Hydrogen-bond-rich ionic liquids as effective organocatalysts for Diels-Alder reactions

Article abstract of DOI:10.1039/c4gc00380b

The synthesis and characterisation of new hydrogen-bond-rich ionic liquids and studies of their catalytic performance in Diels-Alder reactions are described. d-Glucose and chloroalcohols were used as the raw materials and as the sources of hydroxyl groups for the synthesis of ionic-liquid cations, whereas weakly coordinating bis(trifluoromethylsulfonyl)imide was used as the anion. The new ionic liquids were analysed by ¹H and ¹³C NMR spectroscopy and by ESI-MS experiments, which confirmed their structures. In addition, the thermal data of the studied ionic liquids measured by differential scanning calorimetry and thermogravimetric analysis showed that these compounds tend to form a glass at temperatures in the range of -29°C to -16°C and are thermally stable from ambient temperature to at least 430°C, most likely because of the presence of bis(trifluoromethylsulfonyl) imide anions. The performance of the ionic liquids in the model reaction of cyclopentadiene with diethyl maleate or methyl acrylate was investigated. The studied ionic liquids showed high activity even when present in catalytic amounts (4 mol% with respect to the dienophile). An increase in the number of hydroxyl groups present in the ionic liquid structure resulted in higher reaction rates. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00380b

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PAPER
Hydrogen-bond-rich ionic liquids as effective organocatalysts for Diels-
Alder reaction
Karol Erfurta, Ilona Wandzika, Krzysztof Walczaka, Karolina Matuszeka and Anna Chrobok*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
The synthesis and characterisation of new hydrogenꢀbondꢀrich ionic liquids and studies of their catalytic
performance in DielsꢀAlder reactions are described. DꢀGlucose and chloroalcohols were used as the raw
materials and as sources of hydroxyl groups for the synthesis of ionicꢀliquid cations, whereas weakly
coordinating bis(trifluoromethylsulfonyl)imide was used as the anion. The new ionic liquids were
10 analysed by 1H and 13C NMR spectroscopy and by ESIꢀMS experiments, which confirmed their
structures. In addition, the thermal data of the studied ionic liquids measured by differential scanning
calorimetry and thermogravimetric analysis showed that these compounds tend to form a glass at
temperatures in the range of ꢀ29 °C to ꢀ16 °C and are thermally stable from ambient temperature to at
least 430 °C, most likely because of the presence of bis(trifluoromethylesulfonyl)imide anions.
15 The performance of the ionic liquids in the model reaction of cyclopentadiene with diethyl maleate or
methyl acrylate was investigated. The studied ionic liquids showed high activity, even when present in
catalytic amounts (4 mol% with respect to the dienophile). An increased number of hydroxyl groups
present in the ionic liquid structure resulted in higher reaction rates.
pentofuranose unit.12,13 In addition, certain sugarꢀderived
quaternary ammonium salts containing iodides and bromides
have also been described in the literature.14
Currently, only one example of tetraalkylammonium ionic
liquids with a sugar moiety on the anion has been reported: ionic
liquids from Dꢀgalacturonic and Dꢀglucuronic acids.15
1. Introduction
20
Over the past 20 years, ionic liquids, supercritical fluids and
water have become powerful alternatives to conventional organic
solvents. Because of their properties, which include an
undetectable vapour pressure, the ability to dissolve numerous
organic and inorganic substances, high thermal stability, and a
50
The presence of hydroxyl groups in the sugarꢀderived ionic
liquid structures enables them to be highly coordinating solvents,
25 wide liquid phase range, ionic liquids have become one of the
most promising new reaction media.1
55 and they can be used in stereoselective and metal catalysed
reactions. However, the potential utility of these sugarꢀderived
ionic liquids has been investigated for only a few reactions.7 The
utility of ionic liquids for asymmetric induction was investigated
using ionic liquids based on isosorbide in azaꢀDiels–Alder
60 cycloaddition reactions between chiral imines and dienes, and
moderate yields and diastereoselectivities of up to 60% d.e. were
achieved.10 Chiral ionic liquids obtained from 2,3,5ꢀtriꢀOꢀbenzylꢀ
Dꢀribose and Dꢀxylose have been used as coꢀsolvents for the
addition of methylmagnesium chloride to various aromatic
65 aldehydes. The resulting chiral alcohols were recovered in nearly
quantitative yield, but as racemic mixtures.12
The term “green solvents” has been used to describe ionic
liquids because of their negligible vapour pressure. Significant
efforts have been made to design “fully green”, nonꢀtoxic and
30 biodegradable ionic liquids using renewable compounds as the
starting materials. The synthesis of ionic liquids from
environmentally sustainable and renewable raw materials, e.g.,
amino alcohols (choline2 and ephedrine3), hydroxy acids (lactic
and tartaric acids4), amino acids,5 and terpenes,6 is becoming
35 more reasonable compared with the use of compounds derived
from fossil feed stocks.
