N-heterocyclic carbenes as efficient organocatalysts for CO2 fixation reactions

Full text of DOI:10.1002/anie.200901399

Communications
DOI: 10.1002/anie.200901399
Organocatalysis
N-Heterocyclic Carbenes as Efficient Organocatalysts for CO2 Fixation
Reactions**
Yoshihito Kayaki, Masafumi Yamamoto, and Takao Ikariya*
During the last decade, N-heterocyclic carbenes (NHCs) have
attracted attention because of the intensive development of
general synthetic methods and wide applications in the field
of molecular catalysis.[1] The unique properties of the
unsaturated NHC carbon atom, stabilized by the electron-
donating heteroatoms on either side, are highlighted by their
utilization as versatile ligands for transition metals[2] and
organocatalysts[3] as they exhibit strong basicity. Most NHC-
mediated organocatalysis includes the formation of covalent,
CO2 fragment to the unsaturated alcoholic substrate. The fact
that NHCs exhibit similar chemical properties to those of
tertiary phosphanes, prompted us to investigate new CO2
transformation catalyst systems using NHCs. We have
reported some preliminary results in a patent application,[9a]
and herein we report that NHCs (1,3-dialkylimidazol-2-
ylidene; 1) and the corresponding CO2 adducts (1,3-dialky-
limidazolium-2-carboxylates; 2) act as effective catalysts for
carbonate synthesis using CO2 under relatively mild reaction
conditions as shown in Scheme 1.[9b]
=
active intermediates by their addition to C O bonds as the
key step, leading to nucleophilic incorporation of the carbonyl
functional group. Such an apparent s-donor character of
NHCs has also been applied to CO2 capture, and the resulting
imidazolium-2-carboxylates are identified as the typical
NHC–CO2 adducts (Figure 1).[4] Whereas the reverse reaction
Scheme 1. Carboxylative cyclization of propargylic alcohol with CO2.
Figure 1. Imidazolium-2-carboxylates as NHC–CO2 adducts.
We first examined the carboxylative cyclization of 2-
methyl-3-propyn-2-ol (3a, R1 = H; 5.0 mmol) with a catalytic
amount (5.0 mol%) of a parent carbene, 1,3-diisopropyl-4,5-
dimethylimidazol-2-ylidene (1a), under identical reaction
conditions to those used for the nBu3P-catalyzed reaction in
supercritical CO2 (Table 1). When the reaction was carried
out at 1008C and 10.0 MPa for 15 hours, a five-membered
cyclic carbonate, 5-methylene-4,4-dimethyl-1,3-dioxolan-2-
one (4a), was obtained in 80% yield (Table 1, entry 1). The
corresponding NHC–CO2 adduct 2a also showed comparable
has been proven to be decisive for delivering a number of
NHC complexes,[5] in principle, the zwitterionic CO2 adduct
could be utilized as a convenient CO2 carrier to accomplish
CO2 fixation through the nucleophilic incorporation of the
= =
O C O unit as proposed for various Lewis base catalyst
systems.[6]
During our studies on CO2 fixation to produce highly
valuable chemicals, we reported the carboxylative cyclization
of propargylic alcohols catalyzed by nBu3P in supercritical
CO2.[7,8] Although the role of the tertiary phosphane catalyst
has not been fully understood, we proposed a reaction
mechanism involving a putative zwitterionic phosphane–
CO2 adduct which promotes nucleophilic addition of the
Table 1: Carboxylative cyclization of 3a and CO2 (see Scheme 1 for
reaction, R1 =H). [a]
Entry
Catalyst
P [MPa]
T [8C]
Yield [%][b]
1
2
3
4
5
6
7
8
9
10
1a
2a
nBu3P
2a
2a
2a
2a
2b
2c
2d
10.0
10.0
10.0
4.5
2.5
4.5
4.5
4.5
4.5
4.5
100
100
100
100
100
60
40
60
60
60
80
85
99
88
62
90
82
90
99
5
[*] Dr. Y. Kayaki, M. Yamamoto, Prof. Dr. T. Ikariya
Department of Applied Chemistry, Graduate School of Science and
Engineering, Tokyo Institute of Technology
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552 (Japan)
Fax: (+81)3-5734-2637
E-mail: tikariya@apc.titech.ac.jp
[**] This work was financially supported by a Grant-in-Aid for Scientific
Research on Priority Areas (No. 18065007; “Chemistry of Concerto
Catalysis”) and partially supported by the GCOE program from the
Ministry of Education, Culture, Sports, Science, and Technology
(Japan).
