NHC-Catalyzed Radical Prozesses

N-杂环卡宾(NHCs)作为过渡金属的配体和有机催化领域的催化剂，在有机化学中得到了广泛的应用。考虑到有机催化，在大多数情况下，这些转化通过两个电子路径(闭合壳层反应中间体)以离子模式进行。因此，通过离子键形成的NHC有机催化作用也得到了很好的证实。然而，基于NHC的自由基化学，即sigma键的形成是在自由基过程中进行的，到目前为止还没有得到很好的探索。在本项目中，将研究涉及自由基中间体的协同卡宾/光氧化还原催化。当活化的酸衍生物与NHC反应时，容易产生酰氮唑的单电子还原(SET)，从而产生持久的酮基自由基。这类酮基自由基可以与伴随氧化生成的C-自由基发生自由基/自由基交叉偶联反应，从而形成一种新的合成酮的方法。自由基前驱体的SET氧化和ACZ的SET还原将在光氧化还原催化剂上进行，整个转化过程代表氧化还原中性过程。选择性耦合受持久自由基效应(PRE)控制。在这次酮的合成中，将测试各种C-自由基前驱体。除了这种两组分酮的合成，还将研究三组分过程，其中氧化生成的自由基首先加入到烯烃中，然后被acylazolium衍生的酮自由基捕获。氧化生成的以杂原子为中心的自由基也将包括在这些研究中。最后，将尝试控制自由基/自由基耦合过程的立体选择性。

N-Heterocyclic carbenes (NHCs) have been intensively used in organic chemistry either as ligands for transition metals or as catalysts in the field of organocatalysis. Considering organocatalysis, in most cases these transformations proceed in an ionic mode via two-electron pathways (closed shell reactive intermediates). Hence, NHC organocatalysis operating via ionic bond formation is meanwhile very well established. However, NHC-based radical chemistry, where sigma-bond formation proceeds in a radical process, is not well explored to date. In this project, cooperative carbene/photoredox catalysis involving radical intermediates will be investigated. The single electron reduction (SET) of an acylazolium, readily generated upon reaction of an activated acid derivative with an NHC, leads to a persistent ketyl radical. Such ketyl radicals can engage in radical/radical cross coupling reactions with concomitantly oxidatively generated C-radicals, resulting in a novel approach towards ketones. SET-oxidation of the radical precursor and SET-reduction of the acylazolium will be conducted with a photoredox catalyst and the overall transformation represents a redox neutral process. The selective coupling is controlled by the Persistent Radical Effect (PRE). Various C-radical precursors will be tested in this ketone synthesis. Along with such two-component ketone syntheses, three-component processes will be studied as well, where the oxidatively generated radical first adds to an alkene prior to being trapped by the acylazolium-derived ketyl radical. Oxidatively generated heteroatom-centered radicals will also be included in these investigations. Finally, control of the stereoselectivity of the radical/radical coupling processes will be attempted.