**3. Hydrogenation in ionic liquids**

Research into catalytic hydrogenations in ILs began in 1995 with the almost simultaneous work of Chauvin [42] and Dupont [43]. Since, catalytic reactions involving metal complexes in ILs have been actively investigated. Around 300 ILs have been screened and have led to the production of useful products and intermediates [44–46]. The majority of these ILs contained heterocyclic cations, such as pyridinium, imidazolium and polyalkylammonium and recently, synthesized guanidinium, piperidinium, pyrrolium, pyrrolidinium, morpholinium, cholinium, piperazinium and thiazolium. Other ILs had bridged structures, binuclear or polynuclear, zwitterionic, hydrophobic (fluorinated) and chiral derivatives [47].

Gathergood et al. wrote, in 2011, a big chapter entirely dedicated to hydrogenation processes in ILs [48]. This chapter is quite complete. For each hydrogenation reaction reviewed, catalysts and ILs are noted, together associated with the nature of the substrate. A section describing kinetic and thermodynamic studies of hydrogenations in ILs is also presented, as well as the solubility of H2 in many solvents and ILs under 1 atm. Some relevant examples have been identified from this chapter exclusively based on the use of H2 and catalysts (metal complexes or nanoparticles) in ILs.

Concerning the use of transition metal-based catalyst, various Ru- or Rh-based catalysts were used for the hydrogenation of halonitrobenzenes [49], cinnamaldehyde [50], cyclohexanone [51] or hexane [52] in 1-butyl-3-methylimidazolium hexafluorophosphate (bmim PF6) (**Scheme 7**).

**Scheme 7.** Rh- or Ru-catalysed hydrogenations.

**Scheme 6.** Ephedrine- or ampicillin-based ILs.

or nanoparticles) in ILs.

**3. Hydrogenation in ionic liquids**

312 New Advances in Hydrogenation Processes - Fundamentals and Applications

Research into catalytic hydrogenations in ILs began in 1995 with the almost simultaneous work of Chauvin [42] and Dupont [43]. Since, catalytic reactions involving metal complexes in ILs have been actively investigated. Around 300 ILs have been screened and have led to the production of useful products and intermediates [44–46]. The majority of these ILs contained heterocyclic cations, such as pyridinium, imidazolium and polyalkylammonium and recently, synthesized guanidinium, piperidinium, pyrrolium, pyrrolidinium, morpholinium, cholinium, piperazinium and thiazolium. Other ILs had bridged structures, binuclear or polynuclear,

Gathergood et al. wrote, in 2011, a big chapter entirely dedicated to hydrogenation processes in ILs [48]. This chapter is quite complete. For each hydrogenation reaction reviewed, catalysts and ILs are noted, together associated with the nature of the substrate. A section describing kinetic and thermodynamic studies of hydrogenations in ILs is also presented, as well as the solubility of H2 in many solvents and ILs under 1 atm. Some relevant examples have been identified from this chapter exclusively based on the use of H2 and catalysts (metal complexes

zwitterionic, hydrophobic (fluorinated) and chiral derivatives [47].

For these last compounds, Pt- or Pd-based complexes could also be used [53] as well as for the hydrogenation of benzene [54] or benzene derivatives [55] pyridinium ILs (**Scheme 8**).

**Scheme 8.** Pd- or Pt-catalysed hydrogenations of benzene.

PdCl2 was also used by Gathergood et al. [56] in imidazolium ILs, including a readily biodegradable IL [(3-methyl-1-pentoxycarbonylmethyl)imidazolium octylsulphate], for the selective hydrogenation of phenoxyocta-2,7-diene under mild conditions (**Scheme 9**).

**Scheme 9.** Pd-catalysed hydrogenation in the presence of ester-functionalised ILs.

Concerning the asymmetric hydrogenation leading to enantiomerically pure products [57], the source of chiral induction was generally due to the presence of chiral ligands (BINAP or BINAP derivatives) coordinated to a metal catalyst [58–61], Rh- and Ru-based catalysts being generally the favourite candidates [48].

Metallic nanoparticles (NPs) could also be very useful for the hydrogenation processes. However, the knowledge about their formation and stabilisation for hydrogenation reactions in ILs is relatively new [62]. Pd [63, 64], Pt [65], Ir [66, 67], but also mixed Pd/Au NPs [68] were commonly used for (selective) hydrogenation of (poly)alkenes, while Ru-[69] and Ni-NPs [70] remained quite rare and were used for selective or complete hydrogenation of alkenes or arenes in imidazolium ILs (**Scheme 10**).

**Scheme 10.** NPs-assisted hydrogenations.

Scheeren et al. showed the formation of stable and isolable Pt(0)-NPs by reacting Pt2(dba)3 in 1-n-butyl-3-methylimidazolium hexafluorophosphate Bmim PF6 with molecular hydrogen (4 atm) at 75°C [65]. These NPs were very efficient for the hydrogenation of diphenylacetylene in Si-functionalised ILs (**Scheme 11**).

**Scheme 11.** Hydrogenation of diphenylacetylene in the presence of Pt NPs in Si-functionalised ILs.
