Carbon dioxide reactions with alkoxides

Carbon dioxide also can undergo insertion into metal—heteroatom bonds. The following shows the reversible reaction with alkoxides - ... 

Because of thetr electron deficient nature, fluoroolefms are often nucleophihcally attacked by alcohols and alkoxides Ethers are commonly produced by these addition and addition-elimination reactions The wide availability of alcohols and fliioroolefins has established the generality of the nucleophilic addition reactions The mechanism of the addition reaction is generally believed to proceed by attack at a vinylic carbon to produce an intermediate fluorocarbanion as the rate-determining slow step The intermediate carbanion may react with a proton source to yield the saturated addition product Alternatively, the intermediate carbanion may, by elimination of P-halogen, lead to an unsaturated ether, often an enol or vinylic ether These addition and addition-elimination reactions have been previously reviewed [1, 2] The intermediate carbanions resulting from nucleophilic attack on fluoroolefins have also been trapped in situ with carbon dioxide, carbonates, and esters of fluorinated acids [3, 4, 5] (equations 1 and 2)... 

Zinc carbonate reacts with epoxide to form zinc alkoxide, which in turn reacts with carbon dioxide to regenerate zinc carbonate. The most effective catalyst systems were the reaction products between diethylzinc and polyhydroxy compounds such as water or pdyhydric phenols243,244. This copolymer is interesting as a biodegradable elastomer248. ... 

Ito and co-workers observed the formation of zinc bound alkyl carbonates on reaction of carbon dioxide with tetraaza macrocycle zinc complexes in alcohol solvents.456 This reversible reaction was studied by NMR and IR, and proceeds by initial attack of a metal-bound alkoxide species. The metal-bound alkyl carbonate species can be converted into dialkyl carbonate. Spectroscopic studies suggested that some complexes showed monodentate alkyl carbonates, and varying the macrocycle gave a bidentate or bridging carbonate. Darensbourg isolated arylcarbonate compounds from zinc alkoxides as a by-product from work on polycarbonate formation catalysis.343... 

The interactions of dimethyl- and diethylzinc with bulky tris(hydroxyphenyl)methanes, Scheme 86, yielded, depending on the reaction conditions, a variety of alkylzinc alkoxides, featuring two-, three-, and four-coordinate zinc centers. These polynuclear compounds (Figure 63 shows the trinuclear ethylzinc derivative 136) are relatively poor catalysts for the co-polymerization of cyclohexene oxide and carbon dioxide.197... 

The nucleophilic attack by alkoxides, amines, and water is of great interest to homogeneous catalysis. A dominant reaction in syn-gas systems is the conversion of carbonyls with water to metal hydrides and carbon dioxide ("Shift Reaction"), see Figure 2.27. 

Metal-Oxygen Compounds. Trialkyltin alkoxides are remarkable for the variety of addition reactions they undergo with carbonyl and thiocarbonyl compounds. Bloodworth and Davies have reported reactions of tri-w-butyltin alkoxides with isocyanates, carbon dioxide, sulfur dioxide, isothiocyanates, carbon bisulfide, chloral, and ketene. The reactions observed were as follows ... 

Metal-Nitrogen Compounds. Very little work has been done on addition reactions of metal-nitrogen compounds. The trimethyltin dimethylamide apparently does undergo reactions analogous to those of the trialkyltin alkoxides just discussed. For example, the following reactions were observed with carbon dioxide, carbon disulfide, and phenylisocyanate (57) ... 

The insertion of carbon dioxide into a transition metal-oxygen bond, e.g., a metal alkoxide, results in an organic carbonate ester, coordinated in either a monodentate or bidentate manner. Only a limited number of such reactions have been observed, and little mechanistic information is available. The reactions may proceed by interaction of C02 with ROH or RO in solution followed by metal coordination, in a manner similar to the C02 reactions with the early transition metal dialkylamides. Alternatively, direct attack of C02 on the alkoxide oxygen might occur, or a C02 adduct may form as an intermediate. 

Other reports confirm similar reaction of C02 with metal alkoxide compounds. Benzene solutions of the alkoxide compounds Ti(OBu")4, Zr(OBu")4, Zr(OEt)4, Nb(OEt)5, and Fe(OEt)3 were found to absorb carbon dioxide in various amounts (151). Only Zr(OBu")4 reacted with an amount approaching one equivalent per metal complex. The solution ir spectra indicate formation of coordinated organic carbonates. From this evidence, equilibrium formation of the species M(0R) 1(02C0R) under 1 atm of C02 is inferred. 

The known C02 insertion reactions involving metal-carbon bonds have all resulted in carbor. -carbon bond formation with possibly one exception. Infrared spectral and chemical evidence has been presented for the formation of the metallocarboxylate ester Co(C03) (COOEt)(PPh3), n = 0.5-1.0 from the reaction of Co(CO)(C2H5XPPh3)2 with carbon dioxide from Vol-pin s laboratory (68). Although these studies are not conclusive for abnormal C02 insertion, metallocarboxylate esters are well-known compounds which result from the nucleophilic addition of alkoxides on the carbon center in metal carbonyls (69). 

The reactions of C02 with metal-hydroxides, -alkoxides, and -amides to provide metallobicarbonates, -alkyl carbonates, and -carbamates [Eqs. (27)—(29)] in general do not involve activation of carbon dioxide by prior coordination to the metal center. These processes generally entail either... 

In order that an alternating copolymer is produced, the metal alkoxide must undergo faster insertion of carbon dioxide than reaction with a second equivalent of epoxide. If the metal alkoxide reacts with a second epoxide or undergoes decarboxylation reactions, ether linkage(s) may be formed. Ether linkages are undesirable as they compromise the properties of the polymer and reduce the carbon dioxide uptake. 

Carbon dioxide readily undergoes insertion reactions probably via initial C02 complexing, with metal alkoxides, hydroxides, oxides, dialkylamides, and metal hydrides and alkyls 103... 

Insertion reactions involving metal alkoxides are also known. For example, carbon dioxide is known to react with some metal alkoxides as shown in equation (12). The formation of a bidentate ligand is a significant thermodynamic driving force for some of these reactions. The isoelectronic aryl and alkyl isocyanates and carbodiimides can react similarly. Insertion reactions involving alkenes and carbon monoxide are known for platinum alkoxides. 

Reaction of chloroformates with sodium alkyl carbonates, easily available through reaction of carbon dioxide with sodium alkoxides, affords dicarbonates also called pyrocarbonates as shown in scheme 45. 

Table 1. Reaction of Metal Oxides and Alkoxides with Carbon Dioxide...

Metal alkoxides undergo insertion reactions across the M—O bonds (6) when treated with unsaturated substrates such as isocyanates, isothiocyanates, aldehydes, ketones, ketenes as well as with carbon dioxide, carbon disulfide, and sulfur dioxide. 

Examples of addition reactions of tributyltin methoxide are given in Table 14-3. Most of these reactions are reversible for example, the alkyl tin carbonate which is formed by the addition of a tin alkoxide to carbon dioxide eliminates C02 on heating to regenerate the tin alkoxide (equation 14-14). Again, the product of the reaction of an acyl halide with a dioxastannolane is involved in an exchange reaction in which the tin alkoxide adds reversibly to the carbonyl group (equation 14-31).41... 

Metal Alkoxides. Notwithstanding our understanding of the mechanistic aspects operative in insertion reaction of carbon dioxide with metal-hydride and metal-carbon (alkyls or aryls) bonds has... 

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