Mid-valent chromium reagents for controlled radical reactivity

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The catalytic use of transition metals has irrevocably changed the way chemists form carbon-carbon bonds, for applications from the synthesis of pharmaceuticals to the preparation of advanced materials. In the past five years, an increasing number of C-C bond forming reactions have been found to involve carbon-based radicals, highly reactive species that possess an unpaired electron. Many of these processes involve elements (e.g. first row transition metals, Ti-Cu) and/or ligands (e.g. diimines) that are known to be able to accommodate unpaired electrons. However, the relationship between optimal catalytic activity and the electronic structure of the reactive species is presently far from clear. The unmatched ability of chromium complexes to trap carbon-based radicals has been recognized for decades. Current research in the Smith group is focussed on the development of well-defined chromium complexes capable of reversible Cr-R bond homolysis. We have recently demonstrated the use of such catalysts to control the radical reactivity of highly challenging substrates, including the polymerization of vinyl acetate and the catalytic cyclization and reduction of chloro acetals. Future studies described in this proposal explore the use of ligand-based radicals to work with the characteristic radical reactivity of chromium, as well boron and magnesium reagents.