Carbon dioxide insertion reactions

These complexes show an interesting chemistry, e. g. they undergo coupling with ethene to give zirconacyclopentane 71 or with water to give zirconoxane 72, or they can undergo insertion of carbon dioxide with formation of the complexes 73 and 74. In all of these reactions, the pyridine moiety is restored. With acids, complex 73 liberates the corresponding carbonic acids 75 or esters 76. 

Exxon s Flexsorb SE solvents achieve high hydrogen sulfide selectivity by virtue of their molecular structure. These solvents are sterically hindered secondary amines. A bulky molecule is used to shield the available hydrogen radical on the nitrogen atom and prevent the insertion of carbon dioxide. The reaction with hydrogen sulfide is not sensitive to the amine s structure, so the steric hinderance affords higher hydrogen sulfide selectivity. 

Since the insertion of carbon dioxide apparently involves a sequence of reactions, it is instructive to consider the available pathways that can lead to insertion. A simple picture of the available mechanisms for C02 insertion is outlined schematically in (52). There are three components the metal center, the ligand, and the inserting molecule, C02. Any two of these components may be bound together initially. 

The insertion of carbon dioxide into a transition metal-carbon bond is of importance since it forms a new carbon-carbon bond. The general reaction, (55), transforms a coordinated alkyl or aryl group into a coordinated carboxylate. 

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. 

The iodine-catalyzed reaction of aziridines with carbon dioxide leads to 2-oxazolidinones (251). Because carbon dioxide effectively polymerizes ethyleneimine, only low yields are obtained when unsubstituted ethyleneimine reacts with C02. However, direct insertion of carbon dioxide [124-58-9] into aziridines can be accomplished, with better yields, by ethoxycarbonylation of aziridines with subsequent elimination of ethylene under flash vacuum conditions (252). 1- Phenyl aziridine [696-18-4] can react with C02 under antimony [7440-56-0] catalysis to give AI-phenyl-2-oxazolidinone in good yields (253). At low temperatures and with the exclusion of atmospheric humidity, the reaction of ethyleneimine with carbon dioxide produces the unstable ethyleneiminium salt [51645-58-2] of IV-vinylcarbamic acid (254,255). 

The pivotal step in carbon homologation processes using carbon dioxide is the insertion of carbon dioxide into metal-carbon bonds. Hence, this insertion reaction is of paramount importance in the utilization of carbon dioxide as a source of chemical carbon. The generalized reaction is schemat-... 

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. 

Green and co-workerii described the reaction of CO2 with Cp4MejAli M02 (281]. An MoC(0)OAJ system was formed via the insertion of carbon dioxide into an Mo-Al bond. Hydrolysis, extraction with hydrobromic acid, and the addition of aqueous hexafluoruphosphale yielded the carbonyl derivative lCp Mo(CO)Br]PF6, and the reaction of MoCpjHfMgBrCTHF) ] with CO2 yielded MoCpj(CO) (282,283]. 

Insertions of carbon dioxide, sulfur dioxide, and sulfur trioxide yield aluminum carboxylates, sulfinates, and sulfonates, respectively. Treatment of the resulting complexes with aqueous acid yields the corresponding aUcylcarboxylic, alkylsulphinic, and aUcylsulphonic acids. High pressure and temperatures of 220-240 °C are required for multiple insertions of CO2 to yield more than one equivalent of carboxyhc acid per aluminum. Excess aluminum trialkyl must be avoided or the initially formed carboxylate is completely alkylated to a trialkylcarbinol. Reaction of Ets A1 with CO2, for example, gives a 90% yield of triethylcarbinol. 

From a mechanistic point of view the first steps of the catalytic cycle should be similar to the telomerization of butadiene itself (Scheme 2). The catalytic precursor generates the Pd(0) species A that reacts to the bis-(ri -allyl) complex C. The C,C bond formation between two C4 units is followed by insertion of carbon dioxide into a Pd,C bond affording the carboxylate intermediate D. Different pathways have been discussed to describe the multiple product formation (refer to ). Interestingly, a bis-(carboxylato) complex may be prepared directly from the reaction of lactone 1, palladium acetate and P(i-Pr)3. This complex was structurally characterized by Behr and co-workers and shows good activity as catalyst. Reviewing the literature, there are some remarkable facts and open questions of theoretical and technical interest ... 

