**3. Ruthenium complexes for epoxidation of alkene**

Being a transition metal, ruthenium displays several oxidation states that can be readily interchanged. Consequently, ruthenium facilitates the exchange or replacement of ligands in the complexed state, thus mediating access for metal-oxo species. The latter represent the main olefin epoxidation intermediate and are highly useful catalysts because they act as oxygen donor to alkenes. A variety of rutheniumcatalyzed epoxidation catalysts are supplied by homogeneous ruthenium complexes with ligands like porphyrin [18], polypyridyl [19], Schiff base [20], oxazoline [21], and pyrazolyl [22].

In 1984, Balavoine and colleagues suggested that RuCl3, 2,2′-bipyridyl and sodium periodate could be employed in a two-phase reaction medium for alkene epoxidation [23]. Although the mechanism remained unclear, it was possible that a Ru(IV)-0x0 complex, potentially [Ru″'(bipy)&l(O)]', represented the active species. In 1985, Eskanazi and colleagues demonstrated that it was possible to control the rate and stereoselectivity of alkene oxidation by substituting the 2,2′-bipyridyl with different ligands [24].

Fackler and colleagues applied the method of Sonogashira coupling with a bromo-substituted porphyrin and terminal alkyne to create a ruthenium porphyrin epoxidation catalyst of high enantioselectivity and regioselectivity. This catalyst achieved enantioface-selective oxo transfer to alkene functionalized quinolones, pyridones, and amides via non-covalent hydrogen bond interactions [25–26].

In 1998 [27], ruthenium complexes that included pyridine and picoline ligands were used to subject cyclohexene and styrene to catalytic oxidation. Alteration of the oxidant character led to marked variability. Cyclohexene oxidation yielded 2-cyclohexen-l-ol and 2-cyclohexenone when the oxidizing agent was cumenehydroperoxide (CHP), and 2-cyclohexenone when the oxidizing agent was N-methylmorpholine-N-oxide (NMO). Contrary to expectations, it was not epoxide but benzaldehyde that resulted from the oxidation of styrene. Hydrogen bond interactions could facilitate the engagement of the diaxial-dioxoruthenium species displaying catalytic activity with vinyl or alkenyl fragments (**Figure 6**). In this way, oxygen can be supplied to a specific alkene prochiral face [28].

Stoichiometric quantities of Cl2pyNO were used as oxidant to perform the epoxidations in benzene. Epoxidation of n-hydro-3-vinylquinolones was achieved with high enantioselectivity, which was reduced for N-methylated quinolones as one of the two hydrogen bond interactions was lost. Meanwhile, 3,7-divinylquinolone exhibited high regioselectivity, with epoxidation of the vinyl group since it was readily accessible to ruthenium oxocentre. Trans-epoxides with enantioselectivity higher than 90% were obtained by subjecting the 3-alkenyl quinolones to stereospecific and enantioselective epoxidation.

Another study by Man and his group [29] was found that olefin asymmetric epoxidation with ruthenium as catalyst was significantly improved when PhI(OAc)2 was present. The increase in the reaction rate when water was added was two orders of magnitude. It was possible to achieve reactions of aliphatic as well as aromatic olefins, with enantioselectivities being as high as 71% ee.

A novel pentadentatepolypyridine (L5pyr) ruthenium complex [Ru(L5pyr) (CH3CN)]2P was proposed by Hamelin and colleagues. When iodosyl benzene was employed as oxidant, this complex was observed to generate satisfactory amounts of epoxide for cyclooctene and trans-b-methyl styrene. The preparation of the complex involved use of RuCl2 (dmso)2 to reflux L5pyr and subsequent replacement with acetonitrile. Enhanced catalytic activity of [Ru(L5pyr)(CH3CN)]2þ depends greatly on the pentadentate ligand with electron abundance. This is reflected by the fact that epoxidation with lower dentate pyridine analogues [Ru(bpy)2(CH3CN)2]2þ produce suboptimal yields and turnover frequencies [30].

Two Ru(II)-aqua complex catalysts underpinned by oxazoline ligands viz. [RuII(iPr-box-C)(tpm)OH2](PF6)2 and [RuII(iPr-box-O)(tpm)OH2](PF6) were developed in recent times. The preparation of the complexes involved derivation from [RuIIICl3(tpm)] by base catalyzed oxazoline ligand exchange, with the generated chloro complex being subsequently hydrolyze with silver acting as catalyst. Phl(OAc)2 was employed as oxidant to analyze the potential of the two complexes as epoxidation catalysts for trans-stilbene. Results showed that catalyst 1 was associated with 85% epoxide selectivity and catalyst 2 was associated with 81% epoxide selectivity, while the conversion was nearly identical in both cases. Furthermore, catalyst 1 was regioselective for the terminal alkene segment of 4-vinylcyclohexene [31] (**Figure 7**).

*Ruthenium Catalyst for Epoxidation Reaction DOI: http://dx.doi.org/10.5772/intechopen.96466*

**Figure 7.** *Oxazoline ligands [31].*

In another study, a range of olefins were subjected to asymmetric epoxidation with ruthenium as catalyst and TBHP as oxidant. Under catalysis by ruthenium(pyridinebisoxazoline)-(pyridinedicarboxylate) complexes, aromatic and aliphatic olefins produced the equivalent epoxides at ambient temperature in moderate-to-high yields and enantioselectivity as high as 65% ee. The reaction yield and chemoselectivity were markedly enhanced by adding the stoichiometric oxidant in a gradual way [32].

In a similar study [33], olefins were subjected to asymmetric epoxidation with general ruthenium as catalyst and hydrogen peroxide as oxidant. Various aromatic olefins exhibited enantioselectivity as high as 84%. The reaction was successful especially because pyboxazines, a novel group of ligands, were added. It was anticipated that the catalytic behavior of common pybox derivatives harmonized well with such ligands. Furthermore, differences in catalyst structure were diminished by employing two distinct ligands, facilitating refinement of catalytic attributes.

One study undertook the synthesis of bis-facial dinuclear ruthenium complex that included a hexadentate pyrazolate-bridging ligand (Hbimp) and bpy as auxiliary ligands [34]. Additionally, the ability of water and alkene oxidation of this complex was assessed. Various alkenes were successfully subjected to epoxidation under catalysis by the *in situ*-produced bis-aqua complex, {[RuII(bpy) (H2O)]2(μ-bimp)}3+.

The difficulty of ruthenium complex heterogenization stems from the fact that it is necessary to preserve the ligand properties (e.g. lability, enantiopurity, relative orientation) that greatly influence epoxidation reactivity, enantioselectivity, regioselectivity, and chemoselectivity. One possible approach is to anchor the catalyst to a polymer, for which one of the ligands must include a reactive functional group or polymerizable moiety. An additional viable option is to immobilize catalysts in channels of materials of high porosity (e.g. zeolites, molecular sieves), with shape selectivity being afforded by characteristic pore sizes.

To conclude, It was shown that the chemical nature of the solvent, oxidant, type of catalysts and type of the ligand have a significant effect on the catalytic properties and stability of the active species.

#### **Acknowledgements**

Raiedhah Alsaiari would like to express her gratitude to the ministry of education and the deanship of scientific research – Najran University – Kingdom of Saudi Arabia for their support under code number (NU/ESCI/17/061).

## **Conflict of interest**

The author declares no conflict of interest.

*Ruthenium - An Element Loved by Researchers*
