Mid-valent chromium reagents for controlled radical reactivity

过渡金属的催化使用已经不可逆转地改变了化学家形成碳-碳键的方式，应用于从药物合成到先进材料的制备。在过去的五年中，越来越多的C-C键形成反应被发现涉及碳基自由基，这是一种具有未配对电子的高活性物种。其中许多过程涉及已知能够容纳未配对电子的元素(例如第一行过渡金属、钛-铜)和/或配体(例如二亚胺)。然而，最佳催化活性与活性物种的电子结构之间的关系目前还远不清楚。 铬络合物捕获碳基自由基的无与伦比的能力已经被认识到几十年了。目前Smith集团的研究集中在开发能够可逆的Cr-R键均解的定义良好的铬配合物。我们最近展示了使用这种催化剂来控制具有高度挑战性的底物的自由基反应活性，包括乙酸乙烯酯的聚合和氯缩醛的催化环化和还原。这项建议中描述的未来研究探索使用基于配体的自由基来处理铬以及硼和镁试剂特有的自由基反应性。

The catalytic use of transition metals has irrevocably changed the way chemists form carbon-carbon bonds, for applications from the synthesis of pharmaceuticals to the preparation of advanced materials. In the past five years, an increasing number of C-C bond forming reactions have been found to involve carbon-based radicals, highly reactive species that possess an unpaired electron. Many of these processes involve elements (e.g. first row transition metals, Ti-Cu) and/or ligands (e.g. diimines) that are known to be able to accommodate unpaired electrons. However, the relationship between optimal catalytic activity and the electronic structure of the reactive species is presently far from clear. The unmatched ability of chromium complexes to trap carbon-based radicals has been recognized for decades. Current research in the Smith group is focussed on the development of well-defined chromium complexes capable of reversible Cr-R bond homolysis. We have recently demonstrated the use of such catalysts to control the radical reactivity of highly challenging substrates, including the polymerization of vinyl acetate and the catalytic cyclization and reduction of chloro acetals. Future studies described in this proposal explore the use of ligand-based radicals to work with the characteristic radical reactivity of chromium, as well boron and magnesium reagents.