Catalytic transfer hydrogenation in γ-valerolactone-based ionic liquids

aBudapest University of Technology and Economics, Department of Chemical and Environmental Process Engineering, Budafoki str. 8. Budapest, Hungary, H-1111. Tel: +36 463 1263. Fax: +36 1463 3197. e-mail: laszlo.t.mika@mail.bme.hu bEötvös Loránd University, Institute of Chemistry, Pázmány Péter s. 1/a. Budapest, Hungary, H1117. cJános Selye University, Faculty of Education, Department of Chemistry, Bratislavská str. 3322, Komárno SK-94501, Slovakia


Introduction
Typically, the commonly used organic solvents are toxic, have high vapour pressure and negative impact on the environment.Their replacement represents a crucial part in the development of greener and environmentally benign chemical technologies, 1 and preference should be given to solvents with no or low toxicity according to the h principle of green chemistry.1b Recently several alternative solvents e.g.water, 2 uorous solvents, 3 supercritical carbon-dioxide, 4 alcohols, 5 and ionic liquids (ILs) were introduced with signicantly different solvent properties. 6Due to their attractive properties, ILs have attracted considerable attention as environmentally benign reaction media.By the proper variation of their anionic and cationic parts, their physical properties such as vapour pressure, thermal stability, solvation strength can easily be ne-tuned. 7ccordingly, wide range of chemical transformations has been successfully performed in room temperature ionic liquids (RTILs), 8 including industrially important reactions e.g.Friedel-Cras acylation, 9 hydroformylation, 10 and Beckmannrearrangement. 11ydrogenation reaction as one of the most important and widely studied transition metal catalyzed homogeneous transformation 12 was also performed in ionic liquids. 13,14The catalytic transfer hydrogenation of organic substrates is a safer and more energy-efficient alternative operation without using hydrogen gas and can generally be performed in common organic solvents. 15Although, the ionic liquids are widely used as environmentally benign solvents in high-pressure catalysis, there are only a few examples for application in transfer hydrogenation reactions.The combination of the mild conditions of transfer hydrogenation with the advantages of g-valerolactone (GVL)-based ILs could result in an environmentfriendly method for the reduction of organic substrates.In addition, due to the low vapour pressure of GVL based ILs, 14 the risk of environmental emission, therefore the potential hazard to health, could be signicantly reduced compared to conventional organic solvents.
First, Berthold et al. published the application of ILs as reaction media for transfer hydrogenation of ketones and their a,b-unsaturated forms. 16Hermecz et al. reported the reduction of a,b-unsaturated carbonyl derivatives by using [ClRh(PPh 3 ) 3 ] and [Rh(cod)Cl] 2 precursors in imidazolium, ammonium, and phosphonium containing ILs, representing outstanding chemoselectivities depending on the nature of the anions, with TOF ¼ 2.5-20 h À1 at 90 C. 17 Since, the asymmetric hydrogenation of ketones is a key reaction in the synthesis of biologically active molecules, enantioselective transfer hydrogenation reactions were also performed in ILs resulting in enantiomeric excess of 76-96%. 18Dyson 19 and Ohta 18 revealed that ILs do not act only as solvents, they have a special role in the stabilization of the catalytically active species.Chakraborti et al. proved that ionic liquids could have a special role in the activation of organic substrates e.g.ketones and aldehydes via hydrogen bond formation between the IL and C]O group. 20,21t was recently demonstrated, that selective catalytic hydrogenation can be performed in GVL-based ILs. 14 This generation of ILs can easily be prepared from bio-originated GVL, 14,22 which is a proposed sustainable platform molecule. 23The reaction rate could be affected by varying the substituents of the valerate anion and the tetraalkylammonium cation. 14We also reported that various functionalized olens can be selectively reduced in tetrabutylammonium 4-hydroxyvalerate [TBA][HV] ionic liquid with TOF ¼ 20-41 h À1 .Accordingly, [TBA][HV] was selected as solvent for the transfer hydrogenation of acetophenone, a typical model substrate.
We report here the application of GVL-based ILs as reaction media for transfer hydrogenation reactions using various transition metal catalysts.
Initially, to extend the series of GVL-based ionic liquids, novel species were synthesized as follows: ).Since the temperature dependence of the vapour pressure of solvents is a key property, it was determined for novel ILs in the temperature range of 0-85 C. Expectedly, the data obtained was found to be in the same range as measured in case of GVL and the previously published GVLbased ILs 14 as well (Fig. 1).
Furthermore, different catalyst precursors were investigated in the presence of [TBA][HV] and formic acid as hydrogen donor in the transfer hydrogenation of acetophenone at 80 C (Scheme 1).Expectedly, in the absence of any catalyst no conversion was detected.The activities of the Rh-based catalyst precursors were much higher than that of obtained for the Pd-containing system (Table 1), in accordance with previous results.17a,24 The activity of the Shvo catalyst, which was found to be very efficient for the conversion of levulinic acid to GVL, 25 was rather low under these conditions (Table 1, entry 7).The highest activity was achieved in the case of using [Rh(cod) 2 ] + [BF 4 ] À similarly to the conventional hydrogenation reactions. 14The resulted TOF was 20 h À1 which is signicant, compared to literature data reported so far. 17Consequently, further experiments were performed using [Rh(cod) 2 ] + [BF 4 ] À .
