Rhodium-catalyzed hydrogenation of olefins in γ-valerolactone-based ionic liquids

Article abstract of DOI:10.1039/c3gc40360b

γ-Valerolactone-based ionic liquids were successfully used as the catalyst phase for [Rh(cod)₂][BF₄]/RP(C₆H ₄-m-SO₃Na)₂ (R = Me, Pr, Bu, Cp) catalyzed hydrogenation of different olefins. Compared to broadly used ionic liquids e.g. 1-butyl-3-methylimidazolium chloride [bmim][Cl], the turnover frequencies were significantly higher and the reaction was selective for the CC double bonds in the presence of carbonyl, cyano, and phenyl groups. The catalyst was recycled for ten consecutive runs under regular or biphasic conditions without loss of activity. The vapour pressure and viscosity of γ-valerolactone-based ionic liquids were determined as well.

Full text of DOI:10.1039/c3gc40360b

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Rhodium-catalyzed hydrogenation of olefins in
γ-valerolactone-based ionic liquids†
Cite this: DOI: 10.1039/c3gc40360b
Received 19th February 2013,
Accepted 13th May 2013
a
a
a
a
b
Andrea Strádi, Márk Molnár, Mihály Óvári, Gábor Dibó, Frank U. Richter and
c
László T. Mika*
DOI: 10.1039/c3gc40360b
www.rsc.org/greenchem
γ-Valerolactone-based ionic liquids were successfully used as the hydrogenation reaction. It was demonstrated that the appli-
catalyst phase for [Rh(cod) ][BF ]/RP(C -m-SO Na) (R = Me, Pr, cation of higher aliphatic substituents containing ILs, such as
2
4
6
H
4
3
2
Bu, Cp) catalyzed hydrogenation of different olefins. Compared to hexyl and tetradecyl substituted phosphonium salts, resulted
broadly used ionic liquids e.g. 1-butyl-3-methylimidazolium in higher hydrogen concentration in the IL and therefore a
9
chloride [bmim][Cl], the turnover frequencies were significantly higher reaction rate. It was proposed that γ-valerolactone
higher and the reaction was selective for the CvC double bonds (GVL) can be considered as a sustainable liquid for the pro-
1
1
in the presence of carbonyl, cyano, and phenyl groups. The duction of energy and carbon based chemicals including
1
2
catalyst was recycled for ten consecutive runs under regular or tetramethylammonium 4-hydroxyvalerate ionic liquids. The
biphasic conditions without loss of activity. The vapour pressure combination of GVL-based ionic liquids with appropriately
and viscosity of γ-valerolactone-based ionic liquids were deter- designed ligands such as phosphines could result in environ-
mined as well.
mentally friendly alternatives for homogeneous hydrogenation
reactions with higher reaction rate, selectivity and facile cata-
lyst recycling.
The importance of organometallic catalysis combined with
facile catalyst/product separation has been increasing due to
potential applications in greener processes providing value-
added chemicals at the laboratory and industrial scale as well.
One of the elegant approaches for catalyst separation and re-
cycling is the biphasic homogeneous catalysis using alterna-
tive solvents such as water, fluorous and ionic liquids (IL),
and supercritical carbon dioxide. Among these solvents, ionic
liquids have attracted considerable attention as a potential
alternative reaction medium due to their extremely low vapour
pressure, good solvating properties, reasonable thermal stabi-
lity, and easily tuneable physical properties. Accordingly, a
wide range of chemical reactions have been performed in
room-temperature ionic liquids (RTILs).
Hydrogenation as one of the most important and widely
studied reactions of homogeneous catalysis was also per-
formed in ionic liquids. However, the low solubility of hydro-
gen in ionic liquids, which is similar to that in water,
Chauvin et al. reported the [Rh(nbd)(PPh
3 2 6
) ][PF ] catalysed
reduction of pent-1-ene resulting in less than 1% conversion
8a
2
in [bmim][Cl] under 1 bar of H for 2 h. It should be noted
that the hydrogenation of pent-1-ene and cyclohexene exhibi-
1
−1
ted low turnover frequencies (9–152 h ) in [bmim][X] (X =
2
3
4
−
−
−
−
SbF
6
, PF
6
, BF
4
Cl ) type ILs presumably due to the low
5
8
hydrogen concentration in the catalyst phase. Suarez used
[
Rh(cod) ][BF ] for the reduction of cyclohexene with 65% con-
2 4
version, slightly better than that obtained (40%) in the pres-
8b
3 3
ence of ClRh(PPh ) .
