Since when this reaction was discovered in 1969, many catalytic systems have been designed and investigated for the syntheses of organic carbonates and fine tuning of the catalyst was found to strongly influence the yield and selectivity of the process towards one specific product over the competing ones. These catalytic systems might be either homogeneous or heterogenous, which both present positive and negative aspects [5,6]. For instance, heterogeneous catalysts are easily synthesised, low-cost and stable, however they usually require harsher reaction conditions compared to homogeneous catalysts and display lower selectivity. Zinc glutarate (ZnGa) and double metal cyanide (DMC) catalyst are two possible heterogeneous catalyst for the synthesis of this reaction.[6] On the other hands, homogeneous catalyst can be finely tuned to maximise the selectivity towards the desired product under mild conditions; however, their synthesis usually involves multiple steps and the purification procedure in this case might be challenging. A huge range of organometallic complexes have been designed for the synthesis of PCs, among which cobalt, chromium, iron, aluminium, and zinc complexes are, in general, the most active [5]. together with an organometallic complex, the reaction also needs a nucleophile to occur; this might be already present in the complex (bifunctional catalyst) or added to the reaction mixture as a separate species (binary catalyst). Bulky organic salts are commonly used as nucleophiles. Due to their high selectivity, fine tuning, and milder operating conditions, homogeneous catalysts are usually preferred over heterogeneous catalysts [5,6].
For what concerns the mechanism, in general, the coupling reaction between carbon dioxide and epoxides needs to be catalysed to occur and, depending on the catalytic system, the first step that initiates the reaction might either be the coordination of the epoxide or carbon dioxide [7]. The most common mechanism in the case of homogeneous catalysts is the monometallic mechanism [7], in which the first step is usually the coordination of the epoxide to the metal centre; this activates the epoxide and favours the nucleophile to open the epoxydic ring leading to an alkoxide intermediate. At this point, carbon dioxide inserts between the negatively charged alkoxide intermediate and the metal centre to generate a metal carbonate intermediate. This species is now the key intermediate of the reaction as it can undergo intermolecular backbiting to lead the corresponding CCs or it can allow further epoxide and CO2 insertions to yield the desired PCs [7].