**3. Heterogeneous transfer hydrogenation**

With the fast advances in sustainable chemistry, the heterogeneous catalytic systems are profoundly used by industries for large scale production, economics as well as technological point of view. The advantage of heterogeneous catalytic systems over homogeneous is basically the easy handling, recycling and easy separation of the catalyst from the reaction mixture. For heterogeneous transfer hydrogenation, ruthenium catalyst can be immobilized on/in various materials such as nanoparticle, polymers, silica and carbon surfaces [28–30]. Such catalysts can be separated easily from the reaction mixture by simple filtration, centrifuge or applying magnetic force. Despite of this, the ruthenium catalyzed heterogeneous TH mostly limited to the reduction of carbonyl groups only.

#### **3.1 Heterogeneous transfer hydrogenation of carbonyl compounds**

Over last few decades, the field of heterogeneous TH of ketones by employing primary and secondary alcohols as donors in presence of heterogeneous catalysts is growing tremendously. In this context, various research groups have developed several ruthenium-based heterogeneous catalytic systems by altering the incorporation of ruthenium in/on various materials. Due to the importance and demand of catalytic ATH, significant efforts have been dedicated for the development of immobilized forms of the Noyori–Ikariya and other well-established catalysts. In this context, several polystyrene (PS) supported ruthenium complexes (**79**–**82**, **Figure 20**) were prepared initially and their catalytic properties were studied by several groups.

Marcos and co-workers have synthesized Noyori catalyst **79** immobilized on a chlorosulfonylated PS. They used the catalyst in asymmetric transfer hydrogenation of ketones and formic acid as hydrogen source and triethylamine as base at 40 °C [31]. When 0.67 mol% of the catalyst was used, the reduction of acetophenone

#### **Figure 20.** *PS-supported Ru catalysts (79–82) for the ATH of carbonyl groups.*

proceeded smoothly to give the desired product 1-phenylethan-1-ol with *ca.* 97% ee and the conversion was found to be *ca.* 99%. A number of electron donating as well as withdrawing groups showed excellent reactivity with c*a*. 86–99% *ee* values with **79**. Ma and Peng's group independently documented the synthesis of the ruthenium complexes immobilized on various phosphonate-containing single- or doublestranded PS copolymer (**80**–**82**) [32–33]. The catalyst **80** can be efficiently employed for the aqueous ATH of carbonyls using NCOONa-Et3N to give *ca.* 94–98% yields of the desired alcohol with *ca.* 93.9–97.8% *ee,* and *ca.* 100% chemoselectivity [32]. Similarly, ATH of aryl ketone was also achieved with **82** with *ca.* 94% yield and *ca.* 95% *ee* [33]. Interestingly, catalysts **80** and **82** can be easily separated by means of centrifuge from the reaction mixture and were reused without the loss of catalytic efficiency for five consecutive cycles. A comparative reduction of acetophenone using catalyst **79**–**82** was demonstrated in **Table 1**.

Recently, Islam and co-workers have reported the synthesis of simple and efficient PS-supported ruthenium complex (**83**) for the TH of ketone. Both aliphatic and aromatic functionalized ketones showed great conversion to the corresponding alcohols with the yields *ca.* 84–99% using **83** in the presence of KOH and *<sup>i</sup>* PrOH (**Figure 21**) [34]. One of the major issues in a majority of heterogeneous catalyzed TH is the isolation of catalyst from the reaction mixture, which involves tedious filtration or centrifuge. Very recently, magnetic nanoparticles (MNPs) have shown powerful alternates because of the advantages like greater surface area, morphological control, straight forward preparation, and easy separation using magnetic forces. As a result, MNP-immobilized transition-metal catalysts are broadly investigated by the researchers and applied for TH reactions. Verma and co-workers have reported the assembly of the ruthenium incorporated magnetic nanoparticles (Ru@MNPs) having spherical shape and size ranges from 15–30 nm in one pot *via* aggregation of magnetic silica (Fe3O4@SiO2) with binding of RuNPs [35]. The catalytic TH of acetophenone was carried out using Ru@MNPs. In the


