Chiral protic imidazolium salts with a (-)-menthol fragment in the cation: Synthesis, properties and use in the Diels-Alder reaction

Article abstract of DOI:10.1039/c7ra12176h

New chiral protic imidazolium salts containing a (1R,2S,5R)-(-)-menthol substituent in the cation and four different anions (chloride, hexafluorophosphate, trifluoromethanesulfonate and bis(trifluoromethylsulfonyl)imide) were efficiently prepared and exte

Full text of DOI:10.1039/c7ra12176h

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Chiral protic imidazolium salts with a (ꢀ)-menthol
fragment in the cation: synthesis, properties and
use in the Diels–Alder reaction†
Cite this: RSC Adv., 2018, 8, 10318
a
b
Marcin Gano,a Joanna Feder-Kubis
and Jacek Sosnickic
*
Ewa Janus,
´
New chiral protic imidazolium salts containing a (1R,2S,5R)-(ꢀ)-menthol substituent in the cation and four
different anions (chloride, hexafluorophosphate, trifluoromethanesulfonate and bis(trifluoromethylsulfonyl)
imide) were efficiently prepared and extensively characterized. Detailed NMR analysis was performed, and
a comparison of the chemical shifts of the protons and carbons of the imidazolium cation as a function of
the combined anion was discussed. The specific rotation, solubility in commonly used solvents, thermal
properties including phase transition temperatures, and thermal stability were also determined. Three of
the synthesized tertiary salts (Cl, PF6 or OTf anion) were crystalline solids; 1-H-3-[(1R,2S,5R)-
(ꢀ)-menthoxymethyl]-imidazolium bis(trifluoromethylsulfonyl)imide, (ꢀ)[H-Ment-Im][NTf2] was a liquid at
room temperature. The chiral protic salts were used in a Diels–Alder reaction as a test reaction, and the
results were compared with those from aprotic chiral ionic liquids having the same chiral substituent in
the cation (1R,2S,5R)-(ꢀ)-menthol with a bis(trifluoromethylsulfonyl)imide anion. Both protic and aprotic
chiral salts, used in a Diels–Alder reaction, were pure (ꢀ)-enantiomers, which was determined by NMR
with D-TRISPHAT tetrabutylammonium salt as a chiral shift reagent. Protic salts offered distinctly higher
endo/exo ratios than aprotic ones, but an enantiomeric excess was not obtained. The stereoselectivity
reached the same high level even after the fourth recycle of (ꢀ)[H-Ment-Im][NTf2] in the reaction of
ethyl-vinyl ketone with cyclopentadiene at temperature of ꢀ35 ꢁC.
Received 6th November 2017
Accepted 5th March 2018
DOI: 10.1039/c7ra12176h
rsc.li/rsc-advances
mainly of natural origin.5–9 However, chiral cations in ionic
liquids are less common than chiral anions because the
Introduction
ꢁ
synthesis of chiral cations typically requires several steps.
There are only a few reports of protic ionic liquids with
chirality in a cation and a proton located on the nitrogen
atom.10–15 In contrast, protic ionic liquids (PILs) are particularly
important ILs family. They are a hydrogen bond donor,16,17 and
their catalytic utility is accompanied by hydrogen bonding
interactions and Brønsted acidity18,19 that are crucial in many
organic reactions. Many articles and reviews imply the potential
of protic ionic liquids for catalysis and organic synthesis,20,21 as
well as electrochemistry,22,23 chromatography,24 biomass
conversion,25 biocatalysis,26 and protein stabilization. They also
offer a simple synthesis via a combination of amines with
Brønsted acids and the possibility for acidity alterations.18,27
Considering this, the investigation of PILs shown here
evaluates chiral cations and is of great interest because it offers
progress in the context of developing catalysis and organic
synthesis using chiral protic salts as catalysts and solvents.
Therefore, we focus here on the synthesis and properties of
chiral protic imidazolium salts with chirality in the cation.
Three optically active centers in the molecules are associated
with the presence of natural raw material: (1R,2S,5R)-
(ꢀ)-menthol, which belong to the monocyclic monoterpenes.
We also studied the utility of synthesized protic chiral salts as
Ionic liquids (ILs) are salts with melting points below 100 C.
They are very attractive materials distinguished by many factors
including a liquid state across a range of temperatures with
extremely low volatility and the ability to tune viscosity, solva-
tion, polarity, polarizability, and proton-donating/accepting
properties. Therefore, ionic liquids are extensively used in
organic synthesis where they act as alternative solvents and/or
catalysts for new opportunities to conduct both uncatalyzed
and catalyzed processes.1 The chiral version of ionic liquids
with chirality in a cation or anion provides simple access to
chiral solvents. Due to their ionic and highly organized nature,
they are attractive in asymmetric synthesis.2–4 Chiral ionic
liquids have been synthesized based on different raw materials
aWest Pomeranian University of Technology Szczecin, Faculty of Chemical Technology
and Engineering, Institute of Chemical Organic Technology, Pułaski Str. 10, 70-322
Szczecin, Poland. E-mail: ejanus@zut.edu.pl; Tel: +48 91 4494584
b
_
Wrocław University of Science and Technology, Faculty of Chemistry, Wybrzeze
´
Wyspianskiego 27, 50-370 Wrocław, Poland
cWest Pomeranian University of Technology Szczecin, Faculty of Chemical Technology
´
and Engineering, Department of Organic and Physical Chemistry, Al. Piastow 42, 71-
065 Szczecin, Poland
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra12176h
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organocatalysts in the representative Diels–Alder reaction and
1H NMR (CDCl3, 400 MHz, 25 ꢁC) d [ppm]: 0.49 (d, J ¼ 7.0 Hz;
compared the results with those obtained from aprotic analogs 3H; H9 or H10); 0.81–0.99 (m, 9H with two doublets inside at
composed of the same chiral (1R,2S,5R)-(ꢀ)-menthoxymethyl 0.86 (d, J ¼ 7.2 Hz) and 0.92 (d, J ¼ 6.4 Hz); Ha-4, H7, H9 or H10,
fragment in the cation.
