Cascades with carbon dioxide

Carbon dioxide (CO₂) is not just an undesirable greenhouse gas, it is also an interesting source of raw materials that are valuable and can be recycled sustainably. In the journal Angewandte Chemie, Spanish researchers have now introduced a novel catalytic process for converting CO₂ into valuable chemical intermediates in the form of cyclic carbonates.

Getting CO₂ to react is unfortunately not easy. Currently, most research is focused on the conversion of CO₂ into methanol, which can be used as an alternative fuel as well as a feedstock for the chemical industry. Innovative catalytic processes could allow CO₂ to be converted into valuable chemical compounds without taking a detour through methanol, perhaps for the production of biodegradable plastics or pharmaceutical intermediates.

One highly promising approach is the conversion of CO₂ into organic carbonates, which are compounds that contain a building block derived from carbonic acid, comprising carbon atom attached to three oxygen atoms. Researchers working with Arjan W. Kleij at the Barcelona Institute of Science and Technology (Barcelona), the Institute of Chemical Research of Catalonia (Tarragona), and the Catalan Institute of Research and Advanced Studies (Barcelona), have developed a conceptually new process to produce carbonates in the form of six-membered rings, starting from CO₂ and basic, easily accessible building blocks. These cyclic carbonates have great potential for the creation of new CO₂-based polycarbonates.

The starting materials are compounds with a carbon-carbon double bond and an alcohol group (-OH) on a neighboring carbon atom (homoallylic alcohols). In the first step of the reaction, the double bond is converted into an epoxide, a three-membered ring with one oxygen and two carbon atoms. The epoxide is able to react with CO₂ in the presence of a specific catalyst. The product is a cyclic carbonate in the form of a five-membered ring with three carbon and two oxygen atoms. The carbon atom at the "tip" of the five-membered ring is attached to an additional oxygen atom. In the next step, an organic catalyst (N-heterocyclic base) activates the OH group and causes the five-membered ring to rearrange into a six-membered ring. The oxygen atom from the OH group is integrated into the new ring, while one of the oxygen atoms from the original five-membered ring forms a new OH group. However, the reverse reaction also takes place because the original five-membered ring is significantly more energetically favorable, and only a vanishingly small amount of the six-membered ring is present at equilibrium. The trick is to trap the six-membered ring. The new OH group binds to a reagent (acylation) because its different position makes it considerably more reactive than the original OH group.

This newly developed process gives access to a broad palette of novel, six-membered carbonate rings in excellent yields, with high selectivity and under mild reaction conditions. This widens the repertoire of CO₂-based heterocycles and polymers, which are difficult to produce by conventional methods.