1,3-Dialkylimidazolium-2-carboxylates as versatile N-heterocyclic carbene-CO2 adducts employed in the synthesis of carboxylates and α-alkylidene cyclic carbonates

Article abstract of DOI:10.1016/j.tetlet.2008.10.107

1,3-Dialkylimidazolium-2-carboxylate compounds of formulas 1a, 2, and 4 have been synthesized and fully characterized by X-ray spectroscopy quite recently. Up today, these compounds have found some interesting applications as precursors of N-heterocyclic carbenes (NHCs) used as ligands for metal-complexes or in the synthesis of organic compounds and ionic liquids. We have recently reported the use of 1-butyl, 3-methylimidazolium-2-carboxylate and 1,3-dimethylimidazolium-2-carboxylate in a CO₂-transfer reaction to CH₃OH and acetophenone for the synthesis of methylcarbonate and benzoylacetate. The scope of this CO₂-transfer reaction has been expanded to several organic compounds with active hydrogen (acetone, cyclohexanone, and benzylcyanide) for the synthesis of carboxylates of pharmaceutical interest, and to propargyl alcohols for the synthesis of α-alkylidene cyclic carbonates.

Full text of DOI:10.1016/j.tetlet.2008.10.107

Tetrahedron Letters 50 (2009) 104–107
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1,3-Dialkylimidazolium-2-carboxylates as versatile N-heterocyclic
carbene–CO2 adducts employed in the synthesis of carboxylates
and
a-alkylidene cyclic carbonates
*
Immacolata Tommasi , Fabiana Sorrentino
Department of Chemistry, University of Bari, Campus Universitario, via Orabona 4, 70126 Bari, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
1,3-Dialkylimidazolium-2-carboxylate compounds of formulas 1a, 2, and 4 have been synthesized and
fully characterized by X-ray spectroscopy quite recently. Up today, these compounds have found some
interesting applications as precursors of N-heterocyclic carbenes (NHCs) used as ligands for metal-com-
plexes or in the synthesis of organic compounds and ionic liquids. We have recently reported the use of 1-
butyl, 3-methylimidazolium-2-carboxylate and 1,3-dimethylimidazolium-2-carboxylate in a CO2-trans-
fer reaction to CH3OH and acetophenone for the synthesis of methylcarbonate and benzoylacetate. The
scope of this CO2-transfer reaction has been expanded to several organic compounds with active hydro-
gen (acetone, cyclohexanone, and benzylcyanide) for the synthesis of carboxylates of pharmaceutical
Received 25 September 2008
Revised 16 October 2008
Accepted 21 October 2008
Available online 25 October 2008
Keywords:
1,3-Dialkylimidazolium-2-carboxylates
N-Heterocyclic carbenes
Carboxylation reaction
Propargyl alcohols
interest, and to propargyl alcohols for the synthesis of
a
-alkylidene cyclic carbonates.
Ó 2008 Published by Elsevier Ltd.
Carboxylative cyclization reaction
1. Introduction
diisopropylphenyl)imidazolinium-2-carboxylate (7)5 have also
been extensively characterized.
1,3-Dialkylimidazolium-2-carboxylate compounds of formulas
1a,1 2,2 and 43 have been synthesized and fully characterized by
X-ray spectroscopy quite recently (Scheme 1). Analogous com-
pounds of formulas 1b,4 3,5 and 5,5 as well as the 1,3-bis(2,4,6-tri-
methylphenyl)imidazolinium-2-carboxylate (6) and 1,3-bis(2,6-
As N-alkyl and N-aryl substituted imidazol(in)ium-2-carboxyl-
ates are adducts of the corresponding carbene with CO2, they have
been employed in synthetic chemistry both as organocatalysts in
CO2-coupling reactions with epoxides6 and as NHC-transfer agents.
