Fast CO2 sequestration, activation, and catalytic transformation using N -heterocyclic olefins

Article abstract of DOI:10.1021/ja405114e

N-Heterocyclic Olefin (NHO) with high electronegativity at the terminal carbon atom was found to show a strong tendency for CO₂ sequestration, affording a CO₂ adduct (NHO-CO₂). X-ray single crystal analysis revealed the bent geometry of the binding CO₂ in the NHO-CO₂ adducts with an O-C-O angle of 127.7-129.9, dependent on the substitute groups of N-heterocyclic ring. The length of the C _carboxylate-C_NHO bond is in the range of 1.55-1.57 A, significantly longer than that of the C_carboxylate-C _NHC bond (1.52-1.53 A) of the previously reported NHC-CO ₂ adducts. The FTIR study by monitoring the ν(CO₂) region of transmittance change demonstrated that the decarboxylation of NHO-CO₂ adducts is easier than that of the corresponding NHC-CO ₂ adducts. Notably, the NHO-CO₂ adducts were found to be highly active in catalyzing the carboxylative cyclization of CO₂ and propargylic alcohols at mild conditions (even at ambient temperature and 0.1 MPa CO₂ pressure), selectively giving α-alkylidene cyclic carbonates in good yields. The catalytic activity is about 10-200 times that of the corresponding NHC-CO₂ adducts at the same conditions. Two reaction paths regarding the hydrogen at the alkenyl position of cyclic carbonates coming from substrate (path A) or both substrate and catalyst (path B) were proposed on the basis of deuterium labeling experiments. The high activity of NHO-CO₂ adduct was tentatively ascribed to its low stability for easily releasing the CO₂ moiety and/or the desired product, a possible rate-limiting step in the catalytic cycle.

Full text of DOI:10.1021/ja405114e

Article
pubs.acs.org/JACS
Fast CO2 Sequestration, Activation, and Catalytic Transformation
Using N‑Heterocyclic Olefins
Yan-Bo Wang, Yi-Ming Wang, Wen-Zhen Zhang, and Xiao-Bing Lu*
State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116024, People’s Republic of China
S
* Supporting Information
ABSTRACT: N-Heterocyclic Olefin (NHO) with high
electronegativity at the terminal carbon atom was found to
show a strong tendency for CO2 sequestration, affording a
CO2 adduct (NHO−CO2). X-ray single crystal analysis
revealed the bent geometry of the binding CO2 in the
NHO−CO2 adducts with an O−C−O angle of 127.7−129.9°,
dependent on the substitute groups of N-heterocyclic ring.
The length of the Ccarboxylate−CNHO bond is in the range of
1.55−1.57 Å, significantly longer than that of the Ccarboxylate
−
CNHC bond (1.52−1.53 Å) of the previously reported NHC−
CO2 adducts. The FTIR study by monitoring the ν(CO2)
region of transmittance change demonstrated that the
decarboxylation of NHO−CO2 adducts is easier than that of the corresponding NHC−CO2 adducts. Notably, the NHO−
CO2 adducts were found to be highly active in catalyzing the carboxylative cyclization of CO2 and propargylic alcohols at mild
conditions (even at ambient temperature and 0.1 MPa CO2 pressure), selectively giving α-alkylidene cyclic carbonates in good
yields. The catalytic activity is about 10−200 times that of the corresponding NHC−CO2 adducts at the same conditions. Two
reaction paths regarding the hydrogen at the alkenyl position of cyclic carbonates coming from substrate (path A) or both
substrate and catalyst (path B) were proposed on the basis of deuterium labeling experiments. The high activity of NHO−CO2
adduct was tentatively ascribed to its low stability for easily releasing the CO2 moiety and/or the desired product, a possible rate-
limiting step in the catalytic cycle.
anchored to the nucleophilic cobalt(I) ion through a Co−C σ
bond, while the oxygen atoms interact with the alkali cation in a
polymeric structure.3c Soon after, Herskovitz and co-workers
revealed the structure of Rh(diars)2Cl(CO2), in which CO2
trans to the chloride is formally η1-bound toward Rh(I)
(Scheme 1).3d Nevertheless, CO2 prevalently behaves as an
electrophile, since the electrophilicity of the carbon atom is
higher than the nucleophilicity of each of the oxygen atoms. For
example, strong nucleophiles such as the amidines and
guanidines containing nitrogen heterocyclic have been reported
to react with CO2, expectantly affording zwitterionic adducts.
