Besides being an unwanted greenhouse gas, carbon dioxide (CO2) is also an exciting source of raw materials that are useful and can be recycled sustainably.
In the Angewandte Chemie journal, Spanish scientists have now described an innovative catalytic process for transforming CO2 into precious chemical intermediates in the form of cyclic carbonates.
Sadly, it is highly difficult to make CO2 react. At present, a majority of the studies are focused on the transformation of CO2 into methanol, which can be utilized as an alternative fuel and a feedstock for the chemical industry.
Novel catalytic processes could enable the conversion of CO2 into valuable chemical compounds without taking a detour via methanol, possibly for the synthesis of pharmaceutical intermediates or biodegradable plastics.
One highly potential method is the transformation of CO2 into organic carbonates, which are compounds consisting of a building block extracted from carbonic acid, including carbon atom fixed to three oxygen atoms.
Scientists 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 designed a novel process to synthesize carbonates in the form of six-membered rings, beginning from CO2 and fundamental, easily accessible building blocks.
Such cyclic carbonates exhibit a great promise for making new CO2-based polycarbonates.
A carbon-carbon double bond and an alcohol group (-OH) on a neighboring carbon atom (homoallylic alcohols) are used as the starting materials. The initial step of the reaction involves converting the double bond into an epoxide, which is a three-membered ring with two carbon and one oxygen atoms.
The epoxide is capable of reacting with CO2 when a particular catalyst is present. The product is a cyclic carbonate with a five-membered ring that has two oxygen and three carbon atoms.
The carbon atom at the “tip” of the five-membered ring is bound to an additional oxygen atom. In the next step, an organic catalyst (N-heterocyclic base) triggers the OH group and makes the five-membered ring transform into a six-membered ring.
The oxygen atom from the OH group is added to the new ring, and one of the oxygen atoms from the original five-membered ring develops a new OH group. But the reverse reaction also occurs since the original five-membered ring is considerably more energetically favorable, and only a very small amount of the six-membered ring exists at equilibrium.
The strategy is to trap the six-membered ring. The new OH group attaches to a reagent (acylation) since its different position renders it much more reactive compared to the original OH group.
This innovative process paves the way to a wider range of novel, six-membered carbonate rings with higher yields and high selectivity, as well as under mild reaction conditions. This extends the collection of CO2-based heterocycles and polymers, which are hard to synthesize using traditional approaches.
Qiao, C., et al. (2020) Organocatalytic Trapping of Elusive Carbon Dioxide Based Heterocycles by a Kinetically Controlled Cascade Process. Angewandte Chemie International Edition. doi.org/10.1002/anie.202007350.