Technical Field

The present invention relates to oxidation reactions using carbon dioxide.

Background

Carbon dioxide, as a major cause of global warming, has attracted much attention. The anthropogenic source of atmospheric carbon dioxide is mainly derived from burning of fossil fuel (above 60%). An important pathway to cutting global carbon dioxide emissions is its reduction to carbon monoxide. Carbon monoxide produced from reduction of carbon dioxide may be converted to hydrogen by the industrial well- established "water gas shift reaction" (eq. 1). Carbon dioxide may be used as a "green" renewable source of fuel and chemicals by its reduction to carbon monoxide in an economical manner.

CO + H ₂ 0 . CC-2 + H ₂

Due to the high stability of carbon dioxide, splitting the 0=C(0) bond to generate carbon monoxide requires a large energy input. An efficient means for conducting this reaction is by use of the enzyme carbon monoxide dehydrogenase/acetyl-CoA synthase (CODH/ACS). Photoreduction of carbon dioxide as means to utilize solar energy is also of substantial interest and has shown some promise. Electrochemical reduction of carbon dioxide to carbon monoxide has also drawn attention and impressive yields have been observed.

Some metal complexes and metal oxides can abstract an oxygen atom from carbon dioxide to form carbon monoxide in low turnover due to the strong metal-oxygen bonds generated in these reactions. Recently, Sadighi et al (Laitar, David S.; Muller, P. and Sadighi J. P. J. Am. Chem. Soc. 2005, 127, 17196-17197) developed organocopper (I) complexes supported by N-heterocyclic carbene (NHC) ligands, which displayed high turnover numbers and frequencies in the reduction of carbon dioxide to carbon monoxide with diboron reagent.

Metals play an important role in all of these catalytic systems.

Summary of the Invention

In a first aspect of the invention there is provided a method for oxidising a substrate of structure R-CHO, in which R comprises carbon-carbon unsaturation conjugated with the -CHO group, said method comprising exposing said substrate to carbon dioxide in the presence of an N-heterocyclic carbene (NHC). The following options may be used with the first aspect or with any of the subsequent aspects described below. These options may be used either individually or in any suitable combination.

The substrate may be an optionally substituted cirrnamaldehyde. It may be benzaldehyde or a substituted benzaldehyde. The substituted benzaldehyde may have an electron withdrawing group on the aromatic ring to which the -CHO group is attached or it may have an electron donating group thereon.

The NHC may be catalytic. It may be present in a catalytic amount. It may be generated in situ. Thus the method may comprise the step of generating the NHC. It may comprise the step of generating the NHC in situ. The NHC may be generated in situ from the reaction of a suitable precursor, e.g. a nitrogen heterocyclic salt such as an imidazolium salt, with a base. The nitrogen heterocyclic salt may be an N-substituted nitrogen heterocyclic salt, i.e. the imidazolium salt may be an Ν,Ν'-disubstituted imidazolium salt. The base may be a carbonate salt or a phosphate salt. Thus the method may comprise the step of treating an N-substituted nitrogen heterocyclic salt (e.g. an Ν,Ν'-disubstituted imidazolium salt) with a base, e.g. a carbonate or a phosphate salt, to generate the NHC. This step may be conducted prior to the step of exposing the substrate to carbon dioxide in the presence of the NHC.

The method may be conducted in a dipolar aprotic solvent, e.g. dimethylsulfoxide. The method may be conducted under an atmosphere of carbon dioxide. The partial pressure of carbon dioxide may be about 1 to about 10 atmospheres, e.g. about 1 atmosphere. The method may be conducted at about 10 to about 90°C, e.g. about room temperature to about 80°C.

The method may be conducted free of transition metals. It may be conducted free of metals which complex with the NHC. It may be conducted free of metal ions other than those from the base (if used) used to generate the NHC.

In an embodiment there is provided a method for oxidising a substrate which is an optionally substituted cinnamaldehyde or an optionally substituted benzaldehyde, said method comprising exposing said substrate at a temperature of about room temperature to about 80°C to carbon dioxide in the presence of an N-heterocyclic carbene (NHC) which has been generated in situ by reaction of a carbonate salt with an N,N'disubstituted imidazolium salt.