Unfortunately, much effort to isolate and characterize a
zwitterionic adduct between CO2 and a nitrogen base was
unsuccessful, and in most cases bicarbonate salts were
unexpectedly produced owing to the presence of adventitious
water.4 Until recently, the TBD-CO2 adduct from the reaction
of 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) with CO2 was
isolated under strictly anhydrous conditions and clearly
characterized by the X-ray diffraction. The stability of TBD-
CO2 can be ascribed to the H-bond effect between N−H of
TBD and O-atom of the carboxylate anion (Scheme 1).5
INTRODUCTION
■
The utilization of carbon dioxide as a C1 source for the
production of organic chemicals can contribute to a more
sustainable chemical industry.1,2 The key issue is the lack of
effective catalysts to facilitate its activation and subsequent
transformation, since CO2 is such a thermodynamically and
kinetically stable molecule. In 1975, Aresta and co-worker
reported the isolation of the first CO2-based complex,
Ni(PCy3)2(CO2).3a,b The X-ray single-crystal analysis revealed
that the CO2 ligand was coordinated through the carbon atom
and one of the oxygen atoms and thus possessed bent geometry
with a O−C−O angle of 133° (Scheme 1), being distinct from
the nonpolar linear structure of free CO2 at ground state. A
similar bent structure with a O−C−O angle of 132° was also
observed in [Co(I)(nPr-salen)K(CO2)(THF)], in which CO2 is
Scheme 1
Received: May 21, 2013
© XXXX American Chemical Society
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N-heterocyclic carbene (NHC), a stronger nucleophile, was
also found to easily react with CO2 to form NHC−CO2 adduct,
in which a bent geometry with a O−C−O angle of 129−131°
was observed by X-ray single-crystal study.6 Notably, the
NHC−CO2 adduct was shown to be an efficient organocatalyst
for chemical fixation of CO2 to organic compounds.7 Recently,
superbase-based protic ionic liquids were demonstrated to
efficiently capture an equivalent CO2.8 It is noteworthy that the
new metal-free systems of “frustrated Lewis pairs” first unveiled
by Stephan et al.,9 were found to straightforwardly sequester
CO2 binding into frustrated Lewis acid and Lewis base
centers.2d,e,10
Scheme 3. Synthesis of NHO−CO2 Adducts 2a−2g
It is generally known that ene-1,1-diamines (ketene aminals)
as a kind of ylidic olefins possess a strongly polarized CC
double bond and thus made the charge of olefin separation
which can be described through their resonance structure A′)
(Scheme 2).11 Particularly, the incorporation of the nitrogen
shift of methylene H at the same position in 2a−2g clearly
moves downfield. Meanwhile, 13C NMR data of 2a−2g show
strong carboxylate carbon signals at 162.3−164.3 ppm. The
CO stretching frequency of the carboxylate in 2a−2g at
FTIR spectra changes from 1615 cm−1 to 1642 cm−1, obviously
lower than that of NHC−CO2 adducts (1645−1695 cm−1),
dependent on the substitute groups in the imidazolium ring.6f,7a
Furthermore, the structures of compounds 2b, 2c, 2d, 2f, and
2g were solved using single crystal X-ray crystallography
(Figure 1). Selected bond lengths and bond angles are shown
in Table 1. The O−C−O angles of NHO−CO2 adducts are in
the range of 127.7(2)° and 129.9(5)°, indicating the bent
geometry of the binding CO2. The C3−C4 bond length
(1.470(3)∼1.491(5) Å), is significantly longer than that of the
CC bond in 1,3,4,5-tetramethyl-2-methyleneimidazoline
(1.357(3) Å).12b This should be ascribed to the extreme
polarization of the exocyclic double bond of imidazoline ring,
since the polarization process allows for the most resonance
stabilization of the zwitterionic structure consisting of an
imidazolium cation and a carboxylic anion. Correspondingly,
both N1−C3 and N2−C3 bond lengths become shorter in
comparison with the N−C bond in 1,3,4,5-tetramethyl-2-
methyleneimidazoline. Far different from the NHC−CO2
adducts, the dissymmetrical N-alkyl substituent has little effect
on the discrepancy between C5−O1 and C5−O2 bond lengths
of the carboxylate. The distances of C5−O1(2) bonds for the
carboxylate are nearly equivalent for asymmetrical NHO−CO2
adducts, while the biggest discrepancy of only 0.009 Å was
found in the crystal data of compound 2b possessing a
symmetrical substituent at the heterocyclic ring. This result
indicates that the negative charge is uniformly distributed
between the central carbon and the two oxygens. The lengths
of C4−C5 bond in the NHO−CO2 adducts are between
1.549(3) Å and 1.598(6) Å, which are significantly longer than
that of the Ccarboxylate−CNHC bond (1.520−1.530 Å) in the
NHC−CO2 adducts previously reported by Rogers6a and
Louie.6b,f This might mean that the decarboxylation of NHO−
CO2 adducts are easier than that of NHC−CO2 adducts. Note
that the atoms C4, C5, O1, and O2 are nearly coplanar for all
NHO−CO2 adducts. Similarly, the atoms N1, N2, C3, and C4
are also nearly coplanar. The dihedral angle of two planes is
93.8°, 81.7°, 99.6°, 108.8°, and 109.6° for 2b, 2c, 2d, 2f, and 2g,
respectively.
