Utilisation of 1,3-dialkylimidazolium-2-carboxylates as CO 2-carriers in the presence of Na+ and K+: Application in the synthesis of carboxylates, monomethylcarbonate anions and halogen-free ionic liquids

Article abstract of DOI:10.1016/j.tetlet.2005.01.106

1,3-Dialkylimidazolium-2-carboxylate compounds have recently been fully characterised. Here we describe the utilisation of 1,3-dimethyl (1a) and 1-butyl-3-methyl-imidazolium-2-carboxylate (1b) in a carboxylation reaction with transfer of the CO₂ moiety to benzoylacetone and methanol for the synthesis, in high yield, of benzoylacetate and monoalkylcarbonate anions, respectively. Conversely, when compounds 1a and b are reacted with carbonylic substrates lacking C-H active bonds, a product resulting from nucleophilic attack of the 1,3-dialkylimidazol-2-ylidene species on the carbonyl moiety is obtained. The reported reactions can find applications in organic synthesis and in the synthesis of halogen-free ionic liquids.

Full text of DOI:10.1016/j.tetlet.2005.01.106

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 2141–2145
Utilisation of 1,3-dialkylimidazolium-2-carboxylates as
CO2-carriers in the presence of Na+ and K+: application in
the synthesis of carboxylates, monomethylcarbonate anions
and halogen-free ionic liquids
Immacolata Tommasi* and Fabiana Sorrentino
Department of Chemistry, University of Bari, Campus Universitario, via Orabona 4, Bari 70126, Italy
Received 1 September 2004; revised 18 January 2005; accepted 21 January 2005
Abstract—1,3-Dialkylimidazolium-2-carboxylate compounds have recently been fully characterised. Here we describe the utilisation
of 1,3-dimethyl (1a) and 1-butyl-3-methyl-imidazolium-2-carboxylate (1b) in a carboxylation reaction with transfer of the CO2 moi-
ety to benzoylacetone and methanol for the synthesis,in high yield,of benzoylacetate and monoalkylcarbonate anions,respectively.
Conversely,when compounds 1a and b are reacted with carbonylic substrates lacking C–H active bonds,a product resulting from
nucleophilic attack of the 1,3-dialkylimidazol-2-ylidene species on the carbonyl moiety is obtained. The reported reactions can find
applications in organic synthesis and in the synthesis of halogen-free ionic liquids.
ꢀ 2005 Elsevier Ltd. All rights reserved.
1. Introduction
substrates in the presence of Na+ and K+ cations. De-
spite the thermal stability of compounds 1a and b to-
wards decarboxylation in solution we have found that,
by reaction with methanol or benzoylacetone in the
There are very few examples in the literature for the syn-
thesis and characterisation of 1,3-dialkylimidazolium-2-
carboxylates. After the first description by Schossler and
Regitz1 of an 1,3-diphenylimidazolinium-2-carboxylate
Kuhn et al. have reported the synthesis of 1,3-diisopro-
pyl-4,5-dimethyl-imidazolium-2-carboxylate by reduc-
tion of the relevant 1,3-dialkylimidazol-thione with K
and subsequent reaction of the intermediate carbene
with CO2.2 More recently Tkatchenko and co-workers
have published the unexpected formation of 1,3-dialkyl-
imidazolium-2-carboxylates from the reaction of 1-alkyl
imidazoles with dimethylcarbonate3 while Louie and
co-workers have reported the direct carboxylation of a
1,3-dialkylimidazol-2-ylidene with CO2.4
presence of Na+ (NaBF4,NaBPh ) or K+ (KPF6) salts,
4
an easy transfer of the CO2 moiety to the organic sub-
strate is observed to produce effectively monomethylcar-
bonate and benzoylacetate anions,respectively.
Conversely,when 13,-dimethylimidazolium-2-carboxyl-
ate (1a) is reacted with benzaldehyde in the presence
of NaBPh4,product 3 is formed which can be formally
obtained by nucleophilic attack of a 2-ylidene species
on the carbonyl group and transfer of CO2 to the Alco-
holate function.
Here we describe some aspects of the reactivity of 1,3-
dimethylimidazolium-2-carboxylate (1a) and 1-butyl-3-
methylimidazolium-2-carboxylate (1b) towards organic
The reported reactions can find application in the syn-
thesis of monomethyl carbonate,benzoylacetate anions
and halogen-free ionic liquids.
Keywords: CO2-carriers; Carboxylates; Carbonates; Ionic liquids.