Studies about the thermal stability of several NHC–CO2 adducts
carried out by Louie,3 and more recently by Lu and co-workers6
have contributed significantly to the understanding of their reac-
tivity. Notable examples of utilization of compound 1a in chemical
synthesis are the preparation of NHC-metal complexes,7,8 amidates
and thioamidates,9 phosphenium adducts,10 and ionic liquids.11
Several NHC-metal complexes prepared using compounds 3–7
have also been used as catalysts in olefin metathesis,5 cyclopro-
panation reactions,5 and Suzuki–Miyaura couplings.12
The employment of imidazol(in)ium-2-carboxylates in synthe-
sis and catalysis seems interesting5–12 as the generation of NHC
from the corresponding carboxylate 1–7, when applicable, shows
the advantage to start from air- and moisture-stable species.5–8,12
Due to the high moisture and air sensitivity of imidazol-2-yli-
denes, developing new methods for in situ generation of NHC is
of crucial importance. In this context, the literature documents
the development of wide applicable procedures that allow the fac-
ile use of NHCs in a variety of reactions.13–16
Scheme 1.
We have recently synthesized and fully characterized com-
pounds 1a and 1b from 1-alkyl imidazoles and DMC.1 We have also
recently reported that compounds 1a and 1b can be prepared from
* Corresponding author. Tel.: +39 080 5442100; fax: +39 080 5442129.
E-mail address: tommasi@chimica.uniba.it (I. Tommasi).
0040-4039/$ - see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2008.10.107

I. Tommasi, F. Sorrentino / Tetrahedron Letters 50 (2009) 104–107
105
1,3-dialkylimidazolium salts, Na2CO3, and CO2, through a Kolbe–
Smith type reaction,4
ð1Þ
and that they behave as active CO2-carriers, transferring CO2 to
methanol and acetophenone for the synthesis of methylcarbonate
and benzoylacetate salts in high yields (Eq. 2).17
Scheme 3.
62%)23 and the benzylcyanide carboxylation24 affording product 10
(60% yield) were carried out by using a 1:1 ratio of reactants in
CH3CN as the solvent.
ð2Þ
Once having established that compounds 1a and 1b act as CO2-
transfer agent toward methanol and acetophenone (Eq. 2), we
investigated several active hydrogen organic substrates of interest
as their corresponding carboxylates are widely used as intermedi-
ates in chemical industry, and several propargyl alcohols which are
known to react with CO2 undergoing a carboxylative cyclization
While the elucidation of the mechanism of the trans-carboxyl-
ation reaction deserves further investigations, some preliminary
studies seem to indicate that preliminary generation of the free
carbene may occur.17 The carbene (pKa of 22–24 has been re-
ported)25 could react with the organic substrate generating an an-
ion able to react with CO2 (Scheme 3).
reaction to afford a-alkylidene cyclic carbonates.
As far as the role of the metal cation is concerned (Na+ or K+), it
could be involved both in the substrate activation and/or in the
product stabilization.26
2. Carboxylation of ketones and benzylcyanide
Compounds like acetone, cyclohexanone, and benzylcyanide
(pKa ꢀ 20–22)18 can be carboxylated according to the synthetic
procedure shown in Eq. 2, under very mild conditions and in good
yields affording the corresponding carboxylate salts (Scheme 2).
The carboxylates shown in Scheme 2, both in the form of their salts
(–COOM), acids (–COOH), or esters (–COOR), are widely used as
intermediates for pharmaceuticals and agrochemicals.19 The classi-
cal synthesis of compounds shown in Scheme 2 is carried out by
reaction of the organic substrate with strong and expensive bases
like NaNH2, hydrides, naphthalenes, and n-butyl lithium.20
The strong base is required in order to generate the carbon anion
that is able to react with carbon dioxide. Various alternative synthetic
procedures were described inthe scientific literature based on the use
of various catalysts.21 All these alternative synthetic procedures are
affected by unsatisfactory yields, or require high catalyst loading.
The carboxylation of acetone has been carried out according to
Eq. 2 by using the ketone as the co-solvent and reagent. After 70 h
of reaction at room temperature, the sodium salt of 3-oxo-butanoic
acid (8) was isolated in 77% yield.22 The reaction is highly selective
as the formation of the salt of 3-oxoglutaric acid as result of a dou-
ble acetone carboxylation described by other Authors21l was not
observed.