Scheme 2. Resonance Structures of Ene-1,2-diamine and
N,N′-Disubstituents-2-Methylene Imidazoline
atoms into a five-membered ring, named N-heterocyclic olefin
(NHO) shown in Scheme 2, will be advantageous to stabilize a
positive charge, due to the aromatization of the heterocyclic
ring, making the terminal carbon atom of the olefins more
electronegative (resonance structure B ↔ B′). With respect to
their special structures, NHOs were considered as potent
nucleophiles and strong donor ligands.12 A kind of N-
heterocyclic olefin, IPrCH2 [IPr = 1,3-bis(2,6-diiso-propyl-
phenyl) imidazol-2-ylidene], has been demonstrated by Rivard
et al. as a nucleophile to stabilize GeH2 and SnH2 complexes.13
Mayr and co-workers reported the exocyclic olefinic carbon
atoms of the N-heterocyclic olefins attacked a benzhydrylium
ion to give azolium salts.14 More recently, treatment of the N-
heterocyclic olefins with B(C6F5)3 was described by Tamm et
al. to afford classical or abnormal Lewis acid/base adducts.15
The strong nucleophilicity grants NHO to possess great
potential in the application of CO2 capture, activation, and
further transformation. Herein, we for the first time report the
synthesis of various NHO−CO2 adducts and unveil their
geometries by X-ray single crystal analysis. Additionally, these
CO2 adducts were also applied as effective organocatalysts for
CO2 transformation to useful chemicals.
RESULTS AND DISCUSSION
■
A series of NHO−CO2 adducts 2a−2g were prepared in good
yields using a procedure shown in Scheme 3, utilizing the
corresponding N,N′-disubstituted derivatives of 2-methyl
imidazolium iodide 1 as starting material. Compound 1 was
first treated with KH in tetrahydrofuran solvent at room
temperature in the absence of light, affording the intermediate,
N,N′-disubstituent-2-methyleneimidazoline, which was further
transformed into the corresponding NHO−CO2 adduct by
diffusion of CO2 into its tetrahydrofuran solution under strictly
anhydrous conditions. Obvious differences in chemical shift
were observed in the 1H and 13C NMR spectra of compounds 1
and 2. In comparison with the 2-methyl of 1a−1g, a chemical
Generally, the structural data reflects the physical and
chemical properties of organic compounds. With those crystal
data in hand, we then explored the thermal stability of NHO−
CO2 adducts. The thermal stability was studied by means of in
situ FTIR method with monitoring the ν(CO2) region of
infrared spectra using a temperature-controlled high pressure
liquid cell (HPL-TC).7a Because of its better solubility in
CH2Cl2, the NHO−CO2 adduct 2g was first selected to
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Figure 1. Molecular structures of 2b, 2c, 2d, 2f, and 2g with thermal ellipsoids drawn at 30% probability. Hydrogen atoms have been omitted for
clarity.
