Reversible carboxylation of N-heterocyclic carbenes

Article abstract of DOI:10.1039/b311350g

Spectroscopic analysis, thermogravimetric analysis, and cross-over experiments performed on a series of imidazolium carboxylates revealed carboxylation was reversible with N-aryl substituted adducts.

Full text of DOI:10.1039/b311350g

Reversible carboxylation of N-heterocyclic carbenes†
Hung A. Duong, Thomas N. Tekavec, Atta M. Arif and Janis Louie*
Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, UT, USA 84112.
E-mail: louie@chem.utah.edu; Fax: (801) 581-8433; Tel: (801)581-7309
Received (in Cambridge, UK) 17th September 2003, Accepted 28th October 2003
First published as an Advance Article on the web 10th November 2003
Spectroscopic analysis, thermogravimetric analysis, and cross-
over experiments performed on a series of imidazolium
carboxylates revealed carboxylation was reversible with N-aryl
substituted adducts.
carboxylate charge into the imidazole ring as its plane was
perpendicular to the plane of the CO2 group. The crystal structure
also indicated that the methine protons on the isopropyl groups
were twisted inward toward the imidazolylidene plane. This was
also observed in solution as two doublets for the isopropyl methyls
were observed in the 1H NMR spectrum of compound 3. Combined,
this suggested that their rotation was restricted.
Using 13C NMR analysis, signals attributable to the carboxylate
carbon were observed in all complexes. In addition, no free IPr or
CO2 was observed in solution, even after prolonged periods of time,
which indicated a high coordination constant between the two
species.16 Furthermore, the addition of 13CO2 (1 atm) to a solution
of IPrCO2 led to an enhancement in the carbonyl peak of the
IPrCO2 and suggested that the bound CO2 was equilibrating with
free 13CO2. As expected, reintroduction of CO2 (1 atm) attenuated
the signal.
Since their isolation by Arduengo in 1991,1 N-heterocyclic
carbenes (NHCs) have been widely used compounds in several
areas in chemistry. For example, when used as ligands for transition
metals, they often enhance performance and selectivity.2 Independ-
ently, they have been shown to function as nucleophilic organic
catalysts for transesterifications,3,4 acylations,5 and ring-opening
polymerizations.6 Although NHCs are typically air and moisture
sensitive, Waymouth demonstrated how a biphasic system that
generates catalytic amounts of NHCs from imidazolylidene-based
ionic liquids can be used to circumvent the need to use Schlenk
techniques.7 Similar biphasic systems have been recently integrated
with supercritical CO2 to create environmentally friendly separa-
tion systems.8–10 While studying the use of NHCs as ligands for
metal-mediated cycloadditions,11 we discovered that NHCs freely
add CO2 to form the corresponding imidazolium carboxylates.
Herein, we report the synthesis of a series of imidazolium
carboxylates and present evidence which indicates that their
formation is reversible. These results should be of prime im-
portance to the aforementioned programs.
Thermogravimetric analysis provided further evidence for
decarboxylation in 2 and 3. Clean loss of 10.6% of the weight of
There are very few examples in the chemical literature of
synthesizing and studying imidazolium carboxylates. Although
Kuhn demonstrated that the direct addition of CO2 to 1,3-diisopro-
pyl-4,5-dimethylimidazol-2-ylidene (iPrIm) cleanly afforded the
corresponding carboxylate,12 mechanistic details were not pro-
vided. The only other known example was reported by Holbrey in
their “unexpected synthesis” of 1,3-dimethylimidazolium-2-car-
boxylate from the reaction between 1-methylimidazole and dime-
thyl carbonate.13,14
We discovered that the addition of CO2 (1 atm) to iPrIm,
1,3-dimesitylimidazol-2-ylidene (IMes), or 1,3-bis(2,6-diisopro-
pylphenyl)imidazol-2-ylidene (IPr) afforded the corresponding
zwitterionic imidazolium carboxylate quantitatively (Scheme 1,
Pathway A).‡ However, carboxylates 2 (IMes CO2) and 3 (IPrCO2)
were also prepared using a more convenient protocol. Simply
deprotonating the corresponding salts with potassium tert-butoxide
under an atmosphere of CO2 (Pathway B) afforded the products in
high yields (75–95%). All carboxylates were sparingly soluble in
THF, aromatic solvents, and water. However, carboxylates 2 and 3
displayed enhanced solubilities in methanol, DMSO, and chlorin-
ated solvents. Although air-stable as solids, solutions of 2 and 3
decomposed readily in aerated solutions.
Scheme 1 Synthesis of imidazolium carboxylates 1, 2 and 3.
The X-ray crystal structure of carboxylate 3 was determined15
and is shown in Fig. 1.§ The near equivalent C–O bond distances
(1.221(4) and 1.225(4) Å) indicated that the negative charge was
equally distributed between the central carbon (C6) and the two
oxygens (O1 and O2). In contrast, the two C–N bond distances are
substantially different (1.334(4) Å vs. 1.376(4) Å) which suggested
some delocalization of the positive charge between a single N atom
and the aryl ring. There appeared to be no delocalization of the
† Electronic supplementary information (ESI) available: crystal structure
data and ORTEP for 1,3-bis(2,6-diisopropylphenyl)imidazolylium-2-car-
boxylate (3). See http://www.rsc.org/suppdata/cc/b3/b311350g/
Fig. 1 Molecular structure of IPrCO2 (3). Selected bond lengths (Å) and
angles (°): C(6)–O(1) 1.225(4), C(6)–O(2) 1.221(4), C(2)–N(1) 1.334(4),
C(2)–N(3) 1.332(4), O(1)–C(6)–O(2) 129.9(3), N(1)–C(2)–N(3) 107.1(2).
112
C h e m . C o m m u n . , 2 0 0 4 , 1 1 2 – 1 1 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