[a] Reaction conditions: The reaction was carried out in a 50 mL
stainless-steel reactor containing 3a (5.0 mmol) and the catalyst
(0.25 mmol) for 15 h. [b] Determined by 1H NMR methods, using
durene as an internal standard.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200901399.
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catalytic activity (Table 1, entry 2), implying that imidazo-
lium-2-carboxylates can be employed as NHC equivalents.
Although the preceding nBu3P catalyst afforded a satisfactory
result (Table 1, entry 3) under the supercritical conditions, 2a
was found to be operative in a neat, homogeneous phase at
4.5 MPa CO2 and 408C (Table 1, entries 4–7). In contrast, a
sharp drop in the yield of 4a was observed for the nBu3P
system as the reaction temperature fell below 808C
(Figure 2). The advantageous catalytic results obtained with
Scheme 2. Carboxylation of 3a under CO2 atmosphere using 2c with
substrate/catalyst ratios (S/C) of 50 and 100 at temperatures of 50
and 408C, respectively.
opening transesterification[12] of 4a in preference to the
capture of CO2 under moderate conditions.
The isolable NHC–CO2 catalyst 2c provides access to a
variety of five-membered cyclic carbonates (4b–j) from
substrates having internal alkynes (3b–j). As summarized in
Table 2, 2c exhibited better catalyst performance than the
Table 2: Synthesis of Z-4-alkylidene-1,3-dioxolan-2-ones, 4.[a]
Entry
R1
T [8C]
t [h]
Yield [%][b,c]
1
2
3
4
5
6
7
8
9
p-NO2C6H4 (3b)
p-CH3COC6H4 (3c)
p-ClC6H4 (3d)
C6H5 (3e)
p-CH3C6H4 (3 f)
p-CH3OC6H4 (3g)
2-pyridyl (3h)
3-thienyl (3i)
60
60
60
80
80
60
60
60
60
5
5
84 (77)
91 (82)
86 (80)
84 (88)[d]
95 (62)
51[d] (0)
77 (64)
94
Figure 2. Reaction temperature dependence of the yield of 4a for the
&
*
catalyst system of nBu3P ( ; 10.0 MPa) and 2a ( ; 4.5 MPa). Reaction
15
15
15
15
15
45
45
conditions: 3a (5.0 mmol) and catalyst (0.25 mmol) for 15 h.
2a can be attributed to its superior nucleophilic properties. In
fact, the presence of the nBu3P–CO2 adduct derived from
nBu3P and supercritical CO2 (358C, 10.0–20.0 MPa) was not
confirmed by high-pressure NMR experiments.[10] In contrast
to the reaction with nBu3P, the facile formation of the NHC–
CO2 adducts occurred under mild reaction conditions.
Screening tests, using a series of the NHC–CO2 adducts 2
under the solvent-free reaction conditions at 4.5 MPa CO2
and 608C, revealed that substituents on the nitrogen atoms of
the NHC framework delicately influence the catalyst activity.
The reproducible results obtained using 1,3-diisopropylimi-
dazolium-2-carboxylate (2b) and 2a suggests that a substitu-
tent at the 4- and 5-positions of imidazolium ring do not affect
the outcome of the reaction (Table 1, entries 6 and 8). The
best yield of 99% for the product was attained using 1,3-di-
tert-butylimidazolium-2-carboxylate (2c), whereas the diaryl-
substituted NHC–CO2 adduct 2d gave unsatisfactory results
(Table 1, entries 9 and 10). Secondary and primary alcohols,
as well as a homopropargylic alcohol, 2-methyl-4-pentyn-2-ol,
were not transformed even in the presence of the catalyst 2c;
this lack of reactivity is in line with the trends observed for the
nBu3P system.[7]
=
trans-C6H5CH CH (3j)
84
[a] Reaction conditions: The reaction was carried out with 3 (5.0 mmol)
and 2c (0.25 mmol) under CO2 (4.5 MPa). [b] Yield of isolated product.
