Pyrylium sulfonate based ionic liquids

Article abstract of DOI:10.1016/j.tetlet.2011.06.058

Pyrylium salts represent a new group of ionic liquids (ILs) containing a positive charge on the oxygen atom. The novel ILs were obtained starting with 4-pyrones from petroleum feedstock and renewable resources and sulfonic acids. The use of carboxylic aci

Full text of DOI:10.1016/j.tetlet.2011.06.058

Tetrahedron Letters 52 (2011) 4342–4345
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Tetrahedron Letters
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Pyrylium sulfonate based ionic liquids
Juliusz Pernak a, , Anna Swierczynska , Mariusz Kot , Filip Walkiewicz , Hieronim Maciejewski
⇑
a
a
a
b
´
´
a Poznan University of Technology, Faculty of Chemical Technology, Pl. M. Sklodowskiej-Curie 2, 60-965 Poznan, Poland
b Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
Pyrylium salts represent a new group of ionic liquids (ILs) containing a positive charge on the oxygen
atom. The novel ILs were obtained starting with 4-pyrones from petroleum feedstock and renewable
resources and sulfonic acids. The use of carboxylic acids instead of salts resulted in the formation of
cocrystals. The synthesized pyrylium ionic liquids were stable in air and in contact with water and com-
mon organic solvents. The permanganate indices which are characteristic for prepared sulfonates were
also investigated. The pyrylium ionic liquids were useful as immobilizers and dissolving agents in hydro-
silylation reactions.
Received 6 April 2011
Revised 27 May 2011
Accepted 13 June 2011
Available online 21 June 2011
Keywords:
Ionic liquids
Oxonium salts
Cocrystals
Ó 2011 Elsevier Ltd. All rights reserved.
Hydrosilylation
Oxonium ions are widely known as reactive intermediates in
organic reactions or as highly reactive chemical reagents. Trial-
kyloxonium salts with inert counterions (such as BFÀ4 or PF6À) are
very powerful alkylating agents. Oxatriquinane and oxatriquina-
cene, as water-stable versions of these highly reactive salts, have
been synthesized recently.1 The pyrylium cation as a fully unsatu-
rated six-membered heterocycle with one oxygen atom bearing a
positive charge is much less reactive than a trialkyloxonium ion.
For example, it is stable in nucleophilic solvents when the reaction
medium is strongly acidic. However, pyrylium salts are water
unstable. In the literature, different methods are described for syn-
thesizing pyrylium salts, for example, one of the first reactions was
