Guanidinium-based phosphotungstates and ionic liquids as catalysts and solvents for the epoxidation of olefins with hydrogen peroxide

Article abstract of DOI:10.1002/ejic.201100037

Several penta- and hexaalkylated guanidinium-based ionic liquids (GILs) were tested as solvents for the epoxidation of cyclooctene using the Venturello catalyst, [(C₈H₁₇)₃N(CH₃)] ₃[PO₄{WO(O₂)₂}₄], and hydrogen peroxide as the oxidant. Epoxide yields were obtained in a broad range between 13 and 79 % depending on both the anion and the substituents on the guanidinium moiety. Recycling experiments showed that the catalyst can be used at least three times. Furthermore, new guanidinium phosphotungstates with the PW₁₂O₄₀^3- anion were synthesized and characterized. Their catalytic performance was evaluated, and GILs as well as acetonitrile were employed as the solvent. Results were compared to those obtained with the comparable ammonium-based catalysts [NR₄] ₃[PW₁₂O₄₀] (R = C₄H₉, C₆H₁₃) and the analogous imidazolium-based catalyst [BMIM]₃[PW₁₂O₄₀] containing the 1-butyl-3-methylimidazolium cation. Employing different GILs as solvents, similar results were obtained for the guanidinium and imidazolium catalysts but significantly lower epoxide yields were obtained using the ammonium catalysts. On using acetonitrile as the solvent, guanidinium-based catalysts exhibited a better performance than the imidazolium catalyst in the case of linear and branched olefins and vice versa in the case of cyclic olefins. Several guanidinium phosphotungstates and guanidinium-based ionic liquids have been synthesized and characterized. Their catalytic performance in epoxidation reactions with hydrogen peroxide was evaluated and compared to similar ammonium and imidazolium compounds. The reaction systems can be recycled and used in at least three consecutive reactions. Copyright

Full text of DOI:10.1002/ejic.201100037

FULL PAPER
DOI: 10.1002/ejic.201100037
Guanidinium-Based Phosphotungstates and Ionic Liquids as Catalysts and
Solvents for the Epoxidation of Olefins with Hydrogen Peroxide
Loubna Gharnati,[a] Olaf Walter,[a] Ulrich Arnold,[a] and Manfred Döring*[a]
Keywords: Oxidation / Epoxidation / Ionic liquids / Polyoxometalates / Guanidinium salts / Hydrogen peroxide
Several penta- and hexaalkylated guanidinium-based ionic
liquids (GILs) were tested as solvents for the epoxidation of
cyclooctene using the Venturello catalyst, [(C8H17)3N-
(CH3)]3[PO4{WO(O2)2}4], and hydrogen peroxide as the oxi-
dant. Epoxide yields were obtained in a broad range be-
tween 13 and 79% depending on both the anion and the
substituents on the guanidinium moiety. Recycling experi-
ments showed that the catalyst can be used at least three
times. Furthermore, new guanidinium phosphotungstates
were compared to those obtained with the comparable am-
monium-based catalysts [NR4]3[PW12O40] (R = C4H9, C6H13
and the analogous imidazolium-based catalyst
[BMIM]3[PW12O40 containing the 1-butyl-3-methylimid-
azolium cation. Employing different GILs as solvents, similar
results were obtained for the guanidinium and imidazolium
catalysts but significantly lower epoxide yields were ob-
tained using the ammonium catalysts. On using acetonitrile
as the solvent, guanidinium-based catalysts exhibited a bet-
ter performance than the imidazolium catalyst in the case of
linear and branched olefins and vice versa in the case of cy-
clic olefins.
)
]
3–
with the PW12O40 anion were synthesized and charac-
terized. Their catalytic performance was evaluated, and GILs
as well as acetonitrile were employed as the solvent. Results
e.g. the epoxidation of various olefins in 1-ethyl-3-methyl-
Introduction
imidazolium tetrafluoroborate, [EMIM][BF4], catalyzed by
Catalytic oxidation with hydrogen peroxide is a highly
attractive process and rapidly developing topic.[1] With re-
gard to applications, enormous progress was made over the
past few decades, and several benefits such as convenient
handling, the favourable price of the oxidant and the forma-
tion of water as the only byproduct led to the successive
substitution of some traditional oxidants, e.g. organic per-
oxides or inorganic salts, by hydrogen peroxide in several
processes.