In contrast with the conventional high-pressure hydrogenation system, the reaction rate could not be increased by the addition of phosphine ligand.When 10 eq.triphenyl phosphine was added to the reaction mixture, the conversion decreased to 58% under identical conditions.In the presence of the sodium salt of trisulfonated triphenylphosphine (TPPTS), even lower yield (6%) of 1-phenylethanol was obtained.Similar effect was reported for the Pd-catalyzed selective reduction of alkenes. 26o optimize the reaction temperature, we repeated the reduction of acetophenone in [TBA][HV] in the temperature range of 60-90 C. The highest conversion was obtained at 80 C (Fig. 2).
The catalytic performance of [Rh(cod) 2 ] + [BF 4 ] À was also evaluated utilizing different conventional hydrogen donors (Table 2).In the presence of formic acid and sodium formate >99% conversion was obtained, while by using potassium formate, the hydrogenation was incomplete.It should be noted that no ester formation was detected in the presence of HCOOH.No product formation could be detected aer 3 hours by using ammonium formate (which was not even soluble under the applied conditions) and isopropyl alcohol.In accordance with the 7 th principle of green chemistry, formic acid was selected as hydrogen donor since it can be produced from renewable raw materials; 27 moreover the carbon dioxide produced in the reaction is thermodynamically very stable and can be easily removed from the reaction mixture.
It was shown that the reaction rate of the transfer hydrogenation is determined by the concentration of the H-donor. 28The negative effect of acidity on reaction rate was clearly demonstrated in case of asymmetric transfer hydrogenations. 29ccordingly, we investigated this effect in the range of 1.2-6.5 molar excess of HCOOH relative to the substrate (Fig. 3).
The conversion reached a plateau between 5 : 1 and 6 : 1 molar ratio (Fig. 3), similarly to the observed case of asymmetric transfer hydrogenation of ketones, using azeotropic mixture of HCOOH/triethylamine as hydrogen donor.29g In extreme cases, such as when the ratio was over 8 : 1, the conversion decreased   signicantly (below 20%) presumably due to the dilution and increased acidity of the system, which could result in the loss of the catalyst's activity.The exact amount of HCOOH remained aer the completed reaction was determined by 1 H-NMR spectroscopy by using GVL as an internal standard in the presence of 0.3 mmol acetophenone, 0.005 mmol [Rh(cod When the HCOOH to substrate ratio was 1 : 1 and 1.2 : 1, no residual HCOOH was detected aer 3 hours, while in case of 3 : 1 and 5 : 1 ratio 40% decrease of the initial amount of H-donor could be obtained.At ratio 6 : 1 more residual amount of formic acid was detected with full conversion of substrate.Further increasing this ratio resulted a slight decrease of the conversion rate.It is important to note that no water formation was detected.Consequently, due to the decomposition of formic acid, an excess of hydrogen donor has to be used to reach full conversion.It was proven that the GVL-based ILs can be easily ne-tuned by varying the chain length of the alkyl groups on the cation, in signicant changes in viscosity and vapour pressure.In order to determine the inuence of the structure on the performance of the catalytic system, we screened a series of GVL-based ILs having different tetraalkylammonium cations.Table 3 shows, that negligible conversion could be achieved using [TMA][HV], which has relatively high melting point (75-80 C), thus it becomes liquid at the reaction temperature and might be more useful at even higher temperatures.The replacement of methyl to ethyl group resulted in signicantly higher conversion (94%, see Table 3, entry 2).Interestingly, further increase of the length of the alkyl chain on the cation had no dramatic effect on the product formation.The reaction rate reached >99% in case of [TBA][HV] (Table 3, entry 4) similarly as in the conventional hydrogenation reactions using GVLbased ILs containing hydroxyvalerate anion.
Chakraborti's studies indicated that carbonyl group(s) could participate in the hydrogen bond interaction between the oxygen atom of carbonyl species and a hydrogen atom of corresponding ionic liquid to form an activated intermediate that readily reacts with the other substrate of the reaction. 20This observation was also veried for the addition reactions of a,b-unsaturated carbonyl species. 21To investigate the role of the 4-hydroxyl group of the IL's anion in transfer hydrogenation, the reduction of acetophenone was also performed in tetrabutylammonium 4-methoxyvalerate and 4-ethoxyvalerate, (where the 4-OH group was replaced with 4-OMe and 4-OEt ones).In both cases, full conversion was achieved showing that the hydroxyl group has no role in the substrate activation (Table 3, entries 7 and 8).It was established that the C2-hydrogen of the imidazolium cation could activate the C]O group via hydrogen bond formation, as well.20a,b In order to further demonstrate the possible effect of the IL's structure on the efficiency of the system, butylmethyl imidazolium-based ionic liquids [BMIM] + [X] À (X ¼ Cl À , PF 6 À , OH À , octylsulphonate) were investigated for comparison.Unexpectedly, no conversion could be detected under the previously optimized conditions.
