Heterogeneous Pd-Catalyzed Efficient Synthesis of Imidazolones via Dehydrogenative Condensation between Ureas and 1,2-Diols

Abstract

[EN] A heterogeneously catalyzed protocol for the acceptorless dehydrogenative condensation between N,N'-disubstituted ureas and 1,2-diols to afford imidazolones was developed. Palladium nanoaggregates stabilized onto an alumina matrix with suitable acidic properties, namely, [Pd/Al2O3], was designed and successfully applied as efficient and reusable heterogeneous nanocatalyst for this relevant transformation. The methodology developed showed its wide applicability through the synthesis of more than 25 imidazolones with moderate to good yields, reaching a turnover number (TON) of up to 19444 and a initial turnover frequency (TOF0) > 290 h(-1). The active nanostructured catalyst was fully characterized [X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), high-resolution scanning transmission electron microscopy (HR-STEM), energy-dispersive X-ray (EDX), Raman spectroscopy, temperature-programmed reduction (TPR), temperature-programmed desorption (TPD)-NH3, TPD-CO2, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) area], and mechanistic studies were performed. Moreover, other related Pd-based nanomaterials composed of different acidic or basic inorganic supports were synthesized and extensively compared in this reaction. These studies revealed that the presence of Pd nanoparticles with a wide range of sizes (average particle size 2.8 nm) over a metal oxide support with a high density of acid sites is a key point for the good activity of the material, gamma-Al2O3 being the optimum support. Furthermore, a Pd-Zn cooperation effect was described for the dehydrogenative condensation of unactivated 1,2-diols, including ethylene glycol, with ureas. Two Pd-Zn bimetallic materials ([Pd/ZnO] and [Pd(5%)-Zn(5%)/Al2O3) were also designed and characterized properly. These materials, as well as the [Pd/Al2O3] system in combination with catalytic amounts of ZnO, showed good activity and selectivity in the acceptorless dehydrogenative condensation between ureas and unactivated 1,2-diols. The heterogeneous nature of all of the described catalytic systems was demonstrated, and the reusability of the catalysts was proven.