Ha-6 and Ha-3); 1.21–1.28 (m, 1H; H2); 1.36–1.47 (m, 1H; H5);
The Diels–Alder reaction was one of the most noteworthy 1.59–1.67 (m, 2H; Hb-3 and Hb-4); 1.96 (sept d, J ¼ 7.1 Hz and J
because it has great practical applications in the synthesis of ¼ 2.6, 1H; H8); 2.08–2.11 (m, 1H; Hb-6); 3.35 (td, J ¼ 10.6 Hz and
cyclic compounds, which are very oen impossible or difficult J ¼ 4.3 Hz, 1H; H1); 5.65 and 5.89 (d, J ¼ 10,6 Hz, 2H, AB system,
to obtain through other approaches. Oen, it is only one of the H11); 7.33 (t, J ¼ 1.6 Hz, 1H; H12), 7.43 (t, J ¼ 1.6 Hz, 1H; H13);
13
ꢁ
many stages of multistage synthesis including the synthesis of 10.0 (s, 1H; H14). C NMR (CDCl3, 100 MHz; 25 C) d [ppm]:
natural products and biologically active substances in which it 15.67 (C9 or C10); 21.05 (C7); 22.25 (C9 or C10); 22.91 (C3); 25.53
is crucial to obtain an optically pure enantiomer. Therefore, the (C8); 31.31 (C5); 34.14 (C4); 40.30 (C6); 47.87 (C2); 76.64 (C11);
Diels–Alder reaction is a powerful tool in chemists' hands and 79.58 (C1); 119.95 (C13); 120.32 (C12); 135.83 (C14). HRMS
an attractive approach to testing different ionic liquids.
(ESI+): m/z (%) calcd for C14H25N2O: 237.1967, found: 237.1958.
Elemental analysis calc. (%) for C14H25N2OCl (272.81): C 61.635,
H 9.24, N 10.27, found: C 61.54, H 9.37, N 10.30.
Experimental
All materials to the synthesis of chiral protic and aprotic salts,
their purication methods and general preparation methods of
substrates and aprotic chiral salts as well as analytical methods
used to their characterization were included to ESI.†
General procedure for metathesis reaction
The metathesis reaction was conducted in aqueous solution. A
saturated aqueous solution of the respective alkali salt NaOTf,
KPF6, or LiNTf2 (0.120 mol) was added to an aqueous solution of
prepared 1-H-3-[(1R,2S,5R)-(ꢀ)-menthoxymethyl]imidazolium
chloride (27.28 g, 0.100 mol). The reaction mixture was stirred
Materials used in Diels–Alder reaction
at room temperature for 24
h
to produce tri-
Dicyclopentadiene, methyl acrylate (99%), pent-1-en-3-on (ethyl-
vinyl ketone) (97%), were provided from Aldrich. Methyl acry-
late prior to use was sequentially washed with aqueous NaOH
and distilled water. Then it was dried with CaCl2 and fraction-
ally distilled under reduced pressure. It was stored under
nitrogen at 0 ꢁC in dark. Ethyl-vinyl ketone was dried with
MgSO4 and distilled.
uoromethanesulfonate and hexauorophosphate salts, while
bis(triuoromethanesulfonyl)imide was obtained by stirring for
two days. The crude product was then separated—the prepared
salt was precipitated from the reaction mixture as a solid and
ltered. Liquid product was phase-separated. Each protic salt
was washed with distilled water thrice to eliminate the chloride
formed during the metathesis reaction. Each salt was then
dissolved in acetone and stirred for 30 minutes to precipitate
any halide salt residues. These were ltered with a 0.2 mm lter.
To complete the inorganic salt precipitation, the ltrate was
placed at ꢀ5 ꢁC overnight. Another ltration was performed for
precipitation the halide salt, and the nal ltrate was evapo-
rated. Finally, tests with AgNO3 were carried out to determine
whether any chloride remained in the resulting protic salts. The
products were dried for 12 h at 60 ꢁC in a vacuum (0.3 mmHg).
Aerwards, the solid salts were crystallized.
Synthesis of protic imidazolium salts
The 1-H-3-[(1R,2S,5R)-(ꢀ)-menthoxymethyl]imidazolium chlo-
ride, (ꢀ)[H-Ment-Im][Cl] was synthesized following a modied
procedure described elsewhere.28 The 1-(1R,2S,5R)-(ꢀ)-men-
thoxymethylimidazole (70.80 g; 0.3 mol) was introduced into
a glass reactor with a cooling water system mechanical stirring,
reux condenser, and thermometer. A hydrochloric acid solu-
tion 37% (29.60 g; 0.3 mol) was added dropwise to the chiral
substrate under vigorous stirring using a dropping funnel
(approx. 30–45 min.). The process was conducted under
1-H-3-[(1R,2S,5R)-(ꢀ)-Menthoxymethyl]imidazolium
triuoromethanesulfonate, (ꢀ)[H-Ment-Im][OTf]
ꢁ
isothermal conditions at 15–20 C because the acid–base reac-
tion was slightly exothermic. Stirring was continued for 3 hours
at room temperature aer addition of the acid to the 1- Yield: 95.5%. 1H NMR (CDCl3, 400 MHz, 25 ꢁC) d [ppm] ¼ 0.50
(1R,2S,5R)-(ꢀ)-menthoxymethylimidazole.
(d, J ¼ 7.0 Hz, 3H; H9 or H10); 0.83–1.00 (m with two doublets
Aerward, 1,2-dichloroethane (DCE) was added (approx. 300 inside at 0.87 (d, J ¼ 7.2 Hz) and 0.92 (d, J ¼ 6.4 Hz); 9H; Ha-4,
g), and the mixture was distilled under normal pressure until H7, H9 or H10, Ha-6 and Ha-3); 1.23–1.29 (m, 1H, H2); 1.35–1.42
the water–DCE hetero-azeotropic boiling point was reached (73 (m, 1H; H5); 1.61–1.67 (m, 2H, Hb-3 and Hb-4); 1.91–2.00 (m,
ꢁC). Subsequently, the residual DCE was nally evaporated 2H, Hb-6 and H8); 3.27 (td, J ¼ 10.6 Hz and J ¼ 4.3 Hz, 1H; H1);
under reduced pressure to yield a white solid chloride. The 5.57 and 5.67 (d, J ¼ 10,6 Hz, 2H, AB system, H11); 7.36 (t, J ¼
product was crystallized from a mixture of hexane/acetone and 1.7 Hz, 1H; H12); 7.50 (t, J ¼ 1.7 Hz, 1H; H13); 9.12 (s, 1H, H14).
dried in a vacuum drier (0.3 mmHg). The 1-H-3-[(1R,2S,5R)- 13C NMR (CDCl3, 100 MHz; 25 ꢁC) d [ppm] ¼ 15.37 (C9 or C10);
(ꢀ)-menthoxymethyl]imidazolium chloride was prepared with 20.90 (C7); 22.01 (C9 or C10); 22.78 (C3); 25.43 (C8); 31.20 (C5);
93.0% yield (76.4 g; 0.28 mol). In the same way the 1-H-3- 33.97 (C4); 40.06 (C6); 47.70 (C2); 76.90 (C11); 79.96 (C1); 120.20
[(ꢂ)-menthoxymethyl]imidazolium chloride was prepared with (C13); 120.88 (C12); 135.31 (C14). Elemental analysis calc. (%)
90% yield from (ꢂ)-menthoxymethylimidazole as a starting for C15H25F3N2O4S (386.43): C 46.62, H 6.52, N 7.25, found: C
material.
