A systematic investigation of factors influencing the decarboxylation of imidazolium carboxylates

Article abstract of DOI:10.1021/jo901791k

(Chemical Equation Presented) A series of 1,3-disubstituted-2-imidazolium carboxylates, an adduct of CO₂ and N-heterocyclic carbenes, were synthesized and characterized using single crystal X-ray, thermogravimetric, IR, and NMR analysis. The TG

Full text of DOI:10.1021/jo901791k

pubs.acs.org/joc
A Systematic Investigation of Factors Influencing the Decarboxylation of
Imidazolium Carboxylates
Bret R. Van Ausdall, Jeremy L. Glass, Kelly M. Wiggins, Atta M. Aarif, and Janis Louie*
Department of Chemistry, Henry Eyring Building, University of Utah, 315 S. 1400 East, Salt Lake City,
Utah 84112-0850
louie@chem.utah.edu
Received August 28, 2009
A series of 1,3-disubstituted-2-imidazolium carboxylates, an adduct of CO2 and N-heterocyclic carbenes,
were synthesized and characterized using single crystal X-ray, thermogravimetric, IR, and NMR analysis.
The TGA analysis of the NHC-CO2’s shows that as steric bulk on the N-substituent increases, the ability of
the NHC-CO2 to decarboxylate increases. The comparison of NHC-CO2’s with and without methyls at
the 4,5-position indicate that extra electron density in the imidazolium ring enhances the stability of an
NHC-CO2 thereby making it less prone to decarboxylation. Single crystal X-ray analysis shows that the
torsional angle of the carboxylate group and the C-CO2 bond length with respect to the imidazolium ring
is dependent on the steric bulk of the N-substituent. Rotamers in the unit cell of a single crystal of
ItBuPrCO2 (2f) indicate that the C-CO2 bond length increases as the N-substituents rotate toward the
carboxylate moiety, which suggests that rotation of the N-substituents through the plane of the C-CO2
bond may be involved in the bond breaking event to release CO2.
Introduction
has been exploited to quench polymerizations catalyzed by
NHCs.10 More recently, imidazolium carboxylates them-
selves have demonstrated the ability to catalyze reactions
such as the cyclotrimerization of isocyanates11 and the
coupling of epoxides and carbon dioxide.12 Finally, imidazolium
The capture of carbon dioxide by organic compounds has
been a long-standing interest in organic chemistry.1-4
Although the discovery of the reactivity of imidazolidenes
to capture CO2 was discovered a decade ago (eq 1),5,6 this
reaction and the resulting imidazolium carboxylates remain
under-utilized. Initially, imidazolium carboxylates have
been used as an air-stable precursor to imidazolidenes,7
which are both highly synthetically useful ligands for transi-
tion metal catalysts8 and potent nucleophilic organocata-
lysts.9 In addition, the ability for NHCs to react with carbon
dioxide (and other heterocumulenes) to afford stable zwitterions
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DOI: 10.1021/jo901791k
r
Published on Web 09/23/2009
J. Org. Chem. 2009, 74, 7935–7942 7935
2009 American Chemical Society

Van Ausdall et al.
SCHEME 1. Synthesis of Imidazo(in)ium Carboxylates
JOCArticle
carboxylates have been shown to act as a CO2 delivery agent
in the carboxylation of acetophenone.13 Despite these ad-
vances, the utility of imidazolium carboxylates remains
sparse. In an effort to increase the function of these interest-
ing carboxylates, we have synthesized a large array of
imidazolium carboxylates where we have systematically
altered the N-substituent and studied their propensity to
undergo thermal decarboxylation.
Results and Discussion
Synthesis of Imidazolium Carboxylates. A series of imida-
zolium carboxylates were prepared (Scheme 1). The smallest
carboxylate (2a) was synthesized using Crabtree’s modifica-
tion of a literature procedure.7 In all other cases, we found
imidazolium carboxylates (2b-2h) were generated cleanly and
in excellent yields from direct carboxylation of the NHC
precursors. Simple N-alkyl (1b-1e) and -aryl imidazole
(1f-1h) and -aryl imidazolin (1i-1j) carbenes were generated
in situ from deprotonation of the corresponding imidazol(in)-
ium salt with potassium hexamethyldisilylazide (KHMDS) in
toluene. Interestingly, the standard deprotonation methods
such as catalytic amounts of KOtBu with NaH14 generally led
to contaminated carboxylate. Furthermore, when 1 equiv of
KOtBu was used to deprotonate the imidazolium salts, the tert-
butanol that was produced could not be separated effectively
from the carbene. Ultimately, carboxylates that were deter-
mined to be pure by elemental analysis were obtained through
the reaction of carbon dioxide and carbenes that were prepared
in situ via the deprotonation of the imidazol(in)ium salts in
toluene with KHMDS. Importantly, the carbenes were filtered
away from the potassium halide salt byproduct before expo-
sure to carbon dioxide. This step is critical to obtain pure, salt-
free imidazoylium carboxylates (2b-2j, vide infra). N-Alkyl
(5) (a) Kuhn, N.; Niquet, E.; Steimann, M.; Walker, I. Z. Naturforsch., B
1999, 54, 427. (b) Ishiguro, K.; Hirabayashi, K.; Nojima, T.; Sawaki, Y.
