Metalloradical Catalysis for Stereoselective Carbene and Nitrene Transfers

南佛罗里达大学的X. Peter Zhang教授获得了美国国家科学基金会化学分部化学催化项目的奖金，他将发展金属催化，并将寻求立体选择性自由基反应的应用。金属催化为控制C中心和n中心自由基的反应活性和选择性提供了一种全新的方法。稳定的金属自由基，如钴（II）卟啉配合物，能够激活重氮试剂和有机叠氮化物，分别以可控和催化的方式产生C中心和n中心自由基，而氮是唯一的副产物。最初形成的C中心自由基和n中心自由基与钴（II）卟啉络合物保持络合，分别称为钴-碳烯自由基和-亚硝基自由基，它们可以进行自由基加成和原子抽离等常见的自由基反应，但卟啉配体环境有效地控制了反应活性和立体选择性。Co- c /Co- n键的低且合适的键解离能是Co（II）基催化羰基和亚硝基转移成功转化的关键因素。通过卟啉配体与可调的电子、空间和手性环境的支持，这种基于Co（II）的金属催化的一般概念将应用于各种自由基过程的发展，包括一些立体选择性的碳和亚硝基转移。金属催化的概念可能对包括碳、亚硝基和自由基化学在内的几个研究领域产生深远的影响。它的建立将进一步促进开壳金属离子与手性配体的不同组合用于各种立体选择性自由基反应的金属催化剂体系的发展，与离子反应相比，它具有许多固有的合成优势。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，因此它也将对环境产生积极的影响。卟啉配体的生物学和医学相关性，以及手性化合物作为药物、风味和芳香化学品、农业化学品和特种材料的良好实际应用，将使计划中的研究成为向公众推广科学、为K-12教师提供教育资源和激发K-12学生科学兴趣的绝佳主题。

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.