46.75, H 6.66, N 7.16.
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column. The chiral column RT-BetaDEXsa was also used to
check for the enantiomeric excess. Calculations of ee% were
based on the surface area of the respective peaks. A represen-
tative chromatogram is presented in the ESI.†
1-H-3-[(1R,2S,5R)-(ꢀ)-Menthoxymethyl]imidazolium
bis(triuoromethanesulfonyl)imide, (ꢀ)[H-Ment-Im][NTf2]
Yield: 93.7%. 1H NMR (CDCl3, 600 MHz, 25 ꢁC) d [ppm] ¼ 0.51
(d, J ¼ 6.6 Hz, 3H, H9 or H10); 0.80–0.97 (m with two doublets
inside at 0.87 (d, J ¼ 7.0 Hz) and 0.91 (d, J ¼ 6.8 Hz); 9H; Ha-4,
H7, H9 or H10, Ha-6 and Ha-3); 1.20–1.27 (m, 1H; H2); 1.38 (m,
1H; H5); 1.62–1.67 (m, 2H; Hb-3 and Hb-4); 1.95–1.97 (m, 2H,
Hb-6 and H8); 3.25 (td, J ¼ 10.6 Hz, J ¼ 4.2 Hz, 1H; H1); 5.57 and
5.62 (d, J ¼ 10.6 Hz and J ¼ 10.8 Hz, 2H, AB system, H11); 7.45 (s,
1H, H12); 7.50 (s, 1H, H13); 8.75 (s, 1H, H14); 9–13 (br s, 1H,
H15). 13C NMR (CDCl3, 150.2 MHz, 25 ꢁC) d [ppm] ¼ 15.4 (C9 or
C10); 20.96 (C7); 22.05 (C9 or C10); 22.9 (C3); 25.6 (C8); 31.3
(C5); 34.1 (C4); 40.2 (C6); 47.8 (C2); 76.6 (C11); 80.4 (C1); 120.75
(C13); 121.04 (C12); 134.49 (C14); anion: 116.57–122.95 (quartet,
JC–F ¼ 320 Hz). 15N HNMR (CDCl3) d [ppm] ¼ ꢀ208.59 (N–H);
ꢀ189.88. Elemental analysis calc. (%) for C16H25F6N3O5S2
(517.51): C 37.13, H 4.87, N 8.12, found: C 37.29, H 4.98, N 8.01.
When the reaction was conducted in chloroform, 0.2 mmol
of protic chiral salt was rst dissolved in 0.5 mL of chloroform.
Then, 2 mmol of freshly cracked cyclopentadiene was added
followed by 1 mmol of methyl acrylate. The yield of product and
endo/exo ratio were determined based on the 1H NMR spectrum
registered for a reaction mixture. The proton integration data
for the methyl ester group of the methyl acrylate and the
products (2-endo-bicyclo[2.2.1]hept-5-ene-2-carboxylic acid
methyl ester and 2-exo-bicyclo[2.2.1]hept-5-ene-2-carboxylic
acid methyl ester) were used to calculate yield and endo : exo
ratio. A chloroform was removed from taken samples (under
vacuum), and residue was extracted with hexane. The hexane
extract was analyzed by GC on a chiral column.
Gas chromatograph Model 8000 Top Series (ThermoQuest
Italia S.p.A) was equipped with a ame ionization detector and
a 30 m ꢄ 0.53 mm ID ꢄ 1.50 mm df Rxi®-17 column (Restek) or
60 m ꢄ 0.25 mm ID ꢄ 0.25 mm df Stabilwax® (Restek) column
or 30 m ꢄ 0.32 mm ID ꢄ 0.25 mm df RT-BetaDEXsa® chiral
column (Restek) for GC analysis. Hydrogen or helium were used
as the carrier gas.
1-H-3-[(ꢂ)-menthoxymethyl]imidazolium
bis(tri-
uoromethanesulfonyl)imide, (ꢂ)[H-Ment-Im][NTf2] was also
synthesized from 1-H-3-[(ꢂ)-menthoxymethyl]imidazolium
chloride with 92% yield.
1-H-3-[(1R,2S,5R)-(ꢀ)-Menthoxymethyl]imidazolium
hexauorophosphate, (ꢀ)[H-Ment-Im][PF6]
Yield: 96.9%. 1H NMR (CDCl3, 400 MHz; 25 ꢁC) d [ppm] ¼ 0.50
(d, J ¼ 7.0 Hz, 3H; H9 or H10); 0.77–0.98 (m with two doublets
inside at 0.86 (d, J ¼ 7.2 Hz) and 0.91 (d, J ¼ 7.0 Hz); 9H; Ha-4,
H7, H9 or H10, Ha-6 and Ha-3); 1.23–1.29 (m, 1H, H2); 1.40 (m,
1H, H5); 1.60–1.67 (m, 2H; Hb-3 and Hb-4); 1.92–1.99 (m, 2H;
Hb-6 and H8); 3.27 (td, J ¼ 10.6 Hz, J ¼ 4.3 Hz, 1H, H1); 5.55 and
5.61 (d, J ¼ 10.8 Hz, 2H, AB system, H11); 7.37 (t, J ¼ 1.7 Hz, 1H;
H12); 7.46 (t, J ¼ 1.7 Hz, 1H; H13); 8.69 (s, 1H; H14); ꢃ12–16
ATR-FTIR measurements
ATR-FTIR spectra were recorded on a Bruker Alpha Fourier
Transform IR (FTIR) spectrometer equipped with a platinum
ATR single reection diamond-sampling module (Bruker
Optics). The infrared spectra were collected as an average of 24
scans per sample between the wavenumber range of 4000–
360 cmꢀ1 at a resolution of 4 cmꢀ1, controlled by Optics User
Soware (OPUS) version 7.5 (Bruker Optics). Air was used as
reference background spectra. The ATR diamond surface was
cleaned with acetone before each sample was scanned.