Chem. Lett. 2002, 796. (c) Holbrey, J. D.; Reichert, W. M.; Tkatchenko, I.;
Bouajila, E.; Walter, O.; Tommasi, I.; Rogers, R. D. Chem. Commun. 2003,
28. (d) Duong, H. A.; Tekavec, T. N.; Arif, A. M.; Louie, J. Chem. Comm.
2004, 112. (e) Tudose, A.; Demonceau, A.; Delaude, L. J. Organomet. Chem.
2006, 691, 5356.
carbenes that possess a methylated backbone (1aMe-1cMe
)
were prepared from the reduction of the corresponding thio-
urea with potassium and isolated prior to carboxylation.15
Reactions with Water. The stability of the imidazolium
carboxylates toward water was evaluated. When H2O was
introduced to a CD2Cl2 solution of IMeCO2 (2a), IEtCO2
(2b), IiPrCO2 (2c), or ItBuCO2 (2d), protonation occurred
within minutes as indicated by the appearance of a new
singlet at 9.10 ppm in the respective 1H NMR spectra.
Interestingly, an imidazolium carbonate16 was formed that
resulted from decarboxylation in addition to protonation
(eq 2). Rogers and co-workers have prepared 3aOCOOH
through a two-step reaction of 2a and aqueous carbonic acid.
(6) In addition, a dihydroimidazolylidene carboxylate is known; see:
€
Schossler, W.; Regitz, M. Chem. Ber. 1974, 107, 1931.
(7) (a) Voutchkova, A. M.; Appelhans, L. H; Chianese, A. R.; Crabtree,
R. H. J. Am. Chem. Soc. 2005, 127, 17624–17625. (b) Voutchkova, A. M.;
Feliz, M.; Clot, E.; Eisenstein, O.; Crabtree, R. H. J. Am. Chem. Soc. 2007,
129, 12834–12846.
(8) (a) Weskamp, T.; Schattemann, W. C..; Spiegler, M.; Hermann, W. A.
Angew. Chem., Int. Ed. 1998, 37, 2490. (b) Scholl, M.; Ding, S.; Lee, C. W;
Grubbs, R. H. Org. Lett. 1999, 1, 953. (c) Huang, J.; Stevens, E. D.; Nolan,
S. P.; Peterson, J. L. J. Am. Chem. Soc. 1999, 121, 2674. (d) Hoveyda, A. H.;
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Schrock, R. R. Chem.;Eur. J. 2001, 7, 945. (e) Furstner Angew. Chem., Int.
Ed. 2000, 39, 3012. (f) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34,
18. (g) Louie, J.; Gibby, J. E.; Farnworth, M. V.; Tekavec, T. N. J. Am. Chem.
Soc. 2002, 124, 15188. (h) Ho, C.; Jamison, T. F. Angew. Chem., Int. Ed. 2007,
46, 782–785. (i) Veige, A. S. Polyhedron 2008, 27, 3177.
(9) (a) Connor, E. F.; Nyce, G. W.; Myers, M.; Mock, A.; Hedrick, J. L.
J. Am. Chem. Soc. 2002, 124, 914–5. (b) Suzuki, Y.; Yamauchi, K.;
Muramatsu, K.; Sato, M. Chem. Commun. 2004, 2770–2771. (c) Sohn,
S. S.; Rosen, E. L.; Bode, J. W. J. Am. Chem. Soc. 2004, 126, 14370–
14371. (d) Scholten, M. D.; Hedrick, J. L.; Waymouth, R. M. Macromole-
cules 2008, 41, 7399–7404. (e) Merino, P.; Marques-Lopez, E.; Tejero, T.;
Herrera, R. P. Tetrahedron 2009, 65, 1219–1234.
(10) Culkin, D. A.; Jeong, W.; Csihony, S.; Gomez, E. D.; Balsara, N. P.;
Hedrick, J. L.; Waymouth, R. M Angew. Chem., Int. Ed. 2007, 46, 2627–2630.
(11) Duong, H. A.; Cross, M. J; Louie, J. Org. Lett. 2004, 6, 4679–4681.
(12) Zhou, H.; Zhang, W.; Liu, C.; Qu, J.; Lu, Z. J. Org. Chem. 2008, 73,
8039.
In contrast to N-alkyl imidazolium carboxylates, decar-
boxylation does not readily occur when water is added to
(13) (a) Tommasi, I.; Sorrentino, F. Tetrahedron Lett. 2005, 46, 2141–
2145. (b) Tommasi, I.; Sorrentino, F. Tetrahedron Lett. 2006, 47, 6453–6456.
(14) Alcarazo, M.; Roseblade, S. J.; Cowley, A. R.; Fernandez, R.;
Brown, J. M.; Lassaletta, J. M. J. Am. Chem. Soc. 2005, 127, 3290–3291.
(15) Kuhn, N.; Kratz, T. Synthesis 1993, 561–562.
(16) Bridges, N. J.; Hines, C. C.; Smiglak, M.; Rogers, R. D. Chem.;Eur.
J. 2007, 13, 5207–5212.
7936 J. Org. Chem. Vol. 74, No. 20, 2009

Van Ausdall et al.