*
Corresponding author. Tel./fax: +39 080 5442083; e-mail:
tommasi@chimca.uniba.it
0040-4039/$ - see front matter ꢀ 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.01.106

2142
I. Tommasi, F. Sorrentino / Tetrahedron Letters 46 (2005) 2141–2145
2. Synthesis of alkylcarbonate anion and halogen-free
ionic liquids
BPhꢀ4 ),affords the corresponding ionic liquid and benz-
oylacetate through a quantitative transcarboxylation
reaction shown in Eq. 2.10
Both 1,3-dimethyl (1a) and 1-butyl-3-methyl-imidazo-
lium-2-carboxylates (1b) are very stable towards decar-
boxylation at room temperature in most organic solvents
(alcohols,DMF,acetone). At high temperatures
(>100 ꢁC) they behave differently according to the na-
ture of the solvent,5 but they are very stable when heated
in ethylene glycol up to 140 ꢁC.6 Despite their thermal
stability we have found that compounds 1a and b readily
react with dry methanol in the presence of a stoichio-
metric amount of sodium (NaBF4) or potassium (KPF6)
salts quantitatively affording the corresponding 1,3-dial-
kylimidazolium salt and CH3OC(O)ONa according to
Eq. 1.7
ð2Þ
NMR analysis of the crude reaction mixture shows an
almost quantitative conversion of reactants (>95%)
and a high selectivity of the reaction (100%) as benzoyl-
acetate and the ionic liquid are observed as the only
reaction products. Yield in the carboxylation product
is significantly lowered by the presence of adventitious
water. After work-up of the crude reaction mixture the
benzoylacetate salt can be obtained in 81% yield as a
pure product and the corresponding acid can easily be
obtained by acidification in water (at 4–5 ꢁC) and
extraction with ether.11
The time necessary for completion of the transcarboxyl-
ation reaction is limited by the solubility of salt in the
reaction mixture. When 1b is reacted with acetophenone
and NaBF4 (or KPF6) in THF the reaction takes around
70 h for completion because of the low solubility of the
salt in the reaction mixture. Conversely,when 1a or b is
reacted with acetophenone and NaBPh4 in THF,the
reaction are complete in 12 h.12 As can be seen in Eq.
2,imidazolium-2-carboxylates 1a and b act as CO2-car-
riers,which is finally transferred to benzoylacetate.
ð1Þ
After solvent evaporation under reduced pressure,a
mixture of salts is obtained. By addition of dry acetone
the ionic liquids (i.e.,1,3-dimethylimidazolium tetrafluoro-
borate8 or 1-butyl-3-methylimidazolium hexafluoro-
phosphate9) can be quantitatively extracted from the
salt mixture. It is worth noting that if the reaction
reported in Eq. 1 is carried out using 1,3-dialkylimidazo-
lium-2-carboxylates synthesised from 1-alkylimidazoles
and DMC3 (thus,avoiding the use of alkyl halides),
the ionic liquid obtained is completely halogen-free.
We have also shown that 1,3-dialkylimidazolium-2-carb-
oxylates 1a and b can be synthesised from 1,3-dialkylimi-
dazolium salts and CO2.13 Consequently,coupling the
synthesis of the 1,3-dialkylimidazolium-2-carboxylate
using CO2 to the subsequent transfer of the CO2 moiety
to an organic substrate provides a novel route to benz-
oylacetic acid from benzoylacetone and CO2.
4. Reactivity of 1,3-dimethylimidazolium-2-carboxylate
towards benzaldehyde
Reacting 1,3-dimethylimidazolium-2-carboxylate with
benzaldehyde at room temperature in the presence of a
stoichiometric amount of NaBPh4 allows product 3 to
be isolated in 63% yield according to Eq. 3.14
Scheme 1. (a) Reactivity of 3,4-dimethylthiazolium cation, a model of
thiamine.16 (b) Reactivity of 1,3-dimethylimidazolium-2-carboxylate.
ð3Þ
3. Carboxylation of benzoylacetone
Reacting 1,3-dimethyl-(1a) and 1-butyl-3-methylimi-
dazolium-2-carboxylates (1b) at room temperature with
benzoylacetone,in the presence of NaX (X ¼ BFꢀ4 or

I. Tommasi, F. Sorrentino / Tetrahedron Letters 46 (2005) 2141–2145
2143
Product 3 can be isolated as a complex ionic compound.