heterocyclization of 1,5-diketones.2–4 The Balaban–Nenitzescu–
Praill synthesis5 from tertiary butanol and an acid anhydride in
the presence of tetrafluoroboric, perchloric, hexafluorophosphoric,
or trifluoromethanesulfonic acids is the best method for the prep-
aration of pyrylium salts with alkyl substituents.6–8 2,6-Dialkyl-4-
methylpyrylium hexafluorophosphates termed ionic liquids (ILs)
by Balaban9 were obtained in good yields between 60–70% and
characterized by their sharp melting point.10
Table 1
Prepared 4-hydroxypyrylium sulfonates
R1
R2
R3
R4
Yield (%)
Mp (°C)
IMn
c
Salt
1
2
3
4
5
6
7
8
9
Me
Me
Me
Me
Et
Me
Me
Me
Me
H
OH
H
OH
OH
H
OH
H
Me
H
Me
H
H
Me
H
MePh
MePh
Ph
Ph
Ph
Me
Me
Et
96
92
93
96
98
93
94
97
91
115–117
145–147
97–99
9.27
7.13
7.31
6.36
3.28
14.96
10.88
12.36
10.18
143–145
72–74
Liquida
75–78
Me
H
Liquidb
77–79
OH
Et
a
b
c
Density at 20 °C = 1.26 g mLÀ1
Density at 20 °C = 1.22 g mLÀ1
.
.
Permanganate index in mgO2 LÀ1 indicates the quantity of oxygen required for
the chemical oxidation of organic matter present in water.
of pyrylium salts with organic anions. According to our knowledge,
pyrylium ILs have not been used as reaction solvents.
New 4-hydroxypyrylium sulfonates were synthesized in one
step with very good yields of over 90%. 2,6-Dimethyl-4-pyrone,
3-hydroxy-2-methyl-4-pyrone (maltol) and 2-ethyl-3-hydroxy-4-
pyrone (ethyl maltol) were used as bases. Maltol is found in the
bark of young larch trees, pine needles, chicory wood, oils, and
roasted malt. It is used as a fragrant, caramel-like flavor for addi-
tion in foods, imparts a ‘freshly-baked’ flavor (E number E636) to
bread and cakes and the roasting of cocoa and coffee beans. Four
strong sulfonic acids: benzenesulfonic (pKa = 0.714) and p-toluene-
sulfonic (pKa = À1.3414), methanesulfonic (pKa = À1.214) and eth-
anesulfonic (pKa = À1.6115) were used as acids.
ILs, depending on the cation, are divided into: imidazolium,
ammonium, pyridinium, phosphonium, piperidinium, morphol-
inium, and sulfonium ILs.11–13 The positive charge is localized on
the nitrogen, phosphorus, or sulfur atom. Pyrylium salts represent
a new group of ILs containing the positive charge on the oxygen
atom. The pyrylium ILs described by Balaban include anions of
inorganic origin.10 Herein, we report the synthesis and properties
⇑
Corresponding author. Tel.: +48 61 6653682; fax: +48 61 6653649.
E-mail address: juliusz.pernak@put.poznan.pl (J. Pernak).
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.06.058