Table 1. Selected Bond Lengths (Å) and Angles (deg) in Complexes 2b, 2c, 2d, 2f, and 2g
complex
C3−N1 (Å)
C3−N2 (Å)
C3−C4 (Å)
C4−C5 (Å)
C5−O1 (Å)
C5−O2 (Å)
N1−C3−N2 (deg)
O1−C5−O2 (deg)
2b
2c
2d
2f
1.337 (2)
1.333 (2)
1.344 (5)
1.341 (3)
1.340 (2)
1.346 (2)
1.343 (2)
1.346 (5)
1.340 (3)
1.349 (2)
1.480 (2)
1.479 (2)
1.491 (5)
1.470 (3)
1.476 (2)
1.551 (2)
1.549 (3)
1.598 (6)
1.557 (3)
1.568 (2)
1.234 (2)
1.242 (2)
1.226 (5)
1.241 (3)
1.230 (2)
1.243 (2)
1.242 (2)
1.230 (5)
1.241 (3)
1.237 (2)
107.2 (1)
107.5 (2)
107.6 (3)
106.6 (2)
106.7 (1)
128.5 (2)
127.7 (2)
129.9 (5)
128.3 (2)
129.1 (2)
2g
Figure 2. FTIR spectra of 2g in CH2Cl2 at various times at 40 °C.
Figure 3. Plots of transmittance (%) at 1645 cm−1 versus time for the
decomposition of NHO−CO2 adduct 2g at various temperatures.
conduct the thermal stability study. Figure 2 shows the
absorption intensity of asymmetric ν(CO2) vibrations of 2g
at 1645 cm−1 gradually decreases with the extension of time in
CH2Cl2 at 40 °C, which clearly indicates the decarboxylation of
2g. Meanwhile in situ infrared monitoring of 2g in CH2Cl2
solution was conducted at various temperatures to further
investigate the effect of temperature on thermal stability of
NHO−CO2 adducts. At 25 °C, 2g is relatively stable in
CH2Cl2. On the contrary, the decarboxylation of most 2g
molecules appeared within 40 min at 80 °C (Figure 3). Then
the stability difference of NHO−CO2 adducts 2b, 2c, 2d, 2f,
and 2g in CH2Cl2 at 40 °C was investigated (Figure 4). The
result indicates that the decomposition rate was related to the
C4−C5 bond length of NHO−CO2 adduct. Generally, the
longer the C4−C5 bond length is, the less stable the NHO−
CO2 adduct is, except 2d out of the rule.
For a comparison purpose, a typically dissymmetric NHC−
CO2 adduct 3 (1-(2,6-diisopropylphenyl)-3-isopropyl-imidazo-
lium-2-carboxylate) with the same imidazoline structure of
compound 2g was also prepared and its solid structure is shown
in Figure 5. A decarboxylation experiment of in situ infrared
monitoring of 3 in CH2Cl2 solution at 1683 cm−1 was
performed with time at 40 °C (Figure 6). Contrast to the
decarboxylation of most 2g molecules occurred within 2 h, only
small amounts of compound 3 decomposed at the same
condition. These results demonstrate that the decarboxylation
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exploration of CO2 transformation using NHO−CO2 as
potential catalyst become a primary goal of our research.
Chemical fixation of carbon dioxide into useful organic
chemicals is of great interest and has been a long-standing goal
for chemists,16 since CO2 is an abundant, inexpensive, and
renewable C1 feedstock. Previously, NHC−CO2 adducts were
reported as an effective organocatalyst for CO2 transformation.
Because the carboxylate in NHO−CO2 adduct shows lower
stability than that of the corresponding NHC−CO2 adduct, it
perhaps exhibits more excellent reactivity toward some
substrates, especially the nucleophile-promoted reaction. With
kinds of NHO−CO2 adducts in hand, we are of great interest
to explore the transformation of the binding CO2 and further
application of these adducts as potential catalyst for CO2
chemical fixation. We initially studied the carboxylative
cyclization of propargylic alcohols with CO2 to give α-
alkylidene cyclic carbonates, which are versatile intermediates
in organic synthesis and polymer chemistry.17 Usually, the use
of a metallic species18including Ru, Co, Cu, Pd, or Ag as
catalyst is necessary to make this reaction run smoothly.
Although phosphine,19 guanidine,20 or N-heterocyclic car-
benes21 were found to be active in catalyzing this coupling
reaction, the activities were not satisfactory. To our delight, we
initially attempted this coupling reaction of 2-methyl-4-
phenylbut-3-yn-2-ol (4a) with CO2 using 2 mol % 2a to give
a yield of 51% under 60 °C and 2 MPa CO2 pressure (Table 2,
Figure 4. Comparison plots of transmittance for NHO−CO2 adducts
2b, 2c, 2d, 2f and 2g versus time for decomposition in CH2Cl2 at 40
°C.
