**1. Introduction**

Ruthenium was first discovered in 1844 by Karl Ernst Claus and he had named it as Ruthenia (in Latin Russia) in the honor of his motherland. In fact, in 1827, Gottfried Osann had found three new metals from the Ural Mountains and one of these metals was named as Ruthenium. However, its isolation could not be reproduced and hence his claims were withdrawn on these metals. Ruthenium is a noble transition metal with attractive properties and has found uses in different fields of science and technology. From the commercial point of view, ruthenium has been used in a variety of applications such as its alloy with other heavy metals used for voltage regulators, jewelry, fountain pen nibs and electromechanical devices *etc*. Although, ruthenium metal is known ever since the early nineteen centuries, but its first complex was reported in the second half of twentieth century having application in hydrogenation and hydroformylation [1].

Currently, ruthenium complexes are being widely used in academics and industrial purposes such as photosensitizers, biomedical, semiconductor industry as well as catalysts. Some variety reactions, like Diels–Alder, eco-friendly CO2 hydrogenation to hydrocarbon, transfer hydrogenation of unsaturated substrates, oxidation of alcohols, atom transfer radical addition (ATRA), metathesis (ring closing metathesis (RCM)/ring-opening polymerization (ROMP)) are catalyzed by inevitably the ruthenium complexes. Among these reactions, the hydrogenation is one of the most explored reaction. As quoted by Rylander, it is "*one of the most powerful weapons in the arsenal of the synthetic organic chemist*" [2]. This reaction finds immense manufacturing applications in pharmaceuticals, agrochemicals, petrochemicals, food industry, fine chemicals, fragrances as well as bulk chemicals. The process of hydrogenation (HY) or asymmetric hydrogenation (AH) is used to reduce unsaturated substrates (alkenes, alkynes, aldehydes, ketones, esters, imines, nitriles, carbon monoxide *etc*.) in presence of hydrogen gas and catalysts to give enantiomerically-enriched compounds. Alternate strategy for the hydrogenation is the transfer hydrogenation (TH) and asymmetric transfer hydrogenation (ATH) reactions, which require sacrificial hydrogen donor. These hydrogen donors include organic hydrogen source or different azeotropic mixtures with hydrogen acceptor substrates and catalysts in presence or absence of base promoters (NaOH, KOH, Et3N, Cs2CO3 *etc*.) (**Figure 1**). This approach is the most preferred and widely applied due to safe handling and which do not require hazardous pressurized H2 gas or pressure reactors.

The first ruthenium TH catalyst reported was a simple 16e [RuCl2(PPh3)3] complex, which effectively reduce acetophenone in presence of *<sup>i</sup>* PrOH through inner sphere mechanism involving the following steps; i) insertion, ii) reductive elimination, iii) oxidative addition and iv) β-elimination. It was found that small amount of a base facilitates the rate of TH reaction to approximately thousand-fold. Furthermore, the incorporation of basic nitrogen in a ligand, which coordinates directly to Ru (II) is an interesting approach, which was successfully applied by Noyori and coworkers through a half-sandwich chiral SS-**1**, for catalyzed reduction of several aromatic ketones (**2–6**) (**Figure 2**). [RuIICl(TsDPEN)(η<sup>6</sup> -p-cymene/ mesitylene)] (now commercially available) complexes (TsDPEN = N-tosylated-1,2 diamine), in presence of formic acid-triethylamine azeotropic mixture gave good yields of reduced products with enantiomeric excess (ee) (**Figure 2**). It was also correlated that reactivity and enantioselectivity of complexes were found to depend on optimum steric and electronic properties of the arene and TsDPEN ligands. Efficiency of this robust catalyst (R,R-**1**) can be seen by the reduction of multifunctional ketone, which gave R benzylic alcohol (**5**) in *ca.* 92% ee without affecting other functional groups. This conversion was explained through, outersphere mechanism with a six-membered transition state (TS) by concerted hydride transfer process (TS shown in **Figure 2**). To further improve the catalytic property, Noyori and co-workers have modified SS-**1**/RR-**1** with different ligand frames [3]. The significance of these asymmetric hydrogenation studies has been renowned and hence Ryōji Noyori was awarded Nobel Prize in 2001 for his immense contributions

**Figure 1.** *Ru catalyzed transfer hydrogenation reaction in presence of hydrogen donor and acceptor.*

*Recent Advances in Ru Catalyzed Transfer Hydrogenation and Its Future Perspectives DOI: http://dx.doi.org/10.5772/intechopen.96464*

**Figure 2.**

*Structures of selected alcohols (2–6), catalyst Ru-TsDPEN (SS-1) and proposed six membered transition state (TS).*

**Figure 3.** *Selected asymmetric transfer hydrogenation catalyzed, tethered or non-tethered Ru(II) catalysts (7–11).*

in this field. Will and co-workers have further modified SS-**1**/RR-**1** by tethering of the arene ring and diamine (or amino alcohol) ligand to increase the stability of the catalyst, to restrict the rotation of the *η<sup>6</sup>* -arene ring and to yield sterically controlled reduction products. Structures of few efficient catalysts **7**–**11**, are shown in **Figure 3** [4–6].

This chapter discusses recent advances of the homogeneous and heterogeneous TH and ATH reactions and primarily reduction of carbonyls, olefins, imines, nitriles, esters and heterocycles formation with a focus on modification of ligands environment. Furthermore, the mechanistic details were also discussed wherever possible with limitations as well as future perspectives in this important area.
