Synthesis of propiolic acids via copper-catalyzed insertion of carbon dioxide into the C-H bond of terminal alkynes

Article abstract of DOI:10.1002/adsc.201000564

A highly effective copper catalyst has been developed that promotes the insertion of carbon dioxide into the C-H bond of terminal alkynes under unprecedentedly mild conditions. For the first time, propiolic acids can thus be synthesized in excellent yields from alkynes and carbon dioxide in the presence of the mild base cesium carbonate. The catalyst, (4,7-diphenyl-1,10- phenanthroline)bis[tris(4-fluorophenyl)phosphine]copper(I) nitrate, is easy accessible and relatively stable against air and water. Copyright

Full text of DOI:10.1002/adsc.201000564

COMMUNICATIONS
DOI: 10.1002/adsc.201000564
Synthesis of Propiolic Acids via Copper-Catalyzed Insertion of
À
Carbon Dioxide into the C H Bond of Terminal Alkynes
Lukas J. Gooßen,a,* Nuria Rodrꢀguez,a Filipe Manjolinho,a and Paul P. Langea
a
FB Chemie – Organische Chemie, TU Kaiserslautern, Erwin-Schrçdinger-Str. Geb. 54, 67663 Kaiserslautern, Germany
Fax : (+49)-631-205-3921; e-mail: goossen@chemie.uni-kl.de
Received: July 16, 2010; Revised: August 13, 2010; Published online: October 28, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000564.
Abstract: A highly effective copper catalyst has
been developed that promotes the insertion of
À
carbon dioxide into the C H bond of terminal al-
kynes under unprecedentedly mild conditions. For
the first time, propiolic acids can thus be synthe-
sized in excellent yields from alkynes and carbon
dioxide in the presence of the mild base cesium car-
bonate. catalyst,
The
(4,7-diphenyl-1,10-
phenanthroline)bisACHTUNGTERN[NGNU tris(4-fluorophenyl)phosphine]-
copper(I) nitrate, is easy accessible and relatively
stable against air and water.
À
Keywords: carbon dioxide fixation; catalysis; C H
activation; copper; propiolic acids
Scheme 1. Known and proposed propiolic acid syntheses.
lysts. Examples are the direct carboxylation of alk-
AHCTUNGTRENNUNG
ynylmagnesium or -lithium reagents,[6] and the nickel-
or palladium-catalyzed alkylative carboxylation of al-
Propiolic acids are versatile synthetic intermediates, kynes using organozinc reagents under a CO2 atmos-
for example, in cycloaddition or hydroarylation reac- phere.[7] This synthetic strategy is attractive in that it
tions that give access to various heterocyclic deriva- makes use of abundant carbon dioxide as the C1
tives including coumarins, flavones and 3-arylidene-2- building block.[8] However, it requires the synthesis of
oxindole derivatives.[1] Furthermore, they can be used expensive and sensitive organometallic reagents.[9]
in decarboxylative cross-couplings for the preparation
The optimum strategy both from economical and
ecological standpoints would be a one-step catalytic
of alkynylarenes or aminoalkynes.[2]
Traditionally, such compounds are synthesized in carboxylation of terminal alkynes with CO2 under
À
two-step processes from the corresponding alkynes. C H functionalization (Scheme 1).
À
Most effective are the addition of alkynes to formal-
Metal-mediated C H functionalizations have re-
dehyde and subsequent oxidation of the resulting ceived enormous attention within the last years, and
propargylic alcohol,[3] and the oxidative carbonylation great progress has been achieved.[10] The key chal-
of alkynes (Scheme 1).[4] The disadvantages of these lenge in this reaction type is to activate the normally
routes lie in the C1 building blocks employed, namely non-acidic C H bond to an extent that the proton
the relatively high cost of formaldehyde, and the tox- can be abstracted by bases which are many orders of
À
icity and difficult handling of carbon monoxide.
magnitude weaker than the intermediate carbon nu-
À
Other synthetic approaches include the carbonyla- cleophiles. For the C H bonds of arenes, this has
tion of unstable and commercially unavailable alkynyl been achieved, for example, with palladium, rhodium,
halides,[5] and the lithiation of 1-alkynes followed by ruthenium, gold, and copper catalysts. Alkyne C H
À
quenching with chloroformate. Preformed alkynyl- activations have been described in the context of oxi-
metal species have also been coupled with CO2 either dative dimerizations,[11] coupling reactions of alkynes
directly or in the presence of transition metal cata- with arenes,[12] and alkynylations of aldehydes.[13]
Adv. Synth. Catal. 2010, 352, 2913 – 2917
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Lukas J. Gooßen et al.
