Scientists have discovered a way to lock chirality into catalysts to help make enantiomerically pure compounds.

Chirality can be a big headache for organic chemists. One of their biggest challenges is finding ways of steering reactions so only one enantiomer (either the left or right handed form) of a desired molecule is favoured. This is very important in the pharmaceutical industry, for example, when one enantiomer of a drug has harmful side-effects but the other doesn't.

Jik Chin at the University of Toronto, Canada, and Jong-In Hong at Seoul National University, Korea, and colleagues made a cobalt complex based on the salen ligand, which has two phenol groups. Generally, only one phenolic oxygen at a time can coordinate to the metal's centre, which means the complex exists as two chiral forms.

Initially the complex forms an equal mixture of its left and right handed forms. Adding aziridine, a chiral amine, to the complex resulted in a two to one excess of one chiral form in equilibrium, because it stabilises the formation of one phenol-cobalt bond more than the other. Interestingly the team found that when they replaced this chiral amine with a non-chiral amine, benzylamine, its circular dichroism spectrum was unchanged. This hints that aziridine has imprinted chirality onto the metal complex, which it remembers even after the chiral ligand is replaced with a non-chiral one.

However, the researchers found that this memory tended to be short-lived or destroyed by heating. A lasting chiral memory is needed for the complex be any use as a stereoselective catalyst. So the team locked in the chirality by adding acetic anhydride to acylate the free phenolic group once the imprinting agent had done its work.

Graham Lappin of the University of Notre Dame, Indiana, US, said, 'although these chiral reagents show modest selectivities in the reactions that have been examined, there is huge potential to develop the chemistry in novel and exciting ways.' 

Chin agreed, saying, 'obtaining enzyme like selectivity will be a challenge, but obtaining useful stereoselectivity - greater than 98 percent enantiomeric excess - by imprinting is not beyond reach.'

Sarah Corcoran