6
We report here the synthesis and application of GVL-based
ionic liquids, where the anion part is a hydroxyvalerate deriva-
tive (Scheme 1), as solvents for the efficient hydrogenation of
olefins using a rhodium-containing catalyst ([Rh(cod) ][BF ])
4
2
4
7
in situ combined with alkyl-bis(sodium-m-sulfonatophenyl)-
8
,9
24
phosphine RP(C
Scheme 2).
The 4-hydroxyvalerate based ILs can easily be prepared by
6 4 3 2
H -m-SO Na) (R = Me, Pr, Bu, Cp) ligands
1
0
(
notably determines their applicability as solvent for
the combination of GVL and an aqueous solution of the
corresponding tetraalkylammonium hydroxide with excellent
a
Eötvös University, Institute of Chemistry, Pázmány Péter s. 1/a, Budapest, H-1117,
1
2
Hungary
yield and high purity. The new methoxy and ethoxy deriva-
tives were synthesized by reacting the corresponding tetraalkyl-
b
Bayer MaterialScience AG, D-51368 Leverkusen, Germany
c
Budapest University of Technology and Economics, Department of Chemical and
12,13
ammonium hydroxide and alkyl 4-alkoxyvalerate
Environmental Process Engineering, Budafoki str. 8, Budapest, H-1111, Hungary.
E-mail: laszlo.t.mika@mail.bme.hu; Fax: +36 1463 3197; Tel: +36 463 1263
†
(
Scheme 3) with a yield of over 92%.
Being volatile organic compounds (VOC), the temperature
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c3gc40360b
dependence of the vapour pressure is a key property of the
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Fig. 1 Vapour pressure of the GVL-based ILs compared to GVL (the data of
GVL were adopted from ref. 11).
Scheme 1 γ-Valerolactone-based ionic liquids.
Scheme 2 Alkyl-bis(m-sulfonatophenyl)phosphines.
Fig. 2 Viscosities of GVL-based ionic liquids.
Scheme 3 Synthesis of novel GVL-based ionic liquids.
49.86 kJ; [TBA][4HV]: 57.33 kJ; [TBA][4MeOV]: 52.33 kJ; [TBA]-
[
4EtOV]: 53.11 kJ (ESI Fig. S1–S4†) representing slightly higher
1
4
solvents. Thus, the GVL-based ILs were characterized by their values than normal liquids and liquid salts with high melting
vapour pressure before use. Compared to other commercially points. The refractive indexes of the new ILs were similar to the
used oxygenates (e.g. methanol, ethanol, MTBE, or ETBE), values obtained for the previously reported ones.
ionic liquids have a significantly lower vapour pressure.
We investigated the hydrogenation of hex-1-ene (10 mmol)
Expectedly, the temperature dependence of the vapour in the presence of [Rh(cod) ][BF ] (0.004 mmol) under 30 bar
12
2
4
pressure of our ionic liquids correlates with that of GVL. The at 60 °C in [bmim][Cl] (1 mL) as a reference catalyst phase
structure of the cation had a significant effect on the vapour resulting in less than 0.5% conversion. The hydrogenation
pressure (ILs containing tetramethylammonium cation had reaction can be affected by the addition of tertiary phosphine
7
16
the lowest value); however, the influence of the anion was neg- ligands e.g. alkyl-bis(m-sulfonato-phenyl)phosphines. Due
ligible (Fig. 1).
to stronger metal–phosphorous interactions, it was expected
To further characterize the ionic liquids, their viscosities that these electron-donating ligands can enhance the catalyst
were also determined in the temperature range of 20–70 °C stability as well. At the same time, the solubility of the catalyst
(
ESI Table S1†). Fig. 2 indicates that the viscosity values in the apolar product phase will decrease, facilitating an easy
decrease exponentially (with a correlation factor higher than separation.
