Ether derivatives have myriad applications in several areas of chemical industry and academia. Hence, the development of more effective and sustainable protocols for their production is highly desired. Among the different methodologies reported for ether synthesis, catalytic reductive alcohol etherifications with carbonyl‐based moieties (aldehydes/ketones and carboxylic acid derivatives) have emerged in the last years as a potential tool. These processes constitute appealing routes for the selective production of both symmetrical and asymmetrical ethers (including O‐heterocycles) with an increased molecular complexity. Likewise, ester‐to‐ether catalytic reductions and hydrogenative alcohol etherifications with CO2 to dialkoxymethanes and other acetals, albeit in less extent, have undergone important advances, too. In this Review, an update of the recent progresses in the area of catalytic reductive alcohol etherifications using carbonyl‐based compounds and CO2 have been described with a special focus on organic synthetic applications and catalyst design. Complementarily, recent progress made in catalytic acetal/ketal‐to‐ether or ester‐to‐ether reductions and other related transformations have been also summarized.

Scheme 6. Plausible transition-state model and X-ray crystal structure of 3 i. COMMUNICATIONS

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.