Carboxylation of terminal alkynes with carbon dioxide catalyzed by Poly(N-Heterocyclic Carbene)-supported silver nanoparticles

Article abstract of DOI:10.1002/adsc.201100934

An N-heterocyclic carbene (NHC) polymer supported silver nanoparticle catalyst system was developed. The novel nano-composite catalyst demonstrated very high activity and excellent stability and reusability in the carboxylation of terminal alkynes with carbon dioxide at ambient conditions. The unique N-heterocyclic carbene polymer and silver nanoparticle composite structure provided a synergistic effect on activation of terminal alkynes and carbon dioxide that contributed to the high catalytic activity. The poly-NHC-silver catalyst exhibited excellent substrate generality and tolerance to various functionalities. In addition, the catalyst is stable to air and moisture and can be easily recovered and reused. Copyright

Full text of DOI:10.1002/adsc.201100934

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DOI: 10.1002/adsc.201100934
Carboxylation of Terminal Alkynes with Carbon Dioxide
Catalyzed by Poly(N-Heterocyclic Carbene)-Supported Silver
Nanoparticles
Dingyi Yu,a Mei Xuan Tan,a and Yugen Zhanga,*
a
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669, Singapore
Fax : (+65)-6-498-9080; phone: (+65)-6-824-7162; e-mail: ygzhang@ibn.a-star.edu.sg
Received: November 30, 2011; Published online: April 4, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100934.
Abstract: An N-heterocyclic carbene (NHC) poly-
mer supported silver nanoparticle catalyst system
was developed. The novel nano-composite catalyst
demonstrated very high activity and excellent sta-
bility and reusability in the carboxylation of termi-
nal alkynes with carbon dioxide at ambient condi-
tions. The unique N-heterocyclic carbene polymer
and silver nanoparticle composite structure provid-
ed a synergistic effect on activation of terminal al-
kynes and carbon dioxide that contributed to the
high catalytic activity. The poly-NHC-silver catalyst
exhibited excellent substrate generality and toler-
ance to various functionalities. In addition, the cata-
lyst is stable to air and moisture and can be easily
recovered and reused.
tions.[25–27] There are some well-established protocols
for the preparation of alkynyl carboxylic acids, such
as the hydrolysis of bromide and relative derivatives
and the oxidation of alcohols or aldehydes.[28–30] How-
ever, the catalytic carboxylation of terminal alkynes
with CO2 is the most straightforward method for the
synthesis of alkynyl carboxylic acids. Typically, this
type of reaction is facilitated by the insertion of CO2
into a carbon-metal bond. We and Goobenꢀs group
have recently disclosed a copper- or copper-NHC-cat-
alyzed carboxylation of terminal alkynes with CO2
(Scheme 1, A and B).[17,19] Later, we have developed
a transition metal-free method for direct carboxyla-
tion of terminal alkynes with CO2 under heating con-
dition and 2.5 atm (Scheme 1, C).[18] Following that,
a ligand-free Ag(I) catalytic system was reported to
catalyze the carboxylation of terminal alkynes with
CO2 under mild conditions (50–608C, 2 atm)
(Scheme 1, D).[20] In addition, copper-catalyzed cou-
pling CO2 with terminal alkynes and alkyl halides to
form propiolic acid esters was also reported.[31–33]
These elegant methods certainly represented a mile-
Keywords: carbon dioxide; carboxylation; N-het-
ACHTUNGTRENNUNGerocyclic carbenes; silver catalyst; terminal alkynes
The use of CO2 as a C1 building block in organic syn- stone for propiolic acids synthesis. However, there are
thesis has attracted increasing attention since it may still some limitations of these catalytic systems for
provide access to high-value products from a non- carboxylation of terminal alkynes with CO2, such as
toxic, renewable, and low-cost resource.[1–16] Among moderate to good yield, heating and pressure condi-
these procedures, the direct carboxylation of carbon tions and no catalyst reusability was reported.