Sugars appear to be suitable precursors for ionic liquids
because they are among the most abundant and relatively
inexpensive naturally occurring substances available. Research
40 involving sugars for the preparation of ionic liquids has been
limited and has only recently come to the forefront.7 The
following ionic liquids with a sugar moiety as part of the cation
have been reported: ionic liquids derived from methyl Dꢀ
glucopyranose,8 glucoseꢀtagged triazolium ionic liquids,9 ionic
45 liquids obtained from isosorbide,10 monoꢀ and bisꢀammonium
ionic liquids from isomannide,11 and ionic liquids with a
The number of published articles related to the application of
sugarꢀbased ionic liquids in DielsꢀAlder reactions is limited.
However, numerous studies on Diels–Alder reactions in other
70 ionic liquids have shown that both the solvent and the dienophile
structure influence the reaction rate and selectivity.16 The
coordination of the ionic liquid cation to dienophile decreases the
gap between diene HOMO and dienophile LUMO leading to an
increased chemical reactivity.17 The interactions between
75 characteristic groups on the dienophile and ions from the ionic
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DOI: 10.1039/C4GC00380B
liquids have been described.16 The selectivity of Diels–Alder
reactions in ionic liquids appears to be dependent on the
hydrogenꢀbond donor capacity of the ionic liquid; the ions can
the crude product was dissolved in methanol, filtrated,
concentrated under reduced pressure, and purified by column
chromatography (methanol
:
chloroform (3:7)) to give
stabilise the transition state of the Diels–Alder reaction by 60 chloroalkyl glycosides III aꢀc as creamy solids in yields of 50ꢀ
5
forming hydrogen bonds, e.g., with the carbonyl oxygen in the
dienophile. Furthermore, ionic liquids possessing weakly
coordinating bis(trifluoromethylsulfonyl)imide (bistriflamide,
[Tf2N]) anions are one of the most active ionic liquids for the
DielsꢀAlder reaction.
Inspired by the literature, we decided to design new hydrogenꢀ
bondꢀrich ionic liquids based on Dꢀglucopyranoside derivatives
as the cation precursor and the bistriflamide anion. The potential
of these compounds to act as organocatalysts in a DielsꢀAlder
78%, depending on the structure of the chloroalcohol.
2-chloroethyl D-glucopyranoside: α:β= 0.7:0.3; HꢀNMR (400
1
MHz, CD3OD): δ 4.84 (d, J = 3.8 Hz, 0.7 H, Hꢀ1α), 4.32 (d, J =
7.8 Hz, 0.3 H, Hꢀ1β), 4.07 (dt, J = 11.2, 6.0 Hz, 0.3H,
65 OCHHCH2Clβ), 3.94 (dt, J=11.2, 5.6 Hz, 0.7H, OCHHCH2Clα),
3.96ꢀ3.22 (m, 8 H, OCH2CH2Cl, Hꢀ3, Hꢀ4, Hꢀ5, Hꢀ6a,b), 3.39
(dd, J = 8.0, 3.8 Hz, 0.7 H, Hꢀ2α), 3.19 (dd, J = 8.8, 7.8 Hz, 0.3
H, Hꢀ2β); 13CꢀNMR (100 MHz, CD3OD): δ 104.52 (Cꢀ1β),
100.53 (Cꢀ1α), 77.96, 77.93, 74.94, 73.90 (2C), 73.44, 71.63,
10
reaction was demonstrated. This study expands the chemistry of 70 71.50, 70.92, 69.71, 62.68, 62.56, 43.73, 43.59; ESIꢀMS: [M +
15 sugarꢀbased ionic liquids, which is currently in its infancy.
Na]+ calcd: 265.0455, found: 265.0437.
3-chloropropyl D-glucopyranoside: α:β= 0.7:0.3; 1HꢀNMR (400
MHz, CD3OD): δ 4.78 (d, J = 3.8 Hz, 0.7 H, Hꢀ1α), 4.26 (d, J =
7.8 Hz, 0.3 H, Hꢀ1β), 4.00 (dt, J=8.0, 6.0 Hz, 0.3 H,
75 OCHHCH2CH2Clβ), 3.89 (ddd, J=11.0, 7.2, 5.2 Hz, 0.7 H,
OCHHCH2Clα), 3.88ꢀ3.25 (m, 8 H, OCH2CH2CH2Cl, Hꢀ3, Hꢀ4,
Hꢀ5, Hꢀ6a,b), 3.39 (dd, J=9.6, 3.8 Hz, 0.7 H, Hꢀ2α), 3.17 (t, J=
7.8 Hz, 0.3 H, Hꢀ2β), 2.16ꢀ1.98 (m, 2 H, OCH2CH2CH2Cl); 13Cꢀ
NMR (100 MHz, CD3OD): δ 104.47 (Cꢀ1β), 100.22 (Cꢀ1α),
80 78.01, 77.85, 75.05, 73.69, 73.51, 71.71, 71.59, 67.46, 65.53
(2C), 62.72, 62.57, 43.73, 42.67, 34.09, 33.69; ESIꢀMS: [M +
Na]+ calcd: 279.0611, found: 279.0612.
2. Experimental section
2.1. Materials and instrumentation
DꢀGlucose, chloroalcohols (2ꢀchloroethanol, 3ꢀchloropropanol,
and 3ꢀchloroꢀ1,2ꢀpropanediol), 1ꢀmethylꢀ3ꢀbutylimidazolium
20 chloride, sulfamic acid, trialkylamines, lithium bistriflamide, nꢀ
decane, and the dienophiles (diethyl maleate and methyl acrylate)
were commercial materials purchased from SigmaꢀAldrich.