In another embodiment there is provided a method for oxidising a substrate which is an optionally substituted cinnamaldehyde or an optionally substituted benzaldehyde, said method comprising exposing said substrate at about room temperature to carbon dioxide in the presence of 1 ,3 -dimesitylimidazol-2-ylidene.

The invention also encompasses a product obtained by the method of the first aspect. The product may be a carboxylic acid of structure R-COOH, in which R comprises carbon-carbon unsaturation conjugated with the -COOH group. It may be an optionally substituted cinnamic acid. It may be an optionally substituted benzoic acid.

In a second aspect of the invention there is provided a method for reducing carbon dioxide comprising exposing said carbon dioxide to an aldehyde of structure R-CHO, in which R comprises carbon-carbon unsaturation conjugated with the -CHO group, in the presence of an N-heterocyclic carbene (NHC). The method may produce carbon monoxide. The method may comprise the additional step of using the carbon monoxide in a water-gas reaction _, to produce hydrogen.

In an embodiment there is provided a method for reducing carbon dioxide comprising exposing said carbon dioxide at a temperature of about room temperature to about 80°C to a substrate in the presence of an N-heterocyclic carbene (NHC) which has been generated in situ by reaction of a carbonate salt with an N,N'disubstituted imidazolium salt, said substrate being an optionally substituted cinnamaldehyde or an optionally substituted benzaldehyde.

In another embodiment there is provided a method for reducing carbon dioxide comprising:

reacting a carbonate salt with an N,N'disubstituted imidazolium salt so as to produce an N-heterocyclic carbene (NHC); and

exposing the carbon dioxide at a temperature of about room temperature to about 80°C to a substrate in the presence of the NHC, said substrate being an optionally substituted cinnamaldehyde or an optionally substituted benzaldehyde.

The invention also encompasses carbon monoxide produced by the method of the second aspect.

In a third aspect of the invention there is provided the use of carbon dioxide and an N-heterocyclic carbene (NHC) for the oxidation of a substrate of structure R-CHO, in which R comprises carbon-carbon unsaturation conjugated with the -CHO group.

In a fourth aspect of the invention there is provided the use of an aldehyde of structure R-CHO, in which R comprises carbon-carbon unsaturation conjugated with the -CHO group, and an N-heterocyclic carbene (NHC) for the reduction of carbon dioxide. In a fifth aspect of the invention there is provided a method for producing hydrogen comprising:

• exposing carbon dioxide to an aldehyde of structure R-CHO, in which R comprises carbon-carbon unsaturation conjugated with the -CHO group, in the presence of an N-heterocyclic carbene (NHC) so as to produce carbon monoxide; and

• using the carbon monoxide in a water-gas reaction to produce the hydrogen.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings wherein:

Figure 1 is a scheme showing the C0 ₂ splitting reaction with aldehydes catalyzed by NHC;

Figure 2 is a scheme showing the initially expected reaction;

Figure 3 is a scheme showing the actual reaction of cinnamaldehyde and C0 ₂ with NHC as catalyst;

Figure 4 is a scheme showing the proposed catalytic cycle in the reduction of C0 ₂ to form CO;

Figure 5 shows the calculated structures of the identified stationary points in a postulated transition state for the reaction; and

Figure 6 is a scheme showing experiments which examined oxidation of other aldehydes.

Detailed Description of the Preferred Embodiments

The present invention represents an efficient CO? splitting reaction to form CO, using N-heterocyclic carbenes (NHCs) as organocatalysts and unsaturated aldehydes as oxygen acceptors. Thus the invention presents a novel process for catalytic reduction of carbon dioxide to carbon monoxide under ambient conditions using unsaturated aldehydes as reductants and NHCs as organocatalysts. This carbon dioxide splitting reaction provides a new method for carbon dioxide reduction, and represents a significant advance to utilization of carbon dioxide as a renewable "green" source for fuel.