An attempt to isolate the corresponding 2-(a-hydroxy-
benzyl)-1,3-dimethylimidazolium product by acidification
led to decomposition.
philes towards benzaldehyde. The decarboxylation of
compounds 1a and b in CH3OH (Eq. 1) together with
the synthesis of 1-alkylimidazoles and dimethylcarbon-
ate,allow the synthesis of halogen-free ionic liquids that
may find wide application in catalysis.19
It should be noted that product 3 is formally obtained
by nucleophilic attack of an imidazol-2-ylidene species
on the carbonyl carbon of benzaldehyde with subse-
quent transfer of the CO2 moiety to the alcoholate
functionality.
Acknowledgments
Financial support from Italian MIUR (PRIN
2003039774) and the University of Bari (Fondi di Ate-
neo 2002) is gratefully acknowledged.
5. Intermediacy of an imidazolium-2-ylidene species
Whether the observed reactions (Eqs. 1–3) can be ex-
plained on the basis of an in situ generated 1,3-dial-
kylimidazol-2-ylidene reacting,respectively,as a base
or nucleophile towards selected organic substrates,or
whether a different reaction mechanism is involved,is a
matter for detailed investigation. Some preliminary
investigations have shown that reacting 1,3-dimethylimi-
dazolium-2-carboxylate with NaBPh4 in THF-d8 did not
indicate the presence of the free carbene (C2,215 ppm as
reported by Arduengo et al.15) nor that of free CO2 in the
13C NMR spectrum of the reaction solution [within the
sensitivity allowed by the spectroscopic technique].
References and notes
1. Schossler,W.; Regitz,M.
1931.
2. Kuhn,N.; Steimann,M.; Weyers,G.
Chem. Ber. 1974, 107,
Z. Naturforsch.
1999, 54b,427.
3. Holbrey,J. D.; Reichert,W. M.; Tkatchenko,I.; Bouajila,
E.; Walter,O.; Tommasi,I.; Rogers,R. D.
Commun. 2003,28.
4. Duong,H. A.; Tekavec,T. N.; Arif,A. M.; Louie,J.
Chem. Commun. 2004,112.
5. Bouajila,E.; Poinsot,D.; Tommasi,I.; Tkatchenko,I.,
Ô1,3-dialkylimidazolium-2-carboxylates: an entry to halo-
gen-free ionic liquids,heterocarbenes and functionalised
ionic ligandsÕ Green Solvents for Catalysis,Bruchsal,
Germany,October 15–19,2002; p 58.
6. Stability of compounds 1a and b in ethylene glycol.
Approximately 0.170 g 1a or b was solubilised in 5 mL of
dry ethylene glycol in a two-necked Schlenck tube
previously purged with dry nitrogen and equipped with a
reflux system,a gas inlet and a pressure equalising valve.
The clear solution was stirred at 140 ꢁC for 5 h with N2
bubbling and careful exclusion of air. Two samples
(0.70 mL) of the solution was withdrawn before and after
heating at 140 ꢁC for 5 h. 30 lL of ethylene glycol
dimethyl ether (standard purity) was added to each sample
and the 13C NMR spectra were recorded using the inverse
gated decoupling technique. The ratio of the integrals of
the signals of 1a (35.0,122.9,136.1,158.7 ppm) and of 1b
(13.10,36.90,122.15,123.50,140.38,158.54 ppm) to that
of the standard (70.4 and 57.5 ppm) was constant in the
two samples ( 3% accuracy) showing that no significant
decomposition or decarboxylation of the compounds
occurred by heating. It must be noted that significant
decomposition of the compound was observed when
compounds 1a and b were heated at 140 ꢁC in the presence
of oxygen.
Chem.
On the contrary,when compound 1b was reacted with
1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene in anhy-
drous THF-d8 the carboxylate underwent exchange of
CO2 with the diisopropyl-imidazol-2-ylidene.16 Based
on this finding,the intermediacy of a free 2-ylidene spe-
cies in the carboxylation of C–H active compounds
might be proposed. By analogy with the reactivity of
1,3-dimethylimidazolium-2-carboxylate towards benz-
aldehyde (Eq. 3),thiazol-2-ylidenes react with pyruvic
acid to give the 2-(a-hydroxyethyl)-3,4-dimethylthiazo-
lium cation (Scheme 1a).17
Also in this case,based on this analogy,the intermedi-
acy of a free 2-ylidene species able to attack the carbon-
ylic carbon of benzaldehyde (Scheme 1b) affording
product 3 can be proposed.