Table 2. Optimization of Reaction Conditions for the
Carboxylative Cyclization of 2-methyl-4-phenylbut-3-yn-2-ol
a
(4a) with CO2
Figure 5. Molecular structure of the NHC−CO2 adduct 3 with
thermal ellipsoids drawn at 30% probability. Hydrogen atoms have
been omitted for clarity. Selected bond lengths (Å) and angles (°):
C3−C4 1.528(2), C4−O1 1.204(2), C4−O2 1.207(2), C3−N1 1.331
(2), C3−N2 1.342(2), N1−C3−N2 107.7(1), and O1−C4−O2
129.2(2).
b
entry
catalyst (mol %)
T (°C)
P (MPa)
t (h)
yield (%)
1
2
2a (2)
2b (2)
2c (2)
2d (2)
2e (2)
2f (2)
2g (2)
2g (5)
2g (1)
2g (5)
2g (5)
2g (5)
2g (5)
2g (5)
2g (5)
60
60
60
60
60
60
60
60
60
50
70
60
60
60
60
2
2
2
2
2
2
2
2
2
2
2
2
2
1
12
12
12
12
12
12
12
12
12
12
12
6
51
72
64
68
84
70
88
93
70
76
85
72
94
88
94
3
4
5
6
7
8
9
10
11
12
13
14
15
24
12
12
Figure 6. Comparison plots of transmittance at 1645 cm−1 for NHO−
CO2 adduct 2g and at 1683 cm−1 for NHC−CO2 adduct 3 versus time
for decomposition in CH2Cl2 at 40 °C.
4
a
b
Reaction conditions: substrate 4 (3 mmol). Isolated yield.
entry 1). Driven by this result, the carboxylative cyclization of
4a with CO2 as model reaction was chose to optimize reaction
conditions. The results from Table 2 revealed that the
substituents on the nitrogen atoms of the NHO framework
have an obvious influence on the catalyst activity (Table 2,
entries 1−7), the isopropyl-substituted NHO−CO2 adducts
showed higher catalytic activity than those methyl-substitued
adducts, probably due to its higher solubility. The lower catalyst
of NHO−CO2 adduct is significantly easier than that of NHC−
CO2 adduct, which is agreement with the discrepancy in bond
length: 1.568(2) Å of the C4−C5 bond of NHO−CO2 adduct
2g versus 1.528(2) Å of the C3−C4 bond of NHC−CO2
adduct 3. The relatively poor thermal stability of NHO−CO2
adducts probably offers an important chance to serve as a highly
active catalyst for CO2 transformation. Therefore, the
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loading and the reduced temperature have negative effects on
the yield (Table 2, entries 9−11). Interestingly, CO2 pressure
had very limited impact on the yield of the cyclic carbonate
(Table 2, entries 8, 14, and 15).
The formation of carboxylative cyclization product 5a from
2-methyl-4-phenylbut-3-yn-2-ol (4a) was once realized by
MTBD-catalyzed reaction in supercritical carbon dioxide to
give 55% yield.20 The same reaction was reported by Ikariya
and co-workers using 1,3-ditert-butyl-imidazolium-2-carboxylate
as catalyst at an enhanced temperature of 80 °C and a 4.5 MPa
CO2 pressure to obtain 5a in 84% yield.21b For a comparison
purpose of the discrepancy in catalytic activity of NHO−CO2
and NHC−CO2 adducts, the reaction was conducted using
compounds 2g and 3 as catalyst, respectively, since they have
the same substituents in the N atom of imidazole ring. The
results are shown in Table 3. It was found that NHO−CO2
Figure 7. Plot of yield versus time for the carboxylative cyclization of
4f and CO2 catalyzed by 2g and 3 at various temperature. Yields were
determined by H NMR.
1
Table 3. Catalytic Activity Comparison for NHO−CO2
Adduct 2g and NHC−CO2 Adduct 3 toward Carboxylative
should make a contribution to their higher activity in catalyzing
this carboxylative cyclization. Furthermore, a relationship (ρ =
1.876) between the rate constant k of the 4-aryl substituted
propargylic alcohol and the Hammett parameters was
established. The result shows that the carboxylative cyclization
was accelerated by the nucleophilic attack according to the
hammett plot (see Supporting Information, SI, Figure S2).
