## 2 Applications in Chemistry
### Hydrogenation Reaction  
Catalytic hydrogenation of organic carbonates is of synthetic interest as an environmentally benign approach to fundamental synthetic building blocks, such as alcohols and diols [34; 35]. In 2012, [PERSON] and co-workers [36] developed a process to obtain ethylene glycol and methanol via catalytic hydrogenation of cyclic carbonates by using (PNP)RuII (PNP)RuII (PNP) complexes as catalysts (Scheme 6). In this work, ethylene glycol was prepared through insertion of CO2 into epoxides (omega process), as an option for the recycling of CO2. This catalytic process allowed them to obtain a variety of ethylene glycols (EGs) **12** with 96-99% yields, with co-production of methanol (up to 99% yield) from different cyclic carbonates **1**. The reactions proceeded over 4-10 h in THF at 140 \({}^{\circ}\)C under 50 atm of H2 and in the presence of catalytic amounts of the RuII complex **I** (0.02-0.10 mol%) and \({}^{\dagger}\)BuOK (1.0 equivalent relative to the catalyst) in a 125 mL autoclave. It is noteworthy that when using GC **1a** protected with a methyl or a benzyl group, the corresponding EGs **12** were obtained with 99% and 97% yields, respectively, after 4 h. Additionally, it was observed that sterically hindered cyclic carbonates **1** required longer reaction times or higher amounts of the catalyst. The protocol was successfully applied to a six-membered ring carbonate, giving the respective diol after 2 h of reaction. Moreover, the protocol was used in the depolymerization of poly(propylene carbonate) (PPC), resulting in propane-1,2-diol and methanol after 24 h, both with 99% conversion. Similarly, the Ru-lutidine-bridged \(N\)-heterocyclic carbene complex (Ru complex **II**) was used as a catalyst in the hydrogenation of ethylene carbonate **1c** (R = H) (Scheme 6) [37]. In this work, glycol and methanol were obtained with 92% and 42% yields, with 99% conversion of the carbonate under 5 MPa of H2 over 12 h at 130 \({}^{\circ}\)C. A \(N\)-butyl-substituted CNC-pincer ligand and RuHCl(CO)(PPh3)3 were applied to prepare the Ru-NHC complex, formed in situ in the presence of \({}^{\dagger}\)BuOK (10 mol%) as a base. From the optimization studies, the authors concluded that the high catalytic activity might be ascribed to the metal-ligand cooperation through aromatization-dearomatization of the lutidine framework. Furthermore, air and moisture-stable Ru-based complexes were applied for indirect conversion of CO2. In the same line, the pincer pyridine-bridged \(N\)-heterocyclic carbene-ruthenium complex (Ru complex **III**) exhibited high catalytic activity in the hydrogenation of cyclic carbonates **13** (Scheme 7) [38]. When carried out in THF in the presence of K3 PO4 and H2 (50 atm) at 140 \({}^{\circ}\)C, with a catalyst loading of 0.5 mol%, the reaction provided nearly quantitative yields of the corresponding diol **12** and methanol. Even when using a catalytic amount of a weak base and a low catalyst loading, a range of substrates with varying steric congestion underwent the transformation. The production of diols and methanol was reported by using copper-silica nanocomposite catalysts with a uniform Cu dispersion, prepared using a precipitation-gel method (Cu-SiO\({}_{2}\)-PG) (Scheme 8) [39]. This system promoted hydrogenation of cyclic carbonates **1** (87-97% conversion), providing methanol and diols **12** at conversion rates of 29-98% in both batch and fixed-bed continuous flow reactors, under relatively mild conditions. All the cyclic carbonates involved gave the respective diols in high yield, with the exception of GC **1a**, which provided glycerol with 29% yield only. A catalytic process using a basic zwitterionic salt as an alternative to inorganic bases was employed to synthesize glycols through hydrolysis of cyclic carbonates [40].