Gold imidazolium-based ionic liquids, efficient catalysts for cycloisomerization of γ-acetylenic carboxylic acids

Article abstract of DOI:10.1039/b812580e

Ionic liquid stabilized gold(iii) chloride is shown to be a very active catalyst in the cyclization of sterically hindered and unhindered acetylenic carboxylic acid substrates even in the absence of a base. The Royal Society of Chemistry and the Centre Na

Full text of DOI:10.1039/b812580e

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Gold imidazolium-based ionic liquids, efficient catalysts for
cycloisomerization of c-acetylenic carboxylic acids
a
a
b
b
´
Florentina Neat-u, Vasile I. Parvulescu, Veronique Michelet, Jean-Pierre Genet,
Alexandre Goguetc and Christopher Hardacrecd
Received (in Durham, UK) 22nd July 2008, Accepted 8th September 2008
First published as an Advance Article on the web 20th October 2008
DOI: 10.1039/b812580e
Ionic liquid stabilized gold(III) chloride is shown to be a very active catalyst in the cyclization
of sterically hindered and unhindered acetylenic carboxylic acid substrates even in the absence
of a base.
enhance the stability are extremely important for practical
applications.20 These can be in the form of the solvent used or
the nature of the support.21 For example, stabilization of gold
in the form of nanoparticles in ionic liquids by imidazolium
derivatives has been reported.22–25
Introduction
Ionic liquids (ILs) have received significant attention recently
because they exhibit several advantages over molecular
solvents with respect to their environmental impact.1 Catalytic
reactions in ILs have been examined for at least 20 years and
have been used for a wide variety of carbon–carbon bond
forming reactions, which were the first to be undertaken in this
media, and a number of good reviews cover the area of ILs.2–6
The extensive interest stems from the fact that the properties
of ionic liquids may be tuned in order to suit a particular
application by varying the cation–anion combination system-
atically and thereby are useful engineering solvents. In addi-
tion, for chemical reactions, the ionic liquid provides an ionic
environment which can significantly alter the reactivity and
selectivity of processes compared with molecular solvents.7
The transition metal-catalyzed cyclization of 4-alkynoic
acids constitutes a major route8 for the construction of
5-membered exocyclic lactones and has been the subject of a
large number of investigations.9–14 Recently, a very attractive
route to perform this reaction under mild conditions in the
presence of gold was reported.15,16 In our laboratory, the
catalytic properties of AuCl and AuCl3 for gem-substituted
substrates, in the absence of base, was described15 as well as
the use of two heterogeneous systems Au2O317 and Au/beta.18
These systems were found to be active for both substituted and
unsubstituted substrates. In addition, the optimized hetero-
geneously catalyzed system was found to be recyclable.22
Whilst gold has been shown to be highly active, it commonly
