Bond Activation and Catalysis by Main Group Systems

相对惰性的非极性化学键的活化是产生高附加值化工产品的许多催化功能化过程的关键。在约75%的化学品中，目前有75%的化学品在生产的某个阶段需要催化剂，典型的催化剂以较重的后过渡金属为特征，反映了它们对修饰氧化还原过程的适应性。与这些元素的可持续可获得性/成本以及将重金属纳入产品有关的问题意味着，寻找替代催化平台在科学上和经济上都处于前沿。相比之下，主族金属价格便宜，储量丰富，在较轻的元素(如这里最终目标的锗化合物)的情况下，毒性问题较小。近年来，对主族元素化合物的研究强调了具有空缺配位的低价衍生物的可及性，以及具有相对较小能隙的前线轨道能够促进通过氧化加成激活键。因此，一个典型的后过渡金属反应性的基本模式已经被打开，并且可以设想在温和的条件下小分子的活化。利用氧化还原惰性金属(例如，Ca~(2+))在恒定氧化态催化中(例如，通过Sigma-键置换)的补充策略也出现了。因此，开发曾被视为催化惰性的主族元素作为新型催化剂的机会不仅处于科学前沿，而且提供了巨大的增长潜力。关于基于氧化还原的过程，现在已知能够影响E-H键的氧化活化的主基团系统(例如，对于E=H、C、N、O、Si)。然而，能够进行这种插入化学的试剂通常是高活性的亚价物种，而E-H键的活化过程通常会在热力学上非常稳定的氧化状态下生成产物。因此，通过还原再生类似于晚期d-块催化的活性物种的催化周转是很难实现的。然而，我们最近的工作和对15族元素的相关研究表明，主族元素基于n/n+2氧化态的催化循环确实是可行的。这一提议试图带来的均相催化的阶跃变化是打开基于氧化还原化学(氧化加成/还原消除)的主族金属的催化键修饰过程。我们的方法将建立在锡系统令人兴奋的初步结果的基础上，同时最终瞄准以锗为基础的催化剂，后者对环境更有利，但氧化还原前景更具挑战性。在这个项目的生命周期中，我们的目标不一定是在具有重要社会意义的催化转化中立即替代现有的过渡金属系统，而是在一个全新的努力领域为催化剂设计建立基本的基本规则。

The activation of relatively inert non-polar chemical bonds is key to numerous catalytic functionalization processes generating high value-added chemical products. In the region of 75% of all chemicals currently require catalysts at some stage in their manufacture, and typical catalysts feature heavier late Transition Metals, reflecting their amenability to bond modifying redox processes. Issues relating to the sustainable availability/cost of such elements and the incorporation of heavy metals into products, mean that the search for alternative catalytic platforms is at the cutting edge scientifically and economically. Main Group metals, by contrast, are inexpensive, abundant, and in the cases of the lighter elements (such as the germanium compounds ultimately targeted here) less of an issue with regard to toxicity.In recent years, research into Main Group element compounds has highlighted the accessibility of low-valent derivatives with vacant coordination sites, and frontier orbitals with relatively small energy gaps able to facilitate bond activation by oxidative addition. Thus, a fundamental mode of reactivity typical of late Transition Metals has been opened up, and small molecule activation under mild conditions can be envisaged. Complementary strategies utilising redox inert metals (e.g. Ca2+) in constant oxidation state catalysis (e.g. via sigma-bond metathesis) have also emerged. Thus, the opportunity to exploit Main Group elements, once perceived as catalytically inert, as novel catalysts is not only at the cutting edge scientifically, but also offers huge potential for growth. With regard to redox-based processes, Main Group systems capable of effecting oxidative activation of E-H bonds are now known (e.g. for E = H, C, N, O, Si). Reagents capable of such insertion chemistry, however, are often highly reactive sub-valent species, and E-H bond activation processes typically generate products in thermodynamically very stable oxidation states. Catalytic turnover via reductive regeneration of the active species similar to late d-block catalysis is thus difficult to effect. However, our recent work and related research into Group 15 systems gives encouragement that catalytic cycles based on n/n+2 oxidation states for Main Group elements are indeed viable.The step change in homogenous catalysis which this proposal seeks to bring about is to open up catalytic bond modification processes based on redox chemistry (oxidative addition/reductive elimination) to Main Group metals. Our approach will be built on exciting preliminary results for tin systems, while ultimately targeting catalysts based around germanium, which is more environmentally benign, but a more challenging redox prospect. Our goals for the lifetime of this project are not necessarily to produce immediate replacements for existing Transition Metal systems in societally important catalytic transformations, but rather to establish the fundamental ground rules for catalyst design in what is an entirely new area of endeavour.