Cyclopentadiene was obtained as a result of thermal cracking of
dicyclopentadiene, which was purchased from SigmaꢀAldrich.
25 The ionic liquid XIII were synthesised according to established
procedures.18
The structure and purity of all of the synthesised substances
were confirmed by spectral analysis. 1H NMR spectra were
recorded at 400 MHz and 13C NMR at 100 MHz (Varian Unity
30 Inova plus, using TMS as an internal standard). Highꢀresolution
electrospray ionisation mass spectroscopy (ESIꢀMS) experiments
were performed using a Waters Xevo G2 QTOF instrument
equipped with an injection system (cone voltage 50 V; source 120
°C). GC analyses were performed using a Perkin Elmer Clarus
35 500 chromatograph equipped with a SPBTMꢀ5 column (30 m ×
0.25 mm × 0.25 ꢁm); nꢀdecane was used as an internal standard.
Thermogravimetric analysis was performed with a Mettler
Toledo TGA/SDTA 851e/1100 analyser (temperature program
25–600 °C, 20 °C/min, N2 60 ml/min). Differential scanning
40 calorimetry was performed with a Mettler Toledo analyser
(temperature program ꢀ65–250 °C, 20 °C/min, N2 60 ml/min).
Fourierꢀtransform infrared absorption (FTꢀIR) spectra were
recorded with a Fourierꢀtransform infrared spectrometer (IC10,
MettlerꢀToledo) equipped with an ATR diamond probe and
45 operated in transmission mode.
3-chloro-2-hydroxypropyl D-glucopyranoside: α:β= 1:2; 1Hꢀ
NMR (400 MHz, CD3OD) (selected data): δ 5.08 (d, J = 3.9 Hz,
85 0,15 H, Hꢀ1α), 5.05 (d, J = 3.9 Hz, 0.2 H, Hꢀ1α’), 4.50 (d, J = 7.8
Hz, 0,15 H, Hꢀ1β), 4.34 (d, J = 7.8 Hz, 0.5 H, Hꢀ1β’), 4.12ꢀ3.24
(m, 11H, OCH2CH(OH)CH2Cl, Hꢀ2, Hꢀ3, Hꢀ4, Hꢀ5, Hꢀ6a,b); 13Cꢀ
NMR (100 MHz, CD3OD) (selected data): δ 104.76 (Cꢀ1β),
104.72 (Cꢀ1β’), 100.61 (Cꢀ1α), 100.36 (Cꢀ1α’) 46.99, 46.92,
90 46.79, 46.70 (4 OCH2CH2CH2Cl); ESIꢀMS: [M + Na]+ calcd:
295.0561, found: 295.0547.
General method for the synthesis of the tetraalkylammonium
chlorides: Into a twoꢀnecked 250 ml roundꢀbottom flask
equipped with condenser, magnetic stirrer and dropping funnel,
95 an ethanolic solution of trialkylamine (190 mmol) and 30 ml of
methanol were introduced. The flask was closed under an inert
gas atmosphere. The contents of the flask were cooled to 0 °C in
an ice bath, and the chloroalcohol or chloroalkyl glycoside (126.5
mmol) was added. The reaction was stirred at 65ꢀ68 °C for 10 h,
100 and the solvent was subsequently evaporated. The product was
washed five times with 50 ml of diethyl ether and seven times
with 50 ml of acetone until the white solid precipitated. After
drying under vacuum, the tetraalkylammonium chlorides were
obtained as white solids in satisfactory yields (52ꢀ92%).
2.2. General methods
General method for the Fischer glycosylation: Into a 250 ml
roundꢀbottom flask equipped with condenser and magnetic stirrer,
Dꢀglucose (58.9 mmol) and chloroalcohol (223.8 mmol) were
50 introduced. Sulfamic acid (14.2 mmol) was subsequently added at
room temperature. The reaction was stirred at 80 °C for 10 h.
After completion of the reaction, reaction mixture was
concentrated. Next 100 ml of acetone was added. The solvent was
decanted from the precipitate, and the precipitate was washed
55 three times with 10 ml of acetone. The acetone extracts were
collected, the solvent was removed under reduced pressure, and
105 N-[2-(D-glucopyranosyl)ethyl]-N,N,N-trimethylammonium
chloride (IVa): ESIꢀMS [M+] calcd: 266.1604, found: 266.1609.
N-[3-(D-glucopyranosyl)propyl]-N,N,N-trimethylammonium
chloride (IVb): ESIꢀMS [M+] calcd: 280.1760, found: 280.1764.
N-[3-(D-glucopyranosyl)-2-hydroksypropyl]-N,N,N-
110 trimethylammonium chloride (IVc): ESIꢀMS [M+] calcd:
296.1709 , found: 296.1708.
N-[(2,3-dihydroxy)prop-1-yl]-N,N,N-trimethylammonium
chloride: ESIꢀMS [M+] calcd: 134.1181, found: 134.1174.
