Carboxylation of epoxides

Allen JR, SA Ensign (1996) Carboxylation of epoxides to 3-keto acids in cell extracts of Xanthobacter strain Py2. J Bacterial 178 1469-1472. 

It is worth mentioning that both the carboxylation of epoxides and anilines are acid-base reactions, which do not entail redox processes. Therefore a catalyst active in these reactions must provide acid-base functionality. In this perspective, positively charged gold could be the real player, although a co-catalytic or promotion effect of ze-rovalent gold could also be important. Therefore the catalysts for the oxidative carbonylation of aniline, supported on Merck Ion-exchanger IV, could be actually bifunctional. On one side, Au could catalyze the oxidation of CO with O2 to CO2, a reaction for which it is... 

The carboxylation of epoxides (Equation 7.13) has long been known (IG Farben, 1943) [12, 117, 118], and today is available on stream from several production plants. 

Very often, amides such as dimethylformamides (DMF) or dialkylacetamides (DAA) have been used as solvents in reactions where they may themselves promote the carboxylation of epoxides [139], if only to a limited extent. 

As noted above, the carboxylation of epoxides may afford polycarbonates, whether using Al-porphyrin complexes [144,145] or Zn-compounds [146], In fact, the Al-catalysts, which were the first to be described, are currently used in production plants (see Chapter 8). 

Recently, the carboxylation of epoxides was carried out in ionic liquids (ILs) that demonstrate interesting characteristics such as thermal and chemical stability, selective solubility towards organic and inorganic materials, and a high reusability of the catalysts. Taken together, these parameters make ILs useful for this type of application [151-156]. 

Next to oxygenation, zeolite entrapped MePc complexes show also activity in dehydrogenation and hydrogenolysis reactions, and NO reduction [3]. More recently, the carboxylation of epoxides into organic carbonates has been reported [331 (scc scheme). 

Aresta et al. [32, 33] have reported that Nb O, functioning as a Lewis acid, is an active catalyst for the conversion of epoxides into the relevant carbonates and extensively discussed the role of the temperature and solvent on the yield of the carboxylation reaction. These authors have shown that solvents such as DMF or N,N-dimethylacetylamide (DMA) act as promoter of the carboxylation reaction and the cyclic carbonate yield is maximized above 135°C (77% Yp, ). They proposed a mechanism (Figure 8.4a) where the simultaneous interaction of the O-atom of the epoxide with the Nb-center. The attack at the asymmetric carbon by the carbonate moiety formed over Nb O is able to prevent any inversion at the chiral carbon of the epoxide. On the other hand, the solvent (DMF or DMA) could assist the carboxylation of epoxides through a back-attack at the more substituted carbon, preventing any change of configuration at the chiral center in spite of a low conversion yield (Figure 8.4b). Moreover, the addition of a small amount of CH Cl to the reactive system in either DMF alone or DMF in... 

Figure 8.4 Epoxide carboxylation reaction pathway over Nb O catalyst (a) and role of solvent in the carboxylation of epoxides (b) (adapted from Ref. [32]).

Taking into account that the cooperation of Lewis acid-base sites is advantageous for the coupling reaction of CO and epoxides. Yuan et al. [38] attempted to use the combination Mg(OH)Cl with KI as heterogeneous catalyst for the carboxylation of epoxides and found that this... 

The carboxylation of epoxides (6.20) has been known since 1943 but has been exploited only much more recently. It may afford either cyclic carbonates or polymers, depending on the catalyst used [124-126] and the reaction conditions [127-138]. 

Scheme 6.21 Bimetallic Al-O-Al catalysts used in the carboxylation of epoxides. Adapted with...

Scheme 6.22 Reaction mechanism for the carboxylation of epoxides under the action of a bimetallic-Al-complex, Bu4NBr and BU3N. Adapted with permission from [169]. Copyright (2010) John Wiley and Sons...

That solvents may play a key role in the carboxylation of epoxides was proposed by Dibenedetto et al. [109] who showed that both dimethylformamide and diacetamide may also catalyze the carboxylation of epoxides if at a low extent (Scheme 6.23). The active role of DMF has been confirmed in the case of the bromine catalyzed carboxylation of epoxides [171]. DMF has been proposed to attack CO2 (Scheme 6.23) and prompt it to a reaction with epoxides. 

The carboxylation of epoxides may afford either monomers, as discussed above, or polycarbonates for example, Al-porphyrin complexes [175, 176] or Zn-compounds [177] promote the formation of polycarbonates. The pioneering studies of Inoue [178] and Kuran [179] have opened the route to the investigation of the copolymerization of CO2 and epoxides. The key issue here is to master the alternate insertion epoxide-C02. 

The carboxylation of epoxides is strongly dependent on the reaction conditions such as temperature and solvent The use of ionic liquids as reaction medium seems to accelerate the reaction with respect to any other organic solvent, most probably because ionic liquids promote the formation of and/or stabilize polar or ionic intermediates. Heterogeneous catalysts such as oxides [95] or supported ammonium salts [92] or metal complexes [94] work well under these conditions. The solvent can play a key role in such reactions. Amides such as di-methylformamides or dialkylacetamides can themselves promote the carboxylation of epoxides, albeit with a low TON [100]. Most likely this is due to the abil-... 

As noted above, when Al-porphyrin complexes [97] or Zn compounds [98] are used as catalysts for the carboxylation of epoxides, the formation of polymers is observed. A1 catalysts are now used in a plant in China. The mechanism of the polymerization reaction has been studied and the most credited mechanism when Zn compounds are used is shown in Scheme 1.12. The molecular mass of the polymers varies with the catalyst. Primarily propene oxide and styrene oxide have been used so far, with some interesting applications of cyclohexene oxide. It is wished to enlarge the use of substrates in order to discover new properties of the polymers. 

Relatively mild conditions suffice for the regioselective carbonylation (eq 12) and carboxylation of epoxides (eq 13). The... 

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