0
2
.99) for all species, as expected. The measured viscosities at
Consequently, the reduction of four different olefins was
5.1 °C ([TBA][4HV]: 1250 mPa s; [THA][4HV]: 911 mPa s; [TBA]- repeated in [bmim][Cl] in the presence of 10 eq. of L3 (Table 1
[
4MeOV]: 748 mPa s; [TBA][4EtOV]: 835 mPa s) were higher than entry 1); however, an increase of activity could not be detected.
values measured for similar ionic liquids. The activation ener- It was proposed that the reaction rate can also be enhanced by
1
5
gies (E ) for viscous flow of GVL-based ILs were [THA][4HV]: using more apolar ionic liquids as the catalyst phase, which
a
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a
Table 1 Hydrogenation of olefins in different ionic liquids
Substrate
Solvent
Hex-1-ene
Oct-1-ene
Dec-1-ene
Cyclohexene
Time (h)/yield (%)/TOF (h−1)
b
Entry 1
Entry 2
Entry 3
Entry 4
Entry 5
Entry 6
Entry 7
[BMIM][Cl]
[TBA][4HV]
[THA][4HV]
[TMA][4MeOV]
[TMA][4EtOV]
[TBA][4MeOV]
[TBA][4EtOV]
2.0/0.6/0
2.0/0.0/0
4.0/0.0/0
3.0/0.0/0
c
2.0/>99.9/1339
0.5/>99.9/5022
2.0/17.0/200
2.0/70.0/847
0.34/>99.9/5360
0.25/>99.9/7588
2.0/>99.9/1001
1.5/>99.9/1679
2.0/20.1/193
2.0/35.2/313
0.34/>99.9/6933
0.25/>99.9/7605
4.0/>99.9/463
0.75/>99.9/1759
2.0/52.0/379
2.0/50.2/401
2.0/>99.9/721
1.0/>99.9/1536
3.0/56.0/508
3.0/70.0/617
3.0/4.9/44
3.0/4.8/39
3.0/>99.9/941
3.0/>99.9/898
a
b
For experimental details, see ESI Table S2. Turnover frequency (TOF) defined as mol(alkane) per mol(Rh) per hour. Reaction conditions:
c
0
2 4
.004 mmol [Rh(cod) ][BF ], 0.04 mmol L3, pH2 = 30 bar, solvent: 1 mL, T = 60 °C; ESI Fig. S5.
could result in different conversion rates of hex-1-ene to
hexane. The highest activities (TOF = 1339 h− ) were achieved
in the case of L2 (χ = 12.1, θ = 151°) and L3 (χ = 12.5, θ = 153°).
Therefore, L3 was used in further investigations for other
olefins. These results are summarized in Table 1, entry 2. Pre-
sumably more hydrogen gas could be dissolved in the less
polar ionic liquids containing bulky tetrabutylammonium
cations. As a consequence, higher reaction rates were achieved
in the case of all substrates. It is important to note that the
high-pressure NMR measurements cannot provide adequate
integrals to calculate exact hydrogen concentration due to the
relatively high viscosity. Although remarkable turnover fre-
quencies were achieved when [THA][4HV] (containing the
longer hexyl substituent) was used as the catalyst phase
1
(
Table 1, entry 3), it was found to be fully miscible with sub-
Fig. 3 Hydrogenation of hex-1-ene in the presence of different electron donat-
ing sulfonated phosphines. p 2 = 30 bar, T = 60 °C.