N-[(2,3-dihydroxy)prop-1-yl]-N,N,N-triethylammonium chlo-
2
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ride: ESIꢀMS [M+] calcd: 176.1651, found: 176.1650.
N-[(2,3-dihydroxy)prop-1-yl]-N,N,N-tributylammonium chlo-
ride: ESIꢀMS [M+] calcd: 260.2590, found: 260.2589.
chromatography (hexane : ethyl acetate (5:1)) when necessary.
All products were characterised by comparison of their NMR
60 spectra with authentic samples.
General method for the synthesis of the tetraalkylammonium
bistriflamides (V-X): To a solution of tetraalkylammonium
chloride (37.0 mmol) in 2 ml of distilled water lithium
bistriflamide (42.7 mmol) in 2 ml of water was added. The
reaction mixture was stirred at room temperature for 24 h and
then extracted with dichloromethane or acetonitrile. The organic
5
3. Results and discussion
3.1. Synthesis of hydrogen-bond-rich ionic liquids
The goals of this study included the synthesis of ionic liquids
possessing hydroxyl groups in their structure. To design the
10 layer was dried over anhydrous MgSO4. The solution was 65 proper ionic liquids, we selected Dꢀglucose as the raw material.
concentrated under reduced pressure to afford the bistriflamide
salts in 70ꢀ95% yield.
N-[2-(D-glucopyranosyl)ethyl]-N,N,N-trimethylammonium
bistriflamide (V): ESIꢀMS [M+] calcd: 266.1604, found:
15 266.1610, [Mꢀ] calcd: 279.9173, found: 279.9178.
N-[3-(D-glucopyranosyl)propyl]-N,N,N-trimethylammonium
bistriflamide (VI): ESIꢀMS [M+] calcd: 280.1760, found:
280.1765, [Mꢀ] calcd: 279.9173, found: 279.9175.
We used the modified procedure described elsewhere (Scheme
1).19
HO
OH
(I)
IIIa, yield=78%; α:β= 0.7:0.3
IIIb, yield=73%; α:β= 0.7:0.3
IIIc, yield=50%; α:β= 1:2
(IIa-c)
HO
OH
(IIIa-c)
O
O
S
O
O
OH
H2N
HO
OH
O
H2O
OH
OH
Cl
5h, 80°C
Cl
IIa 2-chloroethanol
IIb 3-chloropropanol
IIc 3-chloro-1,2-propanediol
OH
OH
H3C
CH3
CH3OH, N2
15h, 75°C
N
CH3
V, yield=70%
VI, yield=75%
VII, yield=82%
IVa, yield=81%
IVb, yield=78%
IVc, yield=88%
N-[3-(D-glucopyranosyl)-2-hydroksypropyl]-N,N,N-trimethyl-
20 ammonium bistriflamide (VII): ESIꢀMS: [M+] calcd: 296.1709 ,
found: 296.1708, [Mꢀ] calcd: 279.9173, found: 279.9179.
N-[(2,3-dihydroxy)prop-1-yl]-N,N,N-trimethylammonium bis-
(V-VII)
HO
HO
(IVa-c)
[Tf2N]-
O
Cl-
O
Li+[Tf2N]-
CH3
N+
CH3
N+
CH3
O
Li+Cl-
O
OH
OH
OH
CH3OH, RT, 24h
CH3
OH
H3C
H3
C
OH
OH
1
triflamide (VIII): HꢀNMR (400 MHz, CD3OD): 4.16 (m, 1H,
Scheme 1 The synthesis of the ionic liquids using Dꢀglucose as the raw
CHꢀOH), 3.56ꢀ3.32 (m, 4H, CH2), 3.24 (s, 9H, CH3). 13CꢀNMR
25 (100 MHz, CD3OD): 125.97, 122.74, 119.56, 116.37, 69.93,
67.97, 65.22, 54.98. ESIꢀMS [M+] calcd: 134.1181, found:
134.1174, [Mꢀ] calcd: 279.9173, found: 279.9178.
material
70
Glycosylation of Dꢀglucose (I) with the chloroalcohols: 2ꢀ
chloroethanol, 3ꢀchloropropanol, and 3ꢀchloroꢀ1,2ꢀpropanodiol
(II aꢀc) using sulfamic acid as the catalyst yielded the chloroalkyl
glycosides (III aꢀc) in high yields (50ꢀ78%) as a mixture of
glycosides with α:β ratio 0.7:0.3 in case of glucosides III a, III b
75 and 1:2 in case of glucosides III c (trace amount of disaccharide
and trisaccharide chloroalkyl derivatives were detected by MS
analysis, probably as a result of 1,6ꢀglycosidic bond formation in
the reaction conditions). Next, quaternarisation with
trimethylamine gave the ammonium chlorides (IV aꢀc), which
80 have been previously described in the literature.19,8 In this study,
we went one step further by completing the process via
metathesis of the chloride anion, which resulted in the
bistriflamide ionic liquids (VꢀVII). In contrast to the chlorides,
the new ionic liquids are liquids at room temperature.
