C02-epoxide coupling

Buchard A, Kember MR, Sandeman KG, Williams CK (2011) A bimetallic iron(III) catalyst for C02/epoxide coupling. Chem Commun 47 212-214... 

The reaction of epoxides with C02 affords either CCs or polymers [119], and many reports have been made [120-125] and different active catalysts described [126-130] such as alkyl ammonium-, phosphonium-salts and alkali metal halides, in this respect. The main drawbacks here are the need for a high catalyst concentration, a high pressure (5 MPa of C02), and a temperature ranging from 370 to 400 K. The recovery of the catalysts for reuse is also a key issue, and in order to simplify the recovery process various hybrid systems have been developed, an example being that prepared by coupling 3-(triethoxysilyl)propyltriphenylphosphonium bromide with mesoporous silica [131]. In this case, the reaction was carried out in the absence of solvent, under very mild conditions (1 MPa, 263 K, 1 mol% loading of catalyst, 6h), such that the hybrid catalyst could be recovered and recycled several times. 

The direct oxidative carboxylation of olefins has great potential, and many advantages. Notably, it does not require the C02 to be free of dioxygen this is an especially attractive feature, as the cost to purify C02 is extremely high, and may discourage its use. Moreover, the direct oxidative carboxylation of olefins can couple two processes-the epoxidation of olefins, and the carbonation of epoxides. Hence, the process makes direct use of those olefins that are available commercially at low price, and which represent an abundant feedstock. Such an approach also avoids having to isolate the epoxide. 

Scheme 8.1 Coupling reactions of C02 and epoxides to afford polycarbonate and cyclic products.

The scope of this chapter will be to examine discrete, well-characterized metal-based catalysts for the incorporation of C02 as a monomer in the preparation of polymeric materials. The coupling of C02 and epoxides has recently been the subject of several comprehensive reviews, which will not be reiterated herein [17-19]. Instead, attention will be focused on some of the more recent major contributions to this area over the past decade. 

Most often, the extent of completely alternating copolymer formation, expressed as 100% C02 linkages or 50% C02 content, is very high. With regards to the selectivity of the coupling reaction for copolymer versus cyclic carbonate production, two observations are consistently found, regardless of the catalyst. First, aliphatic epoxides are more prone to cyclic carbonate formation than alicyclic epoxides for example, PO affords propylene carbonate more readily than CHO provides cyclohexene carbonate. Second, in either instance, since it has been shown that the activation barriers for cyclic carbonate production are higher... 

The first single-site metal catalyst which was shown to homogeneously catalyze the coupling of epoxides and C02 was (tpp)AlCl (tpp = tetraphenylporphyrin) in the presence of a quaternary organic salt or triphenylphosphine [20]. Although this catalytic system was extremely slow at ambient temperature, copolymers from ethylene oxide, PO, and CHO and C02 were obtained that possessed very narrow molecular weight distributions (polydispersity = 1.06-1.14). The low reactivity of... 

A revitalization of interest in the copolymerization of epoxides and C02 can be traced to studies involving discrete zinc complexes. Several well-defined zinc monomeric and dimeric derivatives have been shown to be effective catalysts for the coupling of CHO and C02 to afford copolymers with high degrees of C02 incorporation. These include reports of sterically encumbering Ms-phenoxide derivatives of zinc [24], a highly fluorinated zinc carboxylate [25], and zinc [3-diiminate complexes [26] (see Figure 8.2). 

Propylene oxide represents a very attractive epoxide monomer for copolymerization with C02, as polypropylene carbonate) is industrially valuable. The low glass transition temperature (Tg) of 313 K, the sharp and clean decomposition above 473 K, and biodegradability of this copolymer are the reasons for its attracting interest in several applications. On a similar basis, H NMR spectroscopy is useful for assessing the coupling products resulting from the reaction of PO and C02 (Figure 8.21). 

In this chapter, some of the essential aspects of the synthesis and characterization of copolymers derived from the coupling of C02 with various monomers, namely, epoxides, oxetanes, and aziridines, have been reviewed. In addition, the use of carbon disulfide as a resource for copolymer production was introduced, and the present understanding of the mechanistic aspects of processes involving cyclic ethers and C02 catalyzed by well-defined metal systems has been emphasized. This knowledge has led to the development of catalytic systems capable of controlling not only the product selectivity but also the regio- and stereoregularities of the resultant copolymers. 

The reaction mechanism for the copolymerization is initiated as shown in Scheme 6.24, left and once the first coupling epoxide-C02 is performed, the growth of the chain should proceed with a perfect alternate insertion of epoxide and CO2. 

M.A. Fuchs, S. Staudt, C. Altesleben, O. Walter, T.A. Zevaco, E. Dinjus, A new air-stable zinc complex based on a 1,2-phenylene-diimino-2-cyanoacrylate ligand as an efficient catalyst of the epoxide-C02 coupling, Dalton Trans. 43 (2014) 2344-2347. 

It is now well estabUshed that the mechanism of the transition metal-catalyzed coupling reactions of CO, and epoxides includes three main steps, epoxides oxidative addition, CO2 insertion, and reductive elimination of carbonates. The important issue of the mechanism is whether first to activate the epoxides or C02- In the following paragraphs, we will discuss a couple of examples to illustrate the mechanistic details. 

Several groups also used NHC-Ag catalysis in order to promote the reaction of a terminal alkyne with an electrophile. Following this process, Sono-gashira couplings and the addition of alkynes to isatins or C02 were described. In the last case, supported NHC-Ag nanoparticles were used. The efficiency of the process was attributed to both the activation of the alkyne by the NHC-Ag complex and the activation of CO2 by a free NHC moiety in a cooperative manner. CO2 insertion has also been exploited in order to synthesize cyclic carbonates and carbamates by NHC-Ag catalysis starting from epoxides, propargyl alcohols or allenyl amines [eqn (11.4)]. ... 

In 2004, Louie reported a convenient route to prepare CO2 adducts by depro-tonating the imidazolium salts with potassium tert-butoxide under an atmosphere of C02. These are convenient free NHC precursors, also used in the preparation of [(NHC)M] complexes (see Chapters 1 and 2 for further details). Alternatively, NHC-CO2 adducts have been used in several CO2 fixation reactions. Lu tracked the progress of the coupling of CO2 with epoxides in imid-azolium-based ionic liquids by in situ IR monitoring with various epoxides. Later, Ikariya and Tommassi extended the substrate scope to propargylic alcohols and aziridines under milder conditions (Scheme 14.26). 

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