Substrate: the substrate used in the present invention has structure R-CHO, in which R comprises carbon-carbon unsaturation conjugated with the -CHO group, such as alkynyl aldehydes, acroleins, alkynyl aldehydes with electron withdrawing groups, acroleins with electron withdrawing groups, or phenylalkynylaldehydes (phenylpropargylaldehydes). In some cases the -CHO group may be replaced by some other oxidisable group such as a phosphine. As the substrate reduces the carbon dioxide to carbon monoxide, it may be regarded as a reductant. R may comprise a double bond or a triple bond or an aromatic structure or more than one of these, conjugated with the -CHO group (i.e. the carbon- carbon unsaturafion may be a carbon-carbon double bond, a carbon-carbon triple bond or an aromatic structure, or it may be a combination of any two or more of these conjugated with each other). It may therefore have structure A(B)C=C(D)-CHO, or AC≡C-CHO, or Ar-CHO, in which A, B and D are, independently, H or some other substituent (which may be, for example Ar, an aliphatic group, and alkenyl group, an alkynyl group, halogen, nitro or some other functional group) and Ar is an aromatic (including heteroaromatic and fused ring aromatic) group (e.g. phenyl, biphenylyl, naphthyl, anthracyl, pyridyl, thiophenyl, indolyl, furyl, pyrrolyl or other suitable aromatic group). In each case the substituents A, B, D and Ar may, independently, be substituted (e.g. by one or more aliphatic groups, Ar groups, halogens, other functional groups etc.). In some embodiments one or both of A and B is an Ar group (as defined above). In some embodiments Ar has a substituent, which may be an electron withdrawing group or may be an electron donating group. If present, the substituent on Ar may be para to the -CHO group or it may be ortho to the -CHO group or it may be meta to the -CHO group. In some embodiments the substrate is Ar-CHO and Ar has an electron withdrawing group. In other embodiments, the substrate is Ar-CHO and Ar has an electron donating group. In the event that the substrate has structure A(B)C=C(D)-CHO, the C=C double bond may be cis or it may be trans. Suitable electron withdrawing groups are well known, and include halogens (F, CI, Br, I), trifluorohalo groups (CF ₃ , CC1 ₃ , CBr ₃ , CI ₃ ), aryl groups (e.g. phenyl, biphenyl), nitro groups, nitrile groups etc. Suitable electron donating groups are also well known, and include alcohols, ethers and amines (e.g. primary, secondary or tertiary amines). Aliphatic groups, referred to above, may be linear, branched, cyclic or a combination of these. They may have from 1 to 20 carbon atoms (3 to 20 for cyclic or branched examples) or 1 to 10, 1 to 6, 1 to 4, 3 to 20, 3 to 10, 3 to 6, 6 to 20, 10 to 20 or 6 to 10 carbon atoms, e.g. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20 carbon atoms. Suitable aliphatic groups therefore include, for example, methyl, ethyl, propyl, isopropyl, butyl, t-butyl, hexyl, cyclohexyl, cyclopentyl and dodecyl. The concentration of the substrate in the reaction mixture may be about 0.1 to about 2 M, or about 0.1 to 1, 0.1 to 0.5, 0.1 to 0.2, 0.2 to 2, 0.5 to 2, 1 to 2, 0.2 to 1, 0.2 to 0.5 or 0.5 to 1M, e.g. about 0.1, 02, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, 1.5, 1.5, 1.6, 1.7, 1.8, 1.9 or 2M. In some cases the substrate may comprise more than one aldehyde group, e.g. 2, 3, 4 or 5 aldehyde groups. In this case the molar ratios discussed herein (e.g. substrate to NHC) should be adjusted so as to reflect ratios per aldehyde group). Thus for example if a 10:1 molar ratio of substrate to NHC is required for a monoaldehyde, a 5:1 ratio would be used if the substrate is a dialdehyde (so that the ratio of aldehyde groups to NHC remains 10:1).