N-[(2,3-dihydroxy)prop-1-yl]-N,N,N-triethylammonium bis-
1
triflamide (IX): HꢀNMR (400 MHz, CD3OD): 4.08 (m, 1H,
30 CHꢀOH), 3.60ꢀ3.31 (m, 10H, CH2), 1.32 (t, 9H, CH3). 13CꢀNMR
(100 MHz, CD3OD): 125.49, 122.73, 119.45, 116.34, 67.27,
65.25, 60.66, 54.83, 7.69. ESIꢀMS [M+] calcd: 176.1651, found:
176.1650, [Mꢀ] calcd: 279.9173, found: 279.9178.
N-[(2,3-dihydroxy)prop-1-yl]-N,N,N-tributylammonium bis-
35 triflamide (X): 1HꢀNMR (400 MHz, CD3OD): 4.06 (m, 1H, CHꢀ
OH), 3.59ꢀ3.22 (m, 10H, CH2), 1.82ꢀ1.56 (m, 6H, CH2), 1.50ꢀ
1.31 (m, 6H, CH2), 1.01 (m, 9H, CH3). 13CꢀNMR (100 MHz, CDꢀ
3OD): 125.98, 122.80, 119.61, 116.42, 67.38, 65.25, 62.21, 60.48,
24.75, 20.71, 13.86. ESIꢀMS: [M+] calcd: 260.2590, found:
40 260.2589, [Mꢀ] calcd: 279.9173, found: 279.9179.
FTꢀIR spectra of all new ionic liquids are available in
Supplementary Information section.
HO
HO
[Tf2N]-
[Tf2N]-
[Tf2N]-
O
HO
O
O
CH3
N+ CH3
O
OH
CH3
N+ CH3
CH3
N+
O
O
OH
OH
OH
CH3
CH3
General method for the Diels–Alder reactions: The reaction
was performed in 5 ml flasks equipped with a stirring bar and a
45 balloon filled with nitrogen. The dienophile (1.0 mmol) and the
ionic liquid (1.0 mmol) were introduced into the flask, followed
by the addition of diene (1.5 mmol) at room temperature. The
reaction mixture was subsequently stirred at room temperature for
5–120 min. The reaction yield and the endo:exo ratio were
50 determined by GC analysis with precision ± 1%. After adequate
period of time, the reaction was stopped by the addition of 5 ml
of dichloromethane. Next, 200 ꢀl of sample was taken, added to
1 ml of dichloromethane and analysed. Each point in figures 1ꢀ4
represents the result from independent reaction. In order to
55 identify, the products were isolated from the reaction mixture by
extraction with dichloromethane. Next, organic layer was
OH
OH
H3
C
CH3
OH
OH
OH
OH
(V) [GlcO(CH2
)
2N111][Tf2N]
[Tf2N]-
(VI) [GlcO(CH2
)
3N111][Tf2N]
(VII) [GlcOCH2CH(OH)CH2N111][Tf2N]
[Tf2N]-
[Tf2N]-
CH3
N+ CH3
R
N+
R
CH3
N+ CH3
H3C
N
H3
C
N+ CH3
HO
[Tf2N]-
HO
R
CH3
OH
CH3
[bmim][Tf2N]
(XIII)
[HO(CH2
)
3N111][Tf2N]
(VIII) [HOCH2CH(OH)CH2N111][Tf2N]
(IX) [HOCH2CH(OH)CH2N222][Tf2N]
(X) [HOCH2CH(OH)CH2N444][Tf2N]
[N1113][Tf2N]
(XII)
(XI)
85
Scheme 2 The ionic liquids used in this study
For purposes of comparison, we sought to prepare ionic liquids
with a few hydroxyl groups incorporated into the alkyl group of
the aglycone and therefore used chloroalcohols as the source of
the hydroxyl groups.20 Ionic liquids possessing two hydroxyl
90 groups and different hydrophobicity properties, such as the
trialkylꢀ2,3ꢀdihydroxypropylammonium bistriflamides (alkyl =
methyl, ethyl, butyl), were designed (VIIIꢀX). The synthesis was
concentrated in
a
vacuum and purified by column
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based on the quaternarisation of trialkylamine with an adequate
amount of chloroalcohol, followed by salt metathesis. In addition,
ionic liquids with one hydroxyl group, i.e., trimethylꢀ3ꢀ
hydroxypropylammonium bistriflamide (XI), and without the
DielsꢀAlder reaction compared to that achieved with ionic liquids
with neat alkyl chains. However, only one hydroxyl group was
considered in the application of trimethylhydroxyethylammonium
bistriflamide21
or
1ꢀhydroxyethylꢀ3ꢀmethylimidazolium
5
ability to form hydrogen bonds, i.e., trimethylpropylammonium 50 bistriflamide.22 Herein, we compare ionic liquids with different
bistriflamide
bistriflamide, were synthesised (XIII).
(XII)
and
1ꢀbutylꢀ3ꢀmethylimidazolium
structures to investigate how the number of hydroxyl groups
influences the reaction course.
The new ionic liquids were soluble in water, slightly soluble in
methanol and acetonitrile, and poorly soluble in ethyl acetate.