13
ꢁ
(br s, 1H, H16). C NMR (CDCl3, 100 MHz; 25 C) d [ppm] ¼
15.4 (C9 or C10); 20.9 (C7); 22.0 (C9 or C10); 22.76 (C3); 25.4
(C8); 31.1 (C5); 34.0 (C4); 40.02 (C6); 47.7 (C2); 77.2 (C11); 79.95
(C1); 120.35 (C13); 121.0 (C12); 134.8 (C14). 31P NMR (CDCl3, 25
ꢁC) d [ppm] ¼ ꢀ12 596 O ꢀ15 529 (sextet, J ¼ 713 Hz).
Elemental analysis calc. (%) for C14H25F6N2OP (382.325): C
43.98, H 6.59, N 7.33, found: C 43.83, H 6.73, N 7.37.
Results and discussion
Synthesis, identication and properties of chiral protic
imidazolium salts with (1R,2S,5R)-(ꢀ)-menthol substituent
group in a cation
Determination of enantiomeric purity of prepared chiral
protic and aprotic salts
Four chiral protic imidazolium salts with different anionic
species have been synthesized by introducing the chiral,
commercially available pool, (1R,2S,5R)-(ꢀ)-menthol into imi-
dazolium cation. The total synthesis route consisted of a few
stages and is outlined in Scheme 1.
To a solution of 3–4 mg of chiral salt in 0.5 mL of CDCl3, were
added 1–3 equivalents of D-TRISPHAT tetrabutylammonium
salt. The registered H NMR spectra were presented in ESI.†
1
Diels–Alder reaction general procedure
The (1R,2S,5R)-(ꢀ)-menthol was rst converted to chlor-
The amount of 1 mmol of chiral ionic liquids was placed in omethyl (1R,2S,5R)-(ꢀ)-menthyl ether, which was next puried
a 4 mL sealed vial followed by 2 mmol of freshly cracked by distillation and used to alkylate the imidazole in the pres-
cyclopentadiene. Aer mixing it, 1 mmol of dienophile was ence of triethylamine to obtain 1-(1R,2S,5R)-(ꢀ)-menthox-
added. The reaction was performed at ꢀ35 ꢁC or 25 ꢁC. The ymethylimidazole. The crystalline chiral amine was then
reaction was monitored by collecting samples, extracting with protonated with hydrochloric acid, and the product was
hexane, and analyzing the extract with GC using the internal recrystallized from a hexane/acetone mixture. Successfully
standard method. The yield of the product and the stereo- protonation was conrmed by HRMS analysis in high resolu-
selectivity were determined via GC analysis with a Rxi-17 tion positive ionization mode. The cation mass agreed with the
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Scheme 1 Synthesis route of the chiral protic imidazolium salts.
mass calculated for the 1-H-3-[(1R,2S,5R)-(ꢀ)-menthoxymethyl] [(1R,2S,5R)-(ꢀ)-menthoxymethyl]imidazolium chloride easily
imidazolium cation.
crystallizes in the form of irregular plates from a hexane-
The synthesized 1-H-3-[(1R,2S,5R)-(ꢀ)-menthoxymethyl]imi- acetone mixture. Salts with a PF6 and OTfꢀ anion from
dazolium chloride, (ꢀ)[H-Ment-Im][Cl] is an excellent reagents a water–ethanol mixture in form of needles and irregular plates,
for obtaining protic salts via a simple exchange reaction. respectively. Only 1-H-3-[(1R,2S,5R)-(ꢀ)-menthoxymethyl]imi-
Therefore, the chloride anion can be changed to other anions dazolium bis(triuoromethylsulfonyl)imide is a liquid at room
such as OTf, NTf2, and PF6 via a metathesis reaction with the temperature.
ꢀ
appropriate salt in aqueous medium. The yield of each
The melting points for the solids were determined using the
synthesis step was over 93% (Table 1). The surfactant content of c-DTA signal from TG analysis. They are 133.5 ꢁC for (ꢀ)[H-
the protic imidazolium salts were determined by a direct two- Ment-Im][Cl], 107.5 ꢁC for (ꢀ)[H-Ment-Im][OTf] and 136.8 ꢁC
phase titration technique (EN ISO 2871-2:2010 standard) for (ꢀ)[H-Ment-Im][PF6] (Table 2). Only the (ꢀ)[H-Ment-Im]
using water as the solvent for (ꢀ)[H-Ment-Im][Cl] and methanol [NTf2] exhibited a glass transition temperature (Fig. 1). The
for all other salts; the range was 99.1 to 99.7%. The remainder reverse heat ow signal was registered by MDSC in subsequent
consists of water. However, the (ꢀ)[H-Ment-Im][NTf2] salt could cooling–heating–cooling cycles. However, no crystalline
not be estimated by the direct two-phase titration technique behavior was observed for this protic ionic liquid at this
because the bis(triuoromethanesulfonyl)imide anion in this temperature range tested. During the heating cycle, the endo-
salt is not interchangeable.
thermal non-reversing signal accompanied the glass transition,
The chiral protic salts were identied and characterized via and this may be attributed to enthalpic relaxation as the
NMR, elemental analysis, optical rotation, and thermogravi- amorphous material approaches equilibrium. The glass tran-
metric analysis. Salt with a NTf2 anion was studied with sition temperature of (ꢀ)[H-Ment-Im][NTf2] was between
modulated differential scanning calorimetry (MDSC) in cooling ꢀ35.99 ꢁC and ꢀ31.07 ꢁC using a midpoint method. This
ꢁ
and heating mode, and its refractive index as a function of temperature was about 20–30 C higher than that determined
temperature was determined. The properties of synthesized for analogous aprotic chiral imidazolium salt with (1R,2S,5R)-
1
chiral protic salts are presented in Table 2. The H NMR, 13C (ꢀ)-menthoxymethyl group in the cation. This is compatible
NMR spectra, TG, DTG, and c-DTA curves registered from with general observations for aprotic and protic ionic liquids.
thermogravimetric and MDSC analysis are attached in the ESI.†
The refractive index values for (ꢀ)[H-Ment-Im][NTf2] in the
ꢁ
temperature range of 20–60 C was also measured. Fig. 2 shows
that the refractive index decreases with an increase in temperature.