undergoes significant deactivation due to the ease by which it
may change its oxidation state and, in the case of nanoparti-
cles, the catalyst particle size leading to instability in the form
of the active catalyst.19 Therefore, modalities which can
In this report the behavior of a 4 wt% Au/beta catalyst and
a series of 1,3-dialkylimidazolium tetrachloroaurate salts in
ionic liquids as catalysts for the cyclization of acetylenic
substrates has been studied. Gold has recently been reported
as a catalyst in ionic liquids in the hydration of phenyl-
acetylene26,27 and the syntheses of substituted 3(2H)-furanones,27
2,5-dihydrofurans28 and substituted indoles.29 However, with
the exception of the Co2(CO)8-catalyzed intramolecular and
intermolecular Pauson–Khand annelation using 1-butyl-3-
methylimidazolium hexafluorophosphate ([C4mim][PF6]) and
tetrafluoroborate ([C4mim][BF4]) ionic liquids as solvents, no
other related reactions to the cyclization of acetylenic sub-
strates have been reported in ILs, to date.30
Experimental
Catalysts preparation
1. Au-beta catalyst. The catalyst was prepared by stirring
1 g of beta zeolite (PQ Corporation) for 3 h with 100 cm3 of 1 M
NH4NO3 at 353 K.22 The slurry was filtered off and carefully
washed with deionized water, dried for 6 h at 333 K and
calcined for 24 h at 773 K. Deposition of gold was performed
using the deposition–precipitation method. The support
(1 g) was added to 100 cm3 of an aqueous solution of HAuCl4
(2.1 ꢀ 10ꢁ3 M) at 343 K which had previously adjusted to a
pH = 8.5 with 0.2 M NaOH. The temperature of the slurry
was maintained at 343 K under vigorous stirring for 3 h.
Thereafter, the sample was filtered off, washed with deionized
water to remove the free chloride and then dried under
vacuum at 333 K for 24 h. The resultant catalyst contained
4 wt% Au as determined by ICP-AES analysis. This catalyst
has been used as a reference material in these experiments.
a Department of Chemical Technology and Catalysis, University of
Bucharest, B-dul Regina Elisabeta 4-12, 030016 Bucharest, Romania.
E-mail: v_parvulescu@chem.unibuc.ro; Fax: +40 21 4010241
b
`
´
Laboratoire de Synthese Selective Organique et Produits Naturels,
ENSCP, UMR 7573, 11 rue P. et M. Curie, F-725231 Paris Cedex 05,
France. E-mail: veronique-michelet@enscp.fr; Fax: +33 1 44071062
c CenTACat School of Chemistry and Chemical Engineering, Queen’s
University, Stranmillis Road, Belfast, Northern Ireland,
UK BT9 5AG
2. Ionic liquids synthesis. Gold ionic liquids were
synthesized using the method reported previously by Hasan
et al.25 Four 1-alkyl-3-methylimidazolium tetrachloroaurate
([Cnmim][AuCl4], n = 2, 4, 6, 18) ionic liquids were prepared
by adding a 10% molar excess of [Cnmim]Cl to tetrachloroauric
d QUILL School of Chemistry and Chemical Engineering, Queen’s
University, Stranmillis Road, Belfast, Northern Ireland, UK BT9 5AG.
E-mail: c.hardacre@qub.ac.uk; Fax: +44 28 90974687
ꢂc
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102 | New J. Chem., 2009, 33, 102–106

