[ad_1]
Even atoms look for a stable bond, the carbon atom even after four. In the form of so-called carbene molecules, however, it has only two bonds and is therefore very reactive – at least as long as no solvent blocks the free binding site. New insights into the reactivity of carbenes as catalysts are now providing computer simulations for Bonn researchers. They can help further improve the efficiency of catalyzed reactions.
Bonn carbenes are highly reactive compounds. This is due to one of their building blocks: a carbon atom – or more precisely, how that atom is integrated into the carbene. A single carbon atom has four so-called "free" electrons. With them, it can usually make four bonds to other atoms, which is usually the case. In Carbenen, however, this sociability is partially unused: only two of the four electrons are involved in links. What remains is a solitary couple who likes to grab a reagent when the opportunity arises
The problem with this: The carbenes are very attractive to many solvents. Their only pair of electrons ensures that they are locally negatively charged. On the other hand, solvents such as water or alcohol contain hydrogen, which is positively charged locally. Due to the electrostatic interaction, he feels attracted to the carbon electrons. The solvent molecule and the carbene thus stick together as two magnets.
Hurried as a high-level footballer
"This mechanism makes it difficult for other molecules to react with carbene," says Dr. Oldamur Hollóczki of the Mulliken Center for Theoretical Chemistry of the University of Bonn. One could compare the situation of the Carbens to that of a midfielder star: as he is always stuck on the heels of an opposing player, he lacks room to beat his fearsome pbadports.
For example, the footballer tries to free himself from this shadow with one or two hooks. The same thing could happen in the world of molecules: "It may be that the hydrogen bond between the carbene and the solvent is broken, leaving enough room for the ruling molecule," suspects Hollóczkis employee Sascha Gehrke. . "However, in our simulation, we were able to show that there was another way."
Triangular relationship instead of exchange of partners
As the researchers' calculations show, some carbenes form particularly strong hydrogen bonds because of their chemical structure. The hydrogen atom of the solvent molecule sticks so tightly to the free carbene electrons that it does not dissolve easily.
In such cases, however, a special intermediate structure may form: in this case, the pair of carbon electrons not only comes into contact with the solvent but also with the reaction partner. Thus two hydrogen bonds are formed, both derived from carbene carbon: one towards the solvent, the other towards the reaction partner
The results allow an improved catalysis
This result is also interesting because it offers new possibilities, the catalyzed reaction influence. So some solvents are too large for this triangular relationship to form. In them, the reaction therefore proceeds much more slowly or even only with heat. Conversely, the results can be used to allow catalysis to occur even at low temperatures. The synthesis would be more sustainable. "Since carbenes play an important role in many areas of organic synthesis, including in the pharmaceutical industry, this point is naturally of immense importance," says Hollóczki.
Up to now, the unusual triangular structure exists only through molecular dynamics software. However, scientists now plan to produce and precisely characterize them.
Original Publication: Sascha Gehrke and Oldamur Hollóczki: Hydrogen Bonding of N-Heterocyclic Carbenes in Solution: Solvent Reorganization Mechanisms. Chemistry – A European newspaper; DOI: 10.1002 / chem.201802286
[ad_2]
Source link