Presumably the proposed interaction could not be established under the applied acidic conditions.Similar observation was reported for the selective reduction of chalcone performed in dialkyl imidazolium type ILs.17a It should be noted that the reduction of the carbonyl group could not be achieved even in tetraalkylphosphonium-based ILs.Analogously, no product formation was detected applying conventional organic solvents such as toluene, methanol, ethanol, propanol, and dimethyl sulfoxide.
To check the activity and stability of the catalytic system, a second portion of acetophenone (0.3 mmol) and HCOOH (1.6 mmol) was added to the mixture of the completed reaction aer 3 hours.It was observed that the catalyst remained active and the substrate was completely converted in the consecutive runs, as well.However, a slight decrease in the activity could be detected in the fourth cycle, in which 4 hours was needed to reach the full conversion.Since [TBA][HV] was miscible with the product, it was extracted from the catalyst phase by pentane (2 Â 0.5 mL) extraction.Subsequently, HCOOH (1.6 mmol) and the substrate (0.3 mmol) were added and the next cycle was started.This procedure was repeated for three times.Although, full conversion was achieved in cycles 1-3, a slight decrease in the activity was detected in the 4 th run (Table 4).Similar remarks were described for Rh-catalyzed transfer hydrogenation of chalcone, where toluene was used for the extraction. 17The isolated yield of 1-phenylethanol was 58%.
As already stated, the electronic properties of the substituents on the carbonyl group had signicant effect on the reactivity.The 4-substituted acetophenone derivatives having electron donating substituent such as methyl or methoxy group, were found to be less reactive compared to acetophenone (Table 5, entries 1-3).Consequently, the results indicated that the introduction of electron withdrawing groups led to higher conversion rate (Table 5, entries 4-6), as expected.The 2-and 3substituted derivatives also showed lower reactivity (Table 5, entries 7 and 8).Identical observations were reported for both conventional and asymmetric transfer hydrogenations. 30It is noteworthy that the reduction of 4-nitroacetophenone led to the formation of 1-(4-nitrophenyl)ethanol.Concerning nonaromatic carbonyl compounds, cyclic ketones were reduced in [TBA][HV] under identical conditions and, as it was predicted, full conversion was achieved (Table 6, entries 1 and 2).Regarding the levulinic acid and its methyl ester, it was revealed that both species can be selectively reduced to gammavalerolactone (Table 6, entries 3 and 4).
Further investigations were focused on the applicability of [Rh(cod) 2 ] + [BF 4 ] À /HCOOH catalyst system for the hydrogenation of different substrates with olenic carbon-carbon double bond, under the optimized conditions.It was shown that alk-1-enes could be quantitatively converted to the corresponding alkanes with TOF ¼ 20 h À1 (Table 7, entries 1-3), as expected.For cycloalkenes full conversions were observed (Table 7, entries 4 and 5), similarly to the catalytic hydrogenation of olens in these ILs using molecular hydrogen.When crotyl bromide and cinnamic acid were subjected to reduction, >99% conversion was achieved for both cases, however, in case of crotyl bromide the reaction was already completed in 1.5 hour (Table 7, entries 6 and 7).For the sterically hindered trans-stilbene slightly lower conversion was observed (Table 7, entry 8).
The selective reduction of a,b-unsaturated carbonyl compounds were investigated by Hermecz et al. in different imidazolium-based ILs using Pd(OAc) 2 , [Rh(cod)Cl] 2 and ClRh(PPh 3 ) 3 as catalyst precursors in the presence of HCOONH 4 or HCOOH as H-donor.17a When 0.6 mmol of 1-cyclohex-2enone in the presence of 3.2 mmol HCOOH and 0.01 mmol [Rh(cod) 2 ] + [BF 4 ] À in 1 mL [TBA][HV] was reduced at 80 C, the complete conversion to cyclohexanone was detected aer 15 min; however, aer 3 hours both the carbon-carbon double bonds and the carbonyl groups were completely reduced to cyclohexanol.The same results were obtained in case of using chalcone as substrate indicating that the hydrogenation of C]C bond is much faster than C]O bond.However, by applying longer reaction time both functional groups were reduced.The reduction of cinnamic aldehyde (0.3 mmol) led to the formation of 3-phenylpropanal with conversion of 66% under identical conditions.1-Butyl-3-methylimidazolium hexauorophosphate, 1-butyl-3-methylimidazolium tetra-uoroborate, 1-butyl-3-methylimidazolium octyl sulfate, chalcone, 1-cyclohex-2-enone, cinnamic acid were purchased from Sigma-Aldrich K. and used as received.1-Butyl-3methylimidazolium chloride, 1-butyl-3-methylimidazolium hydroxide and 1-ethyl-3-methylimidazolium hydroxide were prepared by published methods. 31The water content of the ILs was determined by Karl-Fischer titration.

