**4. Hydrogenation in biobased ionic liquids**

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

314 New Advances in Hydrogenation Processes - Fundamentals and Applications

the favourite candidates [48].

in imidazolium ILs (**Scheme 10**).

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

in Si-functionalised ILs (**Scheme 11**).

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

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

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 2013, Ferlin et al. prepared easily and with good yields, biobased ionic liquids from natural organic-derived anions (L-lactate, L-tartrate, malonate, succinate, L-malate, pyruvate, D-glucuronate and D-galacturonate) by reaction between tetrabutylammonium hydroxide (TBA OH) and an excess of the corresponding acid (**Scheme 3**) [32]. Toxicity assays towards a large panel of bacterial and fungal strains were performed. ILs containing D-glucuronate or D-galacturonate anions were the less toxic whereas TBA L-tartrate and TBA Lmalate were the most toxic biomass derivatives. All ILs were less toxic to *E. coli* than TBA Br. Unfortunately, none of the salt containing biomass-derived anions passes the closed bottle test, nor tetrabutylammonium, tetraethylammonium or tetramethylammonium bromide.

These biobased ILs showed good performance and recyclability (until 10 runs without loss of activity) in catalytic selective hydrogenation of 1,5-cyclooctadiene into cyclooctene at room temperature under atmospheric H2 pressure. In these mild conditions, they were more suitable for selective hydrogenation than commercial imidazoliums or ammonium ILs, which gave cyclooctane as major product (**Scheme 12**) [32].

Proline was also used to prepare easily chiral ionic liquids (CILs) tetrabutylammonium-(S) prolinate, tetrabutylammonium-(R)-prolinate and tetrabutylammonium trans-4-hydroxy-(S) prolinate from aminoacid, still by acido-basic reaction with tetrabutylammonium hydroxide (TBA OH). While all three CILs have low antimicrobial toxicity to a wide range of bacteria and fungi, they did not pass the closed Bottle biodegradation test (**Scheme 13**) [71].

The hydrogenation of double carbon-carbon bonds of α,β-unsaturated ketones was processed under mild conditions with PdCl2 as catalyst in the presence of a CIL and a co-solvent (**Scheme 14**) [71]. The best performance was achieved when isopropanol was used in a cosolvent/CIL ratio equal to 5. Total conversion of isophorone with an enantiomeric excess (ee) up to 47% was obtained, and recyclability of the system was observed for five cycles without loss of reactivity.

**Scheme 12.** Selective hydrogenation of 1,5-cyclooctadiene in the presence of ILs.

**Scheme 13.** Preparation of proline-based ILs.

**Scheme 14.** Selective hydrogenation of α,β-unsaturated ketones under mild conditions in the presence of ILs.

Enantioselective hydrogenation of others α,β-unsaturated ketones was studied in the same mild conditions. Considering the particularly mild conditions, it was found that the method was very effective and competitive by comparison with previous works. Total conversion of pulegone was obtained with a good diastereoisomeric excess (de). Conversion was also complete for (R)-carvone with a very high selectivity of 90%. Hydrogenation occurred in the two carbon-carbon double bonds and an important de of 73% was observed. In the case of progesterone and 4-cholest-3-one, reaction occurred with very good yields and de, especially for 4-cholest-3-one.

Finally, concerning the field of transfer hydrogenation (so without the use of metallic species), γ-valerolactone-based ionic liquids (GVL-ILs) containing hydroxyvalerate anion were prepared according acid base reaction (**Scheme 15**) [72]. Theses ILs were successfully applied as alternative solvents for homogenous catalytic transfer hydrogenation of acetophenone and its substituted forms, but also of functionalised ketones and alkenes.

**Scheme 15.** Preparation of γ-valerolactone-based ILs.

**Scheme 12.** Selective hydrogenation of 1,5-cyclooctadiene in the presence of ILs.

316 New Advances in Hydrogenation Processes - Fundamentals and Applications

**Scheme 14.** Selective hydrogenation of α,β-unsaturated ketones under mild conditions in the presence of ILs.

Enantioselective hydrogenation of others α,β-unsaturated ketones was studied in the same mild conditions. Considering the particularly mild conditions, it was found that the method was very effective and competitive by comparison with previous works. Total conversion of

**Scheme 13.** Preparation of proline-based ILs.

A series of GVL-based ILs associated with tetraalkylammonium cations were tested. Structure of the cation had negligible influence on the catalytic activity. The potential recyclability of the catalytic system was demonstrated in four consecutive cycles, especially for the reduction of acetophenone. The highest conversions were achieved by using [Rh(cod)2] + [BF4] − as catalyst precursor and formic acid as hydrogen donor. The optimal reaction conditions were 80°C and a molar ratio of HCOOH/substrate between 5:1 and 6:1 (**Scheme 16**) [73].

**Scheme 16.** Transfer hydrogenation of substituted acetophenones.