NHC: The NHC may be catalytic. It may be present in a catalytic amount. It may be present relative to the substrate at about 1 to about 20%, or about 1 to 10, 1 to 5, 1 to 2, 2 to 20, 5 to 20, 10 to 20 or 5 to 15%, e.g. about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15 or 20% on a w/w basis. It may be present in the reaction mixture at a concentration of about 0.01 to about 0.2 M, or about 0.01 to 0.1, 0.01 to 0.05, 0.01 to 0.02, 0.02 to 0.2, 0.05 to 0.2, 0.1 to 0.2, 0.02 to 01, 0.02 to 0.05 or 0.05 to 0.1M, e.g. about 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19 or 0.2M. The NHC may be an Ν,Ν'-disubstituted imidazolylidene, e.g. l,3-dimesitylimidazol-2-ylidene. The NHC may be generated in situ. It may be generated in situ from the reaction of a suitable precursor. Suitable precursors include salts of N-substituted heterocycles having at least one (optionally 2 and in some cases 3) nitrogen atom in the ring of the heterocycle. Examples include imidazolium salts, in particular N,N'disubstituted imidazolium salts. The precursor may be any suitable N-heterocyclic material which, when treated with a base, is capable of generating an NHC. The base may be a mild base. It may be a weak base. It may have a pKb of about 1 to about 4, or about 1 to 3, 1 to 2, 2 to 4, 3 to 4 or 2 to 3, e.g. about 1, 1.5, 2, 2.5, 3, 3.5 or 4. It may be for example a carbonate salt or a phosphate salt. It may be at least partially, optionally completely, soluble in the solvent used for the reaction. It may be an alkali metal carbonate or phosphate. It may be a potassium, rubidium or caesium salt. It may be a potassium, rubidium or caesium carbonate or phosphate. The molar ratio of base to precursor may be about 1 to about 10 (i.e. about 1:1 to about 10:1), or about 1 to 5, 1 to 2, 1 to 1.5, 2 to 10, 5 to 10 or 1.5 to 3, e.g. about 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9 or 10. The NHC may be generated (or may be used) in the presence of, optionally under an atmosphere of, carbon dioxide. It may therefore form an association (e.g. a complex or adduct) with the carbon dioxide. The association may be an intermediate or a transition state. It may be unstable or it may be stable. The association of the NHC with carbon dioxide may be the active reagent in the reaction with the substrate. Thus where reference is made in the present specification to exposing the substrate to carbon dioxide in the presence of an NHC, this may be taken to refer to to exposing the substrate to an association of the NHC with carbon dioxide. The NHC may be generated prior to the addition of the substrate, or it may be generated in the presence of the substrate. The N substituents in the NHC (and precursor, if used) may be sterically bulky substituents. Suitable substituents include isopropyl, t-butyl, mesityl (2,4,6-trimethylphenyl), 2- methylphenyl, 2,6-dimethylphenyl, 2,6-dimethyl-4-t-butylphenyl etc.

