Knölker-Type Catalysts for (Asymmetric) Hydrogenation Reactions

The catalytic capabilities of Knölker-type complexes, mostly in the context of (transfer) hydrogenation chemistry, have been intensively studied during the past two decades. This happened mainly in response to the popular request for superseding catalysts that rely on scarce and expensive platinum group metals (PGMs). Indeed, the excellent abundance, very low price, and non-toxicity of Fe render these peculiar organoiron compounds ideal candidates for that purpose. The great structural malleability owing to their modular architecture further adds to the benefits of Knölker-complex-derived catalysts. Moreover, owing to their beneficial redox properties, these cyclopentadienone-tagged Fe complexes are also apt for usage in hydrogen autotransfer (hydrogen-borrowing) reactions. This fact opens up new vistas for atom-efficient and low-waste syntheses of a broad array of amines and more elaborated, pharmaceutically relevant heterocycles. However, the most important trait of Knölker-type complexes is their accessibility to chiral modification upon adjustment of the ligand framework which often encompasses the introduction of planar chirality or C2 symmetrical motifs. Furthermore, the pertinent Fe catalysts are amenable to pairing with enantiopure Brønsted acids such that dual catalysis becomes possible. Hence, promising approaches exist which might enable challenging and highly rewarding asymmetric (transfer) hydrogenation reactions that are effected by Fe complexes.