These values are similar to other reported protic ionic liquids.29
Physical properties of chiral protic imidazolium salts
Of the four synthesized protic salts, three—(ꢀ)[H-Ment-Im][Cl],
(ꢀ)[H-Ment-Im][OTf], (ꢀ)[H-Ment-Im][PF6] are solids. These
solid salts have crystallization conditions elaborated. The 1-H-3-
Temperatures corresponding to
a
5% mass loss (T5-
%
d), obtained for protic salts from thermogravimetric analysis
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Table 1 Structure, abbreviation, yield, and surfactant content of chiral protic imidazolium salts with (1R,2S,5R)-(ꢀ)-menthol substituent
Structure of protic imidazolium salts
Abbreviation, (ꢀ)[H-Ment-Im][X]
(ꢀ)[H-Ment-Im][Cl]
Yielda,b (%)
Surfactant contentc (%)
93.0
99.7
(ꢀ)[H-Ment-Im][OTf]
95.5
99.1
d
(ꢀ)[H-Ment-Im][NTf2]
93.7
—
(ꢀ)[H-Ment-Im][PF6]
96.9
99.6
a
c
Isolated yield aer purication and drying. b Accuracy ꢂ 0.1%. Accuracy ꢂ 0.1%. d Not applicable.
are presented in Table 2. Of the synthesized protic salts, salt decomposition of (ꢀ)[H-Ment-Im][OTf] and (ꢀ)[H-Ment-Im]
withꢁa NTf2 anion is the most thermally stable with a Td5%
¼
[NTf2]. The mass loss percentage in the rst step suggests
222 C. Salts containing PF6 and OTf anions have decomposi- elimination of the menthyl fragment.
tion temperatures that are about 20 ꢁC lower than (ꢀ)[H-Ment-
The solubility of chiral protic salts were studied in
Im][NTf2]. Salt with a chloride anion has the lowest decompo- commonly used solvents (Table 4). All salts were soluble in
sition temperature. The TG curves indicates two-stage alcohols,
dimethylformamide,
dimethylsulfoxide,
Table 2 Properties of the chiral protic imidazolium salts
Phase transition
Protic chiral salt
State at rt
Shape of crystals
Crystallization solvent
temperature (ꢁC)
Specic rotation [a]D20
T5d% (ꢁC)
(ꢀ)[H-Ment-Im][Cl]
(ꢀ)[H-Ment-Im][OTf]
(ꢀ)[H-Ment-Im][NTf2]
Solid
Solid
Liquid
Irregular plates
Irregular plates
—
Hexane–acetone
Water–ethanol
—
Tm ¼ 133.5a
ꢀ100.97, (c 1.040)
155.6
199.4
222.0
Tm ¼ 107.5a
ꢀ82.3, (c 1.001)
ꢀ65.36, (c 1.149)
Tg(h) ¼ ꢀ31.07c,
Tg(2c) ¼ ꢀ32.60d
Tm ¼ 136.8a
b
b
(ꢀ)[H-Ment-Im][PF6]
Solid
Needles
Water–ethanol
ꢀ102.7, (c 0.96)
200.9
a
b
c
Determined as endothermic peak temperature in c-DTA curve registered from TG measurements. Not applicable. Determined as midpoint
d
from MDSC measurements in a heating cycle. Determined as midpoint from MDSC measurements in a second cooling cycle.
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Fig. 1 MDSC reversing heat flow signal registered in cooling–heating–cooling cycle of (ꢀ)[H-Ment-Im][NTf2].
chloroform, dichloromethane, toluene, and acetone but a deshielding effect as a result of imidazole protonation. The
insoluble in hexane. Only (ꢀ)[H-Ment-Im][Cl] dissolves in most signicant deshielding effect is observed at the H-14
water upon heating and is insoluble in ethyl acetate. More- position. Here, the signal is shied to 1.10–2.41 ppm
over, (ꢀ)[H-Ment-Im][NTf2] is the only salt that is soluble in depending on the anion. However, protons H-13, H-12, H-11,
diethyl ether.
and H-1 have a lower increase in chemical shi values—from
0.13 to 0.42 ppm. Changes in the chemical shi of another
protons are minor.
NMR analysis of chiral protic salts
However, the extent of the deshielding effect for these
protons is related to the counter ion that is ascribed the
different electrostatic and steric interactions between the indi-
vidual anions and the cationic head. The structure of the anion
has only a minor effect on the changes in the chemical shi of
H12, H13, H-11, and H-1 protons. For proton H-14, large anions
with a greater steric hindrance such as OTf, PF6 and NTf2 have
less deshielding effect than the chloride anion. In other words,
H-14 is more shielded by large anions. This suggests that the
A comparison of the chemical shis for protons and carbons
in the protic imidazolium salts versus 1-(1R,2S,5R)-(ꢀ)-men-
thoxymethylimidazole indicates protonation of the imidazole
ring. Moreover, the effect of the anion on the chemical shi is
observed. The chemical shis for the selected atoms are listed
in Table 3. The signals of H-14, H-13, H-12, H-11, and H-1
protons in the imidazolium salts are shied to lower eld
versus the starting material [Ment-Im]. This implies a reduc-
tion in electron density surrounding these atoms and
Fig. 2 Refractive index (nD) as a function of temperature for (ꢀ)[H-Ment-Im][NTf2] ionic liquid.
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1
Table 3 Comparison of H and 13C chemical shifts (ppm) for 1-(1R,2S,5R)-(ꢀ)-menthoxymethylimidazole and chiral protic salts composed of
different anions
Position
Compound abbreviation
(ꢀ)[Ment-Im]
14
13
12
11
1
dH (ppm)
dC (ppm)
dH (ppm)
dC (ppm)
dH (ppm)
dC (ppm)
dH (ppm)
dC (ppm)
dH (ppm)
dC (ppm)
7.59
136.8
10.0
135.83
9.12
135.31
8.75
134.49
8.69
134.8
7.08
118.5
7.43
119.95
7.50
120.20
7.50
120.75
7.46
7.05
129.2
7.33
120.32
7.36
120.88
7.45
121.04
7.37
5.32
76.5
5.65 and 5.89
76.64
5.57 and 5.67
76.90
5.57 and 5.62
76.6
5.55 and 5.61
77.2
3.12
73.2
3.35
79.58
3.27
79.96
3.25
80.4
(ꢀ)[H-Ment-Im][Cl]
(ꢀ)[H-Ment-Im][OTf]
(ꢀ)[H-Ment-Im][NTf2]
(ꢀ)[H-Ment-Im][PF6]
3.27
79.95
120.35
121.0
steric effects are signicant. This also demonstrates localization starting menthoxymethylimidazole. The signal of the C-1
of the anion near the two nitrogen atoms of the imidazolium carbon is shied from 73.2 in (1R,2S,5R)-(ꢀ)-menthox-
cation.