[c] Yields in parentheses were obtained from the nBu3P (5mol%) catalyst
system with CO2 (10 MPa), at 1008C for 15 h. [d] Determined by 1H NMR
methods, using durene as an internal standard.
tertiary phosphane. The presence of electron-withdrawing
groups conjugated to the triple bond led to a reduction in the
reaction time or the reaction temperature (Table 2, entries 1–
3). Notably, unlike the nBu3P catalyst, 2c is applicable to the
reaction of 3g, which has a para-methoxyphenyl group
(Table 2, entry 6). The NHC catalyst also tolerates substrates
bearing heterocycles such as pyridine and thiophene (Table 2,
entries 7 and 8). The substrate 3j having an olefinic group at
the acetylenic terminus also provided the desired 5-exo-dig
cyclization product 4j in 84% yield (Table 2, entry 9),
whereas no carbonates were formed from allylic compounds
including 2-methyl-3-buten-2-ol and 2-methyl-4-phenyl-3-
=
buten-2-ol. In each product, the C C double bond was
When the reaction of 3a with 2c was performed with a
lower catalyst loading of 2 mol% under CO2 at atmospheric
pressure and 508C, an acyclic carbonate, 1,1-dimethyl-2-oxo-
propyl 1’,1’-dimethyl-2’-propynyl carbonate (5), was obtained
in 31% yield in addition to a 69% yield of 4a (Scheme 2). An
additional decrease in the catalyst loading to 1 mol% and
using a reaction temperature of 408C gave rise to 5 in 82%
yield (4a: 0% yield). The carboxylative cyclization affording
4a and the subsequent addition of 3a to 4a probably leads to
5 as a 2:1 coupling product of the propargyl alcohol and
CO2.[8b„11] The NHC derived from 2c might promote the ring-
found to have a Z configuration, as determined by NMR
spectroscopy, indicating that the addition to the alkynes
proceeded predominantly in a trans fashion, similar to the
previous carboxylative cyclization.[7,13]
We also examined another carbonate synthesis involving
epoxides 6 and CO2 with a NHC (Table 3).[14,15] By using the
catalyst 2c (5.0 mol%), styrene oxide was successfully con-
verted into the corresponding carbonate within 24 hours
under CO2 (4.5 MPa) at 1008C without using a solvent
(Table 3, entry 1). The product was isolated in 89% yield and
with almost complete selectivity. The cycloaddition of CO2 to
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Table 3: Cycloaddition of CO2 to epoxides 6.[a]
tertiary phosphane catalyst system because of their strong
nucleophilic nature.
Experimental Section
General procedure for the carboxylative cyclization of 3: A 50 mL
stainless-steel autoclave equipped with a magnetic stirring bar was
Entry
R2
P [MPa]
t [h]
Yield [%][b,c]
charged with argon gas in
a desiccator. Catalyst 2a (55 mg,
1
2
3
4
C6H5 (6a)
4.5
2.5
2.5
2.5
24
15
18
40
89 (98)
81
87
0.25 mmol), contained in the reactor, was purged with argon gas to
remove oxygen. 3a (0.5 mL, 5.0 mmol) was introduced into the
autoclave with a syringe while the vessel was purged with argon. The
vessel was charged with CO2 (4.5 MPa) through a cooling apparatus
with an HPLC pump. After stirring for 15 h at 408C, the reaction was
stopped by cooling the autoclave in an ice bath. CO2 was vented and
the autoclave was slowly warmed to room temperature. The reaction
C6H5OCH2 (6b)
ClCH2, (6c)
n-C4H9 (6d)
71
[a] Reaction conditions: The reaction was carried out with 6 (1.5 mmol)
and 2c (7.5ꢀ10À2 mmol) at 1008C. [b] Yield of isolated product.