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3-phenyl-5-methylene-g-butyrolactone (2a), 3-n-butyl-3-ethoxy-
carbonyl-5-methylene-g-butyrolactone (2b), 3-methoxycarbonyl-
5-methylene-3-(30-phenylprop-20-enyl)-g-butyrolactone (2c),
3-allyl-3-methoxycarbonyl-5-methylene-g-butyrolactone (2d), 3-
benzyl-3-ethoxycarbonyl-5-methylene-g-butyrolactone (2e) and
3-methoxycarbonyl-5-methylene-g-butyrolactone (2f) were in
good agreement were those reported previously.19–21 In all
cases, no cyclization is observed in the absence of any of the
gold catalysts irrespective of the solvent used.
Fig. 1 Schematic of the cation of the basic ionic liquid (BIL) used as
solvent for the cyclization of the functionalized acetylenic substrates
1a and 1b.
acid (HAuCl4ꢃ4H2O, Alfa Aesar) resulting in the rapid
formation of a yellow solid. Following heating to above
its melting points with stirring for 0.5 h, the product was
purified by recrystallization from benzene–acetonitrile in the
volume ratio of 4 : 1. [Cnmim]Cl were prepared in house using
standard literature methods.31 Trihexyltetradecylphospho-
nium hydrogen sulfate ([P66614][HSO4]) was supplied by Cytec.
1-Butyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}-
imide ([C4mim][NTf2]) and trihexyltetradecylphosphonium
Results and discussion
Catalysts characterization
The beta zeolite used in these experiments had a surface area
of 464 m2 gꢁ1 and a pore volume of 0.96 cm3 gꢁ1. After the
deposition of gold (4 wt%) the surface area decreased to
383 m2 gꢁ1 and the pore volume to 0.80 cm3
g
ꢁ1. TEM
bis{(trifluoromethyl)sulfonyl}imide
([P66614][NTf2])
were
formed by metathesis from [C4mim]Cl and [P66614]Cl (Cytec),
respectively, according to literature methods.32 The base func-
tionalized ionic liquid 5-diisopropylamino-3-oxapentyl)-
dimethylethylammonium bis{(trifluoromethyl)sulfonyl}imide
(BIL) shown in Fig. 1 was synthesized as previously re-
ported.33 In all cases prior to reaction the ionic liquids were
dried under vacuum at 50 1C overnight. All ionic liquids
contained o0.16 wt% water determined by Karl–Fischer
analysis and o5 ppm halide by suppressed ion chromato-
graphy. All other reagents were used as received.
analysis of this material shown an uniform size distribution
with an average of 3 nm. The characterization of IL-stabilized
gold(III) chloride was examined using TEM, XPS and
EXAFS. Fig. 2 shows the XPS spectra of the Au 4f photo-
electron emission corresponding to the mixture of the catalyst
([C6mim][AuCl4]) with the ionic liquid [C6mim]Cl after the
reaction. The binding energies of Au 4f7/2 and Au 4f5/2 levels
were located at 90.0 eV and 86.3 eV, respectively. EXAFS of
[C6mim][AuCl4] dissolved in [C6mim]Cl showed a single peak
at 0.22 nm associated with 4 chlorine atoms in the first
coordination shell. During reaction, this peak decreases
slightly and a small decrease in the white line of the XANES
is observed. This variation may indicate that chlorine is being
replaced by a lighter element such as coordination by the
substrate during reaction, as would be expected. TEM analysis
is in agreement with the XPS and EXAFS measurements and
showed no nanoparticle formation. These observations are
Catalyst characterization
The solid catalysts were characterized by nitrogen adsorption–
desorption isotherms at 77 K (Micromeritics ASAP 2000) after
out-gassing the samples at 393 K for 12 h.
The XPS spectra were recorded using a Kratos Axis
UltraDLD spectrometer with monochromatic Al-Ka radia-
tion. The data were analyzed using Casa-XPS (v2.3.13)
employing a Shirley-background subtraction prior to fitting.
EXAFS data were collected at the Synchrotron Radiation
Source in Daresbury, UK, using station 9.3. The spectra were
recorded at the Au LIII edge using a double crystal Si(111)
monochromator. Scans were collected and averaged. Data
were processed using EXCALIB which was also used to
convert raw data into energy vs. absorption data. EXBROOK
was used to remove the background. The analysis of the
EXAFS was performed using EXCURV98.34 The gold con-
centration was determined by ICP-AES.
Catalytic tests
Typically 0.26 mmol of acetylenic acid was stirred with 2.5 mol%
[Cnmim][AuCl4] dissolved in 0.5 g [Cnmim]Cl in air at
room temperature, until completion of the reaction. After
the completion of the reaction, the reaction products
were extracted three times with diethyl ether and the
solvent completely removed under vacuum to give the
corresponding lactone. 1H and 13C NMR were recorded
on a Bruker AV 300 instrument operating at 300 Hz to
identify the products. The measured NMR spectra for
Fig. 2 The XPS spectra in the Au 4f region of the [C6mim][AuCl4]
after reaction.
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New J. Chem., 2009, 33, 102–106 | 103