The reaction of cyclopentadiene with diethyl maleate was used
as a model reaction for the kinetic and stereochemical studies of a
10 The physicochemical properties of the obtained ionic liquids were 55 DielsꢀAlder reaction (Scheme 3).
determined. All of the Dꢀglucoseꢀbased ionic liquids were
mixtures of anomers with α:β ratio 0.7:0.3 (ionic liquid V, VI)
and 1:2 (ionic liquid VII). The structures of the new ionic liquids
were confirmed by highꢀresolution ESIꢀMS experiments. The
15 peaks observed in the positive and negative ESI modes confirmed
the presence of bistriflamide anions and an adequate cations in all
of the samples. Additionally, FTIR spectra of ionic liquids are
presented in the Supplementary Information.
Scheme 3 The model DielsꢀAlder reaction used in this study
In primary trials, the ratio of the ionic liquids (VI or VIII) to
60 the dienophile was equimolar (Fig. 1). Consequently, in the
presence of both of the ionic liquids, the complete conversion of
diethyl maleate and a 99% yield of the product with a high ratio
of the endo:exo isomers (13.1–13.5) was obtained immediately
after 5ꢀ10 min. Similar results were obtained for other Dꢀglucose
65 ionic liquids (V and VII), which were not depicted in Figure 1 for
clarity.
The metallic catalyst Yb(OTf)3 in the ionic liquid 1ꢀmethylꢀ3ꢀ
butylimidazolium bistriflamide [bmim][Tf2N] was also very
active. This catalyst is one the most active metallic catalysts used
70 for DielsꢀAlder reactions.23 The reaction was faster but resulted in
a lower endo:exo ratio (9.2). High endo:exo ratios (13.2) were
maintained for the reactions with one –OH group (ionic liquid
XI) and without the –OH group (ionic liquid XII, with a ratio of
13.0, and ionic liquid XIII, with a ratio of 12.5). These reactions
75 required longer reaction times, ranging from 1 h to several hours
for completion.
The influence of the ionic liquids with hydroxyl groups on the
reaction rate is evident, and a large reduction of the reaction time
(from 1 h to 5 min) was achieved by shifting from one to several
80 hydroxyl groups present in the structure of the Dꢀglucoseꢀbased
ionic liquids. Dꢀglucose alone does not catalyse this reaction what
confirms that catalytic activity is not only the result of the
presence of hydroxyl groups in the catalyst structure.
The influence of the length of the alkyl chain in
85 tetraalkylammonium bistriflamide with 2 hydroxyl groups is
presented in Figure 2. The increase in the alkyl chain length
resulted in longer reaction times and only slightly lower endo:exo
ratios (up to 12.5 for ionic liquid X). The effect of the decreased
endo selectivity of the product in the DielsꢀAlder reaction with
90 increasing alkyl chain length of the cation has been described in
the literature.16
The thermal data of the studied ionic liquids as measured by
20 differential scanning calorimetry are summarised in Table 1. The
ionic liquids were heated from ꢀ65 °C to 250 °C, held at 250 °C
before being cooled to ꢀ65 °C, heated again, and finally cooled to
ꢀ65 °C. All of the samples were liquids at temperatures well
below room temperature, and no melting points were detected.
25 These compounds tend to form glasses at temperatures in the
range of ꢀ29 °C to ꢀ16 °C. The data obtained from
thermogravimetric analysis of the ionic liquids are also shown in
Table 1. According to the experimental data, the ionic liquids are
thermally stable from ambient temperature to at least 430 °C for
30 both the tetraalkylammonium ionic liquid with hydroxyl groups
incorporated into the alkyl group of aglycone and the
tetraalkylammonium ionic liquid with Dꢀglucose attached to the
alkyl chain. Moreover, the absence of a glycosyl unit or a
hydroxyl group in the tetraalkylammonium ionic liquid does not
35 decrease the thermal stability of the obtained ionic liquids. This
high thermal stability is most likely due to the presence of the
bistriflamide anion.
Table 1 Thermal properties of the ionic liquids
b
c
Ionic liquid
Tga T50% onset Tdecomp
[°C]
[°C]
[°C]
V [GlcO(CH2)2N111][Tf2N]
ꢀ30
433
454
VI [GlcO(CH2)3N111][Tf2N]
ꢀ20
ꢀ14
ꢀ22
ꢀ39
ꢀ45
435
446
454
441
417
456
460
469
457
431
VII [GlcOCH2CH(OH)CH2N111][Tf2N]
VIII [HOCH2CH(OH)CH2N111][Tf2N]
IX [HOCH2CH(OH)CH2N222][Tf2N]
X [HOCH2CH(OH)CH2N444][Tf2N]
a Glass transition temperature. b Decomposition of 50% of the sample.
40 c Decomposition.
The application of ionic liquids with different structures in the
DielsꢀAlder reaction described in the literature is limited to the
application of the ionic liquid as the solvent in equimolar or
95 greater than equimolar amounts relative to the amount of
dienophile.16 In this study, the possibility of the effective
application of catalytic amounts (up to 4 mol% with respect to the
dienophile) of ionic liquids as organocatalysts was evaluated
(Fig. 3).