As far as the role of Group I cations is concerned,the
activity of Na+ and K+ as promoters of a transcarboxyl-
ation reaction find a precedent in the transcarboxylation
reaction of aromatic carboxylates (Henkel reaction)18
even though this reaction takes place at a much higher
temperature (370–465 ꢁC).
7. The synthesis of compounds 1a and b as described in
Ref. 3 leads to products containing around 5% of 4- and
5-carboxylate isomers. Recrystallisation from CH3OH
allows compounds 1a and b to be obtained with >99%
purity.
8. Procedure for the synthesis of 1,3-dimethylimidazolium
tetrafluoroborate (2a) and sodium monomethylcar-
bonate. 1,3-Dimethylimidazolium-2-carboxylate (6.52 g,
ð4Þ
0.046 mol),5.10 g (0.046 mol) of NaBF and 80 mL of
4
dry CH3OH were stirred as a slurry in a Schlenck tube
previously purged with dry nitrogen,for 12 h at room
temperature. The solvent was then evaporated under
reduced pressure and the white solid residue was extracted
with 5 · 40 mL of dry acetone. The residual CH3O-
C(O)ONa weighed 4.21 g (94.2% yield). The combined
acetone fractions were dried under reduced pressure to
In conclusion,we describe a new aspect of the reactivity
of 1,3-dialkylimidazolium-2-carboxylates as CO2-carri-
ers. Compounds 1a and b were observed to react
selectively with benzoylacetone resulting in an enolisa-
tion–carboxylation reaction and behaving as nucleo-

2144
I. Tommasi, F. Sorrentino / Tetrahedron Letters 46 (2005) 2141–2145
give 7.67 g (89.62% yield) of the ionic liquid 1,3-dimethyl-
stirred for 10 min then extracted with 3 · 20 mL of Et2O.
The combined ether fractions were dried over Na2SO4,
filtered and dried under reduced pressure. Benzoylacetic
acid (3.75 g,88% yield) were recovered. Characterisation
of benzoylacetic acid. Anal. Calcd for C9H8O3: C,65.85;
H,4.91. Found: C,65.19; H,4.97; IR (nujol,KBr): 1649,
imidazolium tetrafluoroborate. Characterisation of
CH3OC(O)ONa: Anal. Calcd for C2H3NaO3: C,24.50;
H,3.08; Na,23.45. Found: C,24.20; H,3.11; Na,23.01;
ꢀ1
IR (nujol,KBr): 1631,1457,1190,1089,825 cm
(spectroscopic data were in full agreement with those of
authentic samples). Characterisation of 1,3-dimethylimi-
dazolium tetrafluoroborate ionic liquid (2a): Anal. Calcd for
C5H9BF4N2: C,32.65; H,4.93; N,15.23. Found: C,32.35;
H,4.98; N,15.09. The ionic liquid contained 36 ppm (w/w)
of Na+. 1H NMR (500 MHz,acetone- d6): d 4.0 (s,6H,
1610 cmꢀ1
;
1.52H,C H2 keto tautomer),5.31 (s,0.24H,C
1H NMR (CDCl3,500 MHz):,
d 3.58 (s,
H enol
tautomer),7.16 (t,2H,C H-meta-Ph, 3JH–H = 7.9 Hz),7.28
H-
3
(t,1H,C H-para-Ph JH–H = 8.0 Hz),7.64 (d,2H,C
3
ortho-Ph, JH–H = 8.0 Hz),12.98 (br s,0.24H,C–O H enol
tautomer); 13C NMR (CDCl3,125 MHz),: d 45.47 (CH2
keto tautomer),86.52 ( CH enol tautomer),127.78 (C-
meta-Ph),128.09 (C- ortho-Ph),133.14 (C- para-Ph),
3
CH3),7.64 (d,2H, JH–H = 1.6 Hz,C 4–H and C5–H),8.96
(s,0.7H due to partial exchange with acetone- d6, C2–H).
Spectroscopic data were in full agreement with data
reported in the literature Holbrey,J. D.; Seddon,K. R.
J. Chem. Soc., Dalton Trans. 1999,2133.
136.34 (C-ipso-Ph),168.75
carbonylic).