Preparation of tetraethylammonium 4-hydroxyvalerate [TEA] [HV]
The mixture of 14.73 g (100 mmol) of tetraethylammonium 4hydroxide (in 25% aqueous solution) and 10.51 g (105 mmol) of g-valerolactone was stirred at room temperature for 1 hour.Aer the reaction was completed, the residual water was removed in vacuo at 80 C. The product was isolated as a colourless, highly viscous liquid.Yield 22.11 g (87.6%). 1 H-NMR (D 2 O) d, ppm: 0.98 (d, 3H, J ¼ 6.5 Hz), 1.07 (t, 12H, J ¼ 7.1 Hz), 1.52 (q, 2H, J ¼ 7.2 Hz), 2.04 (m, 4H), 3.07 (q, 8H, J ¼ 7.1 Hz), 3.62 (m, 1H, J ¼ 6.1 Hz) 13  The vapour pressure of the ionic liquids were measured by using a Rosemount manometer attached to a 25 mL Hastelloy-C high-pressure reactor (Parr Inst., IL, USA) equipped with magnetic stirrer and a thermocouple.The latter was connected to the computer through an AD interface to register the actual temperature.Initially, the reactor was charged with 1 mL of the corresponding ionic liquid, followed by the attachment of the    manometer, and de-aerated at room temperature.The reactor was then placed into ice-water mixture.The exact value of the vapour pressure was recorded when it was stabilized at the desired temperature.The transfer hydrogenation reactions were performed in Hach tubes (100 Â 13 mm) closed with screw-cap.In a typical experiment, 0.005 mmol of the correspondent catalyst was dissolved in 0.5 mL ionic liquid, then 1.5 mmol H-donor and 0.3 mmol substrate was added to the solution.The reaction mixture was stirred at 80 C for 3 h.The conversion and yield were determined by GC analysis and 1 H-NMR spectroscopy.
In order to determine the amount of residual HCOOH in the reaction, equimolar amount of HCOOH and GVL (as internal standard) was added to the mixture of 0.5 mL [TBA][HV] and 0.3 mmol acetophenone.A sample was taken before heating up the mixture and another one aer the reaction was completed.The samples were dissolved in d 6 -acetone, then analyzed by 1 H-NMR.
The NMR spectra were recorded on a Brucker Avance 250 spectrometer.
In the recyclability experiments 0.3 mmol of acetophenone was reduced by using 0.01 mmol of [Rh(cod) 2 ] + [BF 4 ] À in 1 mL [TBA][HV] at 80 C. Aer completing the reaction the product was extracted with 2 Â 0.5 mL pentane.To initiate the following cycle, acetophenone (0.3 mmol) and HCOOH (1.6 mmol) were added to the catalyst phase followed by heating up to 80 C.An intensive gas evolution indicated the start of the reaction.The procedure was repeated 3 times and the isolated products from the consecutive cycles were analyzed by GC.
GC analyses were performed on an HP 5890N instrument with a HP-5 capillary column (15 m Â 0.25 mm) using H 2 as a carrier gas.For the analysis, 5 mL of reaction mixture was dissolved in 1 mL of methylene chloride followed by the addition of 5 mL toluene as internal standard.

Conclusions
g-Valerolactone-based ionic liquids (ILs) containing hydroxyvalerate anion were successfully applied as alternative solvents for homogenous catalytic transfer hydrogenation of acetophenone and its substituted forms, functionalized ketones and alkenes.The highest conversions were achieved by using [Rh(cod) 2 ] + [BF 4 ] À as catalyst precursor and formic acid as hydrogen donor at 80 C, with the molar ratio of HCOOH/ substrate between 5 : 1 and 6 : 1.A series of GVL-based ILs with tetraalkylammonium cations were tested showing negligible inuence of the cation structure on the catalytic activity.The potential recyclability of the catalytic system was demonstrated in four consecutive cycles for the reduction of acetophenone.

Table 4
Reuse of catalyst phase for reduction of acetophenone a

Table 5
Transfer hydrogenation of substituted acetophenones a

Table 6
Transfer hydrogenation of non-aromatic carbonyl species a

Table 7
Transfer hydrogenation of several alkenes a