[9–11]
CH3ReO3
and the epoxidation of cyclooctene in
[BMIM][PF6] catalyzed by different MoVI compounds.[12]
Several epoxidation reactions without a transition metal
catalyst have also been accomplished such as the epoxid-
ation of α,β-unsaturated carbonyl compounds in
[BMIM][PF6]/H2O biphasic system containing NaOH,[13]
the epoxidation of chromones and flavonoids in
a
a
[BMIM][BF4]/H2O/NaOH system,[14] the in situ epoxid-
ation of cyclohexenones with electrogenerated H2O2 in
[BMIM][BF4]/H2O/NaOH,[15] the epoxidation of cyclohex-
enones in [BMIM][PF6] or [BMIM][BF4] with alkaline
H2O2,[16] styrene epoxidation using carboxyl-functionalized
ionic liquids[17] and styrene epoxidation in the presence of
KOH and CO2 employing different ionic liquids as well as
conventional organic solvents.[18]
Recent investigations showed that oxidation reactions
with hydrogen peroxide can be carried out favourably using
ionic liquids as solvents. Such reaction systems afforded
mild reaction conditions, efficient product separation by
distillation or extraction, facile catalyst separation and mul-
tiple use.[2–5] Several examples of transition metal-catalyzed
epoxidation reactions have been reported, e.g. the epoxid-
ation of styrenes and cyclic olefins employing 1-butyl-3-
methylimidazolium tetrafluoroborate, [BMIM][BF4], and
Polyoxotungstates are a well known and efficient class of
catalysts for the epoxidation of olefins with hydrogen perox-
ide.[19–22] Their use in combination with ionic liquids as sol-
vents has been investigated recently. Examples are epoxid-
ation reactions using the ammonium silicotungstate
(Bu4N)3K[γ-SiW10O36(PhPO)2] as the catalyst and micro-
wave heating of the reaction mixtures[23] and epoxidation
reactions catalyzed by the imidazolium (phospho)tungstates
[BMIM]3[PW12O40][24] and [BMIM]4[W10O23].[25] In these
[6]
MnSO4 and an iron(III) porphyrin complex employing a
[BMIM][Br]/CH2Cl2 mixture under biphasic conditions.[7]
Another example is the asymmetric epoxidation of limo-
nene in [BMIM][BF4] catalyzed by Jacobsen’s manganese–
salen complex.[8] Epoxidation reactions with a urea–hydro-
gen peroxide adduct as an oxidant have also been described,
–
cases imidazolium salts of the type [BMIM][X] [X = BF4 ,
[a] IKFT, Karlsruhe Institute of Technology,
Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-
Leopoldshafen, Germany
–
–
–
PF6 , CF3SO3 , N(CF3SO2)2 ] were used as solvents.
In contrast to the widely investigated imidazolium salts,
significantly less work has been dedicated to guanidinium
Fax: +49-721-608-22244
E-mail: manfred.doering@kit.edu
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Guanidinium-Based Phosphotungstates and Ionic Liquids
salts. Similar to imidazolium salts, several guanidinium salts (compounds 2 in Table 1) and hexaalkylated guanidinium
exhibit typical ionic liquid characteristics and can be salts (compounds 3 in Table 1) were investigated. For com-
favourably employed as reaction media[18,26] as well as cata- parison, ammonium phosphotungstates (compounds 4 in
lysts.[27,28] Recently, remarkable progress was made with re- Table 1) and imidazolium phosphotungstate 5 were also
spect to the optimization of synthetic strategies,[29–33] com- employed.
