C-C bond formation with carbon dioxide promoted by a silver catalyst

Article abstract of DOI:10.1002/anie.201201399

A catalytic amount of silver benzoate with 7-methyl-1,5,7-triazabicyclo[4. 4.0]dec-5-ene (MTBD) was an effective catalytic system for the reaction of carbon dioxide with various ketones containing an alkyne group at an appropriate position (see scheme). These reactions afforded the corresponding γ-lactone derivatives in good to high yields under mild conditions. Copyright

Full text of DOI:10.1002/anie.201201399

Angewandte
Chemie
DOI: 10.1002/anie.201201399
Silver Catalysis
À
C C Bond Formation with Carbon Dioxide Promoted by a Silver
Catalyst**
Satoshi Kikuchi, Kohei Sekine, Tomonobu Ishida, and Tohru Yamada*
A range of chemical reactions have been reported to produce
useful chemicals from carbon dioxide, which is an ubiquitous,
abundant, cheap, and nontoxic C1 feedstock.[1] Carbon
dioxide is also used in industry to generate some useful
materials. However, carbon dioxide is thermodynamically
stable and much less reactive than other carbon derivatives
owing to its high oxidation state. Therefore, strong nucleo-
philes and harsh reaction conditions have been required when
carbon dioxide is used in organic syntheses. For example,
Grignard reagents[2] and organolithium compounds[3] are well
known to react with carbon dioxide to afford the correspond-
ing carboxylic acids. Transition-metal catalyzed reactions of
carbon dioxide to produce the corresponding carboxylic acid
and ester derivatives have also been reported. In these
Scheme 1. Silver catalyst and DBU catalyzed the reaction of propargylic
alcohol with carbon dioxide. DBU=1,8-diazabicyclo[5.4.0]undec-7-ene.
systems, however, a boron- or halogen-containing reactant
must be prepared in advance, or a stoichiometric amount of
another metal-containing reducing agent is required.[4] Sev-
eral reactions of CO2 that are catalyzed by metal complexes
under harsh reaction conditions have been reported to afford
more-stable products.[5] In some reactions, the esterification
of products with alkyl halides is required for stabilization or
purification and this process generates waste salts. The
reaction of enolates with CO2 to provide the corresponding
triple bond by the silver catalyst, as a p-Lewis acid, was
essential for these reactions. When the optically active Schiff
base ligand for the silver catalyst was employed, symmetrical
bispropargylic alcohols were converted into the cyclic carbo-
nates through desymmetrization, in high yield and with high
enantioselectivity.[9,10]
À
b-ketocarboxylic acids is a promising C C bond-forming
reaction. However, the b-ketocarboxylic acid products are
thermodynamically unstable, therefore only limited types of
substrates could be used, as otherwise the product readily
converts back into the starting substrate by decarboxylation.[6]
Recently, we reported that a silver catalyst could effec-
In the reaction shown in Scheme 1, we believe the carbon
dioxide would be captured by a hydroxy group to generate the
À
carboxylate intermediate with formation of a new C O bond.
À
The resulting carboxylate could be trapped by the C C triple
À
tively act as a p-Lewis acid to activate the C C triple bond in
bond, which had been activated by the silver catalyst, to
afford the cyclic carbonate. Therefore, we postulated that
when a ketone containing an alkyne group at an appropriate
position is applied to the present reaction system, the derived
enolate would capture carbon dioxide to generate the
corresponding b-ketocarboxylate. The b-ketocarboxylate
propargylic alcohols and thus promote the reaction of carbon
dioxide in the presence of a base to afford the corresponding
cyclic carbonates (Scheme 1, path a)[7] or the corresponding
a,b-unsaturated carbonyl compounds (Scheme 1, path b);[8]
the product obtained depended on the polarity of solvent.[9]
Based on X-ray analysis and NOE experiments, it was
suggested that all cyclic carbonates possessed a Z olefin.
