Insertion involving carbon dioxide

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. 

In recent years there has been a growing interest in the chemistry of carbon dioxide, stimulated by current anxieties about alternative petrochemical feedstocks. One aspect under active exploration involves carbon dioxide activation via coordination to a transition metal, and indeed transition metal ions do form C02 complexes.177 The number of simple and reasonably stable complexes is still relatively small and usually limited to low oxidation state metal ions. There are many systems where C02 is used as a reagent giving rise to systems which, while not true C02 complexes, may simplistically be viewed as the products of insertion into metal-ligand bonds, e.g. reaction (9), where if L = H, formates are produced if L = OH, carbonates or bicarbonates result L = NR2 yields dialkylcarbamates and if L = R, carboxylate products result. Much of this area has recently been reviewed and will not be considered further.149... 

The insertion of carbon dioxide probably involves C02 complexes as intermediates and is known for alkyls, hydrides,204 dialkylamides, dialkylphosphides, hydroxides,205... 

De I squale reported that a series of coordina lively unsaturated nickel complexes, such as Ni(PPli3)2 or Ni(PCy3)2, act as excellent catalysis. A mechanism is proposed consisting of a sequence of oxidative addition, insertion and reductive elimination steps which involve an oxometallocyclobutane intermediate [225], The decisive step is Uie insertion of carbon dioxide mto a metal -oxygen bond. 

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... 

Buhro WE, Chisholm MH, Martin JD, Huffmann JC, Folting K, Streib WE (1989) Reactions involving carbon dioxide and mixed amido-phosphido dinuclear compounds M2(NMe2)4(PR2)(M=M), where M = Mo and W. Comparative study of the insertion of carbon dioxide into metal-nitrogen and metal-phosphoms bonds. J Am Chem Soc 111 8149-8156... 

Figure 5.2 provides a broad summary of current and projected utilization of carbon dioxide. Reactions that use CO2 to produce organic chemicals or intermediates for the chemical industry are summarized from 6 to 11 00 o clock on the diagram. The first examples, such as salicylic acid, are in current practice. Also included in Figure 5.2 are reactions that hold promise for extensive utilization in the future. Many of these involve insertion of carbon dioxide into Y—bonds, often the C—H bond. The products of interest include esters, carbamic esters, salicylic acid, and cyclic carbonates. These reactions... 

The coupling of epoxides and carbon dioxide as catalyzed by metal complexes is generally believed to occur via a coordination-insertion mechanism involving either one or two metal centers. The array of reaction processes that are frequently associated with this transformation are indicated below (Schemes 1 ). 

In general, the insertion reaction of carbon dioxide into metal hydrogen bonds is formally much akin to the analogous process involving olefins (Scheme 1). This analogy is particularly appropriate since the binding of... 

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). 

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. 

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. 

Transamination reactions are catalyzed by carbon dioxide or its salt with dimethylamine, presumably via an insertion process that involves silylcar-bamate formation 34). The catalyzed transamination of Tris with diethylam-ine gives typical results ... 

Ditertiary phosphane complexes of nickel were found to be effective in the formation of pyrone 108 by cyclocotrimerization of alkynes with carbon dioxide. The formation of the nickelacyclopentadiene 105 from two moles of alkyne and a nickel complex is followed by CO2 insertion into a nickel-carbon bond to give the oxanickelacycloheptadienone 106, which then eliminates 108 with intramolecular C—O coupling. Another route involving [4 + 2] cycloadditions of 105 with CO2 in a Diels - Alder reaction to give 107 cannot be ruled out but is less probable because CO2 does not undergo [4 + 2] cycloaddition with dienes. Addition of another alkyne to 105 results in the formation of a benzene derivative (Scheme 38). ... 

The activation of carbon dioxide by homogeneous and heterogeneous metal catalysts, as well as the nature of the stoichiometric insertion processes of these catalysts, are examined. The kinetic and mechanistic aspects of CO2 insertion into the M-X bond of M(C0)5X" complexes (M W, Cr and X H, alkyl, aryl, aryloxy, and alkoxy) is investigated. The mechanism of CO2 insertion in these systems is described as an associative interchange (I ) type mechanism where prior loss of coordinated CO is not involved in the insertion process. 

The reactions of silylphosphines with electron-rich compounds are complex, involving insertion into the Si-P bond as the first step. With carbon dioxide, bis(silyl)phosphines give an equilibrium (equation 20) dominated by the phosphacarbamic acid ester 8 which loses disiloxane if R = 2,4,6-t-Bu3C6H2 (henceforth represented as Ar) to give the phosphaketene 9. This in turn reacts with the silylphosphide 10 to form the 1,3-phosphaallene 11, an analogue of carbodiimides, and also results directly from ArPHSiMe3, n-BuLi and C02 (equation 21)28. 

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