The Diels–Alder reaction,also known as the diene addition reaction, was first discovered and documented in 1928 by the German chemist Otto Paul Hermann Diels and his student Kurt Alder, for which they won the 1950 Nobel Prize in Chemistry. The Diels-Alder reaction is a cycloadgetic reaction in which conjugated dienes react with substituted olefins to form a new ring (hexa-membered ring) composed of six carbon atoms without the need to add or remove any atoms. The reaction can take place even if some of the atoms in the newly formed ring are not carbon atoms. This reaction is one of the most important C-C bond formation methods in organic chemical synthesis reactions, and it is also one of the reactions commonly used in modern organic synthesis. But no scientist has ever implemented the Dills-Alder reaction on the surface of the material, so no one has directly observed its reaction process.
Figure 1 (a) The Dills-Alder reaction, and (b) a more complex Dills-Alder reaction that can be visualized using atomic force microscopy
Recently, scientists have implemented the Dills-Alder reaction on the surface of the material and imaged each stage during the reaction (Reference 1). Diego Peña from the University of Santiago de Compostela in Spain and Leo Gross's group at IBM Research in Zurich, Switzerland, cleverly implemented a relatively complex reaction in the Dills-Alder reaction on the surface of the material (Figure 1(b)). Instead of using three double bonds for cyclic exchange, this reaction uses three triple bonds—a hexadehydrodils-Alder reaction, which has the advantage of producing a flat and highly reactive aromatic hydrocarbon product. To ensure that both reactants were in the correct position, they also chose a special setup that was already part of the same molecule. Interestingly, the molecules of this setup were well studied in solution a few years ago by Yoshito Tobe and his colleagues (Reference 2). Experimental results show that when molecules bound to the surface at high vacuum and low temperature are heated to a higher temperature, they will undergo a Dills-Alder reaction on the surface, and then cooled again and can be studied by scanning probe microscopy.
Figure 2 Study the hexadehydrogen Diels-Alder reaction on a surface using atomic force microscopy
Harry Anderson of the University of Oxford praised the experimental design: "One of the clever things about this work was finding a molecule in which the Dills-Alder reaction within the molecule accelerates by strain, but it is stable enough to sublimate to the surface without triggering the reaction."
The researchers were able to dissect several intermediates and reaction steps from the pathways of reactive aromatic hydrocarbons, which are useful in the field of surface and atomic manipulation chemistry. However, for many fans obsessed with the simplicity of the classic Diels-Alder reaction, it has yet to provide a window into how it works. Michael Gottfried of the University of Marburg in Germany believes that "while this paper is an important step towards using the Diels-Alder reaction in surface synthesis, it remains to be seen how the normal Diels-Alder reaction between two different molecules will proceed on the surface." ”
bibliography:
[1] Castro-Esteban et al, Angew. Chem., Int. Ed., 2021, 60, 26346.
[2] S Nobusue et al, Org. Lett., 2014, 4 p.m., 1940.