Mandelate and prolinate ionic liquids: Synthesis, characterization, catalytic and biological activity

Article abstract of DOI:10.1016/j.tetlet.2011.01.069

A group of quaternary ammonium mandelates and L-prolinates, as ionic liquids, were synthesized and characterized. The prepared salts were soluble in water and showed high surface activity. The described synthesis of L-prolinate was simple and the obtained

Full text of DOI:10.1016/j.tetlet.2011.01.069

Tetrahedron Letters 52 (2011) 1325–1328
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Mandelate and prolinate ionic liquids: synthesis, characterization,
catalytic and biological activity
a
a
a
a
b
´
Jacek Cybulski , Anna Wisniewska , Anna Kulig-Adamiak , Zbigniew Da˛browski , Tadeusz Praczyk ,
Alicja Michalczyk c, Filip Walkiewicz d, Katarzyna Materna d, Juliusz Pernak d,
⇑
a Industrial Chemistry Research Institute, ul. Rydygiera 8, 01-793 Warsaw, Poland
b Institute of Plant Protection, ul. W. Wegorka 20, 60-318 Poznan, Poland
c The Institute of Industrial Organic Chemistry, ul. Annopol 6, 03-236 Warsaw, Poland
d Poznan University of Technology, Faculty of Chemical Technology, pl. M. Sklodowskiej-Curie 2, 60-965 Poznan, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
A group of quaternary ammonium mandelates and
L
-prolinates, as ionic liquids, were synthesized and
Received 28 October 2010
Revised 23 December 2010
Accepted 14 January 2011
Available online 20 January 2011
characterized. The prepared salts were soluble in water and showed high surface activity. The described
synthesis of -prolinate was simple and the obtained ionic liquid contained a chiral anion. -Prolinate in
CH2Cl2 was employed for the asymmetric Michael addition of a ketone to nitrostyrene. A yield of 60%,
enantioselectivity (upto 50% ee), and good diastereoselectivity (syn/anti ratio of up to 90:10) were
obtained for the asymmetric addition of cyclohexanone. These novel ionic liquids proved to be very effec-
L
L
Keywords:
tive anti-microbial and anti-fungal agents, especially didecyldimethylammonium L-prolinate. Addition-
ally, it was found that phytotoxicity can be a useful tool in assessing the optical forms of ionic liquids.
Ionic liquids
Mandelates
Prolinates
Ó 2011 Elsevier Ltd. All rights reserved.
Biological activity
Catalysis
Phytotoxicity
Ionic liquids (ILs) are salts that are generally liquid at ambient
temperatures or melt at <100 °C, have long liquid ranges, and have
low vapor pressures. They have received considerable attention in
recent years due to their potential applications.1 The huge number
of cation–anion combinations permits the design of appropriate ILs
for a particular application. They have been used in many areas of
chemistry including organic2 and polymer synthesis,3 biphasic
catalysis,4 biocatalysis,5 battery systems, electrochemical deposi-
tion, and electrochemistry.6 Consequently, the properties of ILs
have led to a multitude of uses in industry [e.g., BASIL (Biphasic
Acid Scavenging utilizing Ionic Liquids)].7 Recently, ILs as active
pharmaceutical ingredients have been discussed.8
Amino acids represent a very useful class of starting materials
for construction of ionic liquids, especially chiral examples (CILs).
This is the result of their availability in both enantiomeric forms
at reasonable cost. Numerous groups9 have elected to include
either a chiral anion, a chiral cation, or both in the IL.
efficient system for direct asymmetric a-aminoxylation of alde-
hydes and ketones11 as well as Mannich reactions.12 Also, prolina-
mide derivatives modified with IL moieties were synthesized and
studied as organocatalysts in asymmetric aldol reactions in water.13
Here, we report the synthesis, characterization, properties, and
potential application of novel organic salts based on didecyldime-
thylammonium, benzalkonium, and domiphen cations combined
with mandelate (two enantiomers and the racemic mixture) and
the L-prolinate anion.
