Ru-based metalloenzymes for asymmetric C-H functionalization

在过去的半个世纪里，过渡金属催化一直是合成有机化学取得巨大进步的核心。Pd交叉偶联(2010年诺贝尔)、Ru复分解(2005年诺贝尔)、不对称Ru氢化和Fe环氧化(2001年诺贝尔)、铜点击化学以及最近的C-H活化化学等发展为化学家提供了极其强大的工具。然而，除了一些例外，小分子过渡金属催化剂无法提供酶通常能够提供的精致选择性和极高的反应活性。另一方面，自然产生的酶的转化范围只是小分子过渡金属催化剂的一小部分。人工金属酶(AM)是一种金属蛋白质，其中非生物金属辅因子安装在蛋白质支架中。在过去的二十年里，AM的快速发展揭示了扩大酶可用反应工具集的巨大潜力。原则上，人们可以梦想利用均相过渡金属催化剂提供的广泛的反应性，并利用酶的选择性控制、微调和稳健性，提供最终的合成工具。然而，可用作配体的遗传氨基酸的范围很小，不适合最理想的应用。作为另一种选择，研究人员求助于锚定预先组装的辅因子，但在这种方法中，辅因子的准确放置具有挑战性，导致AM的活性低于自由辅因子。因此，从定义明确的小分子过渡金属催化剂过渡到下一代AM版本的技术仍处于起步阶段。在Larrosa小组，我们最近开发了一类新的小分子Ru-催化剂，能够介导芳香族化合物的C-H活化，并使它们与芳基和烷基亲电偶联伙伴官能化。然而，尽管在许多情况下在这些反应中生成了手性化合物(通过形成手性中心或阿托品)，但不可能诱导对映体选择性，从而在所有情况下都导致外消旋产物。此外，当偶联伙伴中有两个或两个以上非常相似的位置可用于反应时，区域和/或化学选择性往往得不到实现。在这个项目中，我们的目标是开发一种新型的AM，它的活性中心包含一个定义明确的Ru-催化剂，它将能够催化具有不同偶联伙伴的芳香族化合物的不对称C-H官能化。为了开发这样的Ru-AM，我们的目标是探索一种新的方法来处理AM：摆脱天然氨基酸，我们将使用先进的蛋白质工程方法来扩大AM可用的配体类型。特别是，我们将在蛋白质中安装一个遗传编码的非天然氨基酸，该氨基酸带有一个合适的Ru的“直接”配体，这将允许形成AM，其中Ru在活性部位的位置可以通过基因操作来控制。这样的AM将能够通过格林小组专门研究的酶的定向进化(2018年诺贝尔)来优化其催化活性。最终，这些新型的AM将用于催化C-H芳基化、烷基化和其他功能化反应，控制区域选择性和对映体选择性，从用于合成的小构件到更大的生物活性分子(如药物)的重要底物。学生将接受最先进的化学催化培训，包括用于设计、合成和研究小分子Ru基过渡金属催化剂的最先进的化学催化技术、有机化学和物理化学技术，以及对非天然氨基酸的遗传编码、酶反应的定向进化和评估。

Transition-metal catalysis has been at the heart of tremendous advances in synthetic organic chemistry over the last half century. Developments such as Pd-cross-couplings (Nobel 2010), Ru-metathesis (Nobel 2005), asymmetric Ru-hydrogenations and Fe-epoxidations (Nobel 2001), Cu-click chemistry and, more recently, C-H activation chemistry have provided extremely powerful tools for chemists. However, with some exceptions, small molecule transition-metal catalysts are unable to provide the exquisite selectivity and extremely high reactivity that enzymes are generally capable of. On the other hand, the range of transformations possible from naturally occurring enzymes is but a fraction of what has been developed for small molecule transition-metal catalysts.Artificial metalloenzymes (AMs) are metalloproteins where an abiotic metal cofactor is installed within a protein scaffold. Over the last two decades, rapid progress in the development of AMs has revealed enormous potential for the expansion of the toolset of reactions available to enzymes. In principle, one could dream of drawing from the vast breadth of reactivities afforded by homogeneous transition-metal catalysts, and harnessing the selectivity control, fine tuning and robustness of enzymes, to afford the ultimate synthetic tools. However, the range of genetically available aminoacids that can be used as ligands is small and not suitable for most desired applications. As an alternative, researchers have resorted to anchoring pre-assembled cofactors, but in this approach the accurate placement of the cofactor is challenging resulting in AMs of lower activity than the free cofactor. Thus the techniques allowing the move from a well-defined small molecule transition-metal catalyst to a next generation AM version are still in their infancy.In the Larrosa group we have recently developed a novel class of small molecule Ru-catalysts that are able to mediate the C-H activation of aromatic compounds and functionalize them with aryl and alkyl electrophile coupling partners. However, while in many cases chiral compounds are generated in these reactions (either through the formation of a chiral centre or atropisomers) inducing enantioselectivity has not been possible, leading in all cases to racemic products. Furthermore, regio and/or chemoselectivity is often not achieved when two or more very similar positions are available for reaction in the coupling partners. In this project we aim at developing a novel AM, containing a well-defined Ru-catalyst in its active site, that will be able to catalyse asymmetric C-H functionalization of aromatic compounds with a variety of coupling partners. To develop such a Ru-AM, we aim at exploring a novel approach towards AMs: moving away from natural aminoacids, we will employ advanced protein engineering methods to expand the type of ligands available to AMs. In particular, we will install in the protein a genetically encoded non-natural aminoacid bearing a suitable 'direct' ligand for Ru, which will allow for the formation of a AM where the position of the Ru in the active site is controllable through genetic manipulation. Such a AM will be amenable of optimization of its catalytic activity through directed evolution (Nobel 2018) of the enzyme on which the Green group specialises. Ultimately, these novel AMs will be used for catalysing the C-H arylation, alkylation and other functionalizations with control on regioselectivity and enantioselectivity, of substrates of importance from small building blocks for synthesis to larger biologically active molecules such as drugs.The student will be trained in state-of-the-art chemocatalysis, including organometallic chemistry, organic chemistry and physical chemistry techniques for the design, synthesis and investigation of small molecule ruthenium based transition-metal catalysts & enzymology, genetically encoding of unnatural aminoacids, directed evolution and assessment of enzymatic reactions