(
COOH),192.5 (C(O)
9. Procedure for the synthesis of 1-butyl-3-methylimidazo-
lium hexafluorophosphate (2b) and potassium mono-
methylcarbonate. 1-Butyl-3-methylimidazolium-2-carboxylate
12. Procedure for the synthesis of sodium benzoylacetate (using
NaBPh4). 1,3-Dimethylimidazolium-2-carboxylate (6.32 g,
0.045 mol),15.43 g (0.045 mol) of NaBPh 4,5.3 mL
(0.045 mol) of benzoylacetone and 70 mL of dry THF
were placed in a Schlenck tube previously purged with dry
nitrogen. The slurry was stirred at room temperature for
12 h during which time an abundant white solid was
deposited. The solvent was reduced to approximately one
half by evaporating in vacuo,then the product was
precipitated with dry acetone. The filtered solid
was washed with 2 · 10 mL of THF and dried in
vacuo. Sodium benzoylacetate (6.12 g,73% yield) was
obtained.
(7.1 g,0.039 mol),7.17 g (0.039 mol) of KPF and 90 mL
6
of dry CH3OH were reacted in a Schlenck tube,previously
purged with dry nitrogen. The reaction mixture was stirred
at room temperature for 12 h. Potassium monomethylcar-
bonate (3.88 g,87% yield) and 1-butyl-3-methylimidazo-
lium hexafluorophosphate (7.99 g,72% yield) were
isolated following the procedure reported above (extrac-
tion of the ionic liquid using 5 · 40 mL of CH3CN) and
fully characterised. Characterisation of 1-butyl-3-methy-
limidazolium hexafluorophosphate ionic liquid (2b),
Anal. Calcd for C8H15PF6N2: C,33.81; H,5.32; N,9.86.
Found: C,33.64; H,5.41; N,9.75. The ionic liquid
contains 29 ppm (w/w) of Na+. 1H NMR (500 MHz,
acetone-d6): d 0.93 (m,3H,CH 2–CH3),1.41 (m,2H,C H2–
13. (a) Tommasi,I; Sorrentino,F.,Proceedings of the 14th
International Symposium on Homogeneous Catalysis,
Munich,Germany,July 5–9,2004; 521; (b) Tommasi,I.,
forthcoming paper.
CH3),1.85 (m,2H,C
H2–CH2–CH3),3.99 (s,3H,N
–
14. Reaction of 1a with benzaldehyde. 1,3-Dimethylimidazo-
lium-2-carboxylate (4.52 g,0.032 mol),11.03 g (0.032 mol)
of NaBPh4 and 3.3 mL (0.032 mol) of benzaldehyde were
suspended in 45 mL of dry THF and the slurry was stirred
at room temperature for 12 h. The solvent was then
evaporated under reduced pressure and the solid residue
was washed with 3 · 10 mL of THF. 70 mL of dry
CH3CN was added to the solid residue (to eliminate
unreacted 1,3-dimethylimidaozolium-2-carboxylate) and it
was then filtered. By evaporation of CH3CN under reduced
pressure 11.9 g (63% yield) of a light yellow solid residue
which was characterised as product 3. Characterisation of
product 3. Anal. Calcd for C37H34BN2NaO3: C,75.52; H,
5.82; N,4.76; Na,3.91. Found: C,74.93; H,5.89; N,4.71;
Na,3.83; IR (nujol,KBr): 1661,1580,1535,1516,1242,
3
CH3),4.31 (m,2H,N 1–CH2),7.62 (s,1H,C 5–H),7.73 (s,
1H,C 4–H),8.90 (s,C 2–H). Spectroscopic data were in full
agreement with data reported in the literature Billard,I.;
Moutiers,Gilles; Labet,Alexandre; Al Azzi,A.; Gaillard,
C.; Mariet,C.; Lutzenkirchen,K. Inorg. Chem. 2003, 42,
1726.