putational[34,35] and structural characterization[29,36] as well
The new guanidinium salts 1a and 1b (Table 1, n = 3)
3–
as a better understanding of their properties.[31,33,37–40] bearing the phosphotungstate anion PW12O40 were pre-
Based on these new insights, the applications of guanidi- pared by ion exchange employing N,N,NЈ,NЈ-tetrahexyl-
nium salts can be vastly extended. Important examples are NЈЈ,NЈЈ-dimethylguanidinium chloride in the case of 1a and
their use as solvents or additives, e.g. for the preparation of the corresponding tetraoctyl compound in the case of 1b
heterocycles,[41] for asymmetric Baylis–Hilman reactions[42] with phosphotungstic acid, H3PW12O40. Spectroscopic data
and for proline-catalyzed asymmetric aldol reactions.[43] are in accordance with the expected structures[48] and X-ray
Separation techniques employing guanidinium-based ionic diffraction analysis of 1a was carried out to elucidate its
liquids (GILs), e.g. the extraction of carbohydrates from solid-state structure (Figure 1). According to the results of
aqueous solutions[44] and the absorption of organic va- XRD analysis the three negative charges of the PW12O40
3–
pours,[45] have also been investigated. Furthermore, their cluster are compensated by three guanidinium counter cat-
potential in liquid crystal applications has been evalu- ions. The cluster itself consists of a central PO4 unit whose
ated.[46,47]
O atoms act as bridges to three W atoms completing their
Within the work presented here we describe the use of distorted octahedral environment to six O atoms. Four of
guanidinium salts in epoxidation reactions with hydrogen
peroxide as the oxidant. Penta- and hexaalkylated GILs
were tested as solvents for the epoxidation of cyclooctene
with a focus on the recyclability of the catalyst systems and
multiple use in consecutive reactions. Furthermore, new
guanidinium phosphotungstates were synthesized and char-
acterized. Their catalytic performance was evaluated and
the results were compared to those obtained with corre-
sponding ammonium and imidazolium salts.
Figure 1. Solid-state structure of 1a (cell content of the indepen-
dent unit of the elementary cell). H atoms and numbering omitted
Results and Discussion
for clarity. W–OP: 242.5(6)–245.2(7) pm, W–OW: 188.7(7)–193.5(6)
pm, W–O(terminal): 168.7(7)–171.2(7) pm; C(central guanidinium
atom)–N: 133(1), 133(1), 135(1), 133(1), 133(1), 135(1), 134(1),
Synthesis and Characterization of Guanidinium Salts
Within this study guanidinium phosphotungstates (com-
pounds 1 in Table 1), pentaalkylated guanidinium salts 132(1), 136(1) pm.
Table 1. Catalysts, solvents and labelling.
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L. Gharnati, O. Walter, U. Arnold, M. Döring
FULL PAPER
these O atoms are involved in the formation of W–O–W GILs, 2a, 2c, 3a, 3b and 3g, small amounts (Յ 1%) of the
bridges and the remaining O atom completes the oxygen diol were detected and when the water insoluble variants,
rich surface of the PW12O403– unit. Bond lengths and angles 2b, 3c, 3d and 3e, were used formation of cyclooctenone (Յ
(Figure 1) are characteristic for this type of oxygen rich 1%) was observed. Time dependence of the reactions was
cluster and in agreement with literature data.[49–56]
investigated and, as an example, the catalytic performance
In the cationic guanidinium counterions of 1a, it can be of 4c in 2a was monitored over 24 h (Figure 2). The cyclo-
noted that the central, planar guanidinium unit exhibits octene conversion increased remarkably within the first five
C–N bond lengths indicating π-electron delocalization in hours and there was still an increase of 3% between 19 and
accordance
with
other
crystallized
guanidinium 24 h. Other reactions showed similar kinetic profiles and
salts.[29,36,57–63] The hexyl substituents at the N-atoms ap- thus reaction times of 24 h were mostly chosen. Usually, the
pear in different conformations in the solid state showing oxidant was completely consumed after 24 h.
their flexible conformational behaviour and build a lipo-
philic environment around the catalytically active metal
oxygen cluster. With these structural findings one could ex-
pect an acceptable catalytic activity in the epoxidation of
cyclooctene.
Guanidinium-Based Ionic Liquids as Reaction Media for the
Epoxidation of Cyclooctene with Hydrogen Peroxide
Numerous guanidinium salts were tested as solvents for
the epoxidation of cyclooctene with hydrogen peroxide cat-
alyzed by the Venturello catalyst, 4c.[64,65] Various water-
soluble and insoluble ionic liquids were employed
(Scheme 1 and Table 2). In all cases control reactions under
the same reaction conditions but without catalyst were per-
formed and no significant reactions were observed.
Figure 2. Cyclooctene epoxidation catalyzed by 4c in 2a as a func-
tion of time.
Obviously, the catalytic performances are largely influ-
enced by the anion used. Considering the pentaalkylated
GILs, 2a, 2b and 2c, which bear the same substituents at
the guanidinium cation, catalytic activities decrease in the
Scheme 1. Epoxidation of cyclooctene catalyzed by the Venturello
catalyst (4c) in different guanidinium salts.