ymethylimidazole to about 80 ppm in 1-H-3-[(1R,2S,5R)-
Notable differences are seen in the 13C NMR spectra of (ꢀ)-menthoxymethyl]imidazolium salts. As a result of proton-
(1R,2S,5R)-(ꢀ)-menthoxymethylimidazole and protic salts ation, there is an increase in the chemical shi values for C-13
(Table 3). Greater changes in the chemical shi are observed for and C-11 carbons. The changes are 1.45–2.25 ppm for C-13 and
the C-1 carbon located in menthyl fragment and slightly smaller 0.1–0.4 ppm for C-11. The ordering of anions according to their
for C-13 and C-11 carbons, connected with a quaternary increasing deshielding capacities for carbons C-1, C-11 and C-
nitrogen atom of the imidazolium ring. Carbons at these posi- 13 are as follows: Cl < OTf < PF6<NTf2. Signals of C-12 and C-
tions are more de-shielded in the imidazolium salts than in the 14 carbons adjacent to the protonated nitrogen atom are
Table 4 Solubilitya of 1-H-3-[(1R,2S,5R)-(ꢀ)-menthoxymethyl]imidazolium salts in the common solvents
Solvent
(ꢀ)[H-Ment-Im][Cl]
(ꢀ)[H-Ment-Im][NTf2]
(ꢀ)[H-Ment-Im][OTf]
(ꢀ)[H-Ment-Im][PF6]
Hexane
Diethyl ether
Acetone
Ethyl acetate
Toluene
Chloroform
Dichloromethane
Water
Methanol
Ethanol
1-Propanol
DMF
ꢀ
ꢀ
+
ꢀ
+
+
+
+
+
+
ꢀ
+
+
+
+
+
ꢀ
ꢀ
+
+
+
ꢀ
ꢀ
+
+
+
ꢀ
+
+
+
+
+
+
+
b
+/ꢀ
ꢀ
ꢀ
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
DMSO
a
+ Soluble at 25 ꢁC. ꢀ Insoluble at 25 ꢁC. b Heating is needed to dissolve.
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Fig. 3 Structures and abbreviations of aprotic chiral ionic liquids.
shied in the spectrum of 1-H-3-[(1R,2S,5R)-(ꢀ)-menthox- with D-TRISPHAT tetrabutylammonium salt. Many research arti-
ymethyl]imidazolium salts in the direction of the higher eld. cles reported that Lacour's D-TRISPHAT anion is an efficient NMR
The signal of C-12 shis from 129.2 ppm in (1R,2S,5R)- chiral shi reagent for chiral cations.30–33 Our rst aim was to
(ꢀ)-menthoxymethylimidazole to about 121 ppm in 1-H-3- demonstrate that this reagent can be used for enantiomeric purity
[(1R,2S,5R)-(ꢀ)-menthoxymethyl]imidazolium salts. The chem- determination of synthesized chiral salts. Effect of D-TRISPHAT
ical shi of the C-14 carbon decreased about 1–2.3 ppm. A addition on chemical shi and expected split of signals of enan-
greater shielding effect was seen for larger anions.
tiomers was presented on the example of protic salt [H-Ment-Im]
An inverse 2D heteronuclear correlation experiment was [NTf2] (Fig. 4) and aprotic salt [C4-Ment-Im][NTf2] (Fig. 5). To
performed for (ꢀ)[H-Ment-Im][NTf2] as the representative ionic observe the differentiation of shis of proton signal from both
liquid. This shows imidazolium ring protonation data as enantiomers, racemic [H-Ment-Im][NTf2] and (+)[C4-Ment-Im]
a function of proton position. In the 1H(15N)HMBCAD spectrum [NTf2] were synthesized and used in these NMR experiments.
(in ESI†), we see two signals of nitrogen atoms in the imidazo-
In Fig. 4 we can see, that already upon addition of 1 equiv-
lium cation (at ꢀ208.59 ppm and ꢀ189.88 ppm). Both signals alent of TRISPHAT, signal of the H14 proton in [H-Ment-Im]
give peak correlations with signals of H14, H13, and H12 [NTf2] split and is shied downeld from 8.740 ppm to 8.821 for
protons of imidazolium ring. However, only the nitrogen atom (+)[H-Ment-Im][NTf2] and 8.804 ppm for (ꢀ)[H-Ment-Im][NTf2].
at ꢀ208.59 ppm correlates with the H15 proton at 12.48 ppm. More equivalents of TRISPHAT is unfounded because showed
Moreover, the signal of H15 proton is a doublet because it broadening of this signal and lower differences in chemical
couples with the adjacent nitrogen atom (N–H group).
shi for both enantiomers. Other signals of [H-Ment-Im][NTf2]
The peak correlation at 5.59 ppm/ꢀ189.88 ppm corresponds were also shied downeld and split, but differences in
to the H11 proton and nitrogen atom to which menthoxymethyl chemical shi were not sufficient and signals of both enantio-
group is attached.
mers were partly overlapped.
1H NMR spectra presented in Fig. 5 for both enantiomers of
aprotic salt (+)[C4-Ment-Im][NTf2] and (ꢀ)[C4-Ment-Im][NTf2]
and their racemic mixture with 1 equivalent of D-TRISPHAT
showed also sufficient split of the signals of H14 proton with Dd
¼ 0.037 ppm. Signals of diastereotopic protons H11 and
Enantiomeric purity of protic and aprotic chiral salts
Enantiomeric purity of the prepared chiral protic (Table 1) and
aprotic (Fig. 3) salts was determined based on NMR experiments
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Fig. 4 1H NMR spectra (400 MHz) of [n-Bu4N][D-TRISPHAT], (ꢂ)[H-Ment-Im][NTf2] with 1, 2 and 3 equivalents of [n-Bu4N][D-TRISPHAT], (ꢀ)[H-
Ment-Im][NTf2] with 1 equivalent of [n-Bu4N][D-TRISPHAT].
protons of methyl group of menthyl were split too and can be imidazolium ionic liquids because the two acidic hydrogen
used to integrate the respective signals of each enantiomer and atoms are directly bound to the imidazolium ring. This allows
measure of enantiomeric purity based on proton integration.
closer spatial approach between the cation and dienophile.38,39
Procedure with 1 equivalent of D-TRISPHAT were carried out Thus, the equilibrium geometries and electronic structures of
for all salts derived from (ꢀ)-menthol and the registered 1H the species in the presence of protic ionic liquid can be changed
NMR spectra were included to the ESI.† It was determined that to affect reactivity and selectivity. Considering the above-
almost all chiral protic and aprotic salts were (ꢀ)-enantiomer of discussed features of the imidazolium cation and previous
100% ee, except of (ꢀ)[C10-Ment-Im][NTf2] purity, which was explanations of its impact on the Diels–Alder reaction, we
83.5% ee.
decided to study the ability to induce enantiomeric excess in the
Diels–Alder reaction product by combining a proton at the
nitrogen atom and the chiral substituent in the imidazolium
ring. Until now, only aprotic chiral ionic liquids with chirality in
an anion40 or cation8 were tested.