#### **Table 1.** *ATH of acetophenone.*

**Figure 21.** *Heterogeneous TH of ketone.*

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

methodology, *<sup>i</sup>* PrOH was used as a hydrogen source along with KOH as base at a temperature of 100 °C under MW irradiation in 30 min to obtain the desired alcohol product with more than 99% yield. A wide range of substituted acetophenones showed great compatibility under optimal conditions to furnish the corresponding alcohols with good yield and selectivity (**Figure 22**). The catalyst can be easily recovered from the reaction mixture by using external magnet.

In 2015, Moores and co-workers used the iron/iron oxide core/shell NPs (FeCSNPs) as heterogeneous support for the synthesis of Ru-magnetic nanoparticles (Ru@FeCSNPs) [36]. The catalyst Ru@FeCSNPs was used as the catalyst of choice for the transfer hydrogenation of carbonyl compounds using KOH as base and *<sup>i</sup>* PrOH as hydrogen donor cum solvent at 100 °C (**Figure 23**). Aryl ketones bearing both electron-donation as well as electron withdrawing groups were converted to their corresponding alcohols very smoothly. The catalyst was found to be highly selective for the keto group over aldehyde or nitro functional group.

Although numerous catalysts and methods have been developed for the Rucatalyzed heterogeneous TH, the exact mechanism is still unclear. However, several mechanistic studies lead to two possible pathways for the metal catalyzed TH reaction; (a) monohydride transfer mechanism and (b) dihydride transfer mechanism. It is believed that, both pathways are possible for Ru-catalyzed transfer hydrogenation [37]. The possible catalytic cycle for the catalytic TH of keto group shown in **Figure 24** uses *<sup>i</sup>* PrOH as the source of hydrogen. As shown in **Figure 24**, in the mono hydride mechanism, *<sup>i</sup>* PrOH in presence of base form the alkoxide ion which in turn react with the metal to form the active metal-alkoxide species **I**. The metal metal-alkoxide give the reactive metal hydride intermediate **II**, which react with the

**Figure 22.** *Ru@MNPs-catalyzed transfer hydrogenation of ketones.*

**Figure 23.** *Ru@FeCSNPs catalyzed TH of acetophenone.*

**Figure 24.** *Possible general mechanisms for the heterogeneous TH reactions: (A) monohydride CTH; (B) dihydride CTH.*

keto group to transfer the hydride ion to the carbonyl carbon and result in the formation of substrate–metal alkoxide intermediate **III**. Finally, another molecule of *i* PrOH was reacted to form the product and regenerate the active catalytic species **I**. Similarly for the dihydride mechanism, both the protons of reducing agent got transfer to form the metal dihydride complex **IV**, which in turn react with the reactant carbonyl group to form the desired product along with regeneration of the metal catalyst.

#### **3.2 Heterogeneous transfer hydrogenation of nitro group**

The substituted aromatic amines act as important intermediates in the field of pharmaceuticals as well as agrochemicals. They also show great versatility in the production of dyes, polymers, herbicides and cosmetics. The aromatic nitro compounds can be easily converted to the corresponding aromatic amines *via* catalytic transfer hydrogenation. Lu and co-worker documented the use of ionic liquid (1 hexadecyl-3-methylimidazolium bromide) as a support in the synthesis of MCM-41-type mesoporous silica (OMS-IL) [37]. In their methodology, they used Ru nanoparticles immobilized on OMS-IL (Ru/OMS-IL) by transfusing OMS-IL with a RuCl3 in water for reducing nitroarenes with good selectivity (**Figure 25**). Both mono and poly substituted nitroarenes were transformed into their respective

