Chiral porous solids converted into catalysts could provide a versatile new way to trigger reactions, say chemists in the UK.
University of Liverpool chemists, Matthew Rosseinsky and co-workers, have developed a way to install catalytic activity within the pores of crystalline solids called metal organic frameworks (MOFs). Rosseinsky made a copper-aspartate based MOF, where the inherent chirality of aspartate (an amino acid) ensures a chiral environment within the solid's pores. To convert the MOF into a chiral catalyst, Rosseinsky treated it with acid, protonating the aspartate to give a catalytically active form.
The chiral nature of the catalytic pores meant the product was formed with a modest enantiomeric excess
'We have been working for several years on the development of chiral crystalline porous solids,' said Rosseinsky. 'Amino acids are attractive chiral components as they are cheap and have a variety of functional groups.'
The reaction used by the Liverpool group to test their catalyst was opening an epoxide with methanol. The chiral nature of the catalytic pores meant the product was formed with a modest enantiomeric excess - one of two possible mirror-image products was formed in slight preference over the other.
'This is a very nice piece of work,' commented Randall Snurr, who reserches MOFs at Northwestern University, Evanston, US. 'On paper, you can imagine a variety of ways to combine chirality and catalysis in MOFs. But in practice, very few groups have achieved it.'
'MOF catalysis has the potential to marry up the best features of homogeneous and heterogeneous catalysts - a solid catalyst that is easy to recover, with well defined, identical catalytic sites,' added Snurr.
In fact, the catalytic groups in the copper-aspartate based MOF are only stable within its pores, and the homogenous phase equivalents cannot be isolated, said Rosseinsky.
'This work is about proof of principle,' added Rosseinsky, 'making the catalytic site in a solid that cannot be made in a homogeneous catalyst. We'll need other examples of chiral porous solids, probably based on amino acids or oligopeptides, to generate sites that have better enantioselectivity. The synthetic possibilities in porous amino acid based materials are extremely broad - we will explore routes to larger pore and multiple metal systems, for example.'
James Mitchell Crow