Theoretical analysis of the silver catalytic system with N-
methylpyrimidine as a model base revealed that the transition
state would provide the Z-exo-alkene product.[9] Conse-
À
would then be trapped by the activated C C triple bond to
afford the corresponding stable lactone without the formation
of any waste sideproducts (Scheme 2).[11] Herein, we report
À
a catalytic C C bond formation with carbon dioxide; this
À
reaction involves C C triple bond activation by a silver
À
quently, it was confirmed that the activation of the C C
catalyst to afford the corresponding lactone under mild
reaction conditions.
Several metal catalysts were initially investigated for the
reaction of ketone 1a, as the model substrate, in DMSO in the
presence of DBU (2.0 equiv) under 1.0 MPa CO2 pressure
(Table 1). The reactions did not proceed in the absence of the
metal salt (Table 1, entry 1). Palladium, copper, and gold(I)
[*] Prof. Dr. S. Kikuchi, K. Sekine, T. Ishida, Prof. Dr. T. Yamada
Department of Chemistry, Keio University
Hiyoshi, Kohoku-ku, Yokohama 223-8522 (Japan)
E-mail: yamada@chem.keio.ac.jp
[**] We thank Prof. Dr. Takuya Kochi, Keio University, for help with X-ray
analysis.
À
salts, which were expected to activate the C C triple bond,
hardly worked for this reaction (Table 1, entries 2–5). When
a gold(III) salt was employed as the catalyst, the dihydrofuran
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201201399.
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Table 2: Examination of various bases and solvents.[a]
Entry
Base
Solvent
Yield [%][b]
2a
3a
1
2
3
4
5
6
7
DMAP
iPr2NEt
TBD
MTBD
MTBD
MTBD
MTBD
MTBD[c]
MTBD[c]
DMSO
DMSO
DMSO
DMSO
DMI
DMA
DMF
DMF
DMF
6
0
7
48
10
59
72
89
91
–
–
–
6
0
4
4
6
4
8
9[d]
[a] Reactions were carried out in 1.0 mL of solvent with 0.125 mmol of
substrate 1a in the presence of AgOBz (20 mol%) and 2.0 equivalents of
base under 1.0 MPa CO2 pressure. [b] Yields of the isolated products.
[c] 4.0 equivalents of MTBD were used. [d] The reaction was performed at
258C. DMI=1,3-dimethyl-2-imidazolidinone, DMA=N,N-dimethylacet-
amide.
À
Scheme 2. The postulated reaction with carbon dioxide through C C
bond formation.
Table 1: Investigation of several metal salts.[a]
Entry
Metal Salt
Yield [%][b]
2a
3a
in DMI afforded product 2a was in low yield, whereas the
reaction in DMA gave the product 2a in moderate yield
(Table 2, entries 5, 6). Among the solvents examined, DMF
was found to be the most suitable as the reaction in this
solvent afforded the product 2a in the highest yield (Table 2,
entry 7). 4.0 equivalents of MTBD were sufficient to promote
the reaction to afford the corresponding b-ketolactone 2a in
89% yield (Table 2, entry 8).[13] At lower reaction temper-
atures the formation of the by-product furan 3a was sup-
pressed, thus the yield of 3a decreased although a longer
reaction time was required.[13] Under the optimized condi-
tions the product 2a was obtained in 91% yield at 258C in
48 h (Table 2, entry 9).
1
2
3
4
5
6
7
None
0
0
0
0
0
0
13
5
Pd(OAc)2
Cu(OTf)2
AuCl
AuCl/AgOBz
AuCl3
trace
0
0
0
trace
40
AgOBz
[a] Reactions were carried out in 1.0 mL of solvent with 0.125 mmol of
substrate 1a under 1.0 MPa CO2 pressure. [b] Yields of the isolated
products. Bz=benzoyl, DMSO=dimethyl sulfoxide, Tf=trifluorome-
thanesulfonyl.