**Figure 25.** *Reduction of nitro arenes via ruthenium-catalyzed TH.*

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

**Figure 26.**

*Ru10%@NSG catalyzed reduction of nitroarenes.*

$$\bigotimes\_{\text{0}} \bigotimes\_{\text{0} \text{ } \text{N} \text{W} \text{H}\_{2}\text{O}\_{4}} \xrightarrow[\text{180°C}]{\text{Ru/NiFe}\_{2}\text{O}\_{4}} \bigotimes\_{\text{0}} \bigotimes\_{\text{1} \text{80} \text{ } \text{°C}} $$

#### **Figure 27.** *Conversion of furfural to 2-methylfuran.*

anilines in high yields (**95a**-**95j)**. The most highlighted advantages of Ru/OMS-IL are: they show high catalytic activity as well as chemoselectivity. The catalysts are also highly stable and can be easily recovered from the reaction mixture. They showed great reactivity towards the reduction of the functionalized nitro compounds to the aromatic amines in the presence of ethanol as the solvent and hydrazine hydrate as a hydrogen donor and exhibited catalytic activity up to six cycles.

Dabiri and co-workers documented the use of graphene oxide and RuCl3 as the starting precursors and thiourea as a reducing doping agent for the preparation of ruthenium nanoparticles supported on nitrogen and sulfur-doped 3-D graphene (Ru@NSG) nanohybrid through a one-pot hydrothermal method [38]. The catalytic efficiency of ruthenium-nanohybrid was compared with the reduction of nitroarenes to the analogous anilines using NaBH4 as hydrogen doner in 1:1 ethanol/water solvent at room temperature (**Figure 26**). A broad range of functionalized nitro arenes were transformed to their corresponding aniline derivatives in decent yields.

#### **3.3 Miscellaneous heterogeneous transfer hydrogenation**

Furfural (FFA) derived from biomass is a promising energy source for the future biorefinery and is industrially produced *via* the dehydration of xylose and arabinose [39]. Over the last few decades, its synthesis received a great interest from the researchers. In this context, Liang and co-workers reported the synthesis of 2 methylfuran (MF) from furfural by using Ru/NiFe2O4 by catalytic TH using isopropanol as hydrogen donor under mild conditions. At 180 °C and 2.1 MPa nitrogen, the transformation of furfural was achieved up to c*a*. 97%, whereas MF was formed in *ca.* 83% yield (**Figure 27**) [40]. Additionally, the catalyst showed excellent activity up to five consecutive cycles.

#### **4. Conclusion and future perspectives**

The simple operational procedure, the mild reaction conditions with high catalytic activity and selectivity make the TH reactions an attractive alternative to direct hydrogenation using H2 gas. This research field is growing rapidly due to the high demand for the development of sustainable and green chemistry point of view. Recently, significant developments of Ru-catalyzed both TH and ATH of carbonyl, olefine, nitro and nitrile groups have been achieved. This improvement was perceived in several aspects, such as design of ligands or stabilizers to improve the reaction efficiency, exploration of "green" hydrogen source, generalization of reaction in water, enhancement in asymmetric synthesis, broadening of substrate diversity, and study of reaction mechanisms. Addition to these, TH has been explored in the syntheses of numerous compounds, in particular fine chemicals, bioactive molecules, agrochemicals, and products bearing multi-functional groups.

Although remarkable developments have been made in Ru-catalyzed TH reactions, many challenges and problems remain in most of the reported results. For example, majority of the reported reactions cannot be applicable for the practical and industrial applications. The catalytic results of ATH are not much promising compared to the direct asymmetric hydrogenations. The TH and ATH reactions of imines, olefins, and nitroarenes are very less efficient than that of ketones and are still not explored properly. The use of Ru-catalyzed heterogeneous TH and ATH is still under developed and mostly limited to the ketone group. However, at the present time, new findings are boosting the field by addressing these challenges which indicate TH has a bright future.

This chapter described the recently developed homogeneous and heterogeneous ruthenium catalysts for different substrates ketones, imines, olefins, nitriles, esters and nitroarenes. Attention was focused on mechanistic characteristics of TH/ATH with different ligands frame and their effects on ATH reaction rate.