3. Synthesis of a-alkylidene cyclic carbonates
As reported in Eq. 2, compounds 1a and 1b have been used in
the carbonation of CH3OH for the synthesis of CH3OC(O)OꢁM+
(M+ = Na+, K+) and ionic liquids.17 As an extension of the study on
the reactivity toward alcohols, we investigated the reaction of
1,3-dimethylimidazolium-2-carboxylate and 1-butyl, 3-methyl-
imidazolium-2-carboxylate with propargyl alcohols of formulas
11a–d as it is known that these alcohols can react with CO2 under-
going a carboxylative cyclization reaction to give the correspond-
ing cyclic carbonates 12a–d (Eq. 3).27
a-Alkylidene cyclic
carbonates 12a–d are useful intermediates to oxazolidinones,
b-oxopropyl carbonates, carbamates, and furanone derivatives.28
ð3Þ
The trans-carboxylation reaction of cyclohexanone affording
selectively 2-oxo-cyclohexan-1-carboxylate product (9) (yield
Scheme 2.

106
I. Tommasi, F. Sorrentino / Tetrahedron Letters 50 (2009) 104–107
Table 1
a
Synthesis of a-alkylidene cyclic carbonates 12a–d from propargyl alcohols 11a–d and CO2
Entry
Alcohol
Substituents
Catalyst
Carbonate
Yieldb (%)
1
2
3
4
11a
11b
11c
11d
R
R
1 = R2 = CH3
1,3-Dimethylimidazolium-2-carboxylate
1,3-Dimethylimidazolium-2-carboxylate
1-Butyl, 3-methylimidazolium-2-carboxylate
1-Butyl, 3-methylimidazolium-2-carboxylate
12a
12b
12c
12d
77
75
1 = CH3, R2 = C2H5
R1–R2 = –(CH2)5–
51
R
1 = CH3, R2 = C6H5
73 (67)c
a
Reaction conditions: 10.3 mmol of alcohol; 0.79 mmol of catalyst (catalyst loading 7.7%); 60 bar CO2; 100 °C; 15 h.
Gas-chromatographic yield.
Isolated yield.
b
c
The reaction depicted in Eq. 3 requires a catalyst. Several tran-
Considering the carboxylation reaction reported in Eq. 1, it is
worth to note that Na2CO341 is stoichiometrically consumed while
we have shown that the imidazolium moiety can be recycled sev-
eral times without significant decomposition (Scheme 4).4
We consider, thus, the imidazolium cation acting as a recyclable
specie for the synthesis of carboxylates using CO2.
sition metals have been successfully employed including Pd,29
Ru,30 Co,31 Cu,32,33 and Ag,34 but the reaction is also promoted by
tertiary phosphines.35,36
An overview of data reported in the literature about this reac-
tion allows to conclude that (i) tertiary propargyl alcohols are more
reactive substrates;37 (ii) higher yields are usually obtained when
R1–R2 (Eq. 3) are represented by linear alkyl groups (alcohols
11a–b, 65–99% yield), while substrates with R1–R2 represented
by bulky (alcohol 11c, 58–75% yield) or phenyl groups (alcohol
11d, 32–50% yield) gave lower yields.29–36
4.2. Synthesis of a-alkylidene cyclic carbonates
By comparison with data reported in the Japanese Patent, 1,
3-dimethylimidazolium-2-carboxykate and 1-butyl, 3-methylimi-
dazolium-2-carboxylate show a good activity. Moreover, catalyst
1b shows a better specificity for the less reactive 2-pheny-l,3-bu-
tyn-2-ol (11d) substrate.
Very recently a Japanese Patent38 reported the use of imidazol-
and imidazolin-2-ylidenes with bulky N-alkyl and N-aryl substitu-
ents and their CO2-adducts as catalysts in the synthesis of
carbonate 12a (obtained in 88% yield). These data confirm that
NHCs compete with phosphines for donating properties39 and cat-
alytic activity, and open the question about the role of the NHC in
the carboxylative cyclization reaction.