JOCArticle
N-aryl imidazolium carboxylates. Specifically, when H2O
was added to a solution of 2h in CD2Cl2, the signature
imidazolium proton signal at 9.10 ppm that was observed
with the alkyl carboxylates 2a-2d did not appear. Instead, a
new set of signals for the aryl and backbone protons was
observed alongside the original signals for starting carbo-
xylate, 2h. The backbone, ortho, and meta aryl protons all
moved downfield, shifting from 7.17 to 7.83 ppm, 7.28 to
7.37 ppm, and 7.51 to 7.6 ppm, respectively. We tentatively
assign this species to an imidazolium carboxylate where the
carboxylate is hydrogen-bonded to a water molecule (eq 3).
The same phenomenon was observed when H2O was added
to aryl carboxylate IMesCO2 (2g).
TABLE 1. IR Stretching Frequencies of Imidazol(in)ium Carboxylates
The addition of water to 2h and 2a in the presence of
NaBPh4 led to rapid and smooth decarboxylation (eq 4).
IPrBPh4 and IMeBPh4, which both possess a distinct acidic
1
proton (∼9 ppm in the H NMR spectrum), and sodium
carbonate were formed quantitatively.
IR Frequency Analysis. The imidazolium carboxylates
display distinct carbonyl stretching frequencies (Table 1).
N-Alkyl imidazolium carboxylates (2a-2e) have COO-
asymmetric stretching frequencies that are in the low to
mid 1600 cm-1. A slight trend between the stretching fre-
quencies and the size of the N-substituent was observed. As
the alkyl substituent changes from Me to Et to iPr (2a, 2b,
2c), the CdO stretching frequencies gradually increase.
However, when the alkyl substituent is replaced with the
t
bulky Bu group (2d), the CdO stretching dramatically
decreases by 37 cm-1. The hybrid NHC-CO2 2e has an
intermediate frequency of 1647 cm-1, between IMeCO2
(2a) and ItBuCO2 (2d).
FIGURE 1. TGA of alkyl NHC-CO2’s 2a-2d.
N-Aryl imidazolium carboxylates have higher CdO
stretching frequencies than their N-alkyl counterparts. Inter-
estingly, the stretching frequencies of IMesCO2 (2g) and
IPrCO2 (2h) are almost identical, which suggests that the
carbon-oxygen bond is less affected by the ortho substitu-
ents (i.e., the methyl of the IMesCO2 and the iPr of the
IPrCO2) than in the N-alkyl series (i.e., 2a vs 2c).
(TGA). During the investigation, we found that the amount
of imidazolium carboxylate that was subjected to TGA had a
profound effect on the results. For example, the TGA
analysis of larger samples of IPrCO2 (2h) afforded higher
decarboxylation temperatures than smaller samples.17 As
such, subsequent TGAs were consistently performed with
3.5 mg of imidazolium carboxylate.
Modification of the imidazolium backbone, through
either methylation or saturation, does not have a large
influence on the CdO stretching frequencies. For example,
TGA of N-Alkyl Imidazolium Carboxylates (2a-2d). The
TGA of a series of N-alkyl NHC-CO2’s where the size of the
N-alkyl substituent was increased in size (i.e., Me (2a), Et
the stretching frequency of 2b and 2bMe differ by only 3 cm-1
.
t
(2b), iPr (2c), and Bu (2d)) is shown in Figure 1. It is clear
In addition, methylation of the backbone causes the CdO
stretching frequency to increase for 2aMe (relative to 2a) but
causes a decrease for 2cMe (relative to 2c). Imidazolinium
carboxylates (2i and 2j) displayed stretching frequencies that
were only 5 cm-1 higher than their unsaturated analogues.
Thermogravimetric Analysis. The imidazolium carboxy-
lates were each evaluated by thermogravimetric analysis
that an increase in substituent size leads to a decrease
in decarboxylation temperature. At the two extremes, IM-
eCO2 (2a) begins to decompose at 162 °C, whereas ItBuCO2
(2d) loses CO2 and decomposes at a much lower temperature
(17) See Supporting Information for the TGA data with IPrCO2 at varied
masses.
J. Org. Chem. Vol. 74, No. 20, 2009 7937

Van Ausdall et al.
JOCArticle
TABLE 2. Calculated pKa’s of Carboxylate Precursor Carbenes with
Decarboxylation Temperatures of the Corresponding NHC-CO2
0
FIGURE 2. TGA analysis of alkyl NHC-CO2 s 2aMe-2cMe and
2a-2c.
(71 °C) for a difference in decarboxylation temperatures of
91 °C. Only IEtCO2 (2b) does not seem to follow this trend
and undergoes decarboxylation at 128 °C, i.e., 12 °C lower
than the decarboxylation temperature of IiPrCO2 (2c). Inter-
estingly, only ItBuCO2 displays a biphasic TGA curve
suggesting that a short-lived intermediate, presumably ItBu,
is generated. Nevertheless, CO2 was detected via mass spec-
trometry at the onset of weight loss in each TGA analysis of
2a-2d.
TGA of N-Alkyl Imidazolium Carboxylates Possessing a
Methylated Backbone (2aMe-2dMe). The collective TGA
spectra of the 4,5-dimethyl N-alkyl NHC-CO2 compounds
2aMe-2cMe as well as 2a-2c are shown in Figure 2 and are
summarized in Table 2. In general, imidazolium carboxy-
lates possessing increasing steric hindrance of the N-alkyl
substituent displayed lower decarboxylation temperatures.