derivative 3a,[12] which would be formed by the direct
intramolecular cyclization of the enol derived from ketone
1a, was obtained in 13% yield although the desired product
was not obtained at all (Table 1, entry 6). Among the catalysts
tested, a silver salt was the most effective catalyst for this
reaction to produce the g-lactone 2a (Table 1, entry 7).[13]
Taking into account the acidity of the a proton of 1a,
several bases were screened (Table 2).[13] When DMAP was
used the corresponding product 2a was obtained in 6% yield
(Table 2, entry 1), but when Hꢀnigꢁs base was used no product
was formed (Table 2, entry 2). In the previous work, amidine-
type bases such as DBU were effectively employed to afford
the cyclic carbonates in high yield. These observations were
supported by DFT calculations on the reactions.[9] Based on
these results, various amine bases, such as TBD and MTBD,
were examined. When MTBD was employed as a base
(Table 2, entry 4), the product 2a was obtained in good yield
(48%), whereas TBD was not an effective base for this
reaction (Table 2, entry 3). In the preliminary examination of
solvents, nonprotic polar solvents were found to promote the
reaction smoothly.[13] Several different nonprotic polar sol-
vents were next examined (Table 2, entries 5–7). The reaction
The substrate scope of this reaction was investigated
under the optimized catalytic reaction conditions (Table 3).
Initially the reactions of substrates that were derived from
acetophenone and had a range of alkyne substituents were
examined (Table 3, entries 1, 3–8). The reactions of substrates
having phenyl (1a), p-tolyl (1b), m-methoxyphenyl (1c), p-
fluorophenyl (1d), and p-trifluoromethylphenyl (1e) groups
on the alkyne proceeded under the optimized reaction
conditions to achieve high yields, irrespective of the elec-
tron-withdrawing or electron-donating nature of the group on
the phenyl ring. When the catalyst loading was decreased to
from 20 mol% to 10 mol%, the reaction of ketone 1a with
carbon dioxide gave the lactone 2a in good yield (Table 3,
entry 2), although a longer reaction time was needed for the
reaction to go to completion.[13] The reaction of substrates 1 f
and 1g, which bear an alkyl substituent on the alkyne, were
also catalyzed in the presence of 6.0 equivalents of MTBD to
afford the corresponding product 2 f and 2g in good yields
(Table 3, entries 7 and 8) under 2.0 MPa CO2 pressure.
Substrates derived from aromatic ketones were subjected to
this catalytic system under the optimized reaction conditions
(Table 3, entries 9–14). The p-tolyl ketone derivative 1h was
2
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Chemie
Table 3: AgOBz and MTBD system was applied to the reaction of several
substrates (1a–1o).[a]
(2m) in 77% yield when 6.0 equivalents of MTBD were used
(Table 3, entry 14). The ketone having a cyclobutyl substitu-
ent (1n) was also converted efficiently into the corresponding
product (2n) in good yield under the same catalytic system
with 2.0 MPa CO2 pressure (Table 3, entry 15). The substrate
bearing a cyclopentyl group (1o) was also applied to this
catalytic system and the product (2o) was obtained in 36%
yield (Table 3, entry 16).
The present catalytic system was also applied to aliphatic
ketone derivatives (1p, 1q, and 1r; Scheme 3) at 508C under
2.0 MPa CO2 pressure. Substrate 1p was converted into the
corresponding g-lactone 2p in 58% yield. The yield of g-
Entry
Product
t
[h]
Yield
[%][b]
1
R1 =
H (2a)
H (2a)
p-Me (2b)
m-MeO (2c)
p-F (2d)
48
240
96
96
72
48
91
85
90
87
79
90
2[c]
3
4
5
6
p-CF3 (2e)
7[d]
8[d]
R2 =
R3 =
Ph (2 f)
70
72
77
83
CH2CH3 (2g)
9
p-Me (2h)
p-CF3 (2i)
p-MeO (2j)
m-MeO (2k)
o-MeO (2l)
72
84
79
69
77
76
10[e]
11[f]
12[f,g]
13[f,g]
192
144
144
220
Scheme 3. The reaction of aliphatic ketone derivatives (2p–2r).[14]
DMF=N,N-dimethylformamide.
