Carbon dioxide insertion into

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 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. 

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. 

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. ...

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>. 

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). 

Other Molecules. —Carbon Dioxide. Carbon dioxide insertion into a rhodium-hydrogen bond seems a likely step in the conversion of mixtures of amines, carbon dioxide, and hydrogen to formamides, catalysed by RhCl(PPh3)3." ... 

The presumed mechanism of the formation of the 6-lactone is shown in Figure 26. Two molecules of butadiene combine to a Cs-chain and form a palladium-bis-n -allyl complex which is in equilibrium with a ri. ql-complex. Carbon dioxide inserts into the palladium-carbon bond yielding a carboxylate complex. The oxygen of the carboxylate group and the allyl group react and form the 6-lactone by a cycli-zation step. 

The proposed reaction mechanism for the formation of the two co-pro-ducts is shown in Equation 9. Butadiene, isoprene and the palladium complex form two different bis-n -allyl complexes, in which the isoprene unit is both head- and tail-connected to butadiene. Carbon dioxide inserts into both complexes to yield a carboxylate species, but the insertions occur only on the n -allyl part steming from the butadiene unit perhaps by electronic reasons. Finally, the two novel lactones are formed by reductive elimination. 

The interpretation of the formation of the Ci3-lactone requires a sequence of mechanistical pathways which are unknown so far in rhodium-catalysis. Two proposals for the mechanism were given in Equation 12. The mechanism of path B is similar to that shown for palladium catalysis. A rhodium Cg-carboxylate complex is formed which under further incorporation of butadiene could yield the lactone. In the mechanism of path A three molecules of butadiene react with the starting rhodium compound forming a C- 2 Chain, which is bound to the rhodium by two n -ally1 systems and one olefinic double bond. Carbon dioxide inserts into one of the rhodium allyl bonds thus forming a C- 3-carboxyl ate complex, which yields the new C-13-lactone. 

Reaction pathways with n-Bu2Sn(OR)2 (R=CH3, CH2CH3, 01(013)2) have been examined by DFT modeling. " ° A transition state similar to that described in Scheme 21.13c has been proposed, highlighting a binuclear activation on two adjacent tin centres.Isolation and characterisation by single-crystal X-ray diffraction analysis of dibutyltin(iv)-C02 adducts revealed that carbon dioxide inserts into Sn-OR and Sn-O-Sn bonds (Scheme 21.15). 

Numerous examples of carbon dioxide insertion into the M-N bond of transition metal amides [78-84] are described in the literature. In comparison, however, mechanistic studies in this area are relatively few. 

In early work, Inoue and Yokoo [121] found that carbon dioxide insertion into the Al-N bond of Et2Al-NEt2 was accelerated by tertiary amines such as, for instance, 1-methyl-imidazole, N-methylbenzimidazole, or pyridine. The role of the base was explained by taking into account that... 

Aresta M, Quaranta E, Tommasi I, Mascetti J, Tranquille M, Borowiak M (1998) Formation of peroxocarbonates Ifom L3Rh(Q2)Cl and L2Ni(C02) a unique reaction mechanism with carbon dioxide insertion into the 0-0 bond. Stud Surf Sci Catal 114 677-680... 

Pinkes JR, Steffey BD, Vites JC, Cutler AR (1994) Carbon dioxide insertion into the iron-zirconium and mthenium-zirconium bonds of the heterobimetallic complexes Cp(CO)2MZr (Cl)Cp2 direct production of the p-Ti (C) Ti (0, 0 )-C02 compounds Cp(C0)2MC02Zr(Cl) Cp2. Organometallics 13(l) 21-23... 

Carbon Dioxide.—The sole example of carbon dioxide insertion into a transition-metal-carbon bond is afforded by the reaction of Ti(A -C6H5)a-Pha with carbon dioxide, which produces (16). The suggested mechanism is... 

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