Reaction conditions: The method may be conducted in a solvent. The solvent may be capable of dissolving the reagents. It may be capable of dissolving the substrate. It may be capable of dissolving the base and precursor (if used). Suitable solvents include dipolar aprotic solvents, e.g. dimethylsulfoxide, dimethylformamide, hexamethylphosphoramide, tetrahydrofuran, tetrahydropyran etc. Mixtures of these solvents, or of other suitable solvents, may also be used. The solvent(s) may be dried before use. It may be dried by use of a dessicant, e.g. an anhydrous salt (such as anhydrous sodium carbonate) or a molecular sieve. The method of the invention may be conducted under an atmosphere comprising carbon dioxide. It may be conducted under an atmosphere consisting essentially of carbon dioxide. In some cases one or more other gases may be mixed with the carbon dioxide. In this case the one or more other gases may be inert under the conditions of the reaction. Suitable other gases include nitrogen, helium, neon, argon etc. The partial pressure of carbon dioxide may be about 1 to about 10 atmospheres, or about 1 to 5, 1 to 2, 2 to 10, 5 to 10 or 2 to 5 atmospheres, e.g. about 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9 or 10 atmospheres. In some embodiments the reaction is conducted under an atmosphere consisting essentially of carbon dioxide at a pressure of about 1 atmosphere. In other embodiments the carbon dioxide (optionally mixed with the one or more other gases) may be passed through the reaction mixture. It may be sparged into the reaction mixture. It may be bubbled through the reaction mixture. It should be recognised that the reaction described herein can generate carbon monoxide, and that as a consequence the atmosphere will contain increasing levels of carbon monoxide as the reaction progresses. The composition of the atmosphere described above therefore applies primarily at the commencement of the reaction. The concentration of carbon monoxide in the atmosphere will depend on the volume of the reaction chamber, the experimental details, the stage of the reaction and the amount of substrate used. The method may be conducted at about 10 to about 90°C, or about 10 to 80, 10 to 60, 10 to 40, 10 to 20, 20 to 90, 20 to 80, 20 to 60, 20 to 40, 50 to 90 or 50 to 80°C, e.g. about 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85 or 90°C. It may be conducted at about room temperature, or at about room temperature to about 80°C. The reaction may be conducted for about 10 minutes to about 6 hours, or about 0.5 to 6 hours, or about 1 to 6, 2 to 6, 4 to 6, 1 to 4, 1 to 2 or 2 to 4 hours, e.g. about 10, 20, 30, 40 or 50 minutes, or about 1, 2, 3, 4, 5 or 6 hours. In some cases, particularly with relatively unreactive substrates, the reaction may be conducted for longer than 6 hours, e.g. about 12, 18, 24, 36, 48, 60, 72, 84, 96, 108, 120, 132 or 144 hours. In these cases, reaction may be promoted by use of higher temperatures (e.g. about 60 to about 90°C). In the event that the NHC is generated in situ prior to addition of the substrate, the generation of the NHC may be conducted at the same temperature as the reaction with the substrate, or it may be conducted at a different temperature. In the latter case, the temperature at which the NHC is generated and the temperature at which the substrate is reacted may each, independently, be as described above. The method may be conducted free of transition metals. It may be conducted free of metals which complex with the NHC. It may be conducted free of metal ions other than those from the base, if used, used to generate the NHC. It may be conducted free of heavy metals. It may be conducted free of toxic metals. This may facilitate purification of a product of the reaction. The method may be conducted free of (i.e. in the absence of) oxidants other than carbon dioxide. It may be conducted free of (i.e in the absence of) oxygen. It may be conducted free of (i.e in the absence of) air.

Products: The product may be a carboxylic acid of structure R-COOH, in which R comprises carbon-carbon unsaturation conjugated with the -COOH group. The nature of R has been described earlier. The product may have the same structure as the substrate with the exception that the -CHO group is replaced by (i.e. converted to) a -COOH group. It may therefore be an oxidation product of the substrate. The oxidation product may be produced in a yield of about 20 to about 100% yield, or about 50 to 100, 70 to 100, 80 to 100, 80 to 95, 70 to 95, 50 to 95, 20 to 95, 70 to 80 or 80 to 95%, e.g. about 20, 30, 40, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100% yield. They yield may depend on the nature of the base and precursor used to make the NHC, the nature of the substrate and the reaction conditions. The method may comprise the step of separating the oxidation product (and/or other products produced in the reaction) from the reaction mixture. Separation may comprise one or more of: solvent evaporation, HPLC separation, GC separation, column chromatographic separation, recrystallisation, extraction into base (optionally followed by acidification of the basic extract), solvent extraction etc. Concurrently with the production of the products described above, the carbon dioxide which takes part in the reaction may be reduced to form carbon monoxide. The carbon monoxide may be produced in approximately equimolar amounts to the carboxylic acid. The carbon monoxide may be subsequently used to generate hydrogen by reaction with water using the water gas shift reaction. This is commonly catalysed by iron oxide, which may be used in combination with chromium oxide. Another commonly used catalyst is copper on a support which comprises zinc oxide and aluminum oxide. The reaction may be conducted at about 150 to about 400°C, or about 150 to 300, 150 to 200, 200 to 400, 300 to 400 or 200 to 300°C, e.g. about 150, 200, 250, 300, 350 or 400°C. In some cases the reaction may be conducted in two stages. These may be at different temperatures (each being within the above ranges). They may use different catalysts. As the water gas shift reaction uses carbon monoxide to reduce water to hydrogen, the carbon monoxide is oxidised to carbon dioxide. Thus if the carbon monoxide which is produced initially from carbon dioxide (by catalysed reaction with the substrate as described above) is subsequently used in the water gas shift reaction, the overall reaction will represent a reduction of water by the substrate to hydrogen catalysed by carbon dioxide and NHC.

