Unexpected CO2 splitting reactions to form CO with N-heterocyclic carbenes as organocatalysts and aromatic aldehydes as oxygen acceptors

Article abstract of DOI:10.1021/ja909038t

(Chemical Equation Presented) The catalytic reduction of carbon dioxide to carbon monoxide under mild conditions using aromatic aldehydes as reductants and NHCs as organocatalysts was developed. This carbon dioxide splitting reaction provides a new method for metal-free carbon dioxide reduction and steps forward in utilizing carbon dioxide as a renewable "green" source under mild conditions. On the other hand, this reaction also shows a new economical way to oxidize aromatic aldehydes under mild conditions with carbon dioxide and could be applied in pharmaceutical synthesis.

Full text of DOI:10.1021/ja909038t

Published on Web 12/29/2009
Unexpected CO2 Splitting Reactions To Form CO with N-Heterocyclic
Carbenes as Organocatalysts and Aromatic Aldehydes as Oxygen Acceptors
Liuqun Gu and Yugen Zhang*
Institute of Bioengineering and Nanotechnology, 31 Biopolis Way, The Nanos, Singapore 138669
Received October 22, 2009; E-mail: ygzhang@ibn.a-star.edu.sg
Scheme 1. CO2 Splitting Reaction with Aldehyde Catalyzed by
Carbon dioxide, as the main cause of global warming, attracts
much attention around the world. The anthropogenic source of
atmospheric carbon dioxide is mainly from the burning of fossil
fuel (>60%).1 An important pathway to cut global carbon dioxide
emission is its reduction to carbon monoxide.2 Carbon monoxide
could easily convert water to hydrogen via the industrial well-
established “water gas shift reaction”.3 Carbon dioxide may thus
be utilized as a “green” renewable source for making fuel and
chemically reduced to carbon monoxide in an economical way.4
Due to the high stability of carbon dioxide, to split the OdC(O)
bond and generate carbon monoxide requires large energy input.5
The current carbon dioxide splitting methods could be classified
into four categories: (1) Enzyme carbon monoxide dehydrogenase/
acetyl-CoA synthase (CODH/ACS);6 (2) Photoreduction;7 (3)
Electrochemical reduction;8 (4) Use of metal complexes or metal
oxides to abstract the oxygen from carbon dioxide to form carbon
monoxide in low turnover.9 Recently, Sadighi group developed
organocopper(I) complexes supported by N-heterocyclic carbene
(NHC) ligands in the reduction of carbon dioxide to carbon
monoxide with diboron reagent.10 Metals play an important role
in all of these catalytic systems. But reduction of carbon dioxide
with organocatalysts remained widely undeveloped until our group
reported the first hydrosilylation of carbon dioxide using NHC
catalyst11-13 under mild conditions recently.14 In our efforts to look
for cheaper and more accessible reductants for CO2 reduction, the
new reaction for carbon dioxide splitting into carbon monoxide with
aromatic aldehydes as oxygen acceptors was successfully developed
(Scheme 1). To our best knowledge, this is the first case in the
reduction of carbon dioxide to form carbon monoxide using
organocatalysts.
Initially, the reaction between cinnamaldehyde and imidazolium-
carboxylate (Imes-CO2) was investigated, and carbonate and/or
anhydrate were expected as products. Carbon dioxide (1 atm, in
balloon) was firstly introduced into 1,3-dimesitylimidazol-2-ylidene
(IMes) (0.05 mmol in 1 mL of DMF) to generate carboxylate (IMes-
CO2)12 and followed by the addition of cinnamaldehyde (0.5 mmol)
at room temperature. However, the expected products were not
obtained, and instead, four different products were observed.
Saturated acid and lactone dimer were generated respectively from
the internal redox reaction,15 and dimerization16 of cinnamaldehyde
occurred as reported in literatures. The presence of novel oxidized
product 2a revealed the existence of oxidant, and carbon dioxide
seemed the only possible oxidant in the reaction mixture.17,18 To
prove the role of NHC and carbon dioxide, several control reactions
were performed. Without carbon dioxide, strong bases (1 equiv)
could lead to self-redox reactions in the absence/presence of NHC,