Long chain alkyl quaternary ammonium mandelates and
prolinates were obtained by metathesis reactions. The structures
of the cations are shown in Scheme 1, and those of the anions in
Scheme 2. Salts with chiral anions were prepared from (R)-(ꢀ)-
mandelic and (S)-(+)-mandelic acids and L-proline. The racemic
mixture was also synthesized when mandelic acid was used. The
products were dried and the potassium chloride by-product was
The main strategy to prepare economic CILs is via substrates de-
rived from the chiral pool. Our research with (1R, 2S, 5R)-(ꢀ)-men-
thol showed that this compound was very suitable for the synthesis
of CILs.10
L-Proline in combination with ILs has proved to be an
⇑
Corresponding author. Tel.: +48 61 665 3682; fax: +48 61 665 3649.
E-mail address: juliusz.pernak@put.poznan.pl (J. Pernak).
Scheme 1. Structures of the cations.
0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2011.01.069

1326
J. Cybulski et al. / Tetrahedron Letters 52 (2011) 1325–1328
Table 2
Thermal stabilities of the ILs
IL
Tg (°C)
Tm (°C)
Tonset5% (°C)
Tonset50% (°C)
1
2
3
4
5
6
7
8
9
ꢀ57.1
ꢀ46.1
ꢀ48.9
ꢀ60.5
ꢀ49.5
ꢀ32.1
ꢀ35.9
ꢀ51.0
ꢀ22.4
ꢀ26.1
ꢀ22.1
—
—
—
—
—
—
—
204
156
212
166
175
202
187
128
210
205
204
232
226
273
253
207
210
238
194
286
267
266
Scheme 2. Structures of the anions.
removed using anhydrous methanol. This metathesis reaction was
simple and the products were obtained in good yields over 86%.
ꢀ21.3
—
In the case of L-proline we did not use an anion exchange resin
10
11
—
—
in contrast to the method of Ohno and co-workers.9a The proper-
ties of the synthesized salts, which contain large ammonium
cations (didecyldimethylammonium [DDA], benzalkonium with
Tg—glass transition temperature, Tm—melting point, Tonset—decomposition
temperature.
C12H25 and C14H29 alkyl groups equal to 60% and 40%, respectively,
[BA], and domiphen [N,N-dimethyl-N-(2-phenoxyethyl)-1-dode-
canaminium] [DOM]) are summarized in Table 1.
All mandelates with the [DOM] cation (9, 10, and 11) and
[BA][L-PRO] were obtained as waxes and the remainder were li-
where: Cmax is the surface excess concentration at the saturated
interface, R is the gas constant and T the absolute temperature, c
is the concentration of the salt.
From Cmax, the minimum surface occupied by a molecule at the
interface Amin can be calculated from the equation:
quid. The results obtained allow the synthesized eleven salts to
be classified as ILs. Eight are chiral ionic liquids (CILs) with the spe-
cific rotations listed in Table 1. The synthesized ILs are air and
moisture stable and soluble in water and organic solvents: metha-
nol, dichloromethane (except 8), toluene (except 8), acetone (ex-
cept 4 and 8), and THF (except 4, 8, 9, 10 and 11).
The water content of the ILs, determined by Karl–Fischer mea-
surements, was found to be less than 500 ppm. The salts were
characterized by 1H and 13C NMR spectroscopy and elemental
analysis.
1
Amin
¼
;
CmaxNA
where: NA is the Avogadro number.
The critical micelle concentration (CMC) and the corresponding
surface tension (cCMC), the surface excess Cmax and area per mole-
cule Amin, are summarized in Table 3.
For the studied IL aqueous solutions, the surface tension de-
creased from the water value to a minimum located between
26.1 and 36.4 mN mꢀ1. After this point the surface tension reached
a plateau region. In the case of [BA] and [DOM] the solutions
The ammonium mandelates begin to decompose at about
155 °C (Table 2). ILs with the prolinate anion were less stable:
[DDA][
respectively. A glass transition was found for all the studied ILs,
but a melting point at ꢀ21.3 °C was observed only for [BA][
L-PRO] and [BA][L-PRO] decomposed at 166 °C and 128 °C,
manifested the highest cCMC values of 36.4 and 36.2 mN mꢀ1
,
L
-
respectively. This demonstrated that [DDA] exhibited more pro-
nounced intermolecular hydrophobic interactions, making it easier
to form aggregates in water than was the case for [BA] and [DOM].