20.8, 125.0, 129.5, 131.1, 134.5, 138.6, 140.8, 149.2. IR (KBr): 1675 cm21
(vs). mp: 315 °C (dec). Anal. Calc. for C22H24N2O2: C, 75.83; H, 6.94; N,
8.04. Found: C, 75.57; H, 6.98; N, 7.89%.
3: dH(300 MHz, CD2Cl2) 1.22 (d, 12H, 3JH–H = 6.9 Hz, CH(CH3)2), 1.28
3
(d, 12H, JH–H = 6.6 Hz, CH(CH3)2), 2.53 (sept., 2H, CH), 7.19 (s, 2H,
NCHCHN), 7.35 (d, 4H, 3JH–H = 7.8 Hz, ArH), 7.55 (t, 2H, 3JH–H = 7.8
Hz, ArH). dC(75.6 MHz, CD2Cl2): 23.4, 24.3, 29.4, 122.9, 124.3, 130.8,
132.8, 145.1, 147.6, 152.3. IR (KBr): 1678 cm21 (vs). mp: 216 °C (dec).
Anal. Calc. for C28H36N2O2: C, 77.74; H, 8.39; N, 6.48. Found: C, 77.71;
H, 8.34; N, 6.42%.
General procedure for the synthesis of 2 and 3 through carboxylation of
the imidazolium salts: An oven-dried two-neck round-bottomed flask
equipped with a magnetic stir bar, septum, gas adapter and balloon was
evacuated and filled with CO2. A solution of the imidazolylidene salt and
potassium tert-butoxide (1.2 equiv.) was then added and the reaction was
stirred at room temperature for 2–6 hours. For the synthesis of 2, the solvent
was removed and the residue was extracted with CH2Cl2. The solution was
filtered through Celite and concentrated in vacuo. Subsequent washing with
Et2O afforded the imidazolium carboxylate as a white solid in 95% yield.
For the synthesis of 3, hexane was added to induce precipitation. The
precipitate was collected via filtration and dissolved in CH2Cl2. The
solution was then filtered through Celite and concentrated in vacuo.
Subsequent washing with Et2O afforded the imidazolium carboxylate as a
white solid in 75% yield.
Fig. 2 TGA of IPrCO2 (3) under N2 atmosphere. Ramp rate = 5 °C
min21
.
IPrCO2 3 (corresponding to the mass of CO2) was observed
between 136 and 164 °C (Fig. 2). Decomposition occurred at 250
°C. IMesCO2 2 appeared to have a higher stability toward
decarboxylation as weight loss began at 187 °C and continued until
decomposition ensued (203 °C). No decarboxylation was observed
for iPrImCO2 (1) which decomposed at 160 °C.
Crossover experiments between free imidazolylidenes and their
carboxylates revealed a trend in their relative stabilities (Equations
1–5). The addition of iPrIm to either IMesCO2 (2) or IPrCO2 (3) led
to quantitative formation of iPrImCO2 (1) and IMes or IPr,
respectively. The reverse addition of IMes or IPr to iPrImCO2 (1)
gave no reaction. Similarly, the addition of IMes to IPrCO2 (3) led
to the quantitative formation of free IPr and IMesCO2 (2), but the
reverse reaction was not observed. Thus, the relative order of
adduct stability appeared to be: iPrImCO2 (1) > IMesCO2 (2) >
IPrCO2 (3).17 The same trend in relative stabilities was extracted
from our TGA data.
§ Crystal data for C28H36N2O2 3: M
= 432.59, monoclinic, a =
12.6385(7), b = 12.5779(8), c = 16.6868(12) Å, b = 92.993(2)°, U =
2649.0(3) Å3, T = 150 K, space group P21/n, Z = 4, m(Mo-K) = 0.71073
Å, 9106 reflections collected, 4822 unique (Rint = 0.0721) which were used
in all calculations. The R1 was 0.1638 and the final wR(F2) was 0.2314 (all
data). Single crystals of 3 were grown from slow diffusion of CO2 into a
solution of IPr in THF at room temperature. The crystals were mounted in
inert oil and transferred to a cold gas stream in the diffractometer. The
structure was solved by a combination of direct methods and heavy atoms
using SIR 97 and refined by full-matrix least-squares on F2. CCDC 219086.
See http://www.rsc.org/suppdata/cc/b3/b311350g/ for crystallographic data
in .cif or other electronic format.