Otherwise, the scope with respect to other propargylic
alcohols substrates was then explored under the optimized
reaction conditions. As shown in Table 4, various terminal and
a
Cyclization of Propargylic Alcohols with CO2
b
entry
R
catalyst
yield (%)
1
2
3
4
5
6
7
C6H5(4a)
2g
3
93 (5a)
43 (5a)
56 (5b)
12 (5b)
78 (5c)
trace (5c)
90 (5d)
42 (5d)
40 (5d)
93 (5e)
63 (5e)
trace (5e)
94 (5f)
4-MeOC6H4(4b)
4-MeC6H4(4c)
3-pyridyl(4d)
2g
3
Table 4. Carboxylative Cyclization of Various Propargylic
Alcohols with CO2 to Give α-alkylidene Cyclic Carbonates
2g
3
a
by 2g
2g
2g
3
c
8
9
d
10
4-CF3C6H4(4e)
2g
2g
3
c
11
d
12
b
entry
substrate
R1
R2
R3
yield (%)
d
13
4-CH3COC6H4(4f)
2g
2g
3
c
c
1
2
3
4
5
6
7
4g
4h
4i
H
Me
Me
Me
Ph
>98
14
65 (5f)
d
H
87
83
66
50
83
84
15
trace (5f)
H
−(CH2)5−
−(CH2)5−
Me
a
Reaction conditions: 2g or 3 (0.15 mmol), substrate 4 (3 mmol), 60
b
c
4j
Ph
Ph
°C, 12 h, 2 MPa CO2. Isolated yield. Reaction conditions: 2g (0.15
4k
4l
Ph
mmol), substrate 4 (3 mmol), rt, 0.5 mL CH2Cl2, 1 atm CO2.
d
2-pyridyl
4-ClC6H4
Me
Me
Me
Me
Reaction conditions: 2g or 3 (0.15 mmol), substrate 4 (3 mmol), rt,
4m
0.5 mL CH2Cl2, 2 MPa CO2.
a
Reaction conditions: 2g (0.15 mmol), substrate 4 (3 mmol), 60 °C,
b
c
12 h, 2 MPa CO2. Isolated yield. Yield determined by GC
spectroscopy.
adduct 2g was superior to NHC−CO2 adduct 3 in catalyzing
the carboxylative cyclization of CO2 with various propargyl
alcohols. Furthermore, taking the carboxylative cyclization of 4f
and CO2 as an example, simple kinetic studies of the two
catalyst systems by monitoring the reaction at various time
points using 1H NMR spectroscopy revealed that the rate using
2g as catalyst was obviously higher than that with NHC−CO2
adduct 3 system whether at 25 or 60 °C (Figure 7). It is worthy
of mention that with 4e or 4f as substrate, the carboxylative
cyclization with CO2 performed smoothly at ambient temper-
ature (Table 3, entries 10−15), a rare example of organo-
catalyzed CO2 fixation at 25 °C. Notably, 5e and 5f were
obtained in moderate yields under 1 atm CO2. In comparison
with NHC−CO2 adduct, the instability of NHO−CO2 adducts
from the above crystal data and decarboxylation experiment
internal propargylic alcohols smoothly underwent the carbox-
ylation cyclization reaction and were converted into the
corresponding alkylidene cyclic carbonates in moderate to
excellent yields.
We are greatly interested to note that only a slight difference
in structure between NHO−CO2 adduct 2g and NHC−CO2
adduct 3 resulted in the significant discrepancy of catalytic
activity for the carboxylative cyclization reaction. The higher
reactivity associated with NHO−CO2 adduct was tentatively
ascribed to the low stability of the Ccarboxylate−CNHO bond,
demonstrated by X-ray single crystal analysis and FTIR study.