–
following order: CF3SO3– ϾC6F13PF5– ϾC2H5OSO3 . With
respect to the hexaalkylated GILs, 3b, 3c, 3d, 3e and 3g,
bearing the same or at least very similar substituents at the
guanidinium moiety, catalytic activities decrease in the or-
Table 2. Epoxidation of cyclooctene catalyzed by the Venturello
catalyst (4c) in different guanidinium salts.
–
der: BF4– ϾPF6– ϾCF3SO3– ϾN(CF3SO2)2 . Considering
Entry
Solvent
Solubility in H2O
Epoxide yield [%]
the GILs with the trifluoromethanesulfonate anion, 2a, 3a,
3b and 3g, the catalytic performance of the system based
on the pentaalkylated GIL 2a is superior to systems based
on the hexaalkylated GILs, 3a, 3b and 3g. A drop from
76% conversion employing 3a to 21 and 38% employing 3b
and 3g was observed and indicates a significant influence of
the substituents at the guanidinium cation on the catalytic
performance.
1
2
3
4
5
6
7
8
9
2a
2b
2c
3a
3b
3c
3d
3e
3g
+
–
+
+
+
–
–
–
+
79[a]
48
30
76
21
13
64
56
38[b]
Comparatively high conversions were observed using the
water-miscible GIL 3a and the immiscible GILs 3d and 3e.
[a] 5% cyclooctene diol byproduct. [b] Reaction time: 68 h.
Considerable fluctuations were observed for the cyclo- Recycling experiments were carried out using these three
octene conversions, which could be due to the activity of the GILs and the catalyst was tested in three consecutive reac-
catalyst system, the viscosity of the solvent, the miscibility tions (Table 3). After each catalytic run the product was
and phase behaviour of the components as well as the solu- separated from the reaction mixture by repeated extraction
bility of the catalyst in the reaction system. Highest conver- with hexane and the residue was diluted with CH2Cl2. The
sions were obtained using water soluble GILs 2a (Table 2, hexane solution and the CH2Cl2 phase were filtered
entry 1, 83%) and 3a (entry 4, 76%). In all reactions epox- through MnO2/Al2O3 and analyzed by GC. After removal
ide selectivities were 99% and higher except for those using of CH2Cl2 in vacuo, the resulting residue was subjected to
2a. Employing 2a led to the formation of significant the next run without further purification. Employing the
amounts of cyclooctene diol as a byproduct and the epoxide GILs 3d and 3e, similar epoxide yields of around 90% were
selectivity reached only 95%. Employing the water soluble observed and the epoxide yield in the second run remained
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Guanidinium-Based Phosphotungstates and Ionic Liquids
the same as in the first. Employing 3a, the epoxide yield were low and reached only 59% in the second run of the
increased from 88 in the first run to 97% in the second. reactions catalyzed by 4a. Significant amounts of byprod-
Third runs were performed after storage of the catalyst sys- ucts, cyclooctene diol and cyclooctenone, were detected in
tems for two months and significantly lower epoxide yields the initial runs and epoxide selectivity increased in the fol-
were obtained. No significant amounts of byproducts were lowing reactions to 99% and higher. In contrast, cyclooc-
detected in any of these reactions.
tene conversions were higher in reactions catalyzed by 4c
(Table 3 and Table 4) and epoxide selectivities were 99%
and higher. However, cyclooctene conversions decreased
from 87 in the first run to 53% in the third.
Table 3. Recovery and repeated use of the Venturello catalyst (4c)
in different guanidinium salts.
Solvent
Epoxide yield in each run [%][a]
The analogous catalyst with the [BMIM] cation (catalyst
5, Table 1) was tested by Zhao et al. using [BMIM][PF6]
as solvent.[24] Employing a catalyst loading of 0.3 mol-%,
1.5 equiv. of H2O2, a reaction time of 1 h and a reaction
temperature of 60 °C, cyclooctene oxide yield was 87% with
99% epoxide selectivity. 31P NMR spectra indicated the for-
mation of the Venturello anion PO4[WO(O2)2]43– as the cat-
alytically active species during these reactions and it was
suggested that formation of this species was promoted by
using [BMIM][PF6] as the solvent. Corresponding salts
1
2
3[b]
3a
3d
3e
88
90
87
97
90
87
67
68
53
[a] The catalyst was activated in a previous run. [b] The catalyst
was not used during a period of two months between runs 2 and
3.