For example, decarboxylation began at 182 °C for IMeMe-
CO2 (2aMe) and at 139 °C for IiPrMeCO2 (2cMe).
An interesting effect caused by the methylation of the
backbone of the NHC-CO2’s was also observed. The TGAs
of 2aMe displayed a 20 °C increase in the decarboxylation/
decomposition temperatures over 2a (i.e., 182 and 162 °C,
respectively). In contrast, 2cMe and 2c, both of which possess
isopropyl N-substituents, have almost identical decarboxy-
lation/decomposition temperatures (i.e., a difference of only
1 °C, entries 4 and 5, Table 2).
0
FIGURE 3. TGA analysis of aryl NHC-CO2 s 2g-2j, 2gMe, and
2hMe
.
stability of the Ccarbene-CO2 bond provided by the extra
electron density at the carbene.
TGA of N-Aryl Imidazolium Carboxylates (2g-2j,
2gMe-2hMe). With NHC-CO2’s 2g-2j, 2gMe, and 2hMe in
hand, the TGAs of structurally similar, but electronically
different, aryl NHC-CO2’s could be compared (Figure 3). As
noted above, methylation of the backbone on the imidazole
ring results in an increased pKa. In the computational study
by Yates, saturation of the backbone purportedly leads to a
loss in aromaticity in the imidazole ring thereby also result-
ing in a higher pKa. The NHC-CO2 series (2h, 2hMe, and 2j)
possessing N-(2,6-diisopropyl)phenyl substituents (i.e., IPr)
displays decarboxylation temperatures that correlate di-
rectly to the increased electron density in the imidazole ring.
Computations performed by Yates et al. have shown that
methylation of the backbone increases the basicity of a
carbene relative to that of the unsaturated parent carbene
(Table 2).18 As electron density of a particular carbene
increases, the NHC-CO2 would most likely possess a stron-
ger Ccarbene-CO2 bond, which would result in an elevated
decarboxylation temperature. Thus, the higher decarboxyla-
tion temperature and inferred increased C-CO2 bond
strength of 2aMe (182 °C) relative to that of 2a (162 °C)
may be attributed to the higher pKa of the parent carbene
(i.e., 1aMe versus 1a). Although 1cMe has a higher pKa
For example, both SIPrCO2 (2j) and IPrMeCO2 (2hMe
)
possess higher decarboxylation temperatures than IPrCO2
(2h). SIPrCO2 (2j) decarboxylates at 136 °C and IPrMeCO2
(2hMe) decarboxylates at 120 °C, while IPrCO2 (2h) decar-
boxylates at 108 °C. Biphasic decomposition was observed in
all cases, similar to what was observed in the TGA analysis of
ItBuCO2 (2d). The biphasic decomposition suggests that a
stable intermediate is formed after decarboxylation. Indeed,
when the TGA of IPrCO2 (2h) was interrupted at 108 °C, the
1H NMR analysis of the residual solid displayed a spectrum
identical to an authentic sample of the parent NHC, IPr.
Thus, in some cases, decarboxylation occurs at a lower
temperature than the decomposition of the parent carbenes.
When the N-substituent is replaced with (2,4,6-trimethyl)-
phenyl groups (i.e., IMes), the decarboxylation tempera-
ture for IMesMeCO2 (2hMe), which possesses a methylated
backbone, is once again higher than for IMesCO2 (2h) (193
than 1c, their corresponding carboxylates, 2c and 2cMe
,
decarboxylate at almost identical temperatures. Thus,
as the N-substituents become larger, the steric bulk of
the highly branched N-substituents overrides the enhanced
(18) Magill, A. M.; Cavell, K. G.; Yates, B. F. J. Am. Chem. Soc. 2004,
126, 8717–8724.
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Van Ausdall et al.
JOCArticle
TABLE 3. IR, Decarboxylation Temperature and C-CO2 Bond
Lengths of the NHC-CO2’s
C-CO2
length
bond (A)
νCOasym
-CO2
temp (°C)
entry
carboxylate
(cm-1
)
˚
1
2
3
4
5
6
7
8
9
10
11
12
13
45
15
2a
32
2c
2d
1653
1654
1666
1629
1647
1669
1657
1662
1675
1675
1674
1678
1683
1680
1683
162
128
140
71
1.523
NA
NA
NA
2e
117
182
144
139
129
155
193
108
136
156
120
NA
2aMe
2bMe
2cMe
2f
2g
2gMe
2h
2hMe
2i
2j
1.521
1.535
1.536
1.525-1.544
NA
0
FIGURE 4. TGA analysis of asymmetric NHC-CO2 s 2e and 2f.
NA
1.536
1.542
NA
stretching frequencies, and C-CO2 lengths are listed in
Table 3.
