Exploring radical relay catalysis in artificial hemoproteins

Project aim: This project aims to translate homogeneous radical relay catalysis into synthetically useful enzymatic processes. The project will use artificial hemoproteins bearing radical metal centres to drive challenging and highly desirable reactivity - including enantioselective C-H fluorination. By studying the experiments with cutting edge EPR techniques, fundamental insights into the performance of the complex biocatalysts will be gained, enabling further optimisation towards application.
Complexes of first row transition metals (Fe, Co, Mn) are well-known for their ability to catalyse challenging chemical transformations via redox-neutral radical relay mechanisms. Whilst radical relay-type mechanisms are known in nature (e.g. vitamin B12 and FeS clusters) the scope and applicability is vastly limited compared to reactions achievable by homogeneous catalysts. Recently, Fe-centred hemoproteins have been adapted to catalyse new radical relay reactions, with close control over the stereoselectivity (Science, 2021, 374, 1612-1616). In addition to this, the Rowbotham Group has developed routes to hemoprotein scaffolds containing metalloporphyrin cofactors where the native Fe is replaced by metals such as Co and Mn. The cobalt(II) centred-proteins are of particular interest, because the unpaired electron in the dz2 orbital is well-situated for performing controlled radical chemistry (Org. Lett., 2020, 22, 3601-3606). Translation of the catalysis into the enzymatic sphere opens up the possibility of performing directed evolution - thereby enabling unprecedented control over regio- and stereo-selectivity. A range of reactivity will be targeted (including cylcopropanation, aziridation, deuteration and C-H amination), but with particular emphasis on C-H fluorination. Here, preliminary work in the Rowbotham Group has shown that N-fluoroamides can be used as a route to amidyl radicals, which in turn enable selective C-H fluorination to be achieved. This challenging reaction, which is without precedent in Nature, is highly desirable for the clean chemical synthesis of a range of pharmaceutical and agrichemical compounds.
The complex environment of enzyme-driven reactions can make the processes difficult to follow however, and this hinders optimisation and troubleshooting. One method that will enable the proposed Co(II)-catalysed radical relay reactions to be studied in-depth is Electron Paramagnetic Resonance (EPR), a non-invasive spectroscopy method that is sensitive only to centres containing unpaired electrons (for example radicals and metal centres) and for this reason it is insensitive to much of the complexity of biocatalytic reaction conditions. Given that Co(II) porphyrin complexes possess a doublet ground state, arising due to the low-spin d7 configuration imparted by the ligand field, EPR has proven to be a valuable tool for monitoring the reaction mechanism. The EPR spectrum of cobalt is very sensitive to both its oxidation state and the environment and thus measurements of the cobalt EPR spectrum will allow the proportion of cobalt centres in each state to be determined as the reaction progresses. In addition, it may be possible to observe and identify radical reaction intermediates. The Bowen group are specialists in EPR methodology, working in collaboration with the EPSRC funded National Research Facility for EPR hosted in the Photon Science Institute at the University of Manchester.