strates except dec-1-ene. It should be emphasized that the mis-
cibility of liquids usually depends on small molecular
differences, for instance the length of the alkyl chain(s). The
H
results in higher hydrogen concentration in ILs as determined influence of the anion’s polarity on the hydrogenation was
9
by Dyson et al. Accordingly, the GVL-derived ionic liquids con- further investigated by using 4-hydroxy-, 4-methoxy-, and
1
2
taining apolar alkyl chains (Scheme 1) were used as the cata- 4-ethoxyvalerate as the anion. In addition, the replacement of
lyst phase for the hydrogenation reactions. The polarity of the –OH group reduced the potential reactivity of the valerate
+
these solvents can be easily fine tuned by varying the chain part of the IL. In the case of [TMA] cation, the conversion was
length of the alkyl groups (R and R ) and a proper solvent and low (Table 1, entries 4–5); however, when it was replaced for
1
2
+
substrate combination could result in a biphasic catalytic [TBA] instead, outstanding turnover frequencies were
system.
obtained (Table 1, entries 6 and 7). Due to their immiscibility
In the case of [TMA][4HV] (R = H and R = CH ) a white with C –C hydrocarbons, both [TBA][4MeOV] and [TBA]-
1
2
3
6
10
solid salt was formed (mp. 75–78 °C). The replacement of the [4EtOV] provided a real biphasic catalytic system; moreover, a
methyl substituents of the cation to butyl groups resulted in significant increase in the TOF was detected. Catalyst activity
[
TBA][4HV], a viscous colourless liquid. When it was used as was also tested on the reduction of 2,3-dimethylbuta-1,3-diene
the catalyst phase, the reduction of hex-1-ene in the presence (10 mmol) as a conjugated diene in [TBA][4HV] under biphasic
of [Rh(cod) ][BF ] under 30 bar H at 60 °C for 2 h (TOF = conditions (30 bar H at 60 °C). While 60% of the substrate
02 h ) led to the formation of 36% hexane. The yield can be was hydrogenated after 3 hours, full conversion could be
2
4
2
2
−
1
6
increased by the addition of 10 eq. of phosphine ligand (L1– achieved after 6 h with product distribution: 34.5% 2,3-
L4) under identical conditions (Fig. 3). The rate of the hydro- dimethylbut-1-ene, 38.8% 2,3-dimethylbut-2-ene, and 26.7%
genation is highly dependent on both the electronic (χ) and 2,3-dimethyl-butane (ESI Fig. S6†). It should be noted that 2,3-
1
7
the steric properties (θ) of the phosphines as defined by dimethylbuta-1,3-diene is fully miscible with [THA][4HV]. To
1
8
Tolman. For example, the rate of the Ru-catalyzed reduction study the effect of the presence of phenyl groups on the hydro-
of levulinic acid to GVL was strongly affected by the ligand genation, (E)-stilbene (5 mmol dissolved in 1.5 mmol toluene)
1
6
structure. While the electronic parameters of L1–L4 are was reduced in [TBA][4HV] (1 mL) using [Rh(cod) ][BF ]
2
4
similar, the significant difference in steric parameters (θ) (0.004 mmol) and L3 (0.04 mmol) under 30 bar H at 60 °C for
2
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6
h, which resulted in 80% conversion. No reduction of the cf. hex-1-ene in ESI Table S3†). Similarly to the biphasic
benzene ring was detected, as expected. system, no L3 was detected in the product after extraction. The
The feasibility of the catalyst recycling was determined hydrogenation of methyl 3-pentenoate was completed in the
under biphasic conditions by performing the experiments case of both [TBA][4HV] and [THA][4HV] (Table 2, entry 2); the
using hex-1-ene (10 mmol), [TBA][4HV] (1 mL), [Rh(cod) ][BF ] product was extracted with pentane. When acrylonitrile or cin-
2
4
(
0.004 mmol), 0.04 mmol of L3, 30 bar H
conversion of hex-1-ene to hexane was achieved after 2 hours conditions, only CvC double bonds reacted (Table 2, entries 3
Table 1, entry 2). After removing the product phase, the cata- and 4). No reaction of the cyano and formyl groups and no
2
at 60 °C. Complete namaldehyde was subjected to hydrogenation under identical
(
lyst phase was reloaded into the autoclave and mixed with the polymerization of acrylonitrile were detected, not even after a
same amount of hex-1-ene. The reduction was initiated by 6-hour reaction. It was observed that ionic liquids could par-
pressurizing the reactor to 30 bar at 60 °C. The beginning of ticipate in specific interaction with the substrate preventing
1
9
the reaction was indicated by a significant initial pressure the reduction of a functional group e.g. CvO. Importantly,
drop. Quantitative formation of hexane was obtained without both acrylonitrile and propionitrile had an effect on the cata-
2
0
loss of activity even after ten consecutive cycles. The product lyst activity via coordination to the active Rh species. This
phases were analyzed by ICP-MS to determine the metal leach- effect could be modified by changing the reaction temperature.
