Reductive eliminations – trends across the d block

在过渡金属催化的C−C、C−CF3和C−F偶联反应中，还原消除是产物形成的步骤。这些转化在有机合成中的价值和潜力是真正突出的。为了以合理的方式优化这些反应，通常需要对操作催化循环，特别是还原消除的基本步骤进行详细的机理分析。除了钯和铂介导的反应，这种洞察力在很大程度上是缺失的。这种缺乏了解的原因是，大多数相关的过渡金属络合物不稳定，这阻碍了它们的分离和深入研究。该项目旨在通过探测分离在气相中的离子络合物的单分子反应性来绕过这个问题。在以前的工作中，申请人已经证明了电喷雾电离在将完整的有机金属离子从溶液转移到气相中的适用性，在气相中，质量选择确保了明确的反应物离子群。这些离子的碰撞活化和由此产生的碎片离子的质谱学检测提供了关于反应物离子经历还原消除或竞争过程的趋势以及这些反应的相对障碍的详细信息。该项目将包括中、晚3d金属以及选定的晚4d和5d金属的络合物，并确定控制它们单分子反应性的趋势。同时，系统考察了络合物的总电荷、聚集态、金属中心的氧化态、配体的配位以及金属结合的有机取代基的电子和空间性质对还原消除的影响。此外，这些实验还将得到量子化学计算的补充，以获得额外的能量和结构信息。因此，这些研究将极大地促进我们对还原消除的基本理解，并有望为改进实际重要的C−C、C−CF3和C−F偶联反应提供指导。

Reductive eliminations constitute the product-forming step in transition-metal catalyzed C−C, C−CF3, and C−F coupling reactions. The value and potential of these transformations for organic synthesis are truly outstanding. To optimize these reactions in a rational manner, detailed mechanistic insight into the operating catalytic cycles in general and the elementary step of the reductive elimination in particular is needed. Except for the case of palladium- and platinum-mediated reactions, such insight is largely missing. This lack of understanding results from the instability of most of the relevant transition-metal complexes, which has prevented their isolation and in-depth examination. The project aims at circumventing this problem by probing the unimolecular reactivity of ionic complexes isolated in the gas phase. In previous work, the applicant has demonstrated the suitability of electrospray ionization for transferring intact organometallic ions from solution into the gas phase, where mass selection ensures a well-defined population of reactant ions. Collisional activation of these ions and mass-spectrometric detection of the resulting fragment ions afford detailed information on the tendencies of the reactant ions to undergo reductive eliminations or competing processes and the relative barriers of these reactions. The project will include complexes of the medium and late 3d metals as well as selected late 4d and 5d metals and determine the trends that control their unimolecular reactivity. At the same time, it will systematically examine how the overall charge of the complexes, their aggregation state, the oxidation state of the metal center, the coordination of ligands as well as the electronic and steric properties of the metal-bound organyl substituents affect the reductive elimination. The experiments will be moreover complemented by quantum-chemical calculations to obtain additional energetic and structural information. Thus, the studies will greatly advance our fundamental understanding of reductive eliminations and promise to provide guidelines for improving practically important C−C, C−CF3, and C−F coupling reactions.