À
nucleophiles using CO2 as the electrophile with C C
Herein, we report a poly-NHC supported silver
bond formation represents one of the most attractive nanoparticle (poly-NHC-Ag) system that can serve as
and straightforward protocols. Recently, the carboxy- an excellent and reusable heterogeneous catalyst for
lation of terminal alkynes with CO2 to produce pro- the carboxylation of terminal alkynes with CO2
piolic acids has been developed by several (Scheme 1, E). The reaction can be carried out in the
groups.[17–20] The ubiquity of alkynyl carboxylic acids presence of poly-NHC-Ag nanocomposite under am-
is demonstrated by their tremendous utility as a syn- bient conditions to afford excellent yields of propiolic
thon in organic synthesis as well as being one of the acids with a wide range of functionality.
most important types of compounds in medicinal
Previously, we have reported a poly-N-heterocyclic
chemistry.[21–24] Furthermore, the propiolic acids can carbene (NHC) and copper(I) catalytic system for the
also be used for the preparation of alkynylarenes or carboxylation of terminal alkynes with CO2.[17] The
aminoalkynes via decarboxylative cross-coupling reac- synergistic effects of the organocatalyst (NHC) and
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Scheme 1. Protocols for synthesis of substituted propiolic acids from terminal alkynes and CO2.
organometallic catalyst make the composite catalyst persed in poly-NHC polymer material (Figure 1). The
much more active, especially for arylalkynes with conversion of poly-imidazolium into poly-N-heterocy-
electron-withdrawing groups which are typically inner clic carbene was confirmed by Fourier transform in-
toward copper catalysts. Later, Luꢀs group reported frared and solid state NMR spectroscopy (see the
a simple AgI catalyst that is active toward aryalkynes Supporting Information, Figure S1).[34] The silver con-
with either electron-withdrawing groups or electron- tent in this poly-NHC-Ag nano-composite is 6.8 wt%.
donating groups under relatively mild conditions With that, the catalytic activity of poly-NHC-Ag
(508C, 2 atm) even with very low catalyst loading.[20] nanocomposite was evaluated in the carboxylation re-
With the intention to develop a high efficient catalyst action of 1-ethynylbenzene (1a) with CO2 under am-
that is active toward a general and wide range of bient conditions (Table 1). Initial screening experi-
alkyne substrates and yet stable and reusable, a poly- ments gave very promising results. Up to 98% yield
NHC/Ag nanoparticle composite catalyst was de- of the desired propiolic acid product was obtained
signed and synthesized. The synergistic effect of NHC with 0.3 mol% of poly-NHC-Ag catalyst under ambi-
and Ag nanoparticles was proposed to play a major ent condition in 20 h (Table 1, entry 4). Under the
role in the catalytic cycle.
same reaction conditions, silver on carbon (Ag/C) and
The poly-imidazolium material was synthesized AgI (5 mol%) only gave 40% and 54% of the desired
based on the literature method.[34–36] The poly-NHC- product. AgI (5 mol%) with N-heterocyclic carbene
supported silver nanoparticle (poly-NHC-Ag) materi- ligand (Imes) (10 mol%) gave 70% product yield
al was prepared by reacting the poly-imidazolium (Table 1).
with AgNO3 in hot dimethyl sulfoxide (DMSO).[34]
Inspired by this high catalytic activity, the stability
Transmission electron microscopy (TEM) image and the reusability of poly-NHC-Ag nano-composite
shows that silver nanoparticles (3–5 nm) were dis- catalyst was further studied. The poly-NHC-Ag cata-
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Carboxylation of Terminal Alkynes with Carbon Dioxide
Figure 1. Preparation of poly-NHC-Ag nano-composite and the transmission electron microscopy (TEM) image of the nano-
composite.
Table 1. Carboxylation of 1-ethynylbenzene with CO2.[a]
Entry
Catalyst (mol% Ag)
Temperature [oC]
CO2 [atm]
Time [h]
Yield [%][b]
1
2
3
4
5
6
Ag@C (5.0)
Ag@C (5.0)
Ag@C (5.0)
Ag@P-NHC (0.3)
AgI-IMes (5.0)
AgI (5.0)
25
25
50
25
25
25
1.0
1.0
2.0
1.0
1.0
1.0
20
36
20
20
20
20
40
48
68
98
70
54
[a]
Reaction conditions: 1-ethynylbenzene (1.0 mmol), Cs2CO3 (1.2 mmol), DMF (5 mL).
GC yields.