Table 1. Development of the catalyst system.[a]
COMMUNICATIONS
Stoichiometric investigations suggest that CO2 can
À
insert into the Cu C bond of preformed alkynylcup-
rates.[14] However, the reverse reaction, a decarboxyla-
tion of the resulting copper propiolates, proceeds rap-
idly even at low temperatures (358C). As the operat-
ing temperatures of known (de)carboxylation cata-
lysts is much higher than that (ꢀ1008C), it has so far
been possible to perform catalytic carboxylation reac- Entry
tions only when the products are continuously re-
1
Cu(I)
catalyst
CO2
[bar]
T
[8C]
t
Yield
[%]
G
[h]
moved by a trapping reaction.[15]
1
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1b
1a
1a
1a
1b
1b
CuI/Phen
CuI/diPhPhen
I
I
I
I
I
I
1
1
1
1
1
1
1
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
100
100
100
80
50
50
50
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
8
8
8
8
8
8
8
8
2
2
2
2
2
2
2
2
2
2
8
8
8
8
8
8
52
64
74
80
92
93
65
85
52
53
43
43
49
52
46
22
58
85
99
9
In the course of our work on decarboxylative cou-
2
pling reactions,[16] we found phenanthroline/copper
3
4
5
complexes to be highly active catalysts for the extru-
sion of CO2 from aromatic carboxylic acids.[17] It ap-
peared reasonable to assume that these catalysts
might set new standards also for the reverse reaction,
6[b,c]
7[c]
8[c]
À
the insertion of CO2 into C H bonds.
9[c]
I
We probed this hypothesis using the model reaction
of 1-octyne (1a) with carbon dioxide according to the
scheme in Table 1. Indeed, using 1 mol% of a cop-
per(I) phenanthroline complex as the catalyst, modest
turnover was observed already at ambient CO2 pres-
sure in the presence of 2 equivalents of the mild base
Cs2CO3 at 1008C (Table 1, entry 1).
10[c]
11[c]
12[c]
13[c]
14[c]
15[c]
16[c]
17[c]
18[c]
19[c]
20
II
III
IV
V
VI
VII
VIII
IX
X
X
X
–
The yield was substantially increased when using
copper/4,7-diphenyl-1,10-phenanthroline complexes as
catalysts, which are also the most active protodecar-
boxylation catalysts (entries 2 and 3). The use of pre-
formed phenanthroline phosphine copper(I) nitrate
complexes, which are easily accessible on a multigram
scale as well as being air-stable and easy to handle,
was also beneficial.
A stepwise reduction of the reaction temperatures
from 100 to 508C led to a step-up in the yields using
catalyst I, which confirms that at higher temperatures,
the equilibrium is shifted towards the starting materi-
als (entries 4 and 5). At 508C, near-quantitative turn-
over was achieved; below this temperature, the yield
dropped as the reaction rate became too low. Under
optimized conditions, the amount of base could be re-
duced further, from 2.0 to 1.2 equivalents (entry 6).
We next evaluated the reaction of phenylacetylene
(1b) as a particularly challenging substrate. Its corre-
sponding carboxylate is so labile towards decarboxy-
21
0
0
0
0
22[d]
23[c]
24[c,d]
I
I
X
[a]
Reaction conditions: 1.0 mmol alkyne 1, 1 mol% Cu(I)
source, 1 mol% ligand, 2.0 mmol Cs2CO3, 3.0 mL DMF.
Yields determined by GC analysis of the methyl esters
generated by treatment of the crude mixture with methyl
iodide, and using n-tetradecane as the internal standard.
DMF=N,N-dimethylformamide;
Phen=1,10-phenan-
throline; diPhPhen=4,7-diphenyl-1,10-phenanthroline.
2 mol% Cu(I) source.
1.2 mmol Cs2CO3.
[b]
[c]
[d]
Without CO2.
The results confirmed 4,7-diphenyl-1,10-phenan-
lation that it is present in only 65% in the equilibrium throline to be the most effective N,N-ligand. Among
mixture under the previously optimized conditions the phosphines tested, the moderately electron-rich
(entry 7). A further reduction of the temperature did tris(4-fluorophenyl)phosphine was optimal (entry 18).
not improve the yields but, rather, led to sluggish Both sterically crowded (e.g., JohnPhos) and particu-
turnover even when increasing the CO2 pressure to larly electron-rich phosphines (e.g., tricyclohexylphos-
5 bar (entry 8). An even more active catalyst was phine) were inferior. The best catalyst, 4,7-diphenyl-
clearly needed for this and related substrates. We 1,10-phenanthroline)bis
therefore systematically varied the ligands in the ne]copper(I) nitrate (X) led to an 85% yield of the
phenanthroline phosphine Cu(I) nitrate complexes product 3b after 2 h, and near quantitative yields after
(Figure 1) and, for better comparison of their activi- 8 h (entries 18 and 19).
