New “diagonal” approach for the reductive functionalization of CO2 to make building blocks for chemicals and fuels
|Approaches to recycling transformations of CO2 as alternatives to petrochemical methods. Source: Das Neves Gomes et al. Click to enlarge.|
French scientists working with Thibault Cantat at the Institut Rayonnement Matière de Saclay in Gif-sur-Yvette have introduced a new approach for the conversion of carbon dioxide into both useable building blocks for chemical synthesis and new fuels.
To date, there have been two different approaches for the utilization of carbon dioxide, Cantat says: a “vertical” approach, and a “horizontal” approach. In a paper in the journal Angewandte Chemie, Cantat and his colleagues propose a third, “diagonal” approach. The “vertical” approach, in one in which the carbon dioxide is reduced (the oxidation state of the carbon atom is reduced by the formal replacement of oxygen with hydrogen). This results in molecules such as methanol or formic acid, which can be converted into fuels. These products have a higher energy content than carbon dioxide, but only a handful of chemicals can be produced this way.
In the horizontal approach, the carbon atom is functionalized (it forms new bonds to oxygen, nitrogen, or other carbon atoms). The oxidation state stays the same, and the energy content is not increased. This does not produce fuels, but chemicals that are useful building blocks for chemical syntheses, such as urea. The French team tried a compromise approach, a combination of both methods to make a “diagonal” approach.
...petrochemicals (hydrocarbons) are extensively used as fuels and basic reagents to access other chemicals, because they are energetic and easy to derivatize. Novel methods for CO2 recycling that aim to compete with petrochemistry require new processes that combine both reduction of CO2 and formation of C—C, C—N, and C—O bonds to enlarge the spectrum of compounds directly available from CO2.
Ideally, these “diagonal transformations” are catalytic and proceed in a single step to ensure energy economy and a positive carbon balance; yet, viable examples remain scarce and lack generality because the functionalization is induced by the reductant itself. In this context, we have investigated three-component systems where CO2 is reacted, in a single step, with a functionalization reagent and a reducing reagent which can be modified independently. Herein we report novel diagonal transformations for CO2 recycling, including an unprecedented organocatalytic synthesis of formamides from CO2.—Das Neves Gomes et al.
In the diagonal method, the carbon dioxide is both reduced and functionalized in one step. This allows the synthesis of a much greater number of chemicals, directly from CO2. This reaction requires three things:
- a reducing agent (e.g. a silane);
- an organic molecule to be attached to the carbon atom of the carbon dioxide (e.g. an amine); and
- a special catalyst that catalyzes both the reduction and the functionalization.
The successful catalyst is a special organic base consisting of a nitrogen-containing ring system. Cantat says that varying the reaction partners should allow the manufacture of a whole series of chemical compounds that are normally obtained from petrochemical feedstocks, such as formamide derivatives, which are important intermediates for both chemical and pharmaceutical industries.
Christophe Das Neves Gomes, Olivier Jacquet, Claude Villiers, Pierre Thuéry, Michel Ephritikhine, and Thibault Cantat (2011) A Diagonal Approach to Chemical Recycling of Carbon Dioxide: Organocatalytic Transformation for the Reductive Functionalization of CO2. Angewandte Chemie International Edition DOI: 10.1002/anie.201105516