Shahab Mortezaei, Noelle R. Catarineu, and James W. Canary J. Am. Chem. Soc.
Various new catalysis paradigms are being developed in modern chemistry, such as cooperative catalysis, cascade catalysis or proximity catalysis. However, new work presented by Canary and co-workers features a new concept: templating of organocatalytic ligands by a metal centre, generating a rigid, chiral complex capable of carrying out asymmetric catalysis. The most remarkable feature of this catalytic complex is the ability to switch the enantioselectivity of the catalyst by altering the oxidation state of the metal.
Previous work from the group has revealed a copper ligand complex that switches ‘handedness’ of pseudo-helical ligand conformation upon one electron transfer to or from the metal centre; alternating between Cu(II) (right handed) and Cu(I) (left handed). The ligands are based on L-methioninol and the handedness of the structures is derived from the differing affinity of the copper for O– and S-ligands in different oxidation states.
In this piece of work, the ligands are further functionalised with organocatalytic urea moieties. The group show the ligand to be competent in catalysing the Michael addition of malonate to a nitroalkene in the absence of the copper. Without copper there is no selectivity, but complexes of the ligand with either Cu(I) or Cu(II) result in clear selectivity for R or S respectively.
When the Cu(II) catalyst is used in the reaction, isolated, reduced with ascorbate to Cu(I) and resubmitted to reaction conditions the yields and ee are consistent but the selectivity reversed. Whilst neither the yields nor selectivities are outstanding as yet, the concept opens new possibilities for developments in tuneable catalysis.