As the steric bulk of NHC N-substituents has been often re-
ported to influence significantly their reactivity,13,15 we decided
to investigate the activity of compounds 1a and 1b as catalysts of
the carboxylative cyclization of terminal propargyl alcohols. Re-
sults of our study are summarized in Table 1. Interestingly, with re-
spect to yields obtained by using different catalysts,40 a higher
yield was obtained by using the less reactive 2-phenyl, 3-butyn-
2-ol (entry 4, 67% isolated yield). In all experiments, catalyst load-
ing was 7.7%, and selectivity in carbonates 12a–d was >99%.
Acknowledgments
Financial support from University of Bari (Fondi di Ateneo
2005–2006) and MIUR (PRIN 2006031888_001) is gratefully
acknowledged.
References and notes
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Scheme 4.

I. Tommasi, F. Sorrentino / Tetrahedron Letters 50 (2009) 104–107
107
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27. Synthesis of a-alkylidene cyclic carbonates: In a case experiment: 0.79 mmol of
1,3-dialkylimidazolium-2-carboxylate and 10.3 mmol of the propargyl alcohol
were placed, under nitrogen atmosphere, into a glass reactor of a magnetically
stirred stainless steal autoclave and added with 4 mL of dry CH3CN. After
closing the autoclave, it was pressurized by CO2, and the mixture was heated to
100 °C for 15 h under continuous stirring (PCO = 60 bar). The reaction was then
stopped by cooling and depressurizing the au2toclave, and the reaction mixture
was analyzed via GC. The products were isolated by silica gel column
chromatography. Carbonates 12a–d are known compounds. Spectroscopic
data for products were in agreement with those reported in the literature:
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carboxylate (3.38 g, 18.5 mmol), dry NaI (2.78 g (18.5 mmol) dry acetone
(25 mL), and CH3CN (10 mL) were reacted, under nitrogen atmosphere at room
temperature for 70 h. The initially clear solution after a few hours became
cloudy with copious white precipitate and a light yellow supernatant. The
reaction mixture was then dried in vacuo, and the residual solid washed with
CH3CN (2 ꢂ 20 mL) and dry acetone (2 ꢂ 20 mL). The white solid obtained
(1.78 g, 77.3%) was characterized as sodium salt of the 3-oxo-butanoic acid.
Anal. Calcd for C4H5NaO3: Na, 18.53. Found: Na, 18.32. IR (Nujol, KBr): 1712,
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1604 cmꢁ1
;
1H NMR (500 MHz, D2O): d, protons due to –CH2– group were not
observed because of fast exchange with D2O, 2.14 (s, CH3–); 13C{1H} NMR
(101 MHz, D2O,): d, 29.8 (CH3–), 52.9 (broad multiplet because of fast exchange
with D2O), 175.3 (CH2–C(O)O–), 201.1 (CH3–C(O)–CH2).
23. Carboxylation of cyclohexanone. In
a
Schlenk flask, 1-butyl, 3-
17.5 mmol), NaI (2.65 g,
36. Kayaki, Y.; Yamamoto, M.; Ikarija, T. J. Org. Chem 2007, 72, 647–649.
37. Note: most of data reported in the literature indicate that primary and
secondary propargylic alcohols do not undergo carboxylative cyclization
reaction. Recently Jiang et al.: Jiang, H.-F.; Wang, A.-Z.; Liu, H.-L.; Qi, C.-R.
Eur. J. Org. Chem. 2008, 2309–2312. reported the carboxylative cyclization of
several secondary alcohols in SC–CO2.
38. Note: Kayaki, H.; Ikariya, T. JP 2006137733, 2006; Chem. Abstr. 145, 27996;
These authors have used imidazol-2-ylidenes and imidazolin-2-ylidenes with
tBu, iPr, 2,4,6-trimethylphenyl N-substituents as well as some thiazol-2-
ylidenes. The internal alcohol 2-methyl-4-phenyl-3-butyn-2-ol gave the
corresponding cyclic carbonate in 75% yield under 100 bar CO2 pressure. Our
study was focused on the more reactive terminal propargylic alcohols.
39. (a) Poyatos, M.; McNamara, W.; Incarvito, C.; Clot, E.; Peris, E.; Crabtree, R. H.