An increase in the size of the N-alkyl substituent causes the
N-C2 bond length to decrease. Specifically, this bond length
1.535
˚
decreases from 1.359 to 1.341 to 1.336 A in IMeMeCO2
and 155 °C, respectively). However, the saturated analogue
SIMesCO2 (2j) decarboxylates at 156 °C, a temperature that
is not markedly different than the decarboxylation tempera-
ture of IMesCO2 (2h). Thus, simple pKa effects may not be
the only factor that determines the decarboxylation ability of
the NHC-CO2 complexes. Alternatively, the difference in
electron density of saturated and unsaturated NHCs may
not be significant. Indeed, Nolan et al. reported the CO
stretching frequencies of various saturated and unsatured
NHC-ligated metal carbonyl were almost identical suggest-
ing similar σ-donor capabilities of saturated and unsaturated
NHCs.19 Furthermore, conflicting reports regarding the
effect of saturation on the electron density of the NHC
exist.20
(2aMe), IEtMeCO2 (2bMe), and IiPrMeCO2 (2cMe), respec-
tively. In contrast, the N1-C2-N2 bond angle steadily
increases from 105.32° in 2aMe to 108.02° in 2cMe. Less of
an effect is observed on the C6-O bond lengths. For
example, IEtMeCO2 (2bMe) and IiPrMeCO2 (2cMe) both pos-
sess a C6-O bond length of 1.244 A, although this bond
length is significantly longer than the C6-O bond length of
2aMe (1.230 A). A similar phenomenom is observed with the
˚
˚
C2-C6 bond lengths. that is, the C2-C6 bond lengths of
IEtMeCO2 (2bMe) and IiPrMeCO2 (2cMe) do not greatly differ
˚
(1.535 A and 1.536 A, respectively) but are significantly
larger than the C2-C6 bond length of 2aMe (1.521 A). A
˚
˚
direct correlation exists between the C2-C6 bond lengths
and decarboxylation/decomposition temperature. 2aMe has
both a significantly smaller bond length and a higher dec-
arboxylation temperature than 2bMe and 2cMe (Table 4). How-
ever, the differences in C2-C6 bond lengths as well as
TGA of Asymmetric Imidazolium Carboxylates 2e and 2f.
The TGA of asymmetric imidazolium carboxylates IMeIt-
BuCO2 (2e) and ItBuIPrCO2 (2f) were also evaluated
(Figure 4). Not surprisingly, decarboxylation/decomposi-
tion of IMeItBuCO2 (2e) occurred at 117 °C, a temperature
in between the decarboxylation temperatures of IMeCO2
(2a) and ItBuCO2 (2d). In addition, a one-step decarboxy-
lation/decomposition was observed for 2e. However, the
low decomposition temperature of 2e relative to other N-
alkyl imidazolium carboxylates 2a-2c suggests that the
one tert-butyl group plays a significant role in the lowering
the decarboxylation temperature of 2e. Despite the in-
decarboxylation temperatures are small between 2bMe and 2cMe
.
Methylation of the backbone appears to cause predictable
changes to the structure. Both N-C2 bond lengths of
IMeMeCO2 (2aMe) and IPrMeCO2 (2hMe) are longer by
0.014 A than their unmethylated counterparts IMeCO2
(2a) and IPrCO2 (2h). In addition, both C6-O bond lengths
of IMeMeCO2 (2aMe) and IPrMeCO2 (2hMe) are shorter by
˚
0.010 A. Interestingly, less of a difference is observed bet-
ween the C2-C6 bond lengths (i.e., 0.002 and 0.006 A
differences, respectively).
Saturation of the backbone also appears to cause changes
to the structure. For example, the N-C2 bond lengths of
SIPrCO2 (2j) is 0.009 A shorter than that of IPrCO2 (2h). The
˚
t
˚
creased bulkiness of the Bu group relative to the 2,6-
diisopropylphenyl group,19 the decarboxylation tempera-
ture of 2f was strikingly similar to that of IPrCO2 (2h).
However, no prolonged carbene intermediate was ob-
served.
˚
˚
C6-O bond lengths of SIPrCO2 (2j) is 0.015 A longer than
Single Crystal X-ray Analysis. The structures of 2aMe
,
that of IPrCO2 (2h). However, no distinct difference is
observed in the C2-C6 bond length of SIPrCO2 (2j) and
IPrCO2 (2h). Thus, saturation appears to have the opposite
effect on the structures of the imidazolium carboxylates than
methylation of the backbone.
2bMe, 2iMe, and 2f were solved using single crystal X-ray
crystallography. Selected bond lengths, bond angles,
dihedral angles, and structures of all solved NHC-
CO2’s are listed in Table 4.5a,c,11 For comparison, a
summary of all of the decarboxylation temperatures, IR
As steric bulk is introduced into the N-substituents of
an NHC-CO2, the cant of the CO2 moiety becomes
more pronounced with regard to the imidazole ring. The
(19) Dorta, R.; Stevens, E. D.; Scott, N. M.; Costabile, C.; Cavallo, L.;
Hoff, C. D.; Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 2485–2495.
(20) (a) Khramov, D. M.; Rosen, E. L.; Er, J. A. V.; Vu, P. D.; Lynch,
V. M.; Bielawski, C. W. Tetrahedron 2008, 64, 6853. (b) Khramov, D. M.;
Lynch, V. M.; Bielawski, C. W. Organometallics 2007, 26, 6042.
carboxylate torsional angles for NHC-CO2’s 2aMe, 2bMe
,
and 2cMe are 22.40°, 49.16°, and 68.96°, respectively. As
the torsional angle of the carboxylate moiety of the
J. Org. Chem. Vol. 74, No. 20, 2009 7939

Van Ausdall et al.