ACHTUNGTRNE[NUNG tris(4-fluorophenyl)phosphi-
ACHTUNGTRENNUNG
ties, tested them at incomplete conversions by reduc-
ing the reaction times (entries 9–18).
A control experiment showed that these conditions
are optimal only for aryl-substituted alkynes, while
for alkyl-substituted alkynes, the previous system
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Synthesis of Propiolic Acids via Copper-Catalyzed Insertion of Carbon Dioxide
Table 2. Carboxylation of terminal alkynes.[a]
Product
Yield
[%]
Product
Yield
[%]
95
85 (96)
97
Figure 1. Copper(I) complexes employed for optimizing the
catalyst system; JohnPhos=2-(di-tert-butylphosphino)bi-
phenyl.
62 (87)
(67)[b]
(entry 20) remains the best. Further experiments con-
firmed that for both protocols the carboxylation
works only in the presence of copper, and that car-
bonate salts alone are not sufficient as sources of CO2
(entries 21–24).
73 (87)
98
81
99
99
Having thus identified a complementary set of ef-
fective protocols, we next tested the generality of the
new carboxylation reaction by applying it to various
terminal alkynes. As can be seen from Table 2, the
first catalyst system allows the smooth conversion of
various alkyl-substituted alkynes, whereas the second
is generally applicable to a broad range of aryl-substi-
tuted alkynes. The corresponding propiolic acids were
isolated by crystallization. Alternatively, they were
converted into esters and purified by column chroma-
tography, the latter method serving to verify that
moderate yields were caused primarily by non-opti-
mized crystallization procedures. Only in some cases,
e.g., when the products contained basic nitrogen het-
erocycles (3m), the free acids could not be isolated
and had to be converted into esters to allow their sep-
aration from the reaction mixture.
63 (84)
86
(75)
87
Due to the mild conditions, many functional groups
including ethers, halogens, trifluoromethyl and alken-
yl groups were tolerated. Unprotected NH or OH
groups appeared to be incompatible with the proce-
dure.[18] However, products with free hydroxy groups
such as 3f can be accessed starting from the corre-
sponding silyl ethers, as the TMS groups is cleaved
during the reaction work-up.
73 (86)
62 (85)
[a]
Reaction conditions: Method A: alkyl-substituted al-
kynes: 1.0 mmol of alkyne 1, 2 mol% of Cu(I) source,
1.2 mmol of Cs2CO3, CO2 (1 bar), 3.0 mL of DMF, 508C,
16 h. Method B: (hetero-)aryl-substituted alkynes:
1.0 mmol of alkyne 1, 1 mol% of Cu(I) source, 1.2 mmol
of Cs2CO3, CO2 (5 bar), 3.0 mL of DMF, 358C, 16 h; iso-
lated yields. Yields in parentheses correspond to the n-
hexyl esters generated by treatment of the crude mixture
with 2.0 mmol of 1-bromohexane. For further details see
Supporting Information.
We assume that the reaction starts with the coordi-
nation of the copper to the alkyne, acidifying the sp3-
H and allowing deprotonation by the added base
under formation of an alkynyl copper complex. It is
reasonable to assume that the phosphine ligand disso-
ciates in the course of these reaction steps. In the
[b]
Using (2-propynyloxy)trimethylsilane 1f as the starting
material.
À
next step, the CO2 inserts into the C Cu bond. Final-
Adv. Synth. Catal. 2010, 352, 2913 – 2917
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Lukas J. Gooßen et al.
COMMUNICATIONS
room temperature, the CHCl3 was removed under vacuum
to afford a yellow solid which was further purified by recrys-
tallization from CH2Cl2 and Et2O, yielding the correspond-
ing copper(I) complexes.