The CMC locations obtained with the plots of [DDA][R-MAN],
[DDA][S-MAN], and [DDA][MAN] were almost identical. A similar
situation was observed for [BA][R-MAN], [BA][S-MAN], and [BA]-
[MAN] solutions.
As far as the cCMC values are concerned, they are dependent
mainly on the cationic form of the IL, but the anion is also impor-
tant. The area per molecule Amin of [DDA] forms with [MAN] was
PRO]. In the case of the pure enantiomers and racemic mixtures
of [DDA] and [BA] mandelates it was observed that pure R and S
forms showed higher glass transitions in comparison to those of
racemic mixtures (Table 2). This phenomenon could be explained
by better and faster packing of the system in the case of pure enan-
tiomers, which was also observed by Pham.14 This phenomenon is
not observed in the [DOM] mandelates because the differences in
glass transition temperature values are small.
The ionic liquids obtained were also studied as possible surface
active compounds. Surface excess concentrations Cmax were calcu-
higher than that of the corresponding [
the molecules of ILs containing [ -PRO] were more tightly packed
at the water–air interface. [DDA][MAN] has a lower CMC value
and a lower CMC, resulting in better surface properties than [BA]-
[MAN]. The CMC values of aqueous solutions of [DDA] and [DOM]
L-PRO] form, indicating that
lated from the slope of the linear portion of the
the Gibbs isotherm:
c
ꢀ log c plots using
L
c
1
dc
Cmax ¼ ꢀ
ꢁ
RT dðlncÞ
Table 1
Properties of the prepared ILs
Specific rotationa
½ ꢂ
a 2D0
IL
Cation
Anion
Yield (%)
Densityb (g mLꢀ1
)
Viscosityc(cP)
1
2
3
4
5
6
7
8
9
[DDA]
[DDA]
[DDA]
[DDA]
[BA]
[BA]
[BA]
[BA]
[MAN]
[R-MAN]
[S-MAN]
[L-PRO]
[MAN]
[R-MAN]
[S-MAN]
[L-PRO]
[MAN]
[R-MAN]
[S-MAN]
92
86
90
95
98
91
98
97
93
97
97
—
0.952
0.952
0.952
0.938
1.015
1.015
1.015
Wax
5.73 ꢃ 103
3.68 ꢃ 103
3.80 ꢃ 103
12.70 ꢃ 103
18.40 ꢃ 103
21.20 ꢃ 103
22.40 ꢃ 103
—
ꢀ31.1
+31.0
ꢀ26.3
—
ꢀ32.7
+32.3
ꢀ30.6
—
[DOM]
[DOM]
[DOM]
Wax
Wax
Wax
—
—
—
10
11
ꢀ30.9
+30.4
a
b
c
In MeOH c ꢄ 1.0.
At 20 °C.
At 25 °C.

J. Cybulski et al. / Tetrahedron Letters 52 (2011) 1325–1328
1327
Table 3
CMC, surface tension (
c
CMC), surface excess concentration (Cmax), and area per molecule (Amin) of the prepared ILs in aqueous solution at 25 °C
IL
CMC (mmol Lꢀ1
)
cCMC (mN mꢀ1
)
Cmax
(l
mol mꢀ2
)
Amin (10ꢀ19 m2)
[BA][MAN]
[BA][R-MAN]
[BA][S-MAN]
[BA][L-PRO]
[DDA][MAN]
[DDA][R-MAN]
[DDA][S-MAN]
[DDA][L-PRO]
[DOM][MAN]
[DOM][R-MAN]
[DOM][S-MAN]
2.399
2.512
2.510
2.818
0.661
0.708
0.631
2.520
1.318
1.259
1.247
36.3
36.2
36.4
30.3
26.4
26.1
26.6
27.5
36.2
36.0
36.3
7.91
6.63
7.98
5.25
4.69
5.11
5.06
6.39
7.40
9.13
8.24
2.10
2.51
2.23
3.16
3.54
3.25
3.28
2.60
2.24
1.82
2.12
Table 4
The effect of CIL catalysts on the asymmetric Michael addition
were lower, or of the same order as the CMC values of cationic sur-
factants.15 These results indicate that ILs with [DDA], [BA], and
[DOM] self-assemble easily in aqueous solutions.