The inventors have found a carbon dioxide splitting reaction to form carbon monoxide with N-heterocyclic carbene (NHC) as organocatalyst and aromatic aldehydes as oxygen acceptors in very mild conditions (Fig. 1). This is the first case in the reduction of carbon dioxide to form carbon monoxide with organocatalysts.

N-heterocyclic carbenes are well established as organocatalysts and ligands in organic synthesis. It is known that NHCs can react with carbon dioxide to form comparatively stable imidazolium carboxylates and these carboxylates could also react with some functional groups producing carbonates under pressure and heating conditions. However reduction of carbon dioxide with environment-friendly organocatalysts remained unknown until the present inventors developed the first hydrosilylation of carbon dioxide using NHC catalysts, which mediated the effective conversion of carbon dioxide to methanol under mild conditions. In seeking for cheaper and more accessible reductants for C0 ₂ , a new reaction for carbon dioxide splitting into carbon monoxide with aromatic aldehydes as oxygen acceptors was successfully developed.

Examples

Initially, the reaction between cinnanialdehyde and imidazolium-carboxylate (Imes- C0 ₂ ) was investigated and carbonate 2a and/or anhydrate 2b were expected as products (Fig. 2). Carbon dioxide (latm, in balloon) was firstly introduced into 1,3- dimesitylimidazol-2-ylidene (IMes) (0.05 mmol in 1 ml of DMF) to generate carboxylate (IMes-C0 ₂ ) and followed with the addition of cinnamaldehyde (0.5 mmol) at room temperature. However, the expected products (2a and/or 2b) were not produced and instead, four different products were observed (3a, 3bb, 3cc, 3dd Fig. 3). Saturated acid 3bb and lactone dimer 3cc, 3dd were presumably generated from an internal redox reaction and by dimerization of cinnamaldehyde. The presence of novel oxidized product 3 a revealed the existence of an oxidant, and carbon dioxide appeared to be the only possible oxidant in the reaction mixture. No product of cinnnamaldehyde reduction was detected, so a self-redox reaction was excluded. Further optimization by increasing the amount of bases and prolonging the reaction time could afford 3a in excellent yield (Table 1, entry 5).

To clarify the role of NHC and carbon dioxide, several control reactions were performed. Strong bases (1 eq) led to self-redox reactions in the absence or presence of NHC, and cinnamaldehyde disappeared within 2 hrs in the absence of carbon dioxide. However, no reaction took place in the presence of strong bases and carbon dioxide (cinnamaldehyde was added after the strong base mixture was flushed with C0 ₂ and kept stirring for 2 hrs under C0 ₂ balloon). These results indicated that the strong bases reacted with carbon dioxide and to form weaker bases, which could not initiate the self-redox reactions. 3a was only produced in the system with NHC as catalyst and in the presence of base and carbon dioxide. Based on these findings, other weak bases were also tested, including common carbonates and phosphates. With cesium carbonate, potassium carbonate and potassium phosphate, the reactions afforded 3a in moderate to excellent yields. Lower conversion was observed when sodium carbonate was used due to the weaker basicity. Replacing DMF with DMSO as reaction solvent led to a significant suppression of the side reactions, which is thought to be due to the higher solubility of inorganic bases in DMSO than in DMF.

It was considered that carbon dioxide was reduced to carbon monoxide, as shown in Fig. 4. Thus in the proposed mechanism, NHC reacts with carbon dioxide, resulting in imidazolium carboxylate 5. Carboxylate 5 attacks the 2-position of cinnamaldehyde generating the hypothesised intermediate 6. The nitrogen on the imidazolium carboxylate interacts with the hydrogen from 2-position carbon on cinnamaldehyde and the electron pair moves to the newly formed carbon-oxygen bond ([1,5] H shift). Subsequently, base traps the FT ^" on nitrogen of 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 7, and with C0 ₂ the imidazolium carboxylate is reformed. It should be noted that there is no stable structure associated with the combination of NHC and CO, other than a non-bonded weakly interacting (van der Waals) complex intermediate 7.

The production of carbon monoxide was confirmed by the reduction of PdCl? solution. Thus the residual gas in the balloons after overnight reaction was transferred into PdCl ₂ solution (clear red solution) in a three-necked flask with negative pressure and the mixture was shaken for 30 mins and subsequently stirred for another 2 hrs. The formation of black solid (Pd) precipitate on the stirring bar and the inside of flask was observed clearly, and demonstrated the formation of carbon monoxide.