Synthesis of (4,7-Diphenyl-1,10-phenanthroline)bis-
AHCTUNGTREG[NNNU tris(4-fluorophenyl)phosphine]copper(I) Nitrate (X)
Complex X was prepared from bisACTHNUTRGNEUGN[tris(4-fluorophenyl)phos-
À
Scheme 2. Carboxylation of a heterocyclic C H bond.
phine]copper(I) nitrate (XVII) (758 mg, 1.00 mmol), 4,7-di-
phenyl-1,10-phenanthroline (339 mg, 1.00 mmol) and tris(4-
fluorophenyl)phosphine (316 mg, 1.00 mmol) affordding X
as a yellow solid; yield: 1.40 g (97%). 31P NMR (162 MHz,
ly, the carboxylate ion is replaced by an alkyne mole-
cule or, alternatively, by the phosphine.
CDCl3):
d=19.84
(s,
2P);
anal.
calcd.
for
The catalysts also allow the carboxylation of het-
C60H40CuF6N3O3P2: C 66.1, H 3.7, N 3.8; found: C 65.4, H
3.8, N 4.0.
erocycles that have
a similar pKa to alkynes
(Scheme 2). However, we discontinued these investi-
gations when a publication appeared in which Nolan
et al. disclosed a mechanistically related carbene gold
hydroxide-catalyzed carboxylation of heterocycles.[19]
Their work suffices to demonstrate that the reaction
concept outlined herein is not limited to alkynes but
is likely to open up general perspectives for carboxyl-
ic acid synthesis.[20]
In conclusion, new copper-based carboxylation cat-
alysts have been developed. By lowering the activa-
tion barrier for the carboxylation of terminal alkynes,
the carboxylation/decarboxylation equilibrium can be
shifted towards the carboxylated products to an
General Procedure for the Carboxylation of Terminal
Alkyl-Substituted Alkynes (Table 2)
An oven-dried, nitrogen-flushed, 10-mL vessel was charged
with
(4,7-diphenyl-1,10-phenanthroline)bis(triphenylphos-
phine)copper(I) nitrate (I) (19.7 mg, 0.02 mmol) and cesium
carbonate (782 mg, 2.00 mmol). Under an atmosphere of ni-
trogen, the degassed DMF (3.00 mL) was added and the
mixture was stirred at room temperature for 5 min. After
purging the reaction vessel with CO2, the alkyne (1a, 1c–g)
(1.00 mmol) was added via syringe. The resulting mixture
was stirred for 12 h at 508C at ambient CO2 pressure. At the
end of the reaction time, the mixture was cooled down to
extent that an isolation of free propiolic acids is possi- room temperature, diluted with H2O and extracted with n-
hexane (3ꢂ20.0 mL). Then the aqueous layer was acidified
with aqueous HCl (1N, 10.0 mL) to afford a colorless solid
which was further purified by recrystallization from H2O
and EtOH. In those cases where no solid was formed, the
aqueous layer was further extracted with ethyl acetate (3ꢂ
20.0 mL). The combined organic layers were washed with a
dilute aqueous solution of LiCl and brine, dried over
MgSO4, filtered and the volatiles were removed under
vacuum to afford the corresponding acids 3a, 3c–g which
were further purified by recrystallization from H2O and
EtOH.
ble for the first time.
Experimental Section
General Procedure for the Synthesis of Copper
Phosphine Complexes
An oven-dried, nitrogen-flushed, 50-mL Schlenk tube was
charged with 2.00 mL of EtOH and heated to reflux. Under
an N2 atmosphere, the phosphine (3.00 mmol) was then
slowly added until it completely dissolved. To this, cop-
General Procedure for the Carboxylation of Terminal
Aryl-Substituted Alkynes (Table 2)
ACHTUNGTRENNUNGper(II) nitrate trihydrate (242 mg, 1.00 mmol) was added,
portionwise, over 20 min. After complete addition, the reac-
tion mixture was stirred under reflux for 30 min. During this
time a gradual formation of a precipitate was observed. The
resulting solid was collected by filtration, washed sequential-
ly with EtOH (2ꢂ10.0 mL) and cold (08C) Et2O (2ꢂ
10.0 mL), transferred to a flask, and dried at 2ꢂ10À3 mmHg
to provide the corresponding phosphine copper complexes.
