Due to its intrinsic beauty and unique properties, bis(eta(5)-cyclopentadienyl)iron or ferrocene continues to attract the attention of chemists even after seven decades from its discovery. One of the particularly attractive and active fields is the preparation of planar-chiral ferrocene derivatives, which found manifold use as auxiliary ligands in enantioselective transition metal catalysis and organocatalysis. This chapter briefly illustrates the historical context and recent trends in this area, paying particular attention to the development of synthetic routes leading to planar-chiral ferrocenes.
The catalytic capabilities of Knolker-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 Knolker-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 Knolker-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 Bronsted 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.
The synthesis of heterocycles from renewable starting materials is a desirable goal for chemical research, as heterocycles have many applications in pharmaceuticals, material chemistry, and natural products. Recently, there has been a notable focus on utilizing earth-abundant 3d-transition-metal catalysts in contemporary catalysis, serving as a viable alternative to noble metals. This chapter provides an in-depth discussion of the recent advancements in 3d-transition metal-catalyzed acceptorless dehydrogenative coupling (ADC) reactions for the construction of diverse heterocyclic compounds. These reactions offer an efficient and environmentally friendly approach to the synthesis of valuable heterocyclic compounds.
The development of 3d metal-catalyzed molecular transformations has been a key focus of research in recent decades. One significant advancement is the discovery of the homogenous iron, cobalt, and manganese-catalyzed (de)hydrogenation processes. Among these redox transformations, the “Borrowing Hydrogen” (BH) principle, also known as hydrogen auto-transfer, stands out as an elegant and eco-friendly method that facilitates the self-transfer of hydrogen between reaction molecules and intermediates, eliminating the need for external hydrogen donors or acceptors. This concept allows for the eco-friendly use of alcohols, such as methanol, as environmentally benign C1 synthons for the alkylation of organic molecules, including pharmaceutically relevant candidates. In this context, the methyl group represents one of the most prevalent carbon fragments in small-molecule drugs. In this book chapter, we summarize the discovery and recent advancements in the use of 3d metal complexes for (multi)methylation of organic compounds using methanol via the hydrogen borrowing methodology. Additionally, we discuss current limitations, challenges, and the future prospects of this field.
Dehydrogenative processes for the catalytic up-conversion of alcohols into higher value products have continued to receive sustained attention over the past few years with a drive towards the application of more sustainable transition metals (i.e. earth-abundant metals) within these processes. This chapter discusses the recent developments in the field of metal-ligand cooperative (MLC) and 3d transition metal catalysts for alcohol reformation to various products such as esters, amines, acids, ureas, polyureas and polyethyleneimines. These MLC facilitated dehydrogenative processes are cost-effective and atom-economic routes to alcohol reformation products, often only producing hydrogen as the only by-product.
