Zhou et al.
SCHEME 1. Synthesis of N,N′-Disubstituted
Imidazol(in)ium-2-carboxylates
are air- and moisture-sensitive, more stable NHCs adducts such
as haloalkane,9 alcohol,10 silver,11 and CO2 adduct precursors12
have been developed to thermally liberate NHCs more conve-
niently. Among these compounds, NHC-CO2 adducts (for
example, N,N′-disubstituted imidazol(in)ium-2-carboxylates) can
efficiently transfer NHCs to transition metal complexes with
release of CO2 under mild conditions.12 Likewise, Louie and
co-workers found that carboxylate groups of N,N′-disubstituted
imidazol(in)ium-2-carboxylates can easily exchange with free
CO2 in solution. Usually, decarboxylation of NHC-CO2 adducts
was thought to be a key step in this transformation. Therefore,
the study on stability of NHC-CO2 adducts appears to be
extremely necessary and may offer new opportunity for their
further applications.
FIGURE 1. Structures of the N,N′-disubstituted imidazol(in)ium
carboxylates.
From a structural point of view, NHC-CO2 adducts N,N′-
disubstituted imidazol(in)ium-2-carboxylates can be considered
to be zwitterionic compounds formed by NHCs nucleophilic
attack on the weak electrophilic carbon center of the CO2
molecule (Scheme 1), just like amidine- or guanidine-CO2
adducts. It has been reported that sterically hindered strong
organic base 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD)
could catalyze the coupling reaction of CO2 and epoxides to
provide corresponding cyclic carbonates. In this system, CO2
was thought to be first activated through the formation of a
zwitterionic adduct MTBD-CO2 and thereby adding to the
epoxide via nucleophilic attack of carboxylate group.13 This
observation also stimulated us to explore the feasibility of the
zwitterionic compounds NHC-CO2 as organic catalyst for the
above-mentioned reaction.
FIGURE 2. FTIR spectra of 1b (IPr-CO2) in CH2Cl2 at various times
at 50 °C.
Results and Discussion
N,N′-Disubstituted imidazol(in)ium-2-carboxylates 1a-1e
(NHCs-CO2 adducts) shown in Figure 1 were prepared by
reaction of the corresponding NHCs with CO2 according to the
literature procedures.12 Although they exhibit good stability in
solid state at room temperature, these NHCs-CO2 adducts
dissolved in organic solvents were prone to decomposition via
decarboxylation, especially at elevated temperature. Because of
their higher solubility in CH2Cl2 compared with other prepared
NHC-CO2 adducts, thermal stability of 1,3-bis(2,6-diisopropy-
lphenyl)imidazolium-2-carboxylate (IPr-CO2, 1b) and 1,3-
bis(2,6-diisopropylphenyl)imidazolinium-2-carboxylate (SIPr-
CO2, 1e) was investigated through in situ monitoring of the
ν(CO2) region of the infrared spectrum utilizing a temperature-
controlled high pressure liquid cell (HPL-TC). Figure 2 displays
the decarboxylation profile of 1b in CH2Cl2 with time at 50
°C. The absorption intensity of asymmetric ν(CO2) vibrations
(1692 cm-1) gradually decreases with prolonging time, which
clearly indicates the decarboxylation of NHC-CO2 adduct 1b.
In comparison to 1b with an unsaturated imidazolinium ring,
1e with a saturated counterpart exhibits higher thermal stability
(Figure 3).
Herein, we report the thermally stability of various NHC-
CO2 adducts N,N′-disubstituted imidazol(in)ium-2-carboxylates
(Figure 1) and their catalytic activities toward the coupling
reaction of CO2 with terminal epoxides.
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To assess the effect of temperature on thermal stability of
NHC-CO2 adducts in solution, in situ infrared monitoring of
1b in CH2Cl2 solution was conducted at various temperatures
(Figure 4). As anticipation, temperature has a pronounced effect
on thermal stability of the NHC-CO2 adducts. At 12 °C, 1b is
very stable in CH2Cl2, and no obvious change in intensity at
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8040 J. Org. Chem. Vol. 73, No. 20, 2008