J. Pernak et al. / Tetrahedron Letters 52 (2011) 4342–4345
4343
Table 2
OH
O
O
The physicochemical properties of the synthesized sulfonates
R2
R1
H
R2
R1
H
R4SO3H
a
b
c
R4SO3
-
+
Sulfonate
Tg
Tm
Tonset
+
R3
O
1
2
3
4
5
6
7
8
9
—
—
—
—
—
146.5
83
102.5
120
78
297
270
320
168
270
293
242
280
248
R3
Scheme 1. Prepation of the pyrylium sulfonates.
À57.5
—
Several examples are presented in Table 1, which were prepared
according to the reaction shown in Scheme 1. Of the nine sulfo-
nates synthesized, seven were crystalline compounds and the
other two were liquids. Six are ILs and three are solids, with melt-
ing points above 100 °C. The liquid products obtained were dried
under vacuum and stored over P4O10. The water content, deter-
mined by Karl-Fischer measurements was found to be less than
500 ppm.
—
77.5
—
À65
—
63
a
b
c
Glass transition temp determined by DSC.
Melting point on heating determined by DSC.
Decomposition temp determined from onset to 50 wt % mass
loss.
All the synthesized sulfonates were characterized by 1H and 13
C
Table 3
The prepared 4-pyrone/carboxylic acid cocrystals
NMR and elemental analysis. The structure of sulfonate 4 was con-
firmed by two-dimensional NMR. The crystal structure of 4 was
determined (Fig. 1). Furthermore, the autostereographic projection
was investigated for the same compound (see Supplementary
data).
The sulfonates synthesized were insoluble in diethyl ether,
hexane, and tetrachloromethane, but were soluble in chloroform,
DMSO, acetone, water, DMF, and low-molecular weight alcohols.
Furthermore, they were poorly soluble in toluene. The physico-
chemical properties of the prepared sulfonates are presented in Ta-
ble 2. The pyrylium sulfonates obtained were thermally stable and
all showed 50% thermal decomposition at around 250 °C (Table 2).
In the case of differential scanning calorimetry analysis (DSC)
only the two liquid salts 6 and 8 exhibited glass transition temper-
atures at À57.5 and À65 °C, respectively.
Cocrystal
R1
R2
R3
Acid
Yield (%)
Mp (°C)
10
11
12
13
14
15
16
Me
Me
Et
Me
Me
Me
Me
H
Me
H
H
H
H
Sal
Sal
Sal
Mal
Ox
95
97
98
96
97
95
93
65–67
82–84
76–78
81–83
135–137
127–129
55–57
OH
OH
OH
OH
OH
H
H
Me
Lac
Gly
When we used carboxylic acids: oxalic (Ox) (pKa = 1.2714), D,L-
malic (Mal) (pKa = 3.4016), salicylic (Sal) (pKa = 2.9817), glyconic
(Gly) (pKa = 3.8317), and lactic (Lac) (pKa = 3.8317
) instead of
sulfonic acids we obtained cocrystals. The prepared cocrystals are
presented in Table 3. These cocrystals manifested sharp melting
points.
The synthesized ILs were air- and moisture-stable salts. The
dynamically growing application of ILs induces misgivings as to
their utilization. We regard their oxidation in an aqueous solution
of KMnO4 as an effective medium. For each of the sulfonates we
were able to investigate the permanganate index (IMn), according
to the analytical standard (EN ISO 8467:2001). The results are
listed in Table 1. For comparison, the estimated values for urea
and pyridine were, respectively, 2.13 and 19.89 [mgO2 LÀ1]. The
values IMn presented were characteristic for the prepared
sulfonates.
The structures of cocrystals were confirmed by 1H and 13C NMR
and elemental analysis. In addition, for 2,6-dimethyl-4-pyrone/sal-
icylic acid 10, two-dimensional NMR, solid-state NMR, and crystal-
lographic analysis were performed (Fig. 2). These solid cocrystals
crystallized from stoichiometric amounts of carboxylic acid and
the 4-pyrone base.
The synthesized cocrystals were stable up to 180 °C and glass
transition temperatures were observed for only two of the cocrys-
tals: 10 and 13 (Table 3).
In these two reactions (using sulfonic or carboxylic acids), pro-
ton transfer took place to give a salt or a cocrystal. The acid ioniza-
tion constant (pKa) is employed for predicting solid form molecular
ionization states.18 In the case of the synthesis of a pyrylium salt
with an organic anion, the pKa value is important: organic acids
with a pKa less than 1 form salts, and those with a pKa more than
1 yield cocrystals.
Figure 1. The ORTEP drawing of the ionic pair of 3,4-dihydroxy-2-methylpyrylium
benzenesulfonate 4 as present in the crystal. The disordered positions of the
sulfonate and methyl groups are indicated by showing one set of atoms bonded and
the other not bonded to the central atom. The thermal ellipsoids are drawn at the
50% probability level. The CCDC reference number for this IL is 798798.
Figure 2. The ORTEP drawing of the H-bonded molecule of salicylic acid and 2,6-
dimethyl-4-pyrone 10, as present in the cocrystal structure. The CCDC reference
number for this cocrystal is 798797.