ing, which was found in the range of 68–278 ppb (ESI Benzaldehyde was also subjected to the same reaction under
3
1
Table S3†). P-NMR did not indicate the presence of L3 in the identical conditions, and no reaction was observed. To con-
product phase. It can be assumed that the Rh-leaching is inde- clude, our hydrogenation conditions were selective for CvC
pendent from the ligand. The results indicate that L3 modified double bonds in the presence of carbonyl, cyano, and phenyl
Rh catalyst in combination with [TBA][4HV] was stable enough groups.
for successful catalyst recycling for ten consecutive cycles.
Further investigations were focused on the chemoselectivity
of [Rh(cod) ][BF ]/L3 catalysed hydrogenation of different sub-
strates containing reducible functional groups (Table 2). The
2
4
Experimental
reduction of 2 in both [TBA][4HV] and [THA][4HV] gave cyclo- Hex-1-ene, oct-1-ene, dec-1-ene, cyclohexene, benzaldehyde,
hexanone with a yield of 99.9% after 6 h (Table 2, entry 1). cinnamaldehyde, methyl 3-pentenoate, 2,3-dimethylbuta-1,3-
Although [THA][4HV] was miscible with the product, cyclo- diene, cyclohex-2-en-1-one, γ-valerolactone, (E)-stilbene, and
hexanone could be completely extracted with pentane (3 × tetrabutyl-ammonium hydroxide were purchased from Sigma
4
mL) from the catalyst phase. After collecting the product, the Aldrich Kft, Hungary and used as received without further
catalyst phase was reused to evaluate the influence of the purification. Toluene and pentane were obtained from Molar
extraction on the activity. In contrast to results obtained for Ltd. GC analyses were performed on an HP 5890N instrument
catalyst recycling involving an extraction step in Rh-catalyzed with an HP-5 capillary column (15 m × 0.25 μm) using H as a
2
1
9
transfer hydrogenation reactions, full conversion of cyclohex- carrier gas. For the analysis 5 μL of the reaction mixture was
-en-1-one to cyclohexanone was achieved; in addition, no loss dissolved in 1 mL of methylene chloride followed by the
of activity was observed after ten consecutive cycles. The addition of 10 μL toluene as an internal standard. The NMR
2
21
product phases were analysed by ICP-MS in order to determine spectra were recorded on a Brucker Avance 250 spectrometer.
the metal leaching. The results indicated a slightly higher Hydrogenation reactions were performed in a magnetically
Rh-leaching in the range of 0.7–2 ppm (ESI Table S4;† stirred 25 mL high-pressure Hastelloy-C reactor (Parr Inst., IL,
USA) equipped with a manometer, a safety relief and a magnetic
stirring bar using external heating. The viscosity measurements
Table 2 Hydrogenation of different functionalized olefins in GVL-based ionic
were performed on an AntonPaar MCR501 instrument by apply-
a
liquids
−1
ing a 2 °C min temperature gradient. The water content of ILs
a
was determined by Karl–Fischer titration.