Prolinate ionic liquids were also studied as compounds with
catalytic activity. The asymmetric addition of cyclohexanone to
Catalyst
Solvent
Time (h)
55
Yield (%)
76
syn/antia
ee (%)
Ref.
[bmim]BF4
TFAc
88
33
12
16
L
-Proline
Func. ILb
10
69
99
60
99
94
98d
4e
CH2Cl2
51d
trans-b-nitrostyrene was investigated using the
L-prolinates
[DDA][ -PRO] and [BA][ -PRO] (Scheme 3). The reaction proceeded
L
L
Determined by 1H NMR.
Functionalized IL = 1-butyl-3-(pyrrolidin-2-yl)imidazolium tetrafluoroborate.
Trifluoroacetic acid.
Determined by HPLC analysis (Chiralpak AD-H).
[DDA][L-PRO], with benzoic acid as the cocatalyst.
a
b
c
with the use of benzoic acid as a cocatalyst. The best results ob-
tained were compared with the literature data12,16 and are given
in Table 4. Asymmetric Michael addition of a catalyst with
d
e
[DDA][
functionalized IL containing
sis of -prolinates [CATION][
L
-PRO] was better than with
-proline in the cation. As the synthe-
-PRO] is simple and is characterized
L-proline, and worse than a
L
L
L
by high efficiency, it is appropriate to seek a cation, which im-
proves the efficiency of the catalytic activity.
Mandelic acid is a well known compound used as an anti-
bacterial, particularly for the treatment of urinary tract infections.
those with [
For example, mandelate with the [DDA] cation was more phyto-
toxic than with the [BA] cation in contrast to -prolinate ILs.
L-PRO], but the type of cation used was also important.
L
Among the tested ILs, mandelate with the [DDA] cation inhibited
plant growth more than other compounds. In this case the ED50
values ranged between 101 and 326 ppm. Compounds with the
[BA] cation were less toxic, especially with the [R-MAN] anion. In
the case of mandelates with the [DOM] cation, large differences
between the phytotoxicity of the compounds with [R-MAN] and
[S-MAN] anions were found: the ED50 values were 412 and
5412 ppm, respectively.
L-Proline as an osmoprotectant is popular in many pharmaceutical
and biological applications. Didecyldimethylammonium and ben-
zalkonium chlorides are antiseptic/disinfectants and domiphen
bromide is a chemical antiseptic. Starting from these active sub-
strates we expected the product to retain high biological activity.
The minimum inhibitory concentration (MIC) and minimum
bactericidal and fungicidal concentrations (MBC and MFC) were
established and are presented in Table 5. All the studied ILs exhib-
ited high activity against the tested microorganisms. The ILs with
[DDA] (1–4) and [DOM] acted more effectively than the standard,
benzalkonium chloride [BA][Cl]. ILs with the [BA] cation also
worked effectively, but less so than [BA][Cl]. The most effective
Some mandelates, especially [DOM][S-MAN], when used at low
concentrations, promoted the growth of garden cress seedlings
(Fig. 1). L-Prolinate salts such as [BA][[L-PRO] and [DDA][L-PRO]
were characterized by relatively high ED50 values ranging from
1060 to 1400 ppm. Our results showed how strong the impact of
the enantiomer structure was on the phytotoxicity. The phytotox-
icity of the racemate was probably a result of the phytotoxicity of
the individual enantiomers. Figure 1 illustrates that the phytotox-
icity of the racemate was between the values of the R and S forms.