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consistent with the presence of a dissolved Au(III) species with
B4 chlorines in the first coordination sphere.35
Table 2 Au-catalyzed cyclization of functionalized carboxylic acids
Catalytic tests
Table 1 shows the activity of Au/beta in a range of ionic
liquids for the cyclization of the functionalized acetylenic
substrates 1a and 1b.
Although cycloisomerization of substrate 1a did occur in the
BIL and [P66614][HSO4] with conversions of 75 and 70%,
respectively, (Table 1, entries 1, 2), the ILs could not be
separated from the reaction products due to the high solubility
of the ionic liquid–substrate mixture in diethyl ether. In the
case of the reaction performed in [C4mim][NTf2] and
[P66614][NTf2], the ionic liquid could only be partially sepa-
rated from the reactants and products and showed conversion
of the substrate of 80% and 70%, respectively (Table 1, entries
3, 4). In each case, the conversions were obtained from NMR
determination in the ionic liquid. [C6mim]Cl was found to
separate efficiently from diethyl ether and the cycloisomeriza-
tion of 1a (Table 1, entry 5) resulted in the desired compound
2a in 79% isolated yield.
Entry R1
R2
Product Temp./1C Time/h Yielda (%)
1
2
3
4
5
6
7
CO2Et n-Bu 2b
CO2Me Cinnamyl 2c
CO2Me Allyl
CO2Et Bn
CO2Et Bn
RT
40
1
2
2
1
2
1
1
96
90
91
85
95
84
96
2d
2e
2e
2f
RT
RT
RT
RT
RT
CO2Me
Ph
H
H
2a
a
Isolated yield.
were cleanly transformed to the corresponding g-alkylidene
g-butyrolactones. Moreover, similar activity and selectivity
was found for the ionic liquid catalyst compared with
the homogeneous AuCl catalysts system in acetonitrile.15
Irrespective of the alkenyl side chain length the lactones were
isolated in 85–96% yields (Table 2, entries 1–5) even at room
temperature. In all cases, the catalytic amount of gold ionic
liquid used in these reactions was equivalent to the amount
used under heterogeneous conditions. No side reactions were
observed on the alkenyl side chains during the course of the
reaction.
Due to the ease of workup [C6mim]Cl was also examined as
a medium for the cyclization of 1b over Au/beta. From an
1
analysis of the H NMR, a conversion of 91% was obtained
with an isolated yield of the lactone of 58% (Table 1,
entry 6). Similar activities and selectivities were also found in
conventional molecular solvents, such as acetonitrile (Table 1,
entry 7).
The heterogeneously catalyzed reaction results were com-
pared with the use of the ionic liquid as a catalyst in the form
of the tetrachloroaurate based ionic liquids. Since [C6mim]-
[AuCl4] is a solid at room temperature, [C6mim]Cl was used as
solvent. The results are summarised in Table 2.
Using the gold based ionic liquid system, complete trans-
formations of 2-prop-2-ynylmalonic acid monomethyl ester 1f
with an 84% isolated yield (Table 2, entry 6) and of 2-phenyl-
pent-4-ynoic acid 1a with an 96% isolated yield (Table 2, entry 7)
after 1 h at room temperature were obtained. Similar results
were found for the reaction of 1a in the presence of [C2mim]-
[AuCl4], [C4mim][AuCl4] and [C18mim][AuCl4] at room
temperature. These results for unsubstituted substrates are
comparable with those obtained under heterogeneous Au2O3
conditions (3 h),21 or homogeneous AuCl/K2CO3 conditions
(2 h)20 and show significant advantages over other homo-
geneous gold chloride based catalysts.19 In the case of the
homogeneous catalysts, the formation of degradation pro-
ducts or the corresponding methylketone was observed for
the sterically unhindered substrates and the reaction only
occurred in the presence of a base. In the ionic liquid catalyzed
reactions, excellent reactivity was found without the need for
additives. Furthermore, the IL-catalysts were recycled three
times without any loss in conversion or yields.
To date, it has only been possible to exclude a base from the
homogeneous reaction conditions if the two substituents on a
tetrahedral centre were of a significant size, as understood by
the Thorpe–Ingold effect.36 In the case of the ionic liquid
catalyst system, all the g-acetylenic carboxylic acids examined
Table 1 Cyclization of functionalized acetylenic substrate over
Au/beta in a range of ionic liquids
Entry R1
R2
Solvent
BIL
[P66614][HSO4] 2a
[P66614][NTf2] 2a
[C4mim][NTf2] 2a
Product Yielda (Conv.) (%)
A comparison of the homogeneous reaction (Table 2, entries
7 and 1) with that of the heterogeneous reaction (Table 1,
entries 5 and 6) indicates that the former is more active
requiring a lower temperature and resulting in a higher con-
version/yield after 1 h. This is reflected in the turnover
frequencies (TOF) for the homogeneous catalysts compared
with the supported catalysts which are B19.8 hꢁ1 (at RT) and
B1.3 hꢁ1 (at 40 1C), respectively, for the formation of 2b, for
example. The lower rate is unlikely to be due to mass transfer
1
2
3
4
5
6
7
Ph
Ph
Ph
Ph
Ph
H
H
H
H
H
2a
(75)b
(70)b
(70)b
(80)b
[C6mim]Cl
2a
2b
2b
79 (83)
58 (91)
60 (90)
CO2Et n-Bu [C6mim]Cl
CO2Et n-Bu CH3CN
a
b
Isolated yield. IL inseparable from the system.
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104 | New J. Chem., 2009, 33, 102–106

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a Portfolio partnership from the EPSRC and an EU transna-
tional grant. CCLRC are thanked for providing EXAFS
beamtime.
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Scheme 1 The mechanism of cyclization of acetylenic carboxylic acid
using Au3+ derived catalysts. It should be noted that representation of
the gold as [Au] is for illustrative purposes only and will exist in the
form of a [AuClx]y+ complex.
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106 | New J. Chem., 2009, 33, 102–106