Metalloradical Catalysis for Stereoselective Radical Reactions
X. Peter Zhang
Professor of Chemistry
Host: Marion Emmert, Ph.D.
Organic synthesis has been dominated by chemical reactions that are based on two-electron ionic processes, either stoichiometrically or in catalytic fashion. While one-electron radical chemistry is equally rich and has been demonstrated with a number of unique features, its application in organic synthesis has been hampered by several enduring challenges. Over the past decade, my laboratory has been in the process of formulating “Metalloradical Catalysis” (MRC) as a general concept to guide the development of fundamentally new approaches for controlling both reactivity and stereoselectivity of radical reactions. In essence, metalloradical catalysis aims for the development of metalloradical-based systems for catalytic generation of carbon- and nitrogen-centered radicals from common organic compounds without the need of radical initiators or the use of light. The subsequent reactions of the resulting organic radical intermediates, which remain covalently bonded to the metal center, can be selectively controlled by the catalyst. For achieving enantioselective radical reactions via MRC, we have developed a family of unique chiral metalloradical catalysts based on structurally well-defined Co(II) complexes of D2-symmetric chiral porphyrins with tunable electronic, steric, and chiral environments. These Co(II)-based metalloradical catalysts have been shown to be highly effective for a wide range of stereoselective organic reactions, including olefin cyclopropanation, olefin aziridination, C–H alkylation and C–H amination. Due to their distinctive radical mechanisms that involve unprecedented a-metalloalkyl and a-metalloaminyl radical intermediates, the Co(II)-based metalloradical systems enable addressing some long-standing problems in these important organic transformations.