The results obtained show that plants are very effective in assess-
ing the optical forms of compounds.
was [DDA][L-PRO]. Evidently no difference in action between race-
mic and optically pure forms [R-MAN] and [S-MAN] were ob-
served. However, we noted differences in the biological activity
of protic ILs (1-alkylimidazolium and 1-alkoxymethylimidazolium
L
-lactates and DL-lactates) in our earlier paper.17
The impact of ILs toward plants depends on the concentration of
substance and plant species. Moreover, the type of anion or cation
is also an important factor. Studies on the response of duckweed
(Lemma minor) to 1-alkyl-3-methylimidazolium compounds
showed that the length of the alkyl chain influenced the phytotox-
In conclusion, long chain alkyl quaternary ammonium mande-
lates, (R)-(ꢀ)-mandelates, (S)-(ꢀ)-mandelates, and
L-prolinates
were prepared. The reaction conditions were mild, with a simple
work-up procedure and high yields. All of the synthesized com-
pounds were classified as ionic liquids with several being chiral
ionic liquids. All of the obtained ammonium ionic liquids were
air- and moisture stable under ambient conditions. Moreover, the
ionic liquids were soluble in water. Our results indicate that ILs
with [DDA], [BA], and [DOM] self-assemble easily in aqueous solu-
icity.18 In our research with mandelate and
L-prolinate compounds
conducted on garden cress (Lepidium sativum) the inhibition of
plant growth depended both on the cation and anion structure.
The ILs with [MAN] anions were generally more phytotoxic than
tions. We noted that the cCMC values were dependent on the cation
form of the IL. Almost the same surface behavior was observed for
enantiomers and racemates. The CMC values of the synthesized ILs
were similar or lower than those of aqueous solutions of classical
cationic surfactants. Furthermore, the chiral ionic liquids were po-
tential solvents and catalysts for asymmetric reactions.
L-Prolinates
Scheme 3. Asymmetric Michael addition.
were selective catalysts for asymmetric Michael addition.

1328
J. Cybulski et al. / Tetrahedron Letters 52 (2011) 1325–1328
Table 5
MIC, MBC and MFC valuesa for the prepared ILs and [BA][Cl]b
Strain
IL
1
2
3
4
9
10
0.1
0.2
1
11
0.2
0.5
1
4
0.5
1
1
1
0.5
1
1
[BA][Cl]b
M. luteus
MIC
MBC
MIC
MBC
MIC
MBC
MIC
MBC
MIC
MBC
MIC
MBC
MIC
MBC
MIC
MBC
MIC
MBC
MIC
MBC
MIC
MFC
MIC
MFC
<0.1
<0.1
0.2
0.5
0.1
0.2
0.5
1
0.1
0.5
0.5
0.5
0.2
0.5
0.5
1
0.2
0.2
1
<0.1
<0.1
0.1
0.1
<0.1
<0.1
0.1
0.2
1
0.2
0.2
1
0.5
4
1
8
0.5
2
2
8
0.2
0.5
1
S. aureus
2
1
S. epidermidis
E. feacium
M. catarrhalis
E. coli
<0.1
<0.1
0.5
1
0.1
0.5
1
0.1
0.2
1
1
1
8
0.5
1
1
8
1
0.2
0.2
0.5
1
0.2
0.2
0.5
1
0.5
0.5
0.5
2
1
4
<0.1
0.5
16
16
4
1
1
2
31
1
S. marcescens
P. vulgaris
P. aeruginosa
B. subtilis
16
16
16
31
31
62
0.1
0.1
1
31
62
8
16
16
31
2
16
31
8
31
16
62
31
31
31
31
16
62
1
31
62
62
31
31
62
62
1
1
4
31
8
62
16
62
16
31
2
2
1
1
1
62
16
16
31
62
1
1
2
16
1
4
16
31
0.1
0.1
1
0.2
0.5
1
1
1
2
0.5
1
1
C. albicans
R. rubra
2
4
2
2
1
4
1
1
2
2
1
8
4
1
8
31
a
In mg Lꢀ1
Benzalkonium chloride.
.
b
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Acknowledgment
This work was supported by grant BW 32-222/2010 from
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2011.01.069.