At rst, we used the protic chiral ionic liquids in the Diels–
Alder reaction between cyclopentadiene and methyl acrylate.
For comparison, we also conducted this reaction in aprotic
chiral ionic liquids with the same chiral (ꢀ)-menthoxymethyl
substituent in the imidazolium or pyridinium cation and NTf2
anion. Their structures and abbreviations are presented in
Fig. 3. Salts with imidazolium cation represent the biggest class
of ILs with usually low melting point and viscosity.
The aprotic and protic chiral ionic liquids with NTf2 anion
were liquids. They were used as a Diels–Alder reaction medium
without addition of any other solvent. As was shown in Table 5
the reaction between cyclopentadiene and methyl acrylate had
yields of 2-methoxycarbonyl-5-norbornene of 77–89% in chiral
ionic liquids with NTf2 anion. Stereoselectivity endo/exo in the
Diels–Alder reaction in protic and aprotic chiral ionic liquids
Many articles have shown that the proton-donating properties
of ionic liquids are crucial to increasing the stereoselectivity in
Diels–Alder reactions. The hydrogen bond between the cation of
the ionic liquid and the carbonyl oxygen of the dienophile
inuences the endo : exo ratio.34,35 The proton of C2 in the
imidazolium ring and proton at nitrogen atom can be involved
in hydrogen bond formation, but the later interacts more with
the electron-withdrawing group of dienophiles.36 The role of the
anion in these H-bonding interactions is also signicant.37
Most recently, the “clamp” effect was shown as being crucial
to explain some features of the Diels–Alder reaction in ionic
liquids. This implies that the coordination of a dienophile by an
ionic liquid cation is affected by the number and the nature of
hydrogens available in the cation. Based on this theory, the
dienophile is the most strongly “clamped” in protic
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Fig. 5 1H NMR spectra (400 MHz) of racemic mixture of (ꢂ)[C4-Ment-Im][NTf2] with 1 equivalent of [n-Bu4N][D-TRISPHAT] and each of the
enantiomer with 1 equivalent of [n-Bu4N][D-TRISPHAT].
Table 5 The results of a Diels–Alder reaction between cyclopentadiene and methyl acrylate in chiral protic salts
Chiral protic salt
Yield (%)
endo : exo ratio
(ꢀ)[C1-Ment-Im][NTf2]a
(ꢀ)[C4-Ment-Im][NTf2]a
(ꢀ)[C10-Ment-Im][NTf2]a
(ꢀ)[C6O-Ment-Im][NTf2]a
(ꢀ)[Ment-Py][NTf2]a
87
88
83
85
77
89
93
97
97
90
3.8
3.7
3.6
4.1
3.7
4.9
4.3
4.1
3.7
5.5
(ꢀ)[H-Ment-Im][NTf2]a
(ꢀ)[H-Ment-Im][NTf2]b
(ꢀ)[H-Ment-Im][OTf]b
(ꢀ)[H-Ment-Im][PF6]b
[H-Me-Im][NTf2]a,c
a
Conditions of the reaction: 1 : 1 molar ratio of CIL:methyl acrylate; reaction temperature of 25 ꢁC; endo/exo ratio was determined based on GC
b
analysis. 0.5 mL of CHCl3, 0.2 mmol of CIL, 1 mmol of methyl acrylate, 2 mmol of cyclopentadiene. The yield and endo : exo ratio were
determined based on 1H NMR analysis of reaction mixture and integration of protons of methyl ester group. 1-H-3-Methylimidazolium
c
bis(triuoromethylsulfonyl)imide.
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Fig. 6 ATR-FTIR spectra of methyl acrylate (red line), (ꢀ)[C4-Ment-Im][NTf2] (blue line) and mixture of both (1 : 3, v/v) (pink line).
protic chiral ionic liquid (ꢀ)[H-Ment-Im][NTf2] was 4.9 and in band, nC]O for pure methyl acrylate and its mixture with
aprotic chiral ILs between 3.7 and 4.1. However, no enantio- aprotic IL (ꢀ)[C4-Ment-Im][NTf2] (Fig. 6) and protic IL (ꢀ)[H-
meric excess was achieved with both protic and aprotic chiral Ment-Im][NTf2] (Fig. 7). The nC]O band for pure methyl acry-
ILs. In addition, the effect of different anions of the chiral protic late is located at 1725.74 cmꢀ1 which corresponds to non-H-
salts was compared in the reaction carried out in chloroform as bonded carbonyl. Adding protic IL to methyl acrylate we
a solvent, because two chiral protic salts were solids. There were observed two peaks—one at 1721.82 cmꢀ1 and second at
slightly higher endo/exo ratios for the chiral protic salt con- 1712.84 cmꢀ1 which provides an evidence on the H-bond
taining the NTf2 anion (endo/exo 4.ꢀ3) than in the ILs with presence. Furthermore, when aprotic ionic liquid was mixed
another anions—3.7 and 4.1 with PF6 and TfOꢀ respectively.
with methyl acrylate, carbonyl stretching band was shied
The higher stereoselectivity in the protic ionic liquid can be only to 1724.38 cmꢀ1 and can be rather attributed to non-H-
the effect of H-bonding between cation and carbonyl group of bonded carbonyl group.
methyl acrylate. Hydrogen bond can stabilize the LUMO
Interestingly, it was found that the chiral protic IL allowed to
energy of the dienophile providing the endo product as the achieve the same endo/exo stereoselectivity in the reaction
preferred isomer. The evidence for the H-bond interaction between cyclopentadiene and methyl acrylate as previous re-
gave IR spectroscopy and comparison of carbonyl stretching ported by us in chiral pyrrolidinium ionic liquids with
Fig. 7 ATR-FTIR spectra of methyl acrylate (blue line), (ꢀ)[H-Ment-Im][NTf2] (green line) and mixture of both (1 : 3, v/v) (red line).
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range from 4.1 to 4.3, depending on the alkyl substituent in
the cation.38
There is one report on the use the aprotic chiral ionic liquids,
containing terpene group in the imidazolium cation and NTf2
anion.42 Authors obtained endo/exo stereoselectivity between 4.8
and 4.3 in the reaction of cyclopentadiene with acrylic acid,
under similar conditions.