JOCArticle
TABLE 4. Structural Features of 2a, 2aMe, 2bMe, 2cMe, 2i, 2iMe, 2g, and the Rotamer of 2g
NHC-CO2 becomes larger. The decarboxylation tempera-
ture of the NHC-CO2 decreases. The decarboxylation tem-
peratures for 2aMe, 2bMe, and 2cMe are 182, 144, and 139 °C,
respectively. This is further exemplified with IPrCO2 (2h),
where the CO2 group is rotated even more than 2cMe
(i.e., 88°) and possesses a lower decarboxylation temperature
of 108 °C.
Although IPrMeCO2 (2hMe) and SIPrCO2 (2j) possess very
large N-substitutents, methylation and saturation of the
imidazole backbone decrease the cant of the CO2 with
7940 J. Org. Chem. Vol. 74, No. 20, 2009

Van Ausdall et al.
JOCArticle
to residual protiated solvent. IR spectra were collected on a
Bruker Tensor 27 instrument. TGA was performed on a TA
Instruments TGA2050. The TGA data was recorded using
Therma Advantage, ver. 1.14. All TGA analyses were per-
formed in a N2 atmosphere at a heating rate of 5 °C/min. All
glassware was dried in an oven at 130 °C for 24 h prior to use.
Elemental analyses were performed by Midwest Microlabs,
LLC.
Materials. Solvents were purified and deoxygenated by pas-
sing through packed silica columns. All oil from NaH was
removed by thorough washing with hexanes. KOtBu (98%)
was purchased from Sigma-Aldrich and used without further
purification. KHMDS (95%) was purchased from Sigma-Al-
drich and used without further purification. All other reagents
were purchased from standard chemical providers without
further purification, unless specified. All NMR solvents were
thoroughly dried using standard procedures prior to use. The
NHC-CO2 2a was synthesized using a literature procedure.7a
Imidazolium salts 3h and 3i were synthesized using a literature
procedure.21
FIGURE 5. Graph of decarboxylation temperatures versus the
torsional angle of imidazolium carboxylates (i.e., the N1-C2-C6-O1
angle).
General Synthesis A for NHC-CO2’s 2b-2j, 2gMe, and 2hMe
.
respect to the imidazole ring. That is, the carboxylate
torsional angles for IPrMeCO2 (2hMe) and SIPrCO2 (2j) are
46.72° and 57.99°, significantly lower than the 88.14° ob-
served in IPrCO2 (2h). The decreased carboxylate torsional
angles are exemplified in the decarboxylation temperatures.
Indeed, IPrMeCO2 (2hMe) and SIPrCO2 (2j) decarboxylate at
higher temperatures (i.e., 120° for 2j and136° for 2hMe) than
IPrCO2 (2h) (108°).
The imidazolium salt22 (1 equiv) and toluene were put into a 100-
mL round-bottom flask equipped with a stir-bar. To this
suspension was added KHMDS (1 equiv). The solution was
allowed to stir for 2 h before being filtered through Celite. The
carbene solution was then transferred to a 100-mL Schlenk
flask, and CO2 was introduced into the flask. The NHC-CO2
precipitated as a white solid and was collected via filtration,
washed with ether, and dried in vacuo.
2b: General Synthesis A was used with 1,3-diethylimidazo-
lium iodide (0.28 g, 1.2 mmol), toluene (50 mL), and KHMDS
(0.25 g, 1.2 mmol) to afford 1,3-diethylimidazolium-2-carbox-
ylate 2b as a white powder (0.06 g, 35%). 1H NMR (300 MHz,
CD2Cl2): δ 7.05 (s, 2H, HC-CdC-CH), 4.57 (quartet, 4H, J =
7.3 Hz, N-CH2-CH3), 1.491 (t, 6H, J = 7.3 Hz, N-CH2-CH3).
13C NMR (75.6 MHz, CD2Cl2): δ 155.0, 143.4, 120.2, 45.6, 16.2.
IR (KBr) 1654, 1507, 1385, 1350, 1216 cm-1. Anal. Calcd for
C8H12N2O2: C, 57.13; H, 7.19; N, 16.66; O, 19.03; Found: C,
56.79; H, 7.39; N, 16.77; O, 19.97.
2c: General Synthesis A was used with 1,3-diisopropylimida-
zolium iodide (0.72 g, 2.6 mmol), toluene (75 mL), and KHMDS
(0.54 g, 2.6 mmol) to afford 1,3-diisopropylimidazolium-2-
carboxylate 2c as a white powder (0.17 g, 29%). 1H NMR
(300 MHz, CD2Cl2): δ 7.12 (s, 2H, HC-CdC-CH), 5.55
(septet, 2H, J = 6.6 Hz, N-CH-(CH3)2), 1.47 (d, 12H, J = 6.7
Hz, N-CH-(CH3)2). 13C NMR (75.6 MHz, CD2Cl2): δ 155.4,
144.0, 116.6, 51.5, 23.3. IR (KBr) 1666, 1487, 1371, 1329, 1220
cm-1. Anal. Calcd for C10H16N2O2: C, 61.20; H, 8.22; N, 14.27;
O, 16.31; Found: C, 61.35; H, 8.16; N, 14.28; O, 16.51.
Imidazolium carboxylates are more stable when the car-
boxylates lie in the same plane as the imidazolium ring since
this geometry allows for the most resonance stabilization.