and cinnamaldehyde disappeared within 2 h (Table 1). However,
no reaction happened in the presence of both strong bases and
carbon dioxide without NHC catalyst. 2a was obtained as major
product only in the system with NHC as catalyst and in the presence
of base and carbon dioxide. The reaction conditions were further
NHC
Table 1. Background Investigationa
entries
conditions
time (h)
yield (2a %)b
1
3a 10 mol %, 10 mol % KOt-Bu,
1 atm CO2
20 mol % KOt-Bu, 80 mol %
NaH
20 mol % KOt-Bu, 80 mol %
NaH, 1 atm CO2
3a 10 mol %, 20 mol % KOt-Bu,
80 mol % NaH
72
32c
2
3
4
5
2
48
2
37d
N.R. or tracee
18d
90d
3a 10 mol %, 20 mol % KOt-Bu,
80 mol % NaH, 1 atm CO2
72
a The reactions were conducted at 0.5 mmol scale in 1 mL of
c
anhydrous DMF. b Determined by crude 1H NMR.
∼50%
cinnamaldehyde was recovered. d 100% conversion yield of 1a.
e Cinnamaldehyde was recovered.
optimized, and we were pleased to find that the reaction gave the
best results (yield up to 95%) when it was performed in DMSO
with potassium carbonate as additive and 1,3-dimesitylimidazol-
2-ylidene (IMes) as catalyst (Table 2).
It was proposed that carbon dioxide was reduced to carbon
monoxide, if the overall reaction equation is considered (see Scheme
2). NHC reacts with carbon dioxide resulting in imidazolium
carboxylate 4, which attacks the 2-position of cinnamaldehyde,
generating the possible intermediate 5. The nitrogen on imidazolium
carboxylate then traps the hydrogen from the 2-position carbon on
cinnamaldehyde, and the electron pair moves to the newly formed
carbon-oxygen bond, forming transition state 5 ([1,5] H shift).
The possible intermolecular hydrogen interaction (N-H---C) in 5
may help to stabilize the intermediate structure. Subsequently the
base traps the H+ on the nitrogen of the imdazolium ring and results
in carbon dioxide splitting from the oxygen-carbon bond and
formation of cinnamic salt. Meanwhile, carbon monoxide is released
from NHC-CO complex 6, and in the presence of CO2, the
imdazolium carboxylate is regenerated very quickly. It should be
noted that there is no stable structure associated with the combina-
tion of NHC and CO, other than “a non-bonded weakly interacting
(van der Waals) complex” intermediate 6.19 The production of
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914 J. AM. CHEM. SOC. 2010, 132, 914–915
10.1021/ja909038t  2010 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. Reaction Using Other Aldehydes
Table 2. Optimization of the Reactionsa
entries
base
catalyst
time (d)
yield (2a %)b
1
2
3
4
5
6
7
8
9
Cs2CO3
K2CO3
K3PO4
3a
3a
3a
3a
3a
3a
3b
3c
3d
3
4
4
5
4
4
4
4
4
92
70
81
50
95
95
c
Na2CO3
“green” source. On the other hand, this reaction also shows a new
economical way to oxidize aromatic aldehydes under mild condi-
tions using carbon dioxide.
c
Cs2CO3
c
K2CO3
K2CO3
K2CO3
c
40 (1a, 50%)
25 (1a, 75%)
32 (1a, 66%)
c
c
Acknowledgment. This work was supported by the Institute
of Bioengineering and Nanotechnology (Biomedical Research
Council, Agency for Science, Technology and Research, Singapore.
K2CO3
a The reactions were conducted at 0.5 mmol scale in 1 mL of
anhydrous DMF. Determined by crude H NMR. c DMSO as solvent.
b
1
Supporting Information Available: Experimental procedures, NMR
of products, and DFT calculation. This material is avaible free of charge
via Internet at http://pubs.acs.org.
Scheme 2. Catalytic Cycle in the Reduction of CO2 To Form CO
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is depicted in SFigure 2 (SI).
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In summary, we have achieved the catalytic reduction of carbon
dioxide to carbon monoxide under mild conditions using aromatic
aldehydes as reductants and NHCs as organocatalysts. This carbon
dioxide splitting reaction provides a new method for carbon dioxide
reduction and steps forward in utilizing carbon dioxide as renewable
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(21) For calculated structure, see Supporting Information.
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