Artificial metalloenzymes as catalysts in stereoselective diels-alder reactions

Numerous enzymes are useful catalysts in synthetic organic chemistry, but they cannot catalyze the myriad transition-metal-mediated transformations customary in daily chemical work. For this reason the concept of directed evolution of hybrid catalysts was proposed some time ago. A synthetic ligand/transition-metal moiety is anchored covalently or non-covalently to a host protein, thereby generating a single artificial metalloenzyme which can then be optimized by molecular biological methods. In the quest to construct an appropriate experimental platform for asymmetric DielsAlder reactions amenable to this Darwinian approach to catalysis, specifically those not currently possible using traditional chiral transition-metal catalysts, two strategies have been developed which are reviewed here. One concerns the supramolecular anchoring of a Cu(II)-phthalocyanine complex to serum albumins; the other is based on the design of a Cu(II)-specific binding site in a thermostable protein host (tHisF), leading to 4698% ee in a model DielsAlder reaction. This sets the stage for genetic fine-tuning using the methods of directed evolution. DOI 10.1002/tcr.201100043