[c] Yields in parentheses were determined by H NMR methods, using
1
1
durene as an internal standard.
mixture was analyzed by H NMR spectroscopy using durene as an
internal standard. The crude reaction mixture was purified by column
chromatography on silica gel (n-hexane/ethyl acetate = 5:1) and then
the isolated product was subjected to Kugelrohr distillation (80–858C,
18 mm Hg) to yield 4a (526 mg, 82%).
other epoxides, including 2-(phenoxymethyl)oxirane, epi-
chlorohydrin, and 2-butyloxirane, also proceeded at 2.5 MPa
CO2 to afford the desired carbonates 7 in yields ranging from
71 to 87% (Table 3, entries 2–4). Very recently, Lu and co-
workers reported the coupling of epoxides with CO2 catalyzed
by 1,3-diarylimidazolium-2-carboxylates at a higher reaction
temperature of 1208C.[4g]
The cyclic carbonate formations from either 3 or 6 using
the NHC–CO2 adduct can be explained by a mechanism
involving the nucleophilic addition of the imidazolium-2-
Received: March 13, 2009
Published online: May 4, 2009
Keywords: carbene · carbon dioxide fixation · carboxylation ·
.
nucleophilic addition · organocatalysis
ꢀ
carboxylate to either the C C bond or the strained epoxide
ring and subsequent intramolecular cyclization of alkoxide
intermediates (Scheme 3). The significant positive effect of
[1] a) N-Heterocyclic Carbenes in Synthesis (Ed.: S. P. Nolan),
Wiley-VCH, Weinheim, 2006; b) N-heterocyclic Carbenes in
Transition Metal Catalysis (Ed.: F. Glorius), Splinger, Berlin,
2007; c) A. J. Arduengo, Acc. Chem. Res. 1999, 32, 913 – 921;
d) D. Bourissou, O. Guerret, F. P. Gabbꢀi, G. Bertrand, Chem.
Rev. 2000, 100, 39 – 91; e) J. L. Methot, W. R. Roush, Adv. Synth.
Catal. 2004, 346, 1035 – 1050.
[2] a) T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2001, 34, 18 – 29;
b) W. A. Herrmann, Angew. Chem. 2002, 114, 1342 – 1363;
Angew. Chem. Int. Ed. 2002, 41, 1290 – 1309; c) V. Cꢁsar, S.
Bellemin-Laponnaz, L. H. Gade, Chem. Soc. Rev. 2004, 33, 619 –
636; d) C. M. Crudden, D. P. Allen, Coord. Chem. Rev. 2004, 248,
2247 – 2273; e) R. A. Kelly III, H. Clavier, S. Giudice, N. M.
Scott, E. D. Stevins, J. Bordner, I. Samardjiev, C. D. Hoff, L.
Cavallo, S. P. Nolan, Organometallics 2008, 27, 202 – 210; f) F. E.
Hahn, M. C. Jahnke, Angew. Chem. 2008, 120, 3166 – 3216;
Angew. Chem. Int. Ed. 2008, 47, 3122 – 3172; g) K. Araki, S.
Kuwata, T. Ikariya, Organometallics 2008, 27, 2176 – 2178.
[3] a) G. A. Grasa, R. Singh, S. P. Nolan, Synthesis 2004, 971 – 985;
b) V. Nair, S. Bindu, V. Sreekumar, Angew. Chem. 2004, 116,
5240 – 5245; Angew. Chem. Int. Ed. 2004, 43, 5130 – 5135; c) D.
Enders, T. Balensiefer, Acc. Chem. Res. 2004, 37, 534 – 541; d) M.
Christmann, Angew. Chem. 2005, 117, 2688 – 2690; Angew.
Chem. Int. Ed. 2005, 44, 2632 – 2634; e) N. Marion, S. Dꢂez-
Gonzꢃlez, S. P. Nolan, Angew. Chem. 2007, 119, 3046 – 3058;
Angew. Chem. Int. Ed. 2007, 46, 2988 – 3000; f) D. Enders, O.