The calculated free energy profile (DFT, B3LYP/6-31G level) showed that the overall reaction is an exothermic process with a small negative energy difference ΔΕ = -7.0 kcal/mol, equation 2. The structures of the identified stationary points are depicted in Figure 5. This transition state is related to 6 in Fig. 3. It was shown that the hydrogen atom is located between the carboxylate carbon and the NHC nitrogen atoms to form a six membered ring that may stabilize this transition state.

C0 ₂ +

Other NHCs which could form the zwitterionic C0 ₂ adducts were also screened in the carbon dioxide splitting reaction with cinnamaldehyde in the presence of ₂ C0 ₃ at room temperature. NHC catalysts with various substituents can activate this reaction to give cinnamic acid in low to moderate conversion yield. l,3-dimesitylimidazol-2-ylidene (IMes) displayed the best activity (Table 2). With the optimised conditions, various aldehydes were examined as reductants with 10 mol% IMes as organocatalyst. Cinnamaldehyde with methyl carboxylate group (3b, Fig. 6) and benzaldehydes with strong electron-withdrawing groups (3c and 3d, Fig. 6) could split carbon dioxide efficiently. Halogen-substituted benzaldehydes, phenyl benzaldehyde, benzaldehyde and anisaldehyde (3e, 3f, 3h, 3i and 3j Fig. 6) were also able to reduce carbon dioxide, by increasing the temperature to 60°C or 80°C. Terephthalaldehyde was oxidized to 4- formylbenzoic acid with excellent yield at room temperature (3g, Fig. 6). Table 1: Background investigation'

Entries Conditions Time (h) Yield (3a %)

1 4a 10 mol%, 10 mol% KOt-Bu, 1 atm C0 ₂ 72 32 ^c

2 20 mol% KOt-Bu, 80 mol% NaH 2 < 37 ^d

3 20 mol% KOt-Bu, 80 mol% NaH, 1 atm C0 ₂ 48 N.R. or trace ^e

4 4a 10 mol%, 20 mol% KOt-Bu, 80 mol% 2 < 18 ^d

NaH

5 4a 10 mol%, 20 mol% Ot-Bu, 80 mol% 72 90 ^d

NaH, 1 atm C0 ₂ ^a : The reactions were conducted on 0.5 mmol scale in 1 niL anhydrous DMF.

: Determined by crude 1H NMR.

°: Around 50% cinnamaldehyde was not converted.

d: 100% conversion.

e: Cinnamaldehyde was recovered.

Table 2: Optimization of the reactions.

Entries Bases Catalyst Time (d) Yield (3a, %)

1 Cs ₂ C0 ₃ 4a 3 92

2 K ₂ C0 ₃ 4a 4 70

n

D K ₃ P0 ₄ 4a 4 81

4 Na ₂ C0 ₃ ^c 4a 5 50

5 Cs ₂ C0 ₃ ^c 4a 4 95

6 K ₂ C0 ₃ ° 4a 4 95 7 K ₂ C0 ₃ ^c 4b 4 40 (la, 50%)

8 K ₂ C0 ₃ ° 4c 4 25 (la, 75%)

9 K ₂ C0 ₃ ^c 4d 4 32 (la, 66%) ^a : The reactions were conducted on 0.5 mmol scale in 1 mL anhydrous DMF.

b: Determined by crude 1H NMR.

c: DMSO as solvent.

In summary, the inventors have achieved the catalytic reduction of carbon dioxide to carbon monoxide under ambient conditions using aromatic or otherwise unsaturated aldehydes as reductants and NHCs as organocatalysts. This carbon dioxide splitting reaction provides a new method for carbon dioxide reduction and enables utilisation of carbon dioxide as renewable "green" source of fuel. Also, this reaction also shows a new economical and green way to oxidize aromatic or unsaturated aldehydes under mild conditions with carbon dioxide and may be applied in pharmaceutical synthesis or other organic syntheses.

Compared to other systems, this new approach is attractive for its mild reaction conditions and metal-free system.