If the peroxocarbonate complexes are prepared by reaction of dioxygen complexes of transition metals LnM(02)Xm (M = Pd, Pt, Rh, Ir) with carbon dioxide, [la] in principle, two ways are possible, that imply the formal insertion of carbon dioxide into the 0-0 (route 1, Scheme 1) or M-O (route 2, Scheme 1) bond. 

Further evidence for the stereochemistry of the SO2 insertion reaction [reaction (j)] has been obtained . The insertion of carbon dioxide into metal-carbon bonds is being developed with renewed interest, in part because of its role in the greenhouse effect . 

Other reactions of organon Pd(0) is regiochemically distinct I Insertion of carbon dioxide into 1... 

Figure 1. (a) Cell arrangement for metal hydride hydrogen insertion reaction. Left side, acid reduction and hydrogen atom incorporation in palladium (Pd) foil membrane. Right side, electrostatic field for enhancement of carbon dioxide/bicarbonate adsorption on foil membrane, (b.) Blow-up of palladium/hydride foil showing hydrogen insertion into carbon dioxide. 

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°). 

In the formation of formamide by the reaction of amine, hydrogen, and carbon dioxide using Rh, Ir, and Co catalysts, the insertion of carbon dioxide is assumed 121)... 

As discussed in Chapter 4 (Oxazolidinone and Oxazole), guanidines catalyze the insertion of carbon dioxide (CO2) to alkynic bonds [22]. This behaviour may give a hint to trapping CO2 by organobase catalyzed chemical reaction, even to necessary modification of reaction conditions. Hopefully, it is the ideal that the C02-incorporated products could be used as new energy sources. 

Binger and Weintz studied the reaction of the unsubstituted methylene-cyclopropane with carbon dioxide and obtained the 3-methy1-2-buten-4-olide in yields up to 80 % [25,26]. As shown in Figure 10, parallel to the main reaction, the formation of cotrimers, cotetramers and copentamers can not be avoided. A mechanism is proposed, in which the palladium forms a trimethylenemethane intermediate which exists in two tautomeric structures (Figure 11). By insertion of carbon dioxide palladium carboxylate complexes are formed which release the furanone. 

It is of special interest, that the 1,3-diene structure of the starting material is maintained in the product. Formally, this reaction corresponds to an insertion of carbon dioxide into a C-H-bond of piperylene. Using butadiene, the analogous 2,4-pentadienoic acid is formed only in yields up to 25 %, with isoprene no reaction takes place [47]. The different behaviour of the dienes can be explained by the mechanism shown in Figure 18. 

Oxanickelacycles of type la have also been synthesized by an alternative procedure based on inserting of carbon dioxide into the Ni-C bond of certain reactive nickelacycles. Thus, reaction of complex 13 with carbon dioxide leads to die formation of metallacycle 14 resulting from an insertion into the Ni-C(aryl) bond of 13 (Eq. 7). Complex 13 is... 

Synthetic application of aluminum porphyrins afforded a series of interesting reactions. Light irradiation to TPP-Al-Et enhanced the addition reaction of ethyl group to vinyl ketone and polymerization [349] (Fig. 32). Photochemical insertion of carbon dioxide into Al-Et bond was reported in the presence of 1-methylimidazole [352]. Similar insertion of CO2 into the Al-enolate bond was also reported [353,354]. These reactions have attracted much attention in i-elation to fixation of carbon dioxide in artificial photosynthesis as described in Sec. II1.C (Fig. 33). 

Chisolm MH, Extine M (1975) Carbon dioxide exchange reactions involving transition-metal N,N-dimethylcarbamato compounds reversible insertion of carbon dioxide into transition-metal-nitrogen o bonds. J Chem Soc Chem Commun 438-439... 

Chisolm MH, Extine M (1977) Reactions of transition metal-nitrogen a bonds. 4. Mechanistic studies of carbon dioxide insertion and carbon dioxide exchange reactions involving early transition-metal drmethylamido and N, N-dimethylcarbamato compounds. J Am Chem Soc 99 792-802... 

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