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

Abstract

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/Al 2 O 3 ], 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 (TOF 0 ) > 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, temperatureprogrammed reduction (TPR), temperature-programmed desorption (TPD)-NH 3 , TPD-CO 2 , 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, γ-Al 2 O 3 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%)/Al 2 O 3 ]) were also designed and characterized properly. These materials, as well as the [Pd/Al 2 O 3 ] 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.

Synthesis of Nitrogen- and Oxygen-Containing Heterocycles Catalysed by Metal Nanoparticles Reported in 2022

“…Efforts of Arango-Daza et al led to the successful synthesis of imidazol-2-ones (119) using the reactants N,N′disubstituted urea (117) and 3,4-dihydroxy-1-butene or 1aryl-1,2-ethanediols (118) in the presence of PdNPs [Pd/Al 2 O 3 ] (Scheme 33) at 150 °C for 24 h in dibutyl ether in 41-87% yields. 86 Among the studied MNPs on different supports, PdNPs over -Al 2 O 3 were found to afford excellent catalytic efficiency and structural properties. These MNPs were prepared from -Al 2 O 3 and K 2 PdCl 4 , and characterisation of the catalyst was undertaken with the aid of XRD, HR-TEM, STEM, EDX, Raman spectroscopy, XPS, and BET.…”

Zinc Oxide (ZnO): an Amphoteric Metal Oxide with Dehydrogenating Activity

“…23 Cabrero-Antonino, Adam et al reported the synthesis of a variety of imidazolones through the acceptorless dehydrogenative condensation of ureas and diols. 24 This reaction was catalyzed by the combination of Pd/Al 2 O 3 and ZnO, able to mediate the diol dehydrogenation. Moreover, the catalytic mixture proved to be reusable in two catalytic cycles.…”

Cucurbit[6]uril supported β-Ni(OH)2 nanoparticles as a heterogeneous catalyst for the synthesis of quinazolines via acceptorless dehydrogenative coupling of alcohols with nitriles

“…In this perspective, acceptorless dehydrogenative coupling (ADC) of alcohols provides a sustainable and economic pathway using several homogeneous and heterogeneous catalytic systems. 4,[16][17][18][19][20][21] Noticeable advances in the ADC between alcohols and nitriles using homogeneous catalysts of 4d and 5d metals (Ru, Ir and Os) have been observed; 4,[22][23][24] however, reports on the use of heterogeneous catalysts in these reactions are scanty. 4 Presently, there is a quest for replacing these expensive noble metal catalysts with the cost-effective, environment-friendly and earth-abundant 3d transition metal catalysts.…”