10. Procedure for the synthesis of benzoylacetate (using
1-butyl-3-methylimidazolium-2-carboxylate
NaBF4):
(6.55 g,0.036 mol) were reacted under nitrogen with
3.95 g (0.036 mol) of NaBF4 and 4.19 mL (0.036 mol) of
benzoylacetone in dry THF (30 mL) at room temperature
for 70 h. A dense suspension was obtained to which were
added 50 mL of dry CH3CN. The white solid isolated by
filtration was washed with 3 · 30 mL of dry acetone,dried
under reduced pressure and characterised as sodium
benzoylacetate (5.53 g,84% yield). Organic fractions were
collected,dried in vacuo,extracted with 2 · 20 mL of
toluene (to eliminate unreacted acetophenone) and dried
in vacuo 6.89 g (84.8% yield) of 1-butyl-3-methylimidazo-
lium tetrafluoroborate ionic liquid were isolated (charac-
terised as reported before). Characterisation of
PhC(O)CH2C(O)ONa. Anal. Calcd for C9H6NaO3: C,
58.07; H,3.79; Na,12.35. Found: C,57.47; H,3.84; Na,
ꢀ1
;
1H NMR (CD3CN,
1066,837,735,713,615,606 cm
500 MHz): d 3.53 (s,3H, imidazolium–CH3),6.85 (t,4H,
H-para,BPh4, JH–H = 7.2 Hz),7.02 (t,8H,H- meta-BPh4,
3
3JH–H = 7.8 Hz),7.23 (s,2H,C 4 imidazolium–H and C5
imidazolium–H),7.35 (m,8H,H- ortho-BPh4),7.58 (t,2H,
3
3
meta-Ph, JH–H = 7.8 Hz),7.70 (t,1H, para-Ph, JH–H
=
3
6.2 Hz),7.91 (d,2H,
ortho-Ph, JH–H = 7.0 Hz),10.04
(s,1H,C– H); 13C NMR (CD3CN,125 MHz): d 37.03
(s,imidazolium– CH3),123.33 (s,C- para-BPh4),127.0
(s,C- meta-BPh4),129.4 (s,C- meta-Ph),130.3 (s,C- ortho-
12.09; IR (nujol,KBr): 1686,1597 cm ꢀ1; H NMR (D2O,
1
500 MHz): protons due to –CH2– group were not
observed because of fast exchange with D2O, d 7.44 (t,
2H,C H-meta-Ph, JH–H = 7.7 Hz),7.57 (t,1H,C H-para-
Ph),135.64 (s,C-
ortho-BPh4),135.71 (s,C4- and
C5-imidazolium),135.8 (s,C- para-Ph),139.2 (s. C- ipso-
3
1
ipso-BPh4, JCB = 49.3 Hz),161.3
Ph),163.5 (q,C-
(O–C(O)O–),196.89 (s,C–H).
15. Arduengo,A. J.; Dias,R.; Harlow,R. L.; Kline,M.
J. Am. Chem. Soc. 1992, 114,5530.
3
3
Ph, JH–H = 7.48 Hz),7.88 (d,2H,C H-ortho-Ph, JH–H
=
8.23 Hz); 13C NMR (D2O,125 MHz): d 50.01 (q,keto
tautomer, CD2, JH–H = 20.83 Hz),130.18 (C- meta-Ph),
3
130.42 (C-ortho-Ph) 135.74 (C-para-Ph),137.49 (C- ipso-
Ph),176.81 (CH– C(O)Oꢀ),201 (Ph– C(O)–CH2).
11. Synthesis of benzoylacetic acid. Sodium benzoylacetate
(4.80 g ) was suspended in water at 4–5 ꢁC and 35 mL of
10% H2SO4 was added to the suspension. The system was
16. Crossover experiment between 1,3-diisopropyl-4,5-dimeth-
ylimidazol-2-ylidene and compound 1a. Compound 1b
(0.050 g,0.27 mmol) were suspended in 2.5 mL of anhy-
drous THF-d8 and reacted,under nitrogen,with 0.059 g of
1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (0.32 mmol).

I. Tommasi, F. Sorrentino / Tetrahedron Letters 46 (2005) 2141–2145
2145
The analysis of the 13C NMR spectra of the reaction
17. (a) Bordwell,F. G.; Satish,A. V..
1991, 113,985; (b) Breslow,R.
J. Am. Chem. Soc.
J. Am. Chem. Soc.
solution showed the appearance of signals at 215.5 ppm
assigned to the C2 carbon of 1,3-dimethylimidazolium-2-
ylidene and at 155.5 ppm due to the carboxylic carbon of
the 1,3-diisopropyl-4,5-dimethylimidazolium-2-carboxyl-
ate. Signals at 205.9 ppm due to C2 of 1,3-diisopropyl-
4,5-dimethylimidazol-2-ylidene and at 161.2 ppm due to
–COO– of 1-butyl-3-methyl-imidazolium-2-carboxylate
were also observed.
1958, 80,3719; (c) Kluger,R. Chem. Rev. 1987, 87,863.
18. McNelis,E. J. Org. Chem. 1965, 30,1209.
19. (a) See for example: Ionic liquids: industrial application for
green chemistry. ACS Symposium Series; Rogers,R. D.,
Seddon,K. R.,Eds.; ACS: Washington,DC,2002; p 818;
(b) Dupont,J.; de Souza,P. A. Z. S. Chem. Rev. 2002, 102,
3667.