An oven-dried, nitrogen-flushed, 5-mL vessel was charged
with the (4,7-diphenyl-1,10-phenanthroline)bisACHTUNTRGNEU[GN tris(4-fluoro-
phenyl)phosphine]copper(I) nitrate X (10.9 mg, 0.01 mmol)
and cesium carbonate (391 mg, 1.20 mmol). Under an at-
mosphere of nitrogen, the degassed DMF (3.00 mL) was
added and the mixture was stirred at room temperature for
5 min. After flushing the reaction vessel with CO2, the
alkyne (1b, 1h–p) (1.00 mmol) was added via syringe. The
reaction vessel was then placed in a steel autoclave, and
pressurized with CO2 (5 bar). The reaction mixture was
stirred at 358C for 12 h. At the end of the reaction time, the
autoclave pressure was released. The reaction mixture was
diluted with H2O (2.00 mL) and extracted with n-hexane
(3ꢂ20.0 mL). The aqueous layer was acidified with aqueous
HCl (1N, 10.0 mL) to afford a colorless solid which was fur-
ther purified by recrystallization from H2O and EtOH. In
those cases where no solid was formed, the aqueous layer
General Procedure for the Synthesis of Copper(I)
Mixed-Ligand Nitrate Complexes (Figure 1)
An oven-dried, nitrogen-flushed, 50-mL Schlenk tube was
charged with the copper phosphine complex (1.00 mmol)
and 10.0 mL of CHCl3. To this, the phosphine (1.00 mmol)
was added the mixture was stirred until all materials were
completely dissolved. Then, a solution of the N-ligand
(1.00 mmol) in 2 mL of CHCl3 was gradually added over
30 min. After stirring the reaction mixture for 30 min at
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Synthesis of Propiolic Acids via Copper-Catalyzed Insertion of Carbon Dioxide
was futher extracted with ethyl acetate (3ꢂ20.0 mL). The
combined organic layers were washed with a dilute aqueous
solution of LiCl and brine, dried over MgSO4, filtered and
the volatiles were removed under vacuum to afford the cor-
responding acids 3b, 3h–p which were further purified by re-
crystallization from H2O and EtOH.
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(3a)
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An oven-dried, nitrogen-flushed, 100-mL vessel was charged
with
(4,7-diphenyl-1,10-phenanthroline)bis(triphenylphos-
[7] For a review, see: S. N. Riduan, Y. Zhang, Dalton
phine)copper(I) nitrate (I) (590 mg, 0.60 mmol) and cesium
carbonate (11.7 g, 36.0 mmol). Under a nitrogen atmos-
phere, degassed DMF (50 mL) was added, and the mixture
was stirred at room temperature for 5 min. After flushing
the reaction vessel three times with CO2, 1-octyne 1a
(4.47 mL, 30.0 mmol) was added via syringe. The resulting
mixture was stirred at 508C under an ambient CO2 pressure
for 16 h. Once the reaction was complete, the mixture was
cooled to room temperature, diluted with H2O and extract-
ed with n-hexane (3ꢂ20 mL). The aqueous layer was then
acidified with aqueous HCl (1N, 100 mL) and extracted
with ethyl acetate (3ꢂ60 mL). The combined organic layers
were washed with a dilute aqueous LiCl solution and brine,
dried over MgSO4, filtered, and the volatiles were removed
under vacuum. The residue was purified by filtration over
silica gel (500 mg) eluting with ethyl acetate/hexane 1:5, to
afford 3a as a colorless oil; yield: 4.6 g (97%). The spectro-
scopic data (NMR, IR) matched those reported in the litera-
ture for 1-a-nonyoic acid (3a) [CAS: 1846–70–4].
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Acknowledgements
[16] a) L. J. Gooßen, N. Rodrꢀguez, K. Gooßen, Angew.
Chem. 2008, 120, 3144–3164; Angew. Chem. Int. Ed.
2008, 47, 3100–3120; b) L. J. Gooßen, N. Rodrꢀguez, C.
Linder, J. Am. Chem. Soc. 2008, 130, 15248–15249;
c) L. J. Gooßen, C. Linder, N. Rodrꢀguez, P. P. Lange,
Chem. Eur. J. 2009, 15, 9336–9349; d) L. J. Gooßen, N.
Rodrꢀguez, P. P. Lange, C. Linder, Angew. Chem. 2010,
122, 1129–1132; Angew. Chem. Int. Ed. 2010, 49, 1111–
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Linder, Chem. Eur. J. 2010, 16, 3906–3909.
[17] a) L. J. Gooßen, W. R. Thiel, N. Rodrꢀguez, C. Linder,
B. Melzer, Adv. Synth. Catal. 2007, 349, 2241–2246;
b) L. J. Gooßen, F. Manjolinho, B. A. Khan, N. Rodrꢀ-
guez, J. Org. Chem. 2009, 74, 2620–2623; c) L. J.
Gooßen, N. Rodrꢀguez, C. Linder, P. P. Lange, A.
Fromm, ChemCatChem. 2010, 2, 430–442.
We thank the Deutsche Forschungsgemeinschaft and Nano-
Kat for funding, and the A.-von-Humboldt Foundation for a
scholarship to N.R.
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