Niemeier, A. Henseler, Chem. Rev. 2007, 107, 5606 – 5655;
g) S. E. Denmark, G. L. Beutner, Angew. Chem. 2008, 120,
1584 – 1663; Angew. Chem. Int. Ed. 2008, 47, 1560 – 1638.
[4] a) K. Kuhn, M. Steimann, G. Weyers, Z. Naturforsch. B 1999, 54,
427 – 433; b) H. A. Duo, T. N. Tekavec, A. M. Arif, J. Louie,
Chem. Commun. 2002, 112 – 113; c) J. D. Holbrey, W. M. Reich-
ert, I. Tkatchenko, E. Bouajila, O. Walter, I. Tommasi, R. D.
Rogers, Chem. Commun. 2003, 28 – 29; d) H. A. Duong, T. N.
Tekavec, A. M. Arif, J. Louie, Chem. Commun. 2004, 112 – 113;
e) I. Tommasi, F. Sorrentino, Tetrahedron Lett. 2005, 46, 2141 –
2145; f) I. Tommasi, F. Sorrentino, Tetrahedron Lett. 2006, 47,
Scheme 3. Plausible mechanism of the carboxylation catalyzed by the
NHC–CO2 adduct.
the electron-donating alkyl substituents on the NHC nitrogen
atoms implies that the nucleophilic attack of the CO2 moiety,
bound to the NHC, onto the substrates is a possible rate-
limiting step in the catalytic cycle.
In summary, we have demonstrated that NHCs and NHC–
CO2 adducts serve as potent organocatalysts providing
straightforward methods for solvent-free carbonate synthesis,
and pave the way to utilizing CO2 as a nucleophilic fragment
in CO2 fixation. In particular, the use of N,N’-dialkyl-
substituted NHC derivatives has a significant advantage for
the carboxylative cyclization relative to the earlier reported
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Chemie
6453 – 6456; g) H. Zhou, W.-Z. Zhang, C.-H. Liu, J.-P. Qu, X.-B.
Lu, J. Org. Chem. 2008, 73, 8039 – 8044.
Kissling, S. P. Nolan, Org. Lett. 2002, 4, 3583 – 3586; c) G. W.
Nyce, J. A. Lamboy, E. F. Connor, R. M. Waymouth, J. L.
Hedrick, Org. Lett. 2002, 4, 3587 – 3590; d) Y. Suzuki, K.
Yamauchi, K. Muramatsu, M. Sato, Chem. Commun. 2004,
2770; e) T. Kano, K. Sasaki, K. Maruoka, Org. Lett. 2005, 7, 1347;
f) W. Jeong, E. J. Shin, D. A. Culkin, J. L. Hedrick, R. M.
Waymouth, J. Am. Chem. Soc. 2009, 131, 4884 – 4891.
[5] a) A. M. Voutchkova, L. N. Appelhans, A. R. Chianese, R. H.
Crabtree, J. Am. Chem. Soc. 2005, 127, 17624 – 17625; b) S.
Tudose, L. Delaude, B. Andrꢁ, A. Demonceau, Tetrahedron Lett.
2006, 47, 8529 – 8533; c) A. Tudose, A. Demonceau, L. Delaude,
J. Organomet. Chem. 2006, 691, 5356 – 5365; d) A. M. Voutch-
kova, M. Feliz, E. Clot, O. Eisenstein, R. H. Crabtree, J. Am.
Chem. Soc. 2007, 129, 12834 – 12846.
[13] Y. Kayaki, M. Yamamoto, T. Suzuki, T. Ikariya, Green Chem.
2006, 8, 1019 – 1021.
[6] E. R. Pꢁrez, R. H. A. Santos, M. T. P. Gambardella, L. G. M.
de Macedo, U. P. Rodrigues-Filho, J.-C. Launay, D. W. Franco, J.
Org. Chem. 2004, 69, 8005 – 8011, and references therein.
[7] Y. Kayaki, M. Yamamoto, T. Ikariya, J. Org. Chem. 2007, 72,
647 – 649.
