Achiral or racemic molecules with imprintable chiral memory can be transformed into stereoselective catalysts, say scientists. 

Jik Chin at the University of Toronto, Canada, and Jong-In Hong at Seoul National University, Korea, and colleagues have developed a salen based Co(III) complex ±1 with imprintable chiral memory that can be locked in and used for stereoselective catalysis. 

So how does this molecular system remember chirality? The starting cobalt complex is chiral, existing as two enantiomers present in equal amounts (a racemate). When a chiral aziridine was added it coordinated to the metal complex and formed two optical isomers. Initially there were equal amounts of both isomers but this equilibrium shifted as one became more stable than the other. The coordinated chiral amine was then replaced with an achiral amine, benzylamine. Despite the amine being achiral the circular dichroism spectrum (a spectroscopic method used for characterising chiral compounds) changed little after the replacement suggesting that chiral memory has been imprinted on the metal complex with the chiral azididine. 

However, this memory may be short lived or destroyed by heating and a lasting chiral memory is needed if the imprinted complex is to make a good stereoselective catalyst. Using a chiral amino acid as the imprinting agent, Jik Chin and co-workers, were able to lock in this chiral memory by acylating the free phenolic group of the imprinted complex.

'Once imprinted, the imprinted receptor can be used to separate large amounts of amino acids into left and right handed ones,' said Chin. He added, 'in general, it is difficult and expensive to make handed receptors that bind only left or right handed amino acids. By using the technique of imprinting and locking, handed receptors can be made easily and economically.'  

Sarah Corcoran