3.2. Application of the ionic liquids as organocatalysts in the
Diels-Alder reaction
According to the literature, the presence of strongly interacting
groups, such as hydroxyl, carboxyl, nitrile, or benzyl groups, in
45 the ionic liquid cation structure increases the selectivity of the
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DOI: 10.1039/C4GC00380B
amounts relative to the amount of dienophile. The positive effect
of the hydroxyl groups on the reaction rate and the selectivity is
35 clearly visible in Figure 4. Using chiral column we also checked
the possibility of the asymmetric induction in this reaction but
unfortunately racemic forms of endo and exo products were
detected. The best selectivity (7.1) was obtained with the Dꢀ
glucoseꢀbased ionic liquid (V) and with the tetraalkylammonium
40 ionic liquid (7.0) possessing two hydroxyl groups in its structure
(VIII). After 2 h, full conversion of the dienophile was achieved.
The reactions in which other ionic liquids were used as the
reaction medium were slower, and a decrease in the selectivity
was observed (4.2ꢀ5.5).
100
80
60
40
20
0
endo/exo
9.2
XIII+Yb(OTf)3
VI
VIII
XI
13.5
13.1
13.2
CH2Cl2+Yb(OTf)3
9.1
XIII
XII
12.5
13.0
CH2Cl2+ D-glucose 3.1
0
20
40
60
100
Time [min]
Fig. 1 The influence of the amount of hydroxyl groups present in the
ionic liquid (1 mmol) structure on the reaction rate and the selectivity for
the reaction of cyclopentadiene (1.5 mmol) with diethyl maleate
(1 mmol).
80
endo/exo
V
VI
13.0
13.1
9.1
5
CH2Cl2+Yb(OTf)3
60
40
20
0
VII
VIII
XI
XII
XIII
12.8
12.2
4.7
4.3
3.1
100
80
60
endo/exo
0
20
40
60
80
100
120
VIII
IX
X
13.1
12.7
12.5
40
20
0
Time [min]
45
Fig. 3 The influence of catalytic amounts of the ionic liquids (0.04 mmol)
on the rate and selectivity for the reaction of cyclopentadiene (1.5 mmol)
with diethyl maleate (1 mmol).
0
20
40
60
Time [min]
Fig. 2 The influence of the length of the alkyl chain in the structure of
tetraalkylammonium ionic liquids (1 mmol) on the reaction rate and
selectivity for the reaction of cyclopentadiene (1.5 mmol) with diethyl
maleate (1 mmol).
50
Scheme 4 The reaction of cyclopentadiene with methyl acrylate.
10
100
endo/exo
All of the Dꢀglucoseꢀbased ionic liquids and the ionic liquid
with two hydroxyl groups (VIII) were active in the reaction,
whereas the other ionic liquids possessing one ꢀOH group (XI) or
without an ꢀOH group in their structure (XII and XIII) lost their
15 activity, and longer reaction times were required to achieve full
conversion of the dienophile. The metallic catalyst in
dichloromethane was as active as the Dꢀglucose based catalyst. In
addition, the difference in the reaction rates among the Dꢀ
glucoseꢀbased ionic liquids could be observed. The activity
20 decreased in the order V > VI > VII. The longer alkyl chain
linking the Dꢀglucose unit with the tetramethylammonium cation
resulted in longer reaction times to reach full conversion of the
dienophile, with minor differences in selectivity.
V
7.1
7.0
5.5
5.4
4.2
VIII
XI
80
60
40
20
0
XII
XIII
0
20
40
60
80
100
120
Time [min]
The influence of selected ionic liquids on the reaction rate and
25 the product distribution in the reaction of cyclopentadiene with
methyl acrylate was also evaluated (Scheme 4). In the literature,
the highest isomer endo:exo ratio (5.25ꢀ19) was achieved using
chloroaluminate ionic liquids.16 Many other nonꢀmetallic ionic
liquids were tested and resulted in lower endo:exo ratios (4.8ꢀ
30 6.7).
Fig. 4 The influence of the number of hydroxyl groups present in the
ionic liquid (1 mmol) structure on the reaction rate and the selectivity for
the reaction of cyclopentadiene (1.5 mmol) with methyl acrylate (1
mmol).
55
The cycloaddition of cyclopentadiene to diethyl maleate or
methyl acrylate is one of the most studied models for the Diels–
Alder reaction and for reactions using ionic liquids as the reaction
medium.16 In the literature, extensive studies using empirical
In this study, hydrogenꢀbondꢀrich (V, VIII, XI) and hydrogenꢀ
bondꢀpoor (XII, XIII) ionic liquids were used in equimolar
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DOI: 10.1039/C4GC00380B
Table 2 Dꢀglucoseꢀbased ionic liquid V (1 mmol) as catalyst in the Dielsꢀ
Alder reaction of various dienes (1.5 mmol) and dienophiles (1 mmol)
parameters and theoretical calculations were performed to
investigate the correlation of the endo:exo selectivities and
kinetic constants to the solvent properties. Both the cation and the
anion played important roles in the reaction mechanism. In
summary, the factors that can influence the selectivity of the
DielsꢀAlder reaction include the ability to form hydrogen bonds
and the dipolarity/polarisability of the ionic liquid. In addition,
the solvophobic effect can influence the selectivity of the
reaction.
