Metalloradical Catalysis for Stereoselective Carbene and Nitrene Transfers

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With this award from the Chemical Catalysis Program in the Chemistry Division at the National Science Foundation Professor X. Peter Zhang of the University of South Florida will develop metalloradical catalysis and will seek applications for stereoselective radical reactions. Metalloradical catalysis represents a fundamentally new approach for controlling reactivity and selectivity of radical reactions of both C- and N-centered radicals. Stable metalloradicals such as cobalt(II) porphyrin complexes are capable of activating diazo reagents and organic azides to cleanly generate C- and N-centered radicals, respectively, with nitrogen as the only byproduct in a controlled and catalytic manner. The initially formed C- and N-centered radicals, which remain complexed with cobalt(II) porphyrin complexes and are termed as cobalt-carbene and -nitrene radicals, respectively, can undergo common radical reactions such as radical addition and atom abstraction, but with effective control of reactivity and stereoselectivity by the porphyrin ligand environment. Several factors including the low and appropriate bond dissociation energy of Co-C/Co-N bonds are key for the successful turnover of the Co(II)-based catalytic carbene and nitrene transfers. Through the support of porphyrin ligands with tunable electronic, steric, and chiral environments, this general concept of Co(II)-based metalloradical catalysis will be applied for the development of various radical processes, including a number of stereoselective carbene and nitrene transfers.The concept of metalloradical catalysis may have a far-reaching impact in several areas of research, including carbene, nitrene and radical chemistry. Its establishment will stimulate further development of metalloradical catalyst systems with different combinations of open-shell metal ions and chiral ligands for various stereoselective radical reactions, which have a number of inherent synthetic advantages in comparison with ionic reactions. Since metalloradical catalysis can generate radical intermediates catalytically without the need of stoichiometric initiators such as the commonly employed organotin reagents, it will have a positive environmental impact as well. The biological and medical relevance of porphyrin ligands, together with the well demonstrated practical applications of chiral compounds as pharmaceuticals, flavor and aroma chemicals, agricultural chemicals, and specialty materials, will make the planned study an excellent theme to promote science among the general public, to provide education resources for K-12 teachers, and to stimulate scientific interests of K-12 students.