Not surprisingly, our data suggest that decarboxylation
temperatures are directly related to the carboxylate torsional
angles (Figure 5). Specifically, an increase in torsional angle
leads to a decrease in decarboxylate temperature. Steric
interactions can lead to an increase in the torsional angle
and subsequently lead to a decrease in decarboxylation
temperature. Although electronic effects can counteract
some of the steric effects (i.e., IPrCO2 (2h) versus IPrMeCO2
(2hMe) versus SIPrCO2 (2j)), the relationship between the de-
carboxylation temperature and the torsional angle remains.
Conclusion
The synthesis of a series of NHC-CO2’s from the reaction
of N-heterocyclic carbene and carbon dioxide allowed for
a thorough study decarboxylating ability of this class
of molecules. Thermogravimetric analysis, single crystal
X-ray crystallography, and IR analysis were used to study
potential variables involved in a particular NHC-CO2’s
ability to decarboxylate. TGA analysis of the NHC-CO2’s
clearly shows that the decarboxylating ability of an NHC-
CO2 is largely dependent on the steric bulk of the N-
subsituent. The TGA analysis of 2a-2d and 2aMe-2cMe also
shows that electronics in this system can be overridden by
steric bulk. Single crystal X-ray of a series of NHC-CO2’s
indicates that the carbon-carbon bond-breaking event may
be mechanical in nature.
2e: General Synthesis A was used with 1-tert-butyl-3-methy-
limidazolium iodide (0.69 g, 2.6 mmol), toluene (75 mL), and
KHMDS (0.54 g, 2.6 mmol) to afford 1-methyl-3-tert-butylimi-
dazolium-2-carboxylate 2e as a gray powder (0.30 g, 48%). 1H
NMR (300 MHz, CD3CN): δ 7.24 (d, 1H, JH-H = 2.0 Hz,
(CH3-N)(H)CdC(H)(N-C(CH3)), 7.08 (d, 1H, JH-H = 2.0 Hz,
(CH3-N)(H)CdC(H)(N-C(CH3)), 3.754 (s, 3H, (CH3-N)(H)-
CdC(H)(N-C(CH3)), 1.713 (s, 9H, (CH3-N)(H)CdC(H)(N-C-
(CH3)). 13C NMR (75.6 MHz, CD3CN): δ.147.7, 119.6, 116.8,
61.7, 36.1, 29.8. IR (KBr) 1647, 1437, 1385, 1342, 12119 cm-1
.
Anal. Calcd for C9H14N2O2: C, 59.32; H, 7.74; N, 15.37; O,
17.56; Found: C, 59.30; H, 7.62; N, 15.34; O, 17.30.
2f: General Synthesis
A
was used with 1-(2,6-
diisopropylphenyl)-3-tert-butylimidazolium tetrefluoroborate
(0.42 g, 1.1 mmol), toluene (75 mL), and KHMDS (0.24 g, 1.1
mmol) to afford 1-(2,6-diisopropylphenyl)-3-tert-butylimidazo-
Experimental Section
General Procedures. All listed procedures were performed
under a N2 atmosphere unless otherwise stated. 1H and 13C
NMR spectra of pure compounds were acquired at 300 and 75
MHz, respectively, unless otherwise noted and were referenced
(21) Nolan, S. P. U.S. Patent 653688, 2003.
(22) See Supporting Information for synthesis of imidazolium salts.
J. Org. Chem. Vol. 74, No. 20, 2009 7941

Van Ausdall et al.
JOCArticle
1
lium-2-carboxylate as a gray powder (0.32 g, 87%). H NMR
(300 MHz, CD3CN): δ 7.50 (t, 1H, J = 7.8 Hz, para-ArH), 7.28
(d, 4H, J = 7.8 Hz, meta-ArH), 6.88 (s, 2H, HCdCH), 2.44
(sept, 1H, J = 6.6 Hz, CH(CH3)2), 1.81 (s, 9H, C(CH3)3), 1.28
(d, 6H, J = 6.6 Hz, CH(CH3)2), 1.09 (d, 6H, J = 6.9 Hz,
CH(CH3)2). 13C NMR (75.6 MHz, CD3CN): δ 148.9, 146.9,
131.5, 124.7, 120.8, 116.9, 29.9, 29.3, 25.5, 23.2. IR (KBr) 1675,
1632, 1478, 1462, 1321, 1303, 1208 cm-1. Anal. Calcd for
C20H28N2O2: C, 73.14; H, 8.59; N, 8.53; O, 9.74; Found: C,
72.85; H, 8.54; N, 8.42; O, 9.91.
2aMe: General Synthesis B was used with 1,3,4,5-methyl-
imidazolylid (1.16 g, 9.3 mmol) and THF (100 mL) to afford
1,3,4,5-methyl-imidazolium-2-carboxylate as a white solid (1.23
g, 78%). 1H NMR (300 MHz, CD2Cl2): δ 3.89 (s, 6H, N-CH3),
2.16 (s, 6H, H3C-CdC-CH3). 13C NMR (75.6 MHz, CD2Cl2):
155.9, 142.8, 125.6, 33.5, 8.9. IR (KBr): 1669, 1510, 1440, 1423,
1315, 1230 cm-1. Anal. Calcd for C8H12N2O2: C, 57.13; H, 7.19;
N, 16.66; O, 19.03. Found: C, 56.85; H, 7.06; N, 16.76; O, 19.21.
