A chiral sulfoxide ligand has been used to catalyse the isomerization of an achiral ruthenium complex to produce a chiral-at-metal product.
Asymmetric catalysis very often involves a metal centre coordinated with a chiral ligand. The resulting chiral complex can then be used to catalyse the selective formation of a chiral organic product. Coordination compounds can, however, be 'chiral-at-metal', in which the coordination geometry of achiral ligands still results in the formation of a chiral complex. Such complexes can be equally useful in asymmetric catalysis applications, but methods for their selective formation are rare. Now, Eric Meggers and co-workers, from Philipps-Universität Marburg in Germany, have developed a method1 that uses a chiral sulfoxide to catalyse the selective formation of a ruthenium complex.
Meggers and co-workers started with an achiral ruthenium complex, trans-[Ru(bpy)2(MeCN)2]2+. They envisaged that an exchange of the two acetonitrile (MeCN) ligands with a chelating ligand would require an isomerization of the bipyridine (bpy) coordination. The use of a chiral sulfoxide ligand to do this means that the isomerization happens in a stereoselective fashion to result in a diastereomerically enriched — one stereocentre at the sulfoxide, and one at the ruthenium — coordination complex. Next, by adding further bipyridine and acid, the sulfoxide ligand could be removed. This exchange occurs without any isomerization and produces a chiral complex with a single stereocentre at the ruthenium atom.
Finally, reaction conditions were identified that allowed the use of a catalytic quantity (20 mol%) of the chiral sulfoxide. The process therefore uses a small organic molecule to catalyse the formation of a chiral octahedral complex — the opposite of the typical situation seen in asymmetric catalysis.
Gong, L., Lin, Z., Harms, K. & Meggers, E. Isomerization-induced asymmetric coordination chemistry: from auxiliary control to asymmetric catalysis. Angew. Chem. Int. Ed. 10.1002/anie.201003139 (2010).