[8] a) J. Fournier, C. Bruneau, P. H. Dixneuf, Tetrahedron Lett. 1989,
30, 3981 – 3982; b) J. M. Joumier, J. Fournier, C. Bruneau, P. H.
Dixneuf, J. Chem. Soc. Perkin Trans. 1 1991, 3271 – 3274; c) C.
Bruneau, P. H. Dixneuf, J. Mol. Catal. 1992, 74, 97 – 107.
[9] a) Y. Kayaki, T. Ikariya (JST), JP 2006137733, 2006 [Chem.
Abstr. 2006, 145, 27996]; b) During preparation of this paper, a
similar reaction using NHC–CO2 adducts under severe con-
ditions was presented, on the basis of our preliminary patent
results: c) I. Tommasi, F. Sorrentino, Tetrahedron Lett. 2009, 50,
104 – 107.
[10] The experimental setup is presented in our previous report: a) Y.
Kayaki, T. Suzuki, Y. Noguchi, S. Sakurai, M. Imanari, T. Ikariya,
Chem. Lett. 2002, 31, 424 – 425; b) Y. Kayaki, T. Suzuki, T.
Ikariya, Chem. Asian J. 2008, 3, 1865 – 1870.
[11] a) Y. Sasaki, Tetrahedron Lett. 1986, 27, 1573 – 1574; b) Y. Inoue,
J. Ishikawa, M. Taniguchi, H. Hashimoto, Bull. Chem. Soc. Jpn.
1987, 60, 1204 – 1206.
[14] a) D. J. Darensbourg, M. W. Holtcamp, Coord. Chem. Rev. 1996,
153, 155 – 174; b) J. Sun, S.-i. Fujita, M. Arai, J. Organomet.
Chem. 2005, 690, 3490 – 3497; c) T. Sakakura, J.-C. Choi, H.
Yasuda, Chem. Rev. 2007, 107, 2365 – 2387.
[15] In a number of reports on the carboxylation of epoxides using
imidazolium-based ionic liquids as reaction media or catalysts,
the counter anions have been generally recognized as nucleo-
philic activators: a) J. Peng, Y. Deng, New J. Chem. 2001, 25,
639 – 641; b) H. Kawanami, A. Sasaki, K. Matsui, Y. Ikushima,
Chem. Commun. 2003, 896 – 897; c) H. S. Kim, J. J. Kim, H. Kim,
H. G. Jang, J. Catal. 2003, 220, 44 – 46; d) J. Palgunadi, O.-S.
Kwon, H. Lee, J. Y. Bae, B. S. Ahn, N.-Y. Min, H. S. Kim, Catal.
Today 2004, 98, 511 – 514; e) J. Sun, S.-i. Fujita, F. Zhao, M. Arai,
Green Chem. 2004, 6, 613 – 616; f) F. Li, L. Xiao, C. Xia, B. Hu,
Tetrahedron Lett. 2004, 45, 8307 – 8310; g) L.-F. Xiao, F.-W. Lee,
C.-G. Xia, Appl. Catal. A 2005, 279, 125 – 129; h) Y. J. Kim, R. S.
Varma, J. Org. Chem. 2005, 70, 7882 – 7891; i) L.-F. Xiao, F.-W.
Li, J.-J. Peng, C.-G. Xia, J. Mol. Catal. A 2006, 253, 265 – 269; j) Y.
Xie, Z. Zhang, T. Jiang, J. He, B. Han, T. Wu, K. Ding, Angew.
Chem. 2007, 119, 7393 – 7396; Angew. Chem. Int. Ed. 2007, 46,
7255 – 7258; k) J.-Q. Wang, X.-D. Yue, F. Cai, L.-N. He, Catal.
Commun. 2007, 8, 167 – 172; l) J. Sun, S. Zhang, W. Cheng, J.
Ren, Tetrahedron Lett. 2008, 49, 3588 – 3591.
[12] a) E. F. Connor, G. W. Nyce, M. Meyer, A. Mꢄck, J. L. Hedrick,
J. Am. Chem. Soc. 2002, 124, 914 – 915; b) G. A. Grasa, R. M.
Angew. Chem. Int. Ed. 2009, 48, 4194 –4197
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