(1)
(2)
IMesCO2 (2) + iPrIm ? IMes + iPrImCO2 (1)
IPrCO2 (3) + iPrIm ? IPr + iPrImCO2 (1)
1 A. J. Arduengo, R. L. Harlow and M. J. Kline, J. Am. Chem. Soc., 1991,
113, 361.
2 W. A. Herrmann, T. Weskamp and V. P. W. Bohm, Adv. Organomet.
Chem., 2001, 48, 1.
3 G. W. Nyce, J. A. Lamboy, E. F. Connor, R. M. Waymouth and J. L.
Hendrick, Org. Lett., 2002, 4, 3587.
(3)
4 G. A. Grasa, R. M. Kissling and S. P. Nolan, Org. Lett., 2002, 4,
3583.
5 G. A. Grasa, T. Guveli, R. Singh and S. P. Nolan, J. Org. Chem., 2003,
68, 2812.
(4)
(5)
IPrCO2 (3) + IMes ? IMesCO2 (2) + IPr
IMesCO2 (2) + IPr ? no reaction
In conclusion, free N-heterocyclic carbenes were converted to
their respective imidazolylidene carboxylates by introducing them
to CO2. The adducts were found to readily exchange their
carboxylate groups with free CO2 in solution and with relatively
less stable imidazolium carboxylates. Likewise, adduct stability
also affected the onset temperature of a thermally induced
decarboxylation. Collectively, our data indicate that the formation
of imidazolium carboxylates may be reversible under certain
conditions. These results should be considered when working with
imidazolylidene or NHC-based ligands or ionic liquids in the
presence of CO2.
6 E. F. Connor, G. W. Nyce, A. Möck and J. L. Hendrick, J. Am. Chem.
Soc., 2002, 124, 914.
7 G. W. Nyce, T. Glauser, E. F. Connor, A. Möck, R. M. Waymouth and
J. L. Hedrick, J. Am. Chem. Soc., 2003, 125, 3046.
8 T. Welton, Chem. Rev., 1999, 99, 2071.
9 A. M. Scurto, S. N. V. K. Aki and J. F. Brennecke, J. Am. Chem. Soc.,
2002, 124, 10276.
10 F. Liu, M. B. Abrams, R. T. Baker and W. Tumas, Chem. Commun.,
2001, 433.
11 J. Louie, J. E. Gibby, M. V. Farnworth and T. N. Tekavec, J. Am. Chem.
Soc., 2002, 124, 15188.
12 N. Kuhn, E. Niquet, M. Steimann and I. Walker, Z. Naturforsch., Teil B,
1999, 54, 427.
13 J. D. Holbrey, W. M. Reichert, I. Tkatchenko, E. Bouajila, O. Walter, I.
Tommasi and R. D. Rogers, Chem. Commun., 2003, 28.
14 In addition, a dihydroimidazolylidenecarboxylate is known, see: W.
Schössler and M. Regitz, Chem. Ber., 1974, 107, 1931.
15 Carboxylate 3 has a similar structure to 1,3-dimethylimidazolium-
2-carboxylate and 1,3-diisopropyl-4,5-dimethylimidazolium-2-carbox-
ylate (1). A notable difference is that the CO2 groups on the latter
complexes are not perpendicular to the imidazole plane.
16 K. A. Connors, Binding Constants, John Wiley & Sons, New York,
1987.
We gratefully acknowledge the University of Utah and the
Petroleum Research Fund (Type G) for support of this research.
Notes and references
‡
iPrIm, IMes, and IPr were prepared using literature procedures.1,18
General procedure for the synthesis of 1, 2 and 3 through direct
carboxylation of the carbenes: In a Schlenk flask, a solution of N-
heterocyclic carbene in ether was cooled to 278 °C and CO2 (1 atm) was
added via condensation. The solution was warmed to room temperature and
a white precipitate formed. The precipitate was collected by filtration and
washed with ether to afford the imidazolylidene carboxylate in 85, 95, and
90% yields for 1, 2 and 3, respectively.
17 Semi-empirical (PM3) calculations performed using the Titan Software
Program (Wavefunction, Inc., Irvine, CA 92612, USA) support this
trend.
2: dH(500 MHz, DMSO-d6) 2.09 (s, 12H, ArCH3), 2.31 (s, 6H, ArCH3),
7.08 (s, 4H, ArH), 7.87 (s, 2H, NCHCHN). dC(126 MHz, DMSO-d6): 17.0,
18 N. Kuhn and T. Kratz, Synthesis, 1993, 561.
C h e m . C o m m u n . , 2 0 0 4 , 1 1 2 – 1 1 3
113