–
–
Guanidinium and Ammonium Phosphotungstates as
Catalysts for the Epoxidation of Cyclooctene in
Guanidinium-Based Ionic Liquids
with the anions N(CF3SO2)2 and BF4 were also investi-
gated as reaction media but the catalytic performance de-
creased in the order [BMIM][PF6] Ͼ[BMIM][N(CF3-
SO2)2]Ͼ[BMIM][BF4]. It was claimed that the catalyst sys-
tem could be used in five recycling experiments without sig-
nificant decrease in reactivity and selectivity.
The guanidinium phosphotungstate 1a was tested as an
epoxidation catalyst in the GILs 3d, 3e and 3f (Scheme 2
and Table 4). Catalyst recycling experiments comprising
three consecutive reactions were carried out as described
above for 4c. The catalytic performance was significantly
influenced by the GIL. In the case of 3d, cyclooctene con-
version increased from 73 in the first run to 91% in the
third, whereas a decrease from 83 to 75% was observed
using 3e as solvent. Employing the GIL with the saccharin-
ate anion, 3f, conversion increased firstly from 65 to 83%
and decreased subsequently to 77% in the last reaction. Ep-
oxide selectivities were 99% and higher, in all reactions.
Guanidinium Phosphotungstates as Catalysts for the
Epoxidation of Olefins in Acetonitrile
Due to the good solubility of all reaction components in
acetonitrile, it was chosen as solvent for the epoxidation of
several cyclic, linear and branched olefins using the guan-
idinium phosphotungstate 1b as a catalyst. For direct com-
parison, analogous reactions catalyzed by the correspond-
ing imidazolium phosphotungstate 5[24] were also carried
out (Scheme 3 and Table 5). In all cases, test reactions un-
der the same reaction conditions but without catalyst
showed no significant olefin conversion.
Scheme 2. Epoxidation of cyclooctene catalyzed by guanidinium
and ammonium phosphotungstates in GILs.
Table 4. Epoxidation of cyclooctene catalyzed by guanidinium and
ammonium phosphotungstates in GILs.
Scheme 3. Epoxidation of different olefins catalyzed by the phos-
photungstates 1b and 5 in acetonitrile.
Catalyst Solvent Cyclooctene conversion in each run [%][a]
1
2
3
The catalytic performance of 5 in the epoxidation of cy-
clooctene using acetonitrile as solvent has already been de-
scribed by Zhao et al.[24] However, a short reaction time of
1 h and a low catalyst loading of 0.3 mol-% led to an epox-
ide yield of only 1% with 77% selectivity. Only trace
amounts of cyclooctene oxide were observed when MeOH
or CH2Cl2 were employed as solvents. In our own experi-
ments under the same reaction conditions using catalyst 1b,
i.e. a catalyst loading of 4 mol-% and an extended reaction
time of 24 h, an olefin conversion of 74% with 96% epoxide
1a
3d
3e
3f
3e
3e
3e
73 (Ն99)[b]
83 (Ն99)
65 (Ն99)
28 (92)[c]
34 (93)[e]
87 (Ն99)
81 (Ն99)
77 (Ն99)
83 (Ն99)
59 (96)[d]
25 (Ն99)
87 (Ն99)
91 (Ն99)
75 (Ն99)
77 (Ն99)
19 (Ն99)
25 (Ն99)
53 (Ն99)
4a
4b
4c
[a] The catalyst was activated in a previous run. [b] Values in parentheses
are epoxide selectivities. [c] Byproducts: 5% cyclooctene diol and 3% cy-
clooctenone. [d] Byproducts: 3% cyclooctene diol and 1% cyclooctenone.
[e] Byproducts: 3% cyclooctene diol and 4% cyclooctenone.
For comparison, the ammonium-based catalysts[66] 4a selectivity could be reached (entry 5, Table 5). In the case of
and 4b bearing the same anion as 1a were also studied with the analogous guanidinium catalyst 1b a significantly higher
3e as the solvent (Scheme 2 and Table 4). Olefin conversions cyclooctene conversion of 91% with 95% epoxide selectivity
Eur. J. Inorg. Chem. 2011, 2756–2762
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L. Gharnati, O. Walter, U. Arnold, M. Döring
FULL PAPER
Table 5. Epoxidation of different olefins catalyzed by the phospho-
tungstates 1b and 5 in acetonitrile.
To elucidate the nature of the active species, catalysts 1a
and 1b were also investigated by 31P NMR spectroscopy.