In NHC−CO2 adduct catalyzed carboxylative cyclization of
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Scheme 4. Plausible Mechanisms for Carboxylative Cyclization of Propargyl Alcohols with CO2 Catalyzed by NHO−CO2
propargylic alcohols with CO2, Ikariya et al. proposed a
mechanism regarding the nucleophilic attack of the CO2 moiety
bound to the NHC onto the substrates. First, the zwitterionic
compound NHO−CO2 could add to triple bond of propargylic
alcohol by nucleophilic attack. Meanwhile, hydrogen transfer of
alcohol generates the new zwitterions (Ia) (Scheme 4),
alkoxide anion of which attacked the carbonyl carbon to
release the desired product and NHO, which rapidly captures
free CO2 to form the compound NHO−CO2 for finishing a
catalyst cycle. However, because of the instability of NHO−
CO2 adduct, it is possible to release the free NHO from the
decomposition of the adduct, and thus a kinetic equilibrium
exists in the catalytic system, dependent on CO2 pressure and
reaction temperature. NHO with high electronegativity at the
terminal carbon atom could abstract hydrogen of propargylic
alcohol to form the intermediate IIa, which reacts with CO2 to
give the intermediate IIb. Subsequently, the intermediate IIc
can be obtained by an intramolecular ring-closing reaction of
the intermediate IIb, which snatchs the hydrogen of 2-methyl
imidazolium to release the desired product. The obvious
difference of two paths is the hydrogen at the alkenyl position
of cyclic carbonates coming from substrate (see Scheme 4, path
A) or both substrate and catalyst (path B). In our study,
deuterium labeling experiments were carried out to gain insight
into the reaction course (see Scheme 5 and the SI). Due to the
strong H/D exchange between the hydrogen atom of C4, C5 in
the imidazoline ring and −CD2CO2 (see SI Figure S6), the
systhesis of deuterated NHO−CO2 adducted D2-2g was
unsuccessful. D2-2h was obtained by replacing the H atom of
C4, C5 in the imidazoline ring with methyl. Unfortunately, the
H/D exchange between the hydrogen atom of propargylic
alcohols and −CD2CO2 was observed (see SI Figure S10).
Since the mechanism of path B involves the cleavage of a C−H
bond of the intermediate IIc, the measurement of kinetic
isotope effects (KIEs)22 was performed. As shown in Scheme 6,
Scheme 6. KIE Measurement from Intermolecular and
Intramolecular Reactions
whether an intramolecular or intermolecular labeled substrate
and catalyst, the value of KIE reveals a small, perhaps secondary
isotope effect, indicating that path B makes a lower
contribution to the carboxylative cyclization. Otherwise, when
Scheme 5. Deuterium Labelling Substrate, Catalyst, and
Product
13
4f reacted with 13C-labeled 2g under 1 atm CO2,
C
carbonyl
-
labeled 5f was detected (see SI Figure S11), which may imply
that NHO−CO2 adduct was involved in the catalytic reaction.
Compared with NHC−CO2 adduct, the decreased stability of
the NHO−CO2 adduct benefits the departure of the product
from NHO, which might be a possible rate-limiting step in the
catalytic cycle.
F
dx.doi.org/10.1021/ja405114e | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
CONCLUSIONS
AUTHOR INFORMATION
■
■
In summary, we have established a simple procedure for the
synthesis of various NHO−CO2 adducts from N-heterocyclic
olefins. The X-ray single crystal study revealed that the binding
CO2 moiety possesses a bent geometry with an O−C−O angle
of 127.7−129.9°, dependent on the substitute groups of N-
heterocyclic ring. The length of the Ccarboxylate−CNHO bond is in
the range of 1.55−1.57 Å, significantly longer than that of the
Ccarboxylate−CNHC bond of the corresponding NHC−CO2
adducts.
Moreover, the NHO−CO2 adduct was demonstrated to be
an efficient organocatalyst for the carboxylative cyclization of
CO2 and propargylic alcohols at mild temperature and pressure,
selectively giving α-alkylidene cyclic carbonates in good yields.
The catalytic activity is about 10−200 times that of the
corresponding NHC−CO2 adducts at the same conditions. The
plausible mechanism of the carboxylative cyclization was
suggested, with regard to the hydrogen at the alkenyl position
of cyclic carbonates coming from substrate (path A) or both
substrate and catalyst (path B) on the basis of deuterated
labeling experiments.
Corresponding Author
lxb-1999@163.com
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the National Natural
Science Foundation of China (U1162101, 21004007), the
National Basic Research Program of China (973 Program:
2009CB825300) and the Fundamental Research Funds for the
Central Universities (DUT12LK47). X.-B.L. gratefully acknowl-
edges the Chang Jiang Scholars Program (No. T2011056) from
Ministry of Education, People’s Republic of China.
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General Procedure for the Synthesis of NHO−CO2 Adducts.
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* Supporting Information
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General experimental procedures for the synthesis of various
CO2 complexes, as well as their characterizations. X-ray
crystallographic file in CIF format for the structure
determination of NHO−CO2 adducts 2b, 2c, 2d, 2f and 2g,
and NHC−CO2 adduct 3. Synthetic procedures of various α-
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