Chemical Science Blog

Scientists report the first example of Brønsted acid asymmetric catalysis in aqueous solution.

Water is an attractive reaction medium as it is cheap, clean, non-toxic and non-flammable. Also, its high heat capacity makes it ideally suited to exothermic reactions on an industrial scale. There are a number of reports of metal catalysed asymmetric reactions in the presence of water, as well as organocatalytic reactions involving covalent bonding, but until now a non-covalent, asymmetric organocatalytic reaction has remained elusive.

Magnus Rueping and Thomas Theissmann at Aachen University, Germany, performed enantioselective hydrogenation of quinolines using a chiral phosphoric acid catalyst in water. Phosphoric acid forms a hydrogen bond with the quinoline, and directs the dihydropridine hydride donor to a particular face.

This hydrogen-bonding catalysis occurs despite the fact that water is an excellent hydrogen donor, due to the phenomenon of ‘hydrophobic hydration’. Interaction between water molecules at the hydrophobic-hydrophilic interface causes the contact surface between water and substrate molecules to be minimised, reducing the possibility for water to participate in hydrogen-bonding, explains Rueping. The selectivity of the catalyst was further improved by adding bulky organic side chains that create a hydrophobic pocket for the substrate.

A hydrophobic site is created allowing catalysis to take place

Rueping says, ‘Non covalent asymmetric activation in aqueous solution has been considered impossible due to fast proton transfer in protic media. Our solution based on the principle of hydrophobic interaction allowed us to develop a Brønsted acid catalysed reaction in aqueous solution that provides the products in good yields and with excellent enantioselectivities.’

Peter Dalko from the Paris Descartes University in France says, ‘the discovery of the efficient reaction conditions is only part of the cake, since the rational behind the observed selectivity is also worth reflection. Hydrophobic interactions are well known in enzymology and are evoked to explain stereoselectivity in many enzymatic transformations. This concept is now emerging in chemistry as a major paradigm.’

Rueping is confident that it could be carried out on a larger scale. ‘Typically we use one equivalent of the dihydropyridine, but for large scale processes catalytic amounts of hydride donor would be used. Given that recycling of dihydroypyridines in water is already possible, the new Brønsted acid catalysed process in aqueous solution paves the way for large scale application.’

Jacob Bush

Read the full Chemical Science Edge article

Only a single metal centre is needed to catalyse the reduction of oxygen to produce water, opening the door to more efficient fuel cells in the future, say researchers in the US.

Converting solar energy to chemical energy using solar fuel cells and releasing stored energy from hydrogen fuel cells involves two key multielectron redox reactions – oxidising water to evolve oxygen and the reverse, reducing oxygen to water. It is the second reaction that limits the application of hydrogen fuel cells at the moment, as it generally requires expensive metal catalysts, such as platinum.

Cobalt porphyrin catalyst could improve fuel cell technology

Nature achieves similar results in many different catalytic systems using metalloenzymes that contain bi- or multimetallic reaction sites, which has provided inspiration for development of bimetallic porphyrin catalysts. Now Daniel Nocera and colleagues at Massachusetts Institute of Technology have shown for the first time that single centre cobalt porphyrins anchored on carbon nanotubes can efficiently catalyse the reduction of oxygen, as long as they also contain a proton transfer group.

The positioning of the proton transfer group – in this case a carboxylic acid – the correct distance away from the cobalt is essential to stops the catalyst from partially reducing the oxygen, which is often a key problem in maintaining the efficiency of these reactions, explains Nocera.

Nocera’s porphyrins are much more efficient than existing cobalt catalysts and are made easily in two steps, so could invigorate the design of future fuel cells using cobalt over its more costly metal cousins.

Minhua Shao, an expert in fuel cell technologies at UTC Power in the US, believes that the results are ‘important to guide the design and development of non-precious metal electrocatalysts for oxygen reduction reaction in fuel cells’.

This is something Nocera is keen to develop, saying that he is now ‘focusing on improving the catalysts by lowering the amount of energy needed for the reaction’.