[b]
lyst was recycled by centrifugation and filtration of was observed in the hot filtrate solution. The Ag con-
the reaction mixture. The solid residue was washed centration in the filtrate was about 2.5 ppb as moni-
with DMF twice and then directly used as the recy- tored by inductively coupled plasma-mass spectrome-
cled catalyst in the subsequent runs. As shown in try (ICP-MS), which suggested that silver leakage
Figure 2, the catalytic activity of the poly-NHC-Ag (0.0037%) is negligible. In general, the poly-NHC-Ag
nano-composite was maintained in a high level (more catalyst exhibited excellent stability and reusability as
than 93% of yield in each of the 5 runs) although a heterogeneous catalyst for the carboxylation termi-
a slight decrease in activity was observed upon recy- nal alkynes with CO2. The total TON number for this
cling. This decreasing activity may due to the partial catalyst in the first five runs was 1583 and this
loss of catalyst during the recovery process. In our number could be further increased.
heterogeneous reaction systems, no catalytic activity
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the functionality of the aromatic alkynes. Excellent
yields (92–96%) were achieved for both arylalkynes
with electron-donating groups (Scheme 2, 2b–2g) and
also with electron-withdrawing groups (Scheme 2, 2i–
2l). Excellent yields (95%) were also achieved for less
active alkyl alkynes (Scheme 2, 2m and 2n). For both
alkyl- and aryl-substituted alkynes,[37,38] the carboxyla-
tion reaction proceeded smoothly without any side
product formation. It should also be noted that the
poly-NHC-Ag nano-composite catalytic system is tol-
erant to various functional groups, such as OH, CHO,
CN, NO2 etc.
In general, copper(I)- and silver(I)-catalyzed car-
boxylation reactions of terminal alkynes with CO2
follow a similar mechanism as that reported in the lit-
erature.[17,19,20] Silver or copper cations activate the ter-
minal alkyne with base to form a metal acetylide in-
termediate, and then CO2 inserts into the metal-
carbon bond to form the carboxylic acid product.
However, the current poly-NHC-Ag catalyst demon-
strated extraordinary high activity as compared to
Figure 2. Recycling of poly-NHC-Ag nano-composite cata-
lyst for the carboxylation of terminal alkynes with CO2
To investigate the substrate scope and generality of other silver or copper catalysts. It is believed that the
the poly-NHC-Ag catalyst in this carboxylation reac- synergistic effect of the organocatalyst (NHC) and
tion, a variety of different types of terminal alkynes the organometallic catalyst (silver nanoparticle) plays
were examined under the standard conditions: poly- the role here. In the poly-NHC-Ag catalytic system,
NHC-Ag (5.0 mg, 0.3 mol% of Ag), 1.2 equivalents of silver nanoparticles activated the terminal alkyne with
Cs2CO3, 258C and 1.0 atm) of CO2 (see Scheme 2). base to form a silver acetylide intermediate; mean-
As expected, the current catalyst is not sensitive to while the free carbene of poly-NHC activated CO2 to
Scheme 2. Carboxylation of terminal alkynes with CO2. Unless otherwise noted, reaction conditions are as follows: alkyne
(1.0 mmol), P-NHC-Ag catalyst (5.0 mg, 0.3 mol% of Ag), Cs2CO3 (1.2 mmol), CO2 (1.0 atm), room temperature, DMF
(5 mL). GC yields (isolated yields are in parentheses).
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Carboxylation of Terminal Alkynes with Carbon Dioxide
Scheme 3. Possible reaction mechanism for the poly-NHC-Ag nano-composite catalyzed carboxylation of terminal alkynes
with CO2.
form an NHC-carboxylate (Scheme 3).[39–43] The
5 mL) and the mixture was stirred for 30 mins. The mixture
was washed with dichloromethane (3ꢂ5 mL) and the aque-
ous layer was acidified with concentrated HCl to pH 1, then
extracted with diethyl ether (3ꢂ5 mL). The combined or-
ganic layers were dried with anhydrous Na2SO4, filtered and
the solution was concentrated in vacuum affording the pure
product.
NHC-carboxylate then coordinated to a nearby silver
center and this induced the nucleophilic carbanion of
alkyne attacking the carboxylic carbon. Following the
À
new C C bond formation, the CO2 unit was trans-
ferred from the carbene center to the silver center
and the silver acetylide is regenerated by metathesis
with alkyne. The synergistic effect between N-hetero-
cyclic carbene polymer and silver nanoparticle greatly
enhanced the catalytic activity of this nano-composite
catalyst system.
Acknowledgements
This work was supported by the Institute of Bioengineering
and Nanotechnology (Biomedical Research Council, Agency
for Science, Technology and Research, Singapore).
In summary, we have successfully developed an N-
heterocyclic carbene polymer-supported silver nano-
particle catalyst system. The novel nano-composite
catalyst demonstrated very high activity and excellent
stability and reusability in the carboxylation of termi-
nal alkynes with CO2 at ambient conditions. The poly-
NHC-Ag catalyst exhibited excellent substrate gener-
ality and is tolerant to various functionalities. In addi-
tion, the catalyst is stable to air and can be easily re-
covered and reused.
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