In a typical hydrogenation experiment, the reactor was
2 4
charged with [Rh(cod) ][BF ] (1.7 mg, 0.004 mmol) as a catalyst
precursor and 10 eq. of the phosphine ligand (0.04 mmol), dis-
solved in 1 mL (ca. 3 mmol) of ionic liquid, resulting in a light
orange solution. The reaction mixture was pressurized with 30
bar hydrogen gas and heated up to 60 °C. After 1 h stirring
#
1
Substrate
Product
Solvent
Conv. (%)
[TBA][4HV]
>99.9
>99.9
>99.9
>99.9
[
THA][4HV]
[TBA][4HV]
THA][4HV]
[TBA][4HV]
THA][4HV]
[TBA][4HV]
THA][4HV]
2
[
3
4
80.0
94.1
45.3
50.0
[
(
350 rpm) the reaction mixture was cooled down to room temp-
[
erature and the pressure was released. Subsequently, the
corresponding substrate (1 mL, ca. 0.01 mol) was injected into
the reactor. The hydrogenation was started by heating up the
mixture to 60 °C and setting hydrogen pressure to 30 bar and
stirring at 950 rpm. After completion, the pressure was
decreased and the reaction mixture was cooled down to room
5
[TBA][4HV]
THA][4HV]
0
0
[
a
Yields and conversions were equal. Reaction conditions: 10 mmol
][BF ], 0.04 mmol L3, solvent: 1 mL,
substrate, 0.004 mmol [Rh(cod)
time: 6 h, pH2 = 30 bar, T = 60 °C.
2
4
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temperature. If a biphasic system was formed, the phases were Fig. S9–S10†). [TMA][4MeOV] was insoluble in the Karl–Fischer
separated for further analysis. In the case of a single phase solution.
system, the product was extracted by the addition of 3 × 2 mL
pentane. The extracted phases were collected and pentane was 14.65 g (0.1 mol) methyl 4-methoxyvalerate, 25.95 g (0.1 mol)
removed by vacuum. tetramethylammonium hydroxide, 38.93 g (2.16 mol) water.
The recyclability of the catalyst phase was tested by using The product was isolated as a yellow, highly viscous liquid.
hex-1-ene and cyclohex-2-en-1-one. After completing the reac- Yield 37.02 g (99.0%). H O (wt%): 5.2. Refractive index (25 °C):
tion, the product phase was separated from the catalyst phase 1.4652. mp: <−40 °C. λ25°C = 99 μS cm−1
Tetrabutylammonium 4-methoxyvalerate [TBA][4MeOV]:
2
1
.
H-NMR (D O) δ,
2
either by simple decantation or extraction with pentane. The ppm: 0.83 (t, 12H, J = 7.3 Hz); 1.03 (d, 3H, J = 6.2 Hz); 1.24 (sx,
catalyst phase was then mixed again with the same amount of 8H, J = 7.3 Hz); 1.53(m, 8H); 1.64 (m, 2H,); 2.07 (t, 2H, J = 7.8
substrate, pressurized to 30 bar H at 60 °C (no pre-hydrogen- Hz); 3.07 (m, 8H); 3.21 (s, 3H); 3.34 (sx, 1H, J = 6.2 Hz);
2
1
3
ation was applied). The start of the reduction was indicated by
a pressure drop. After reaching a constant value of pressure, 55.56; 58.48 (t, J = 2.7 Hz); 77.42; 183.22 (ESI Fig. S11–S13†).
the same procedure was repeated in the consecutive runs. Tetramethylammonium 4-ethoxyvalerate [TMA][4EtOV]:
The preparation of tetramethylammonium 4-hydroxyvalerate 15.00 g (86 mmol) of ethyl 4-ethoxyvalerate, 7.84 g (86 mmol)
2
C-NMR (D O) δ, ppm: 13.23; 18.48; 19.52; 23.50; 32.20; 33.77;
[TMA][4HV], tetrabutylammonium 4-hydroxyvalerate [TBA][4HV], tetramethylammonium hydroxide, 23.56 g (1.3 mol) water. The
methyl 4-methoxyvalerate, and ethyl 4-ethoxyvalerate was per- product was isolated as a colourless, highly viscous liquid.
1
2,22
formed according to published methods.
2
The purity of the Yield: 19.65 g, 92%. H O (wt%): 7.2. Refractive index (25 °C):
1
13
1
freshly prepared ILs was determined by H- and C-NMR spec- 1.4558. mp: 9–12 °C. H-NMR (D
2
O) δ, ppm: 1.01 (m, 6H, J =
troscopy. The preparation of alkyl-bis(m-sulfonatophenyl)phos- 6.5 Hz); 1.56 (m, 2H); 2.05 (t, 2H, J = 7.8 Hz); 3.03 (s, 12H) 3.45
24
13
phines was performed according to published methods.
