectivity (≥92%), good stability (8 cycles), and broad functional group tolerance (10 substrates) were achieved under very mild conditions (1 bar hydrogen pressure at 303 K) over the developed Pt/MgO@C. Control experiments indicated that a proper amount of basic sites, subnanometric Pt clusters, and a close contact between them synergistically contributed to the extraordinary performance of Pt/MgO@C. Density functional theory calculations showed that the high activity and chemoselectivity of Pt/MgO@C stemmed from the active interfacial sites of Pt/MgO@C simultaneously activating the H₂ and substrate, where the formed stable hydroxyl groups contributed to the enhancement of activity. The combined experimental and theoretical approach identified the formation of the –NO and –NHOH groups during the hydrogenation process, and the former corresponds to the rate-determining step.