Chemists synthesise 'impossible' alcohols
Chemists from the University of Bonn and Columbia University have discovered a novel mechanism in catalysis, allowing the synthesis of certain alcohols in a cheaper and more environmentally friendly way than ever before. Their work has been published in the journal Science.
Alcohols are common chemical compounds which, in addition to carbon and hydrogen, contain at least one OH group. They serve as starting materials for a whole series of chemical syntheses and are often produced directly from olefins by the addition of water (H2O). Olefins are hydrocarbons with a double bond available from oil. The water molecule serves as a ‘donor’ of the OH-group characteristic of alcohols.
This synthesis is simple and efficient, but it can only be used to produce certain alcohols; the so-called ‘Markovnikov alcohols’. The OH group cannot simply be attached to any position of the olefin — one of two positions is excluded.
“We have now found a new catalytic method that can produce exactly these ‘impossible’ alcohols,” said Prof Dr Andreas Gansäuer, from the Kekulé Institute of Organic Chemistry and Biochemistry at the University of Bonn. It was a method that originally emerged in 2013, through collaboration with Prof Dr Jack Norton’s group at Columbia University, but it took almost five years until the synthesis of the so-called ‘anti-Markovnikov alcohol’ using the new catalytic system worked well enough to be published.
The success of the method comes down to the unusual reaction mechanism. Epoxides, common and valuable intermediate products of the chemical industry, serve as starting materials. Epoxides can be produced by adding an oxygen atom (O) to olefins. If they are allowed to react with hydrogen molecules (H2), the oxygen becomes an OH group. Normally, with this approach only Markovnikov alcohols are produced.
“In our reaction, however, we successively transfer the hydrogen in three parts,” said Prof Dr Gansäuer. “First a negatively charged electron, then a neutral hydrogen atom, and finally a positively charged hydrogen ion, a proton. We use two catalysts, one of which contains titanium and the other chromium. This allows us to convert epoxides into anti-Markovnikov alcohols.”
The timing of the entire process must be strictly coordinated — like in juggling, where each ball has to maintain a specified flight duration. To achieve this, the chemists had to synchronise the speed of three catalytic reactions. To this end, they attached the ‘right’ ligands — molecules that control the metals’ reactivity — to the titanium and chromium atoms.
Until now, anti-Markovnikov alcohols have been produced through a so-called hydroboration followed by an oxidation. However, this reaction is relatively complex and not particularly sustainable. The new mechanism, on the other hand, does not produce any by-products and is thus practically waste-free. “Titanium and chromium are also very common metals, unlike many other noble metals that are often used in catalysis,” Prof Dr Gansäuer said.
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