In this study, the presence of a carbonyl acceptor group in the
structure of diethyl maleate or methyl acrylate enables hydrogenꢀ
bond interactions with the ionic liquid, thereby enhancing the
endo selectivity. The course of the reaction of cyclopentadiene
with methyl acrylate was monitored using FTꢀIR spectroscopy;
15 the spectrum of the reaction mixture (cyclopentadiene with
diethyl maleate in the presence of the Dꢀglucoseꢀbased ionic
liquid VI) was taken with the IR probe every 15 s. In Figure 5
spectrum of reaction mixture (after 10, 20, 30 and 60 min.) and
for comparison spectrum of substrates was presented. In general,
20 the entire reaction mixture spectrum is complicated (see the
Supplementary Information). The appearance of a 3400ꢀ3600
[cmꢀ1] band during the reaction, which is characteristic of
hydrogen bonds and the decline of signal from double bond from
diethyl maleate were detected. However this is not a clear
25 evidence for the presence of C=O…H bonds because the Dꢀ
glucoseꢀbased ionic liquids themselves could also form HO…HO
hydrogenꢀbonds (see the Supplementary Information).
Dienophile
Diene
Product Time Yield endo: exo
[h] [%]
cyclopentadiene methacrolein
1
2
3
4
5
6
7
8
1
10
1
1
1
1
1
1
96
85
92
90
96
95
98
99
8.0 : 1
8.2 : 1
1.9 : 1
1.8 : 1
ꢀ
ꢀ
ꢀ
ꢀ
5
cyclopentadiene methylꢀvinyl ketone
cyclopentadiene methyl methacrylate
cyclopentadiene acrylonitrile
cyclohexadiene 1,4ꢀbenzoquinone
cyclohexadiene maleic anhydride
isoprene
isoprene
1,4ꢀbenzoquinone
maleic anhydride
10
CH3
COOCH3
CHO
O
CN
COCH3
2
CH3
3
4
1
O
O
O
O
O
CH3
CH3
O
O
6
O
7
O
5
8
Scheme 5 Products of the reaction of cyclopentadiene and isoprene with
45
various dienophiles.
Conclusions
The possibility of using a renewable and relatively inexpensive
starting material, such as Dꢀglucose, as the target ionic liquid
cation precursor was investigated in this study. In addition,
50 chloroalcohols served as a source of hydroxyl groups for the
construction of the ionic liquid cation. The resulting ionic liquids
were liquids at room temperature because of the presence of the
weakly coordinating bistriflamide anion. The physicochemical
properties of the studied ionic liquids were determined. The
55 thermogravimetric analysis results revealed that the ionic liquids
are thermally stable at temperatures up to 430 °C.
The resulting ionic liquids are able to create a large number of
hydrogen bonds and were used in the DielsꢀAlder reaction of
cyclopentadiene and diethyl maleate. The first approach used
60 equimolar amounts of the ionic liquid with respect to the amount
of dienophile, and showed high activity for the studied ionic
liquids. The new Dꢀglucoseꢀbased ionic liquids were as active as
the metal catalyst with respect to the reaction rates but were
significantly more active than the other ionic liquids tested. The
65 endo selectivity was high in all cases (around 13). An increase in
the length of the alkyl chain in the tetraalkylammonium ionic
liquids caused a decrease in the reaction rate and a slight decrease
in the endo:exo ratio. Similar results were obtained when methyl
acrylate was used as the dienophile.
Fig. 5 FTꢀIR spectra of the reaction course of cyclopentadiene (5 mmol)
with diethyl maleate (5 mmol) in the presence of the Dꢀglucoseꢀbased
ionic liquid VI in catalytic amounts (0.20 mmol).
30
Finally, the most active Dꢀglucoseꢀbased ionic liquid V was
examined in the DielsꢀAlder reaction of various dienes and
dienophiles to determine its practical potentials. As can be seen
35 from Table 2 and scheme 5 the cyclization processes proceeded
very efficiently; in fact, products were readily obtained in high
yields under mild conditions within short reaction times.
70
Encouraged by the successful outcome of the DielsꢀAlder
reaction with hydrogenꢀbondꢀrich ionic liquids, we decided to use
catalytic amounts (4 mol% with respect to the dienophile) of the
ionic liquids, as organocatalysts. With this approach, only the Dꢀ
glucoseꢀbased ionic liquids and the tetraalkylammonium ionic
75 liquid with two –OH groups maintained their high activity, as
indicated by the reaction rate and the endo selectivity.
40
The catalytic system proved to be active in the DielsꢀAlder
reaction of various dienes and dienophiles carried out in the
presence of the most active Dꢀglucoseꢀbased ionic liquid.
80
In conclusion, this study showed that the Dꢀglucoseꢀbased
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DOI: 10.1039/C4GC00380B
ionic liquids are efficient catalysts for DielsꢀAlder reactions.
Acknowledgements
This work was financed by the Polish National Science Centre
5
(Grant no. UMOꢀ2012/06/M/ST8/00030).
Notes and references
a Silesian University of Technology, Faculty of Chemistry,
ul. Krzywoustego 4, 44-100 Gliwice, Poland. Fax: 48 3223 71032;
Tel: 48 3223 72917; E-mail: anna.chrobok@polsl.pl
10 † Electronic Supplementary Information (ESI) available: [FTꢀIR spectra
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