Insertions carbon dioxide

So far, carbon dioxide insertion has found limited use to form C—C bonds. Dialkyl pyrones have been obtained in low yield (up to 9%) from alkylacetylenes, probably via C02 insertion into a metallacycle 198), e.g. ... 

Key words hydrogenation of carbon dioxide, insertion of carbon dioxide into the metal-hydride bond, reductive elimination of formic acid, C-bond metathesis... 

The catalytic cycles for reduction of alkyl and atyl halides using Ni(o), Co(i) or Pd(o) species are interrupted by added carbon dioxide and reaction between the first formed carbon-metal bond and carbon dioxide yields an alkyl or aryl car-boxylate. These catalyses reactions have the advantage of occuriiig at lower cathode potentials than the direct processes summarised in Table 4.14. Mechanisms for the Ni(o) [240] and Pd(o) [241] catalysed processes have been established. Carbon dioxide inserts into the carbon-metal bond in an intermediate. Once the carboxy-late-metal species is formed, a further electron transfer step liberates the carboxy-late ion reforming the metallic complex catalyst. 

Carbon dioxide insertion reactions are potential intermediate steps in catalytic cycles leading to reduction of C02 or its incorporation into organic molecules. Analogies with carbon monoxide chemistry may be drawn, e.g., insertion reactions of carbon monoxide (20) play a key role in both the... 

Carbon dioxide insertion into a rhodium-carbon bond has been found in the reaction of Rh(Ph)(PPh3)3 with C02 under 20 atm of pressure at room... 

Examples of C02 insertion into M-H and M-C bonds are numerous. Less common are instances of carbon dioxide insertion into M-O and M-N bonds. There are excellent reviews which encompass these areas by Eisenberg and Hendriksen (4), Volpin and Kolomnikov (24), Kolomnikov and Grigoryan (25), and Sneeden (26). Hence we will emphasize developments since the time of these reviews (i.e., from 1979 to present) with our perspective being primarily that of understanding mechanistic aspects of these insertion processes. Some overlap with these earlier reviews will necessarily occur during such efforts. 

A significant rate enhancement for the C02 insertion process was noted in the presence of alkali metal counterions (Table I), even in the highly coordinating THF solvent. This rate acceleration was not, however, catalytic in alkali metal counterion, since the once formed carboxylate was observed to form a tight ion pair (76, 77) via its uncoordinated oxygen atom with the alkali metal ion, as evinced by infrared spectroscopy in the v(C02) region. That is, the counterion was consumed during the carbon dioxide insertion reaction. 

Carbon dioxide insertion into the W-C bond of CH3W(CO)j was not retarded by excess carbon monoxide. In other words, as Fig. 10 illustrates, the C02 insertion process does not involve a coordinatively unsaturated intermediate. This observation could only be made when an alkali metal counterion was present, since the rate of C02 insertion was much faster than that of CO insertion under this condition. On the other hand, [PNP][CH3W(CO)5] undergoes CO insertion (80) at a much faster rate than carbon dioxide insertion. Both processes exhibited similar metal (W > Cr) and R(CH3 > C6H5) dependences. 

There are two possible pathways to homologate methanol with carbon dioxide the CO2 insertion path and CO insertion path (Scheme 2). As for the former, Fukuoka et al. reported that the cobalt-ruthenium or nickel bimetallic complex catalyzed acetic acid formation from methyl iodide, carbon dioxide and hydrogen, in which carbon dioxide inserted into the carbon-metal bond to form acetate complex [7]. However, the contribution of this path is rather small because no acetic acid or its derivatives are detected in this reaction. Besides, the time course... 

Formation of peroxocarbonates from L3Rh(02)CI and L2Ni(C02) a unique reaction mechanism with carbon dioxide insertion into the O-O bond. 

The high reactivity of these two classes of catalysts, carboxylate and alkoxide derivatives, has been confirmed by recent work of Coates and co-workers [87]. They reported the synthesis of two new types of Zn diimido complexes (30 and 31) as shown in Scheme 7 and successfully utilized both types of complexes in the copolymerization of CO2 with epoxides. Their high activities and selectivies in regard to the carbon dioxide insertion (up to 96% carbonate linkages) are unprecedented. 

Ligand addition to M2 (OR)e is observed as illustrated by - Mo2 OCH2CMe3)6L2 (L = NMes, PMe2Ph), and W2(OPr-i)6(Py)2. Carbon dioxide inserts into metal-amide and metal-alkoxo bonds with retention of the multiple metal-metal bond ... 

Table I. Second-order rate constants for carbon dioxide insertion into cis-CH3W(C0)uL derivatives. ...

Carbon dioxide is abundant and readily available, but its reaction with transition metal complexes has not been extensively studied. A few examples of carbon dioxide insertion are known. Thus, formic acid can be formed by the insertion of carbon dioxide into the cobalt hydride bond U9>,2°). 

So far no definite carbon dioxide insertion into analogous metal carbon bonds has been reported. A rhodium complex coordinated with carbon dioxide was reported by Iwashita and Hayata 122>. 

Michael type reaction. Chlorosulphonyl isocyanate adds to epoxides to generate the carbon dioxide inserted oxazolidines (74) or dioxalones (75) depending on the substitution pattern of the epoxide. 

Similar photochemical insertion reactions are observed for indium alkylporphy-rins In the presence of pyridine and under irradiation by visible light, carbon dioxide inserts into the carbon-indium o bond leading to stable carboxylate indium porphyrins (Scheme 10). 

Another carbon dioxide insertion reaction is observed with Al (TPP)(OMe) This compound readily and reversibly traps carbon dioxide at room temperature in the presence of 1-methylimidazole. The trapped carbon dioxide is sufficiently activated to react with an epoxide at room temperature, thus producing the corresponding alkylcarbonate. As illustrated in Scheme 11, the cyclic carbonate is considered to be formed - at least partly - by nucleophilic attack on a linear intermediate. Thus the alkoxide aluminum porphyrin-methylimidazole system would be a good catalyst for synthesis of alkylene carbonates from carbon dioxide and epoxides under mild conditions. 

Table 10.10. Rate Constants for Carbon Dioxide Insertion with cis-[WMe(CO)4L] in THF at 25 "C (Reference 95)...

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