The catalysts were dissolved in acetonitrile and a signal typ-
3–
ical of the PW12O40 anion was observed at around
–14 ppm. After addition of H2O2 (50 equiv.) the spectrum
changed and new peaks at around 25, –28 and –44 ppm
emerged in addition to the peak at –14 ppm. The Venturello
anion, which typically shows a peak at around 4 ppm,[24]
was not detected. Thus, the nature of the active species re-
mains unclear and will be investigated in future work.
Conclusions
Guanidinium-based ionic liquids are suitable solvents for
epoxidation reactions with H2O2 as the oxidant. The com-
pounds are oxidation resistant and, employing the Ven-
turello catalyst, the catalyst system can be recovered after
the reaction and used repeatedly. Guanidinium phospho-
tungstates with the PW12O403– anion exhibit remarkable ac-
tivity in epoxidation reactions with H2O2 and their per-
formance is similar to the analogous imidazolium phospho-
tungstate [BMIM][PW12O40]. The corresponding ammo-
nium compounds showed a significantly lower catalytic per-
formance. Recyclable catalyst systems can be obtained by
combining guanidinium phosphotungstate catalysts with
guanidinium-based ionic liquids as solvents. Further experi-
[a] 1% Cyclohexenone and 2% cyclohexanediol as byproducts. [b]
Ketone as major byproduct: Entry 2, 60% (1b) and 35% (5); Entry
9, 13% (1b) and 2% (5). [c] 7% dioxide, 11% diol and 5% ketone. [d] ments will clarify the relationship between the structure of
3% dioxide, 11% diol. [e] Diol as byproduct: Entry 5, 5% (1b) and
guanidinium-based catalysts and solvents and their per-
formance in catalytic epoxidation reactions.
4% (5); Entry 8, 3% (1b) and 4% (5); Entry 10, 14% (1b) and 28%
(5). [f] Diepoxide as byproduct: 15% (1b) and 2% (5). [g] Aldehyde
as byproduct: 3% (1b) and 4% (5).
Experimental Section
General: GC analyses were carried out using an Agilent 6890N gas
chromatograph with FID detector equipped with a DB-5 (5%
phenyl siloxane und 95% methyl siloxane) capillary column. NMR
spectra were recorded with a Bruker Avance 250 or a Varian Unity
INOVA 400 spectrometer. FTIR measurements were carried out
using a Varian 660-IR spectrometer in the transmission mode.
Spectra were recorded from KBr pellets. Melting points were mea-
sured by differential scanning calorimetry employing a Mettler To-
ledo DSC822e device. Tungsten and phosphorus contents of the
catalysts were determined by inductively coupled plasma-atomic
emission spectroscopy with a Varian Liberty 150 instrument. Elec-
trospray ionization-mass spectrometry (ESI-MS) was carried out
using a Hewlett–Packard Series 1100MSD device. X-ray diffraction
data were collected using a Siemens SMART 1000 CCD dif-
fractometer at 200 K. Calculations were performed using
SHELXS-97 and SHELXL-97.[67] Crystal data, data collection
and refinement parameters are summarized in Table 6. CCDC-
808202 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
was observed. However, in the epoxidation of all other cy-
clic olefins a different behaviour was observed and catalyst
5 led to significantly higher conversions compared to 1b.
Conversions of cyclic olefins were usually 70% and higher
with the exception of 2-norbornene (entry 4, Table 5) and
1,5-cyclooctadiene (entry 6, Table 5). In the epoxidation of
these two olefins both catalysts, 1b and 5, gave conversions
in the range of 25 to 41% with high epoxide selectivities of
98% and higher.
Conversely, in the case of linear and branched olefins,
except for trans-2-octene, 1b performed better than 5 with
respect to olefin conversion. Good results were obtained in
the epoxidation of 1-octene with 1b, (entry 7, Table 5), and
olefin conversion increased with increasing number of alkyl
substituents at the double bond thus facilitating electro-
philic epoxidation. Olefin conversion increased from 66%
in the case of 1-octene to 73% in the case of trans-2-octene
(entry 8, Table 5) and to 86% using 2-methyl-2-heptene as
substrate (entry 9, Table 5). In the case of the long-chain
olefin trans-5-decene (entry 10, Table 5) low conversion was
observed, most probably due to steric hindrance. These re-
sults indicate that the catalyst cation has a significant influ-
ence on the catalytic performance of the reaction system.
