“Chemistry” often has a bad image. On closer inspection, however, a modern life without chemical products is inconceivable. From skin creams to fragrances in deodorants, solar cells on the roof, plastics for furniture and vehicles, batteries for cell phones to life-saving medication. We owe all of this to chemical reactions, many of which do not take place spontaneously, but have to be initiated. This is done by means of catalysts – substances that actually cause ingredients that are unwilling to react to form a compound after all.
The importance of catalysis is quickly underestimated. In fact, according to the Nobel Prize Committee, it is associated with around a third of global economic output. Now the committee has decided to honor two pioneers in this field with the highest distinction. The one with converted around 980 000 Euro endowed Nobel Prize in Chemistry goes in equal parts to Benjamin List from the Max Planck Institute for Coal Research in Mülheim der Ruhr and David MacMillan from Princeton University in the USA.
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Small and organic instead of large or toxic
Both discovered before good 20 years independently of each other a new class of catalysts with which chemical processes run faster and more precisely. They are based on specially structured organic molecules, which is why the principle is called organocatalysis. It has a great influence, especially on pharmaceutical research, and makes chemistry overall “greener”, argues the committee.
Up until the breakthroughs by List, MacMillan and their teams, two types of catalysts dominated the chemistry. On the one hand, there are metals. They can take in or give up electrons and are therefore good “matchmakers”. But quite a few of these catalysts are very sensitive and require an environment without oxygen or moisture. This can be managed in the laboratory, but it is more difficult in large industrial plants. In addition, many catalysts are environmentally hazardous heavy metals.
The second group are enzymes, compounds made up of numerous amino acids. They are mainly used for asymmetric catalysis. The point here is to create molecules with a specific shape. Many organic molecules come in two different forms, in which the number and type of atoms are the same, but their spatial arrangement is a mirror image. Like the two hands of a person, each with four fingers and a thumb, but not congruent.
Mirror images with different effects
This “handedness” of two molecules significantly influences their effect. While one expression brings a desired effect, the other can be useless or even harmful. Thalidomide, which was sold under the brand name Contergan, achieved sad fame. In one variant, the molecule had a calming effect and was therefore prescribed as a sleep aid. The mirror-image variant, which was also created during production, caused severe malformations in embryos.
It is therefore very important to only produce the desired molecules in production. Enzymes are good at this, but sometimes there are huge molecules made up of hundreds of amino acids that have to be laboriously produced. That gave Benjamin List an idea. He was at the end of the 1952 at the Scripps Research Institute in La Jolla (California) and worked on catalytic antibodies. He asked himself: Do the amino acids have to be built into these giant molecules in order to have their effect? Or can they do the same thing as independent molecules?
List remembered that it was in the 1970 there were attempts to use an amino acid called proline (see picture) as a catalyst. Things had petered out and List tried again, this time with aldol reactions. The aim is to connect carbon atoms from two different molecules with one another. And it worked great. Even more: The product did not produce two mirror-image molecules in equal parts, but rather one type.
List recognized the potential that proline offered. Compared to metals or enzymes, this simple molecule is a “dream tool,” as the Nobel Prize Committee says. “It’s simple, cheap and environmentally friendly.” And, as the following years have shown, it is just one of many organic molecules that considerably simplify asymmetric catalysis.
It’s not List’s sole credit. At the same time, David MacMillan, just a little further north at the University of California at Berkeley, was working on a similar problem. He had initially researched metallic catalysts and was frustrated. Hardly anyone in industry wanted them because they were so sensitive and expensive. MacMillan began to design organic molecules that – like metals – can accept or donate electrons. This was achieved with the help of a nitrogen atom that was inserted into the carbon-dominated molecule.
The researcher tested this catalyst for the production of carbon rings. Also with great success. MacMillan was certain that there had to be many more organic catalysts and therefore coined the term “organocatalysis”.
It was a “total surprise” and “a milestone” in the knowledge that that catalysis can be done without metals and thus chemical transformations are possible that result in indispensable substances, such as drugs, says Matthias Drieß from the TU Berlin and one of the heads of the Unified Systems in Catalysis Cluster of Excellence (UniSysCat). “And it could also be a key to understanding how life came about.”
At that time, simple organic catalysts such as proline were certainly important. “But that is of course speculative,” says Drieß. “We don’t know what role metals and which organic catalysts played back then when the earth’s atmosphere was completely different and there was hardly any oxygen.”
“An abundance of Reactions and processes ”
The Nobel Prize Committee uses strychnine as an example to calculate how efficient organocatalysis is. Known to most as a bad poison, for chemists it is a kind of Rubik’s cube: How can it be produced in as few steps as possible? 1952, when it was first synthesized, were for 29 chemical reactions necessary and only 0, 0009 percent of the starting material has become strychnine. The rest was garbage. 2011 thanks to organocatalysis only 12 Steps necessary and the process optimized to such an extent that the production is mathematically 7000 – was times more efficient than 1952.
When talking to Stockholm during the press conference, award winner List went one better. The early catalysts were “about a million times less efficient” than the ones available today, he explains with a noticeable euphoria. “The real revolution is only happening now because we have these extremely reactive organocatalysts that can do things that are impossible with enzymes or even the best metal complexes.”
“What we need now , is above all a more sustainable chemistry, ”says TU researcher Drieß. “We can control a multitude of reactions and processes”. But in order to convert carbon dioxide into valuable substances, for example, chains of reactions are necessary that have to be coupled with one another. “In a nerve cell, millions and millions of chemical reactions take place in a coordinated manner. We have to learn that too,” he says. Coupling many catalytic reactions so that in the end they produce what is desired. “How the director of a film arranges the actors in the many scenes in such a way that something useful emerges in the end.”