The melting point of [TMA][4HV] was 75–78 °C. The water 55.13 (t, J = 4.1 Hz); 63.75; 75.20; 182.89 (ESI Fig. S14–S15†).
content of [TBA][4HV] was 4.1% and its conductivity (λ25°C) Tetrabutylammonium 4-ethoxyvalerate [TBA][4EtOV]:
17.49 g (0.1 mol) of ethyl 4-ethoxyvalerate, 25.95 g (0.1 mol)
2
(m, 3H); C-NMR (D O) δ, ppm: 14.44; 18.69; 32.09; 33.36;
was 200 μS cm− (literature value: 199 μS cm ).
1
−1
Preparation of tetrahexylammonium 4-hydroxyvalerate tetramethylammonium hydroxide, 38.93 g (2.16 mol) water.
[THA][4HV]: in a round-bottomed flask, 2.6 g (26 mmol) of The product was isolated as a yellow, highly viscous liquid.
GVL was added to 9.62 g (26 mmol) tetrahexylammonium Yield: 37.61 g 97%. H
hydroxide and 14.38 g water then stirred for 1 hour. After the 1.4632. mp: <−40 °C. λ25°C = 77 μS cm−
reaction was completed, the water was removed by vacuum at ppm: 0.88 (t, 12H, J = 7.3 Hz); 1.06–1.09 (t, 6H, J = 6.6 Hz); 1.29
0 °C. The product was isolated as a yellow, viscous liquid. The (sx, 1H, J = 7.3 Hz); 1.58 (m, 8H); 1.70 (m, 2H,); 2.13 (t, 2H, J =
2
O (wt%): 5.3. Refractive index (25 °C):
1
1
.
2
H-NMR (D O) δ,
8
1
3
product was identified by NMR spectroscopy. Yield: 10.89 g, 7.8 Hz); 3.12 (m, 8H); 3.48 (m, 3H); C-NMR (D
2
O) δ, ppm:
9
1
5%. mp.: <−40 °C. H O (wt%): 2.5, refractive index (25 °C): 13.3; 14.98; 19.23; 19.57; 23.56; 32.64; 33.97; 58.57; 64.18;
.4625. λ25°C = 13 μS cm . H-NMR (D O) δ, ppm: 0.80 (t, 12H, 75.70; 183.32 (ESI Fig. S16–S18†).
2
2
−
1 1
J = 6.5 Hz); 1.07 (d, 3H, J = 6.3 Hz); 1.25 (s, 24H); 1.57 (m,
The vapour pressure of the ionic liquids was measured with
1
0H); 2.12 (dt, 1H, J = 2.9 Hz, J = 7.5 Hz); 3.09 (m, 8H); 3.69 a Rosemount manometer attached directly to the reactor.
1
3
(dd, 1H, J = 6.2 Hz, J = 12.5 Hz); C-NMR (CDCl
3
) δ, ppm: Initially, the reactor equipped with a magnetic stirrer was
1
3.71; 21.80; 22.24; 23.77; 25.83; 31.06; 34.76; 35.88; 58.64; charged with 2 mL of the corresponding ionic liquid; then the
0.20; 68.18; 179.84 (ESI Fig. S7–S8†). manometer was connected to the reactor, de-aerated at room
General procedure for the preparation of tetraalkylammo- temperature and placed in an ice–water mixture. The exact
nium 4-alkoxyvalerates: the alkyl 4-alkoxyvalerates were pre- value of the vapour pressure was recorded when it was stabil-
6
2
3
pared by a published method. In a round-bottomed flask, ized at the adjusted temperature.
equimolar amounts of alkyl 4-alkoxyvalerate and the corres-
ponding tetraalkylammonium hydroxide were mixed followed
by the addition of water at room temperature. The reaction
mixture was stirred for 1 h at 50 °C. The water and alcohol
Conclusions
formed were removed by vacuum at 80 °C. The products were Gamma-valerolactone based ionic liquids were successfully
identified by NMR spectroscopy.