Organometallics 2008, 27, 2128–2136; (b) Kelly, R. A.; Clavier, H.; Giudica, S.;
Scott, N. M.; Stevens, E. D.; Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.;
Nolan, S. P. Organometallics 2008, 27, 202–210.
methylimidazolium-2-carboxylate (3.19 g,
17.7 mmol), and cyclohexanone (3 mL) were reacted in dry CH3CN (25 mL).
The reaction mixture was stirred under nitrogen atmosphere at room
temperature for 70 h, after which the solution was concentrated to dryness
in vacuo. The residual solid was washed with THF (3 ꢂ 15 mL) to remove
excess of cyclohexanone, then with acetone (3 ꢂ 15 mL). A light yellow solid
was obtained (1.79 g, 62.2% yield) characterized as sodium 2-oxo-cyclohexane-
1-carboxylate. Anal. Calcd for C7H9NaO3: Na, 14.01. Found: Na, 13.91. IR (Nujol,
KBr): 1573 and 1697 cmꢁ1 1H NMR (500 MHz, D2O): d, 1.54 (1H) and 1.61 (m,
.
1H) (C(4)–H2), 1.68 (m, 2H, C(5)H2), 2.12 (m, 1H) and 2.19 (m, 1H) (C(6)–H2),
2.38 (m, 1H) and 2.43 (m, 1H, C(3)H2), 3.26 (t, approximately 0.7 (C(1)H).
13C{1H} NMR (101 MHz, D2O): d, 22.64, 26.63, 31.10, 40.98, 61.06 (C1), 177.79
(–C(O)O–), 215.65 (–C(O)–).
24. Carboxylation of benzylcyanide. In a Schlenk flask were reacted, under nitrogen
atmosphere,
1-butyl,
3-methylimidazolium-2-carboxylate
(2.73 g,
14.98 mmol), NaI (2.32 g, 15.01 mmol), and benzylcyanide (1.85 mL,
15.79 mmol) in dry CH3CN (15 mL) at room temperature for 4 days. The
reaction mixture was dried in vacuo, and washed three times with CH3CN
(3 ꢂ 7 mL). A light yellow solid was obtained (1.74 g, yield 60%) characterized
as sodium 2-cyano, 2-phenylacetate salt. Anal. Calcd for C9H6NNaO2: Na, 12.55.
40. Note: yields in 4-methyl-5-methylen-4-phenyl-1,3-dioxolan-2-one (carbonate
12d) reported from other Authors with different catalysts were (Refs. 29–35 of
this Letter): 32% (Dixneuf et al., by using PnBu3: 100 °C, PCO = 50 bar, 20 h,
catalyst loading 8%), 51% (Inoue et al., by using K2CO3/CE, 802°C, PCO = 5 bar,
2
50 h, catalyst loading 25%), 45% (Deng et al., by using CuI/ionic liquid, 120 °C,
PCO = 10 bar, 8 h, catalyst loading 2%). The product was not obtained working
wit2h CuI-polymer supported catalyst in SC–CO2 (Jiang et al.).
Found: Na, 12.40. IR (Nujol, KBr): 1712, 1604 cmꢁ1 1H NMR (400 MHz, D2O,):
;
d, 7.27 (broad signal due to overlapping aromatic protons). 13C{1H} NMR
(101 MHz, H2O): d, 46.25 (C–H tertiary carbon), 119.94 (–CN), 127.82 (ortho-
Ph), 128.32 (para-Ph), 129.15 (meta-Ph), 132.79, (ipso-Ph), 170.8 (–C(O)O–).
25. Alder, R. W.; Allen, P. R.; Williams, S. J. J. C. S. Chem. Soc., Chem. Commun 1995,
1267–1268; Kim, Y.-J.; Streitweiser, A. J. Am. Chem. Soc. 2002, 124, 5757–5761.
26. The role played by the metal cations (Na+, K+) could be quite complex. It is
known that Group I cations can interact with organic substrates in organic
41. Note: it is worthy to note that Na2CO3 itself cannot be used in the direct
carboxylation of compounds with active hydrogen. To the best of our
knowledge in the scientific literature can be found only one example of
direct utilization of K2CO3 in the carboxylation of acetophenone with CO2 by
implementation of the mechanochemistry technique (Haruki, H. JP 55151532,
1980; Chem. Abstr. 1981, 94, 174894).