First-row metal catalysts for controlled radical reactivity

绿色化学以防止污染和减少不可再生资源的消耗为目标，对合成无机化学工作者来说是一个重要的机遇。人们越来越重视开发使用容易获得的过渡金属、在温和条件下操作并使用从可再生资源获得的反应物的催化剂。绿色化学研究的一个关键前沿是发现地球上丰富的第一排过渡金属交叉偶联催化剂，这些催化剂打破了碳氧键或碳氢键，而不是依赖卤化反应物。这个领域通常被认为是困难的，因为它可能涉及自由基化学。与催化有关的第一排金属络合物通常具有不成对的d电子。催化剂容易发生单电子反应，产生有机自由基，特别是在氧化条件下进行的反应。与金属中心结合的有机分子本身可以获得或失去单电子，形成基于配体的自由基。 我的研究计划正在进行的目标是设计控制自由基反应性的第一排金属催化剂。随着催化底物从传统的石化资源转向可再生资源，将需要开发新的方法，在较低的温度下选择性地裂解碳-氧键，而不需要求助于苛刻的试剂。自2000年以来，我的研究小组开发了定义明确的顺磁性铬配合物，用于金属介导的自由基反应。在本研究提案所涵盖的时期内，这项工作将扩展到其他第一排金属催化剂，特别是镍和钛。 与较重的第二和第三排过渡金属同系物相比，第一排金属催化剂的独特和有用的反应活性往往是由于金属介导的自由基反应的参与。尽管人们越来越广泛地认识到这一事实，但积极研究金属介导的自由基活性用于有机合成催化的无机研究小组仍然相对较少。最近对绿色化学的关注似乎可能会推动富含地球的第一排金属催化剂的受欢迎程度继续上升。对于依赖于一种以上金属催化剂的串联并行催化系统，或者与光驱动的光氧化还原催化剂结合运行的串联并行催化系统，了解如何控制和利用它们的自由基反应倾向尤为关键。

With its goals of preventing pollution and reducing consumption of nonrenewable resources, green chemistry represents an important opportunity for synthetic inorganic chemists. There is a growing emphasis on developing catalysts that use readily-available transition metals, operate under mild conditions, and use reactants obtained from renewable resources. A critical frontier of green chemistry research is to discover earth-abundant first-row transition metal cross-coupling catalysts that break carbon-oxygen or carbon-hydrogen bonds instead of relying on halogenated reactants. This field is generally perceived as difficult due to the potential involvement of radical chemistry. The first-row metal complexes relevant to catalysis often have unpaired d-electrons. The catalysts are prone to single-electron reactions that can generate organic radicals, especially for reactions conducted under oxidative conditions. The organic molecules bound to the metal centers can themselves gain or lose single electrons, forming ligand-based radicals. The ongoing objective of my research program is to design first-row metal catalysts that control radical reactivity. As catalytic substrates shift from traditional petrochemical sources to renewable resources, new methods will need to be developed to selectively cleave carbon-oxygen bonds at lower temperatures without resorting to harsh reagents. Since 2000, my research group has developed well-defined paramagnetic chromium complexes for metal-mediated radical reactivity. In the period covered by this research proposal, this work will be extended to other first-row metal catalysts, particularly nickel and titanium. The involvement of metal-mediated radical reactivity is often responsible for the distinctive and useful reactivity of first-row metal catalysts compared to their heavier second- and third-row transition metal congeners. Despite the growing and wide-spread recognition of this fact, there are still relatively few inorganic research groups that are actively investigating metal-mediated radical reactivity for catalysis for organic synthesis. The recent focus on green chemistry seems likely to fuel the continuing increase in popularity of earth-abundant first-row metal catalysts. Understanding how their propensity for radical reactivity can be controlled and exploited is particularly crucial for tandem concurrent catalytic systems that rely on more than one metal catalyst, or that operate in combination with light-driven photoredox catalysts.