used as a solvent for the hydrogenation of olefins by using sul-
Tetramethylammonium 4-methoxyvalerate [TMA][4MeOV]: fonated phosphine-modified Rh-catalyst. The combination of
3
.00 g (21 mmol) methyl 4-methoxyvalerate, 1.87 g (21 mmol) 4-hydroxyvalerate and its O-alkyl derivatives (4-methoxy-vale-
tetramethylammonium hydroxide, 5.6 g (0.31 mol) water. The rate, 4-ethoxyvalerate) anions with tetraalkylammonium
product was isolated as a colourless, highly viscous liquid. cations (methyl, butyl, hexyl) resulted in a novel recyclable
1
Yield of 3.87 g (92%). mp: 30–35 °C. H-NMR (D
2
O) δ, ppm: catalyst phase. The highest turnover frequency was obtained in
1
.01 (d, 3H, J = 6.21 Hz); 1.58 (ddd, 2H, J = 7.3 Hz, J = 13.6 Hz, the case of the reduction of oct-1-ene in tetrabutylammonium
J = 21.0 Hz); 2.06 (t, 3H, J = 7.8 Hz); 3.04 (s, 12H); 3.19 (s); 3.32 4-methoxyvalerate, its value was 6933 h− in the presence of a
1
1
3
(dd, 1H, J = 6.2 Hz, J = 12.3 Hz); C-NMR (acetone-d
6
) δ, ppm: catalyst in situ formed from [Rh(cod)
2 4
][BF ] and butyl-bis-
2
0.00; 35.0; 36.29; 55.91 (t, J = 3.7 Hz); 56.39; 78.07; 178.27 (ESI (m-sulfonatophenyl)phosphine ligand under biphasic conditions.
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The temperature dependence of the vapour pressure of GVL-
based ionic liquids was also determined. It was shown that the
structure of the cations had a significant effect on the vapour
pressure, while the GVL-based anions had a negligible influ-
ence on it.
drecht, 1994; (b) L. Oro, Hydrogenation – Homogeneous, in
Encyclopedia of Catalysis, ed. I. T. Horváth, John Wiley &
Sons, Hoboken, New Jersey, 2003.
8 (a) Y. Chauvin, L. Mussmann and H. Olivier, Angew. Chem.,
Int. Ed. Engl., 1995, 34, 2698; (b) P. A. Z. Suarez,
J. E. L. Dullius, S. Einloft, R. F. de Souza and J. Dupont,
Polyhedron, 1996, 15, 1217.
9
P. J. Dyson, G. Laurenczy, C. A. Ohlin, J. Vallance and
T. Welton, Chem. Commun., 2003, 2418.
Acknowledgements
This work was supported by the Hungarian Scientific Research 10 CRC Handbook of Chemistry and Physics, ed. R. C. Weast,
Fund (grant no. OTKA-CNK 78065). The European Union and
CRC Press, Boca Raton, Florida, 63rd edn, 1982.
the European Social Fund have provided financial support to 11 I. T. Horváth, H. Mehdi, V. Fábos, L. Boda and L. T. Mika,
the project under grant agreement no. TÁMOP 4.2.1./B-09/
Green Chem., 2008, 10, 238.
KMR-2010-0003. The authors would like to thank Prof. János 12 D. Fegyverneki, L. Orha, G. Láng and I. T. Horváth, Tetra-
Rohonczy (Eötvös Loránd University) for the NMR
measurements.
hedron, 2010, 66, 1078.
13 S. A. King, J. Org. Chem., 1994, 59, 2253.
14 Definition of VOC see 40 C.F.R. (Code of Federal Regu-
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zero filling, LB = 0.3.
3
4
(
22 The ionic liquids can also be prepared by applying 5%
excess of γ-valerolactone. The residue of GVL can easily be
removed by vacuum evaporation at 80 °C for 1 h. Due to
the excess of GVL no water soluble tetraalkylammonium
hydroxide was detected in the product (ESI Fig. S19†). If
necessary, the ILs can be further purified by washing with
diethyl ether, pentane, hexane, or cyclohexane.
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Green Chem.
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