Ruthenium-metalloenzymes for catalytic C-H functionalization

The realm of transition-metal catalysis has been a focal point for remarkable advancements in synthetic organic chemistry throughout the past fifty years. Milestones like 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 equipped chemists with potent tools. Despite these strides, the selectivity and reactivity of small molecule transition-metal catalysts often fall short of the precision and potency inherent in enzymes. 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. Enter Artificial metalloenzymes (AMs), a category of 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, envisioning the amalgamation of the extensive reactivity spectrum offered by homogeneous transition-metal catalysts with the selectivity control, fine-tuning, and robustness of enzymes seems like the ultimate synthetic dream. However, limitations arise from the relatively small pool of genetically available amino acids suitable as ligands, hindering their application in many desired scenarios. As an alternative, researchers have turned to anchoring pre-assembled cofactors, yet achieving accurate cofactor placement in this approach remains challenging, resulting in AMs of lower activity compared to their free cofactor counterparts. Consequently, the techniques facilitating the transition 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, alkyl and methyl electrophile coupling partners under ambient conditions. 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, our primary objective is the development of a groundbreaking artificial metalloenzyme (AM) equipped with a precisely defined Ru-catalyst in its active site. The ultimate goal is to enable this innovative AM to catalyze the asymmetric C-H functionalization of aromatic compounds in conjunction with a diverse array of coupling partners.
Our approach to constructing this Ru-AM involves a departure from conventional methods. Instead of relying on natural amino acids, we intend to leverage advanced protein engineering techniques to broaden the scope of ligands available to AMs. The strategy involves incorporating a genetically encoded non-natural amino acid that carries a ligand suitable for Ru. This strategic modification will grant us control over the positioning of Ru within the active site 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.