Concerning epoxide selectivities, both catalysts gave similar
results.
N,N,NЈ,NЈ-tetrahexyl-NЈЈ,NЈЈ-dimethylguanidinium chloride and
N,N,NЈ,NЈ-tetraoctyl-NЈЈ,NЈЈ-dimethylguanidinium chloride were
synthesized as described by Afonso et al.[44] The pentaalkylated
NЈЈ-ethyl-N,N,NЈ,NЈ-tetramethylguanidinium triflate 2a and the
corresponding guanidinium salt with the tris(pentafluoroethyl)tri-
fluoro phosphate anion 2b were obtained from Merck KGaA. The
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Guanidinium-Based Phosphotungstates and Ionic Liquids
(KBr): ν = 2956, 2925, 2903, 2855, 1570, 1546, 1467, 1427, 1079
˜
Table 6. Crystallographic data and refinement details for 1a.
(PO), 986, 977 (W=O), 896, 819 (O–O), 595, 522 (W–O–O) cm–1.
Empirical formula
Formula weight
Crystallization
Crystal size
Space group
Crystal system
Unit cell dimensions
C81H174N9O40PW12
4151.46
acetonitrile
Epoxidation of Olefins: Reactions were carried out in a 10 mL glass
tube equipped with a magnetic stirrer. Typically, reaction tempera-
tures of 60 °C and reaction times of 24 h were chosen. The reported
amounts of catalyst, substrate, internal standard for GC analysis
(decane) and hydrogen peroxide (30% aqueous solution) were dis-
solved in the respective solvent and reactions were monitored by
sampling and GC–MS as well as GC–FID analyses. Product quan-
tification was carried out by comparison of the product mixtures
with calibration curves obtained from standard solutions. Recycl-
ing experiments were carried out by removal of the products by
extraction. Catalyst recovery was achieved by separation of the
ionic liquid phase with the dissolved catalyst and the catalyst solu-
tion was reused in the next catalytic run. The error range of the
given yields, conversions and selectivities was Ϯ4%, mainly due to
GC measurements and slightly diverging product workup.
0.35ϫ0.25ϫ0.2 mm
P21/n (No. 14)
monoclinic
a = 29.338(2) Å
b = 15.504(1) Å
c = 29.514(2) Å
β = 116.459(1)°
12018(1) Å3
Volume
Z
4
Density (calculated)
Wavelength
Temperature
θ-Range
Index ranges
Reflections
2.294 gcm–3
0.71073 Å
200(2) K
1.52Յ θ Յ28.29
–38 Յ h Յ 38, –20 Յ k Յ 20, –38 Յ l Յ 39
collected / unique / observed 143523 / 29524 / 20998 (IϾ2σ)
Largest diff. peak and hole
Refinement method
GooF
2.789 and –2.913 Å–3
Acknowledgments
full-matrix least-squares on F2
1.060
Financial support by the Bundesministerium für Bildung und For-
schung (BMBF) is gratefully acknowledged. We thank Prof. Maas
for providing N,N,NЈ,NЈ-tetraethyl-NЈЈ,NЈЈ-dibutylguanidinium tri-
fluoromethanesulfonate and N,N-dimethyl-NЈ,NЈ-diethyl-NЈЈ,NЈЈ-
bis(2-methoxyethyl)guanidinium trifluoromethanesulfonate, as well
as Prof. Kantlehner for the kind donation of N,N,NЈ,NЈ-tet-
ramethyl-NЈЈ-ethylguanidinium ethyl sulfate.
R indices
R1 = 0.0489 (I Ͼ2σ), wR2 = 0.0989 (all data)
analogous compound with the ethyl sulfate anion 2c was obtained
from Prof. Kantlehner et al. (University of Stuttgart). The hexa-
alkylated guanidinium salts 3a and 3g were purchased from Prof.
Maas et al. (University of Ulm). Further hexaalkylated salts 3b–3f
were prepared according to methods described in the litera-
ture.[44,45,68,69] The ammonium phosphotungstates 4a and 4b,[66] the
Venturello catalyst 4c[64,65] and the imidazolium salt 5[24] were also
synthesized according to literature methods. Other compounds
were commercially available and used without further purification.
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Eur. J. Inorg. Chem. 2011, 2756–2762
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2761

L. Gharnati, O. Walter, U. Arnold, M. Döring
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Received: January 12, 2011
Published Online: May 2, 2011
2762
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© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2756–2762