Metalloradical Catalysis for Selective Radical Reactions

南佛罗里达大学的Peter Zhang教授在化学催化项目的资助下，将金属催化的概念应用于卟啉类钴（II）配合物的新型催化体系的开发，以控制自由基反应的活性和选择性。由于金属催化可以催化生成自由基中间体，而不需要化学计量引发剂，如常用的有机锡试剂，因此该方法将对环境产生积极影响。与通常需要稀有且昂贵的第二和第三行金属离子来催化双电子离子反应相比，基于地球丰富且廉价的第一行过渡金属离子（如钴）的催化剂可以有效地进行单电子自由基反应。因此，金属催化的发展解决了对可持续催化剂日益增长的需求。张教授提出的项目集研究和教学活动于一体。除了为研究生和博士后提供科学培训外，还为高中学生和教师以及本科生提供实验室体验。这些新的催化自由基过程具有机械上的独特性和操作上的吸引力，是构建有机化合物的有价值的合成工具，如生物学上重要的天然产物和药学上有趣的小分子。在对Co（II）基自由基环丙烷化和叠氮化反应的催化机理有了基本认识的基础上，人们正在开发新的催化工艺，包括一系列的自由基反应，以有效地构建三元环丙烷和叠氮嘧啶以外的碳环和杂环。本项目旨在通过设计和合成具有潜在氢键能力的手性卟啉，并以可调的电子、空间和手性环境作为配体作为支撑，实现几种特定类型的Co（II）催化自由基级联过程。这个项目的成功可能会促进基于开壳金属离子和手性配体的不同组合的新的金属催化剂体系的鉴定，这些体系可以催化广泛的立体选择性自由基反应。

AbstractIn 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.