The second experiment was the reaction of cyclopentadiene
with ethyl-vinyl ketone. The effect of the protic ionic liquid, (ꢀ)[H-
Ment-Im][NTf2], on the increase in product rate formation is
signicant (Fig. 8) for this reaction. The reaction time needed to
obtain a high yield (above 90%) was achieved in a two-fold shorter
period of time in the protic ionic liquid than in the aprotic one.
The differences in the reaction rate between imidazolium (ꢀ)[C1-
Ment-Im][NTf2] and pyridinium cation (ꢀ)[Ment-Py][NTf2] was
minimal. In addition, the endo/exo ratio was higher in the protic
ꢁ
ionic liquid with a value of 9.5 at 25 C. This endo/exo ratio was
Fig. 8 The effect of type of chiral protic imidazolium salt on the rate of
product formation and stereoselectivity in the reaction of cyclo-
pentadiene with methyl-vinyl ketone; A: (ꢀ)[H-Ment-Im][NTf2]; B: (ꢀ)
[C1-Ment-Im][NTf2]; C: (ꢀ)[Ment-Py][NTf2]. Conditions of the reaction:
1.1 mmol of chiral ionic liquid; 1 mmol of methyl-vinyl ketone; reaction
temperature of 25 ꢁC; the endo/exo ratio was determined based on
GC analysis.
also higher than obtained in nonchiral protic ionic liquid (ꢀ)[H–
Me-Im][NTf2] (endo/exo 8.4) (Table 6). Not able to facilitate enan-
tiomeric excess we tried to conduct the reaction at lower temper-
ature (Table 6). There are numerous data on enhanced selectivity
of Diels–Alder reaction with lowering of reaction temperature.40,42
Our experiments showed that the endo/exo ratio almost doubled
versus values at room temperature; enantioselectivity has not yet
been achieved. The results suggest that the steric hindrance is too
small to force the formation—albeit in slight excess—of one of the
enantiomers of diastereomeric products.
The results denitely favor ionic liquids as solvents. They
offer reusability, easier product separation, simple immobili-
zation, and catalyst recycling. Therefore, we have undertaken
experiments to recycle (ꢀ)[H-Ment-Im][NTf2]. The products
were separated from the reaction mixture via hexane extraction.
Stereoselectivity reached a similarly high value even on the 5th
synthesis in series (Table 6, entry 2).
hydroxyethyl and (ꢀ)-menthol substituents in the cation.41
However, as was shown by a control experiment carried out in
nonchiral aprotic bis(triuoromethylsulfonyl)imide 1-H-3-
methylimidazolium, (ꢀ)[H–Me-Im][NTf2], the (ꢀ)-menthol
substituent in the protic imidazolium cation gave a little lower
endo/exo ratio than methyl substituent.
Our results in aprotic chiral salts, were similar to the
earlier reported for nonchiral aprotic imidazolium or pyr-
idinium ionic liquids with NTf2 anion, which were in the
Table 6 Effect of chiral ionic liquid and reaction temperature on the stereoselectivity of the reaction between cyclopentadiene and ethyl-vinyl
ketone
Entry
Chiral ionic liquid
Reaction temperature/time
endo : exo ratioa
1
2
3
4
5
6
7
(ꢀ)[H-Ment-Im][NTf2]
(ꢀ)[H-Ment-Im][NTf2]
(ꢀ)[C1-Ment-Im][NTf2]
(ꢀ)[C1-Ment-Im][NTf2]
(ꢀ)[Ment-Py][NTf2]
(ꢀ)[Ment-Py][NTf2]
[H-Me-Im][NTf2]
25 ꢁC/1 h
9.5
ꢀ35 ꢁC/24 h
18.7 (18.5; 19.1; 19.0; 18.9)b
25 ꢁC/2 h
6.2
9.7
5.5
8.9
8.4
ꢀ35 ꢁC/24 h
25 ꢁC/2 h
ꢀ35 ꢁC/24 h
25 ꢁC/1 h
a
Determined by GC. b The stereoselectivities are given in parentheses for the recycling experiments.
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Conclusion
The preparation of new protic salts with a chiral (1R,2S,5R)-
(ꢀ)-menthoxymethyl group attached to one nitrogen atom of
the imidazolium cation was efficiently performed from imid-
azole and (1R,2S,5R)-(ꢀ)-menthol as the main starting mate-
rials. The yield was higher than 90% in each of the four
synthesis stages. We studied the general properties such as
specic rotation, thermal properties, and solubility in solvents
for the chiral protic salts.
Three of these salts (comprising of Clꢀ, CF3SO3ꢀ, or PF6
ꢀ
anions) were crystalline solids and salts with a (CF3SO2)2Nꢀ
anion as a liquid at room temperature. The melting points for
the solid salts (determined based on c-DTA signal from ther-
mogravimetric analysis) were above 100 ꢁC. The modulated DSC
measurements indicated that the liquid salt, (ꢀ)[H-Ment-Im]
[NTf2], tends to supercool and transform into a glassy state;
however, it could not be crystallized or melted. The very slow
diffusion of ions can contribute to a lack of crystal formation.
Thermal stability of the protic salts can be ordered in relation to
the anion with an increasing trend as follows: Cl < OTf <
PF6<NTf2. Comparison of NMR data indicated, the effect of the
anion on the chemical shi of the protons and carbons of 1-H-3-
[(1R,2S,5R)-(ꢀ)-menthoxymethyl]imidazolium
cation.
The
strong effect of the anion was seen for the proton and carbon
located between the two nitrogen atoms of the imidazolium
cation. These are more shielded by the large anions and sug-
gesting localization of the anion near two nitrogen atoms of the
imidazolium cation.
Additionally, the presence of a proton on the nitrogen atom
1
was conrmed by H(15N)HMBCAD.
Comparative studies on the use of chiral protic and aprotic
salts with the same chiral substituent in the cation during Diels–
Alder reactions indicates that the H-bonding catalysis results in
a distinctly higher endo/exo ratios for protic rather than aprotic
salts. The stereoselectivity reached the same high level even aer
the fourth round of recycling for (ꢀ)[H-Ment-Im][NTf2] in the
reaction of ethyl-vinyl ketone with cyclopentadiene at ꢀ35 ꢁC.
However, no enantiomeric excess was obtained for either group.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was nanced by the National Science Centre of
15 L. Zhang, L. He, C. B. Hong, S. Qin and G. H. Tao, Brønsted
acidity of bio-protic ionic liquids: the acidic scale of [AA]X
amino acid ionic liquids, Green Chem., 2015, 17, 5154–
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Poland (grant no. UMO-2012/05/B/ST5/01689).
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