|
A collaboration between researchers in Canada and Korea has demonstrated, using NMR spectroscopy and circular dichroism, and other techniques, the first stereoselective catalyst in which handedness has been locked in following imprinting. This chiral memory effect could be exploited in the development of new catalysts for making enantiomerically pure compounds, such as starting materials for drug, agrochemicals, and polymers. Jik Chin and colleagues at the University of Toronto, Canada, working with Jong-In Hong's team at Seoul National University, Korea to synthesise a cobalt(III) complex of the ligand "salen". Salen is a commonly used ligand in organometallic chemistry. It is a Schiff base formed from a two to one reaction of derivatives of salicylaldehyde and ethylene diamine. Complexes of this ligand are very effective catalysts for a wide range of reactions including epoxidation of alkenes. However, according to Chin and colleagues locking in chirality into a catalyst for stereoselective reactions in which only one of two possible chiral forms of the product is obtained has proved difficult. Now, they have prepared a cobalt-salen complex that forms initially as a racemic, or 50:50, mixture of its left and right handed forms. This compound can exist in two distinct handed forms in which only one phenolic group at a time can bond to the cobalt metal at the compound's centre. NMR shows four distinct peaks due to tertiary-butyl groups affected by this switching of the phenolic bonding to the cobalt. However, this alone is not enough to produce a locked-in stereoselective catalyst because the ligand flips slowly between left and right forms, according to the NMR analysis. However, the team has found that adding another small ligand, aziridine, to the complex can result in the formation of a 2:1 excess of one chiral form in equilibrium as it destabilises the formation of one phenol-cobalt bond in favour of the other. The team then investigated the nature of metal centre using circular dichroism and obtain a strong signal associated with the chirality of the guest ligand that was not observed in the CD of any of the species alone. They suggest that the cobalt complex acts as a reporter or sensor of the chirality of the guest generating or amplifying the CD signal so that it can be observed. Swapping the coordinated chiral amine with an achiral amine, benzylamine does not affect the CD results. This the team explains hints that chirality has been locked into their salen catalyst by the aziridine and is not lost by reacting it with an achiral compound. The team has tested the new chirally locked catalyst in the ring-opening reaction on styrene oxide with N-methylaniline. A product of high chiral purity is obtained, as measured by proton NMR, moreover the unreacted styrene is itself chirally enriched, making it more useful than the racemic starting material for other reactions. Analogous reactions with amino acids can also be used to separate racemic amino acids in to their chiral forms a key step in many drug manufacturing processes. Related links: |
.jpg) Chin, on the one hand, can lock chirality into a catalyst
.jpg) Locked up stereoselective cobalt catalyst |
