In this project funded by the Chemical Catalysis program of the Chemistry Division, Professor Peter Zhang of the University of South Florida applies the concept of metalloradical catalysis for the development of new catalytic systems based on cobalt(II) complexes of porphyrins to control the reactivity and selectivity of radical reactions. Since metalloradical catalysis can generate radical intermediates catalytically without the need for stoichiometric initiators such as the commonly employed organotin reagents, the method will have a positive environmental impact. In comparison with the typical need for rare and expensive second- and third-row metal ions to catalyze two-electron ionic reactions, one-electron radical reactions can be carried out effectively with catalysts based on earth-abundant and inexpensive first-row transition metal ions such as cobalt. As such, the development of metalloradical catalysis addresses the increasing need for sustainable catalysts. The proposed project by Professor Zhang integrates research and educational activities. In addition to scientific training for graduate and postdoctoral students, laboratory experiences for high school students and teachers, as well as undergraduate students, are provided.
These new catalytic radical processes, which are both mechanistically distinctive and operationally attractive, serve as valuable synthetic tools for constructing organic compounds, such as biologically important natural products and pharmaceutically interesting small molecules. On the basis of the fundamental understanding of the catalytic mechanisms of Co(II)-based radical cyclopropanation and aziridination reactions, new catalytic processes that involve a cascade sequence of radical reactions are in development to allow for the efficient construction of carbocycles and heterocycles beyond three-membered cyclopropanes and aziridines. Through the design and synthesis of chiral porphyrins possessing potential hydrogen bonding capability and with tunable electronic, steric, and chiral environments as the supporting ligands, this project aims to achieve several specific types of Co(II)-catalyzed radical cascade processes. The success of this project may prompt the identification of new metalloradical catalyst systems based on different combinations of open-shell metal ions and chiral ligands that can catalyze a broad range of stereoselective radical reactions.