4344
J. Pernak et al. / Tetrahedron Letters 52 (2011) 4342–4345
CH3
H3C Si
CH3
CH3
H3C Si
CH3
CH3
Si
CH3
CH3
Si
CH3
O
O
Si CH3
CH3
O
O
Si
CH3+ CH2=CH(CH2)5CH3
CH2
CH3
H
CH2(CH2)5CH3
Scheme 2. Hydrosilylation of 1-octene.
Figure 3. The yield of product over 10 runs with the recovered catalytic system for the hydrosilylation of 1-octene catalyzed by Pt or Rh complexes immobilized in pyrylium
IL 6.
ILs are recognized as potential media for the immobilisation of
catalysts and were used with considerable success in a wide range
of laboratory scale reactions.19 Having experience with catalytic
systems for hydrosilylation processes, based mainly on rhodium
siloxide complexes immobilized in imidazolium and phosphonium
ILs,20–22 we decided to test pyrylium ILs. In the present study we
employed a rhodium complex [Rh(PPh3)3Cl] and complexes of
platinum at different oxidation states, namely [Pt(PPh3)4],
[Pt(PPh3)2Cl2], and [PtCl4], immobilized in liquid pyrylium sulfo-
nate 6. The obtained systems were used as catalysts for the model
reaction of octene hydrosilylation (Scheme 2).
Our studies showed that immobilisation of the complexes in
sulfonate 6 gave biphasic systems which enabled easy separation
of the products from the catalytic system and its reuse in subse-
quent catalytic runs. In all cases only one product was formed
(the b addition product). Figure 3 shows the yields of product over
10 reaction cycles for the reaction catalyzed by a particular metal
complex immobilized in pyrylium IL 6.
The catalytic activity of most of the systems studied was high
and enabled product yields above 50% to be obtained in subse-
quent runs, except for the catalytic system based on [PtCl4]. How-
ever, the highest catalytic activity as well as high stability and
reproducibility were shown by the systems based on [Rh(PPh3)3Cl]
and [Pt(PPh3)2Cl2], which made it possible to obtain the product in
yields over 90% in 10 subsequent runs.
Pyrylium ILs are characterized by thermal stability and have
very low, but not null vapor pressure. The hydrosilylation process,
employed in our study as a test reaction, is usually carried out
without a solvent. ILs chosen for such an application have to be
of high purity (no contaminants able to poison the catalyst) and
should dissolve and immobilize the metal complex, while main-
taining its high catalytic activity.
interact with the products. The ILs applied in this study have
met all these requirements which resulted in a highly effective
catalytic system for the reaction studied (particularly in the case
of the rhodium siloxide complex). Another advantage of the IL
was its immiscibility with the reactants and ability to form well-
separated phases, which made isolation of the post-reaction mix-
ture from the catalytic system very easy. The hydrosilylation in
the presence of four different complexes of metals indicated differ-
ences in their catalytic performance, particularly when they were
reused. In the case of [Rh(PPh3)3Cl] and [Pt(PPh3)2Cl2] complexes,
immobilization in the IL was excellent and they were well-pro-
tected against leaching and showed high catalytic activity. The
two other complexes were not so efficiently immobilized in the
IL, which resulted in their gradual leaching from the catalytic sys-
tem as reflected by the decreasing product yield. This fact was con-
firmed by results of post-reaction mixture analysis performed by
ICP-OES which showed no presence of Rh or Pt in the case of the
former two complexes, whereas in that of the remaining two com-
plexes the post-reaction mixture obtained after subsequent reac-
tion runs contained platinum.
4-Pyrones, obtained from petroleum feedstock and renewable
resources (maltol), are reactive cyclic compounds, capable of form-
ing both oxonium cation and cocrystals with organic acids. Very
strong sulfonic acids lead to pyrylium ionic liquids due to a proton-
ation reaction. On the other hand, carboxylic acids are too weak to
create ionic bonds, but are strong enough to yield hydrogen bond
based cocrystals. Both kinds of product could be obtained in one
step with high efficiency, and purity. These thermally-stable pyry-
lium ionic liquids are useful as immobilizers and dissolving agents
in hydrosilylation reactions.
Acknowledgments
The pyrylium IL used by us did not play the role of solvent, but
that of a metal complex immobilizing agent. Moreover, the IL
should form a bi- or multiphase system with substrates and prod-
ucts, be stable in the presence of reactive substrates and should not
We are grateful to The Ministry of Science and Higher Education
(Poland) for financial support project No. N 204 165 436 and
Poznan University of Technology – grant 32-172/2011.

J. Pernak et al. / Tetrahedron Letters 52 (2011) 4342–4345
4345
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Supplementary data (synthetic and analytical details, NMR
spectra, crystals) associated with this article can be found, in the
online version, at doi:10.1016/j.tetlet.2011.06.058.
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