14[d]
(2m)
120
77
15[h]
n=1 (2n)
n=2 (2o)
120
48
74
lactone 2q was not satisfactory (31% yield), but the reaction
of ketone 1r with carbon dioxide afforded the corresponding
g-lactone 2r in 59% yield. These ketones possess two
different protons a and a’ to the carbonyl group and thus
two different enolates can be generated, but the g-lactone was
selectively obtained without the formation of any other
lactone derivatives. The reaction to form the five-membered
lactone is the productive pathway,[13] and the corresponding g-
lactone was selectively obtained without any need for control
of the enolization.
16[h,i]
36[j]
[a] The reaction was performed with AgOBz (20 mol%) and MTBD
(4.0 equiv) in 1.0 mL of DMF with 0.125 mmol of substrate 1 at 258C
under 1.0 MPa CO2 pressure. [b] Yield of the isolated products. [c] The
amount of silver catalyst was decreased from 20 mol% to 10 mol%.
[d] The reaction was carried out with 6.0 equivalents of MTBD under
2.0 MPa CO2 pressure. [e] The reaction temperature was 58C. [f] The
reaction was performed at 108C under 2.0 MPa CO2 pressure. [g] AgOBz
(40 mol%) was used. [h] The reaction was carried out under 2.0 MPa
CO2 pressure. [i] AgOAc (40 mol%) was used instead of AgOBz at 608C.
[j] 47% of ketone 1o was recovered and the corresponding furan was
obtained in 6% yield.
À
The geometry of the C C double bond in the lactone
derivative 2h was confirmed by X-ray analysis to reveal the
Z isomer as the sole product (Figure 1). All other lactone
derivatives were also selectively obtained as the Z isomers
based on NOE experiments.
a good substrate for this reaction and the corresponding
product 2h was obtained in good yield (Table 3, entry 9). The
substrate bearing a p-trifluoromethylbenzoyl group 1i was
also a good substrate, producing the corresponding product 2i
in good yield, although a longer reaction time and low
reaction temperature were required (Table 3, entry 10). This
catalytic system catalyzed reaction of methoxy-substituted
ketone derivative 1j, but the yield was not satisfactory. The
yield of the product 2j was improved to 69% at lower
temperature (108C) under 2.0 MPa CO2 pressure (Table 3,
entry 11). The reactions of other methoxyacetophenone
derivatives 1k and 1l proceeded in the presence of AgOBz
(40 mol%) at 108C under 2.0 MPa CO2 pressure and the
corresponding products (2k and 2l) were obtained in good
yields (Table 3, entries 12–13). The 1-naphtyl derivative (1m)
was transformed into the corresponding lactone derivative
Figure 1. Single-crystal structure of 2h.[15] Thermal ellipsoids are shown
at 50% probability.
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In this study, we successfully developed a catalytic C C
bond-forming reaction with carbon dioxide; this reaction
employed catalytic silver benzoate in the presence of MTBD
to afford lactone derivatives in good-to-high yields under mild
reaction conditions. This reaction system could be applied to
aliphatic ketone derivatives and the corresponding lactone
was obtained without any control of the formation of enolate.
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À
The geometry of the C C double bond of the products was
confirmed to be Z based on X-ray analysis and NOE
À
experiments. Further applications of the silver-catalyzed C
C bond-forming reaction with carbon dioxide are underway
with various organic compounds.
Received: February 20, 2012
Published online: && &&, &&&&
À
Keywords: carbon dioxide · C C bond formation ·
homogeneous catalysis · silver · synthetic methods
.
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[13] See the Supporting Information.
[14] The other products were furan derivatives and the recovered
starting ketone 1.
[15] CCDC 827765 (2h) contains the supplementary crystallographic
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4
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Angewandte
Chemie
Communications
Silver Catalysis
S. Kikuchi, K. Sekine, T. Ishida,
T. Yamada*
&&&& — &&&&
À
C C Bond Formation with Carbon
Dioxide Promoted by a Silver Catalyst
A catalytic amount of silver benzoate with
7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-
ene (MTBD) was an effective catalytic
system for the reaction of carbon dioxide
with various ketones containing an alkyne
group at an appropriate position (see
scheme). These reactions afforded the
corresponding g-lactone derivatives in
good to high yields under mild condi-
tions.
Angew. Chem. Int. Ed. 2012, 51, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!