2bMe: General Synthesis B was used with 1,3-diethyl-4,5-
methyl-imidazolylid (0.24 g, 1.5 mmol) and THF (50 mL) to
afford 1,3-diethyl-4,5-methyl-imidazolium-2-carboxylate as a
white solid (0.16 g, 52%). 1H NMR (300 MHz, CD2Cl2): δ
4.45 (quart, J = 7.2 Hz, 4H, NCH2,CH3), 2.20 (s, 6H,
CH3CdCH3), 1.38 (t, J = 7.22 Hz, 6H, NCH2CH3). 13C
NMR (75.6 MHz, CD2Cl2): 155.8, 142.5, 124.7, 41.9, 15.9,
8.7. IR (KBr): 1657, 1503, 1456, 1323, 1274, 1209 cm-1. Anal.
Calcd for C10H16N2O2: C, 61.20; H, 8.22; N, 14.27; O, 16.31.
Found: C, 61.41; H, 8.27; N, 14.36; O, 16.41.
Reaction of IPrCO2 with H2O. A heterogeneous solution of
NHC-CO2 in CD3CN was made, and a background spectrum
was obtained. To the sample was added 25 equiv of H2O and
mixed thoroughly, forming a homogeneous solution. Another
spectrum was obtained. The spectra indicate that there is a small
interaction with H2O as noted above. All of the liquid in the
sample was then removed in vacuo, and another spectrum was
obtained with CD3CN, showing only NHC-CO2 peaks.
2gMe: General Synthesis A was used with 1,3-bis(2,4,6-
trimethylphenyl)-4,5-dimethyl-imidazolium chloride (0.50 g,
1.3 mmol), toluene (75 mL), and KHMDS (0.27 g, 1.3 mmol)
to afford 1,3-bis(2,4,6-trimethylphenyl)-4,5-dimethyl-imidazo-
lium-2-carboxylate as a white powder (0.38 g, 75%). 1H NMR
(300 MHz, CD3CN): δ 7.09 (s, 4H, m-ArH), 2.35 (s, 6H, p-ArH),
2.10 (s, 12H, o-Ar-CH3), 1.91 (s, 6H, CH3-CdC-CH3-N). 13C
NMR (75.6 MHz, CD2Cl2): 154.5, 145.9, 140.8, 135.6, 130.8,
129.8, 124.8, 21.5, 17.8, 8.9. IR (KBr): 1674, 1494, 1301 cm-1
.
Anal. Calcd for C24H28N2O2: C, 76.56; H, 7.50; N, 7.44; O, 8.50.
Found: C, 76.37; H, 7.42; N, 7.48; O, 8.63.
2hMe: General Synthesis A was used with 1,3-bis(2,6-
diisopropylphenyl)-4,5-methyl-3-imidazolium chloride (0.50 g,
1.1 mmol), toluene (75 mL), and KHMDS (0.22 g, 1.1 mmol) to
afford1,3-bis(2,6-diisopropylphenyl)-4,5-methyl-3-imidazolium-
1
2-carboxylate, (0.40 g, 75%). H NMR (300 MHz, CD2Cl2):
δ 7.540 (t, 2H, J = 6.8 Hz, 8.4 Hz, para-ArH), 7.31 (d, 4H,
J = 7.8 Hz meta-ArH), 2.38 (septet, 4H, J = 7.05 Hz, IPr-
CH-(CH3)2), 1.95 (s, 6H, CH3-CdC-CH3), 1.24 (d, 12H,
J = 7.0 Hz IPr-CH-(CH3)2), 1.22 (d, 12H, J = 6.9 Hz IPr-
CH-(CH3)2). 13C NMR (75.6 MHz, CD2Cl2): 153.2, 146.6,
145.3, 135.5, 130.9, 126.9, 124.8, 29.7, 24.3, 23.9, 9.6. IR
Reactions of Carboxylates þ MX with H2O. Carboxylates
(1 equiv) were mixed with any MX (M = Li, Na, or K; X =
BPH4, BF4, Cl, or I) salt (1 equiv) in CD3CN. A background
spectrum was obtained prior to insertion of deoxygenated,
deionized H2O (10 equiv). H NMR showed the acidic imida-
zolium proton at ∼9 ppm.
1
(KBr): 1683, 1549, 1467, 1298 cm-1
. Anal. Calcd for
C30H40N2O2: C, 78.22; H, 8.75; N, 6.08; O, 6.95. Found: C,
77.95; H, 8.54; N, 6.05; O, 6.71.
Acknowledgment. We gratefully acknowledge the Depart-
ment of Energy and the NSF (Career Award) for supporting
this research. We thank Professor Joel Miller for the use of
his TGA instrument.
General Synthesis B for NHC-CO2’s 2aMe-2cMe. Carbenes
1aMe-1cMe were synthesized via potassium reduction of the
appropriate thiourea following a literature procedure.15 The
appropriate carbene (1 equiv) was dissolved in THF in an airless
flask, and the N2 atmosphere was removed and replaced with
CO2. The NHC-CO2 precipitated out of solution upon CO2
introduction. The reaction was allowed to stir for 2 h before
filtering the white precipitate away.
Supporting Information Available: Additional experimental
1
procedures, X-ray, H NMR, 13C NMR, and IR data for all
compounds and CIF files. This material is available free of
charge via the Internet at http://pubs.acs.org.
7942 J. Org. Chem. Vol. 74, No. 20, 2009