Iron- and Cobalt-Catalyzed Hydrogenation and Cross-Coupling Reactions

The application of sustainable, abundant and non-toxic catalytic processes becomes more and more important. Especially 3d-metals like iron and cobalt based catalysts constitute to be an interesting alternative to well-established noble metal catalysts. But several limitations (e.g. lifetime, selectivity, etc.) still prevent their industrial application. Mechanistic understanding is the key for further catalyst adjustments. Chapter 1 contains a review about two of the most important techniques that can be used to distinguish between homotopic and heterotopic catalysts. The highlighted so called in operando techniques are kinetics, reaction progress analyses and poisoning experiments. Since there is not a single test that can proof homo- or heterotopicity of catalytic system, always various tests should be performed to validate results. In chapter 2, a facile iron-catalyzed deallylation protocol has been described. The described protocol utilizes iron(II)-chloride as pre-catalyst under mild reaction conditions (0 °C). A selective deallylation process occurs in less than one hour for a variety of compatible substrates. Tolerable functional groups are halides, olefins, esters, methylthio, allylamine, and benzylether groups. The only by-products (propene, pentene, CO2, etc.) obviates the need for laborious product separation. The use of NHC-ligands enhanced the reaction rate. Beside allyl ethers, also benzylic ethers and carbonates could be successfully cleaved. Chapter 3 contains a practical iron-catalyzed protocol for the chemoselective crosscoupling of alkenyl acetates with Grignard reagents. The combination of iron(II)-chloride and Grignard reagents under mild reaction conditions (0 °C) enabled the selective cross-coupling of alkenylacetates as electrophiles with nucleophilic Grignard reagents. Thermodynamically preferred deprotonation and acylation reactions are suppressed by the highly active catalyst system. A post-isomerization (even to thermodynamically more favored olefins) does not occur. Several primary and secondary Grignard reagents can be utilized and numerous functional groups, like halides, nitriles, esters, acetals, ethers and thioethers are tolerated. Kinetic studies and poisoning experiments support a homogeneous mechanism. An observed secondary KIEs suggest a rate-determining coordination of the conjugated alkenyl acetate. The catalytic hydrogenations of alkenes, ketones and imines with homoleptic iron- and cobalt arene complexes are described in chapter 4. For the first time bis(anthracene)cobaltate and bis(anthracene)ferrate, have been utilized in catalytic hydrogenation reactions. Bis(anthracene)cobaltate was highly active in the hydrogenation of alkenes, ketones, and imines(1–5 mol% cat., 1–10 bar H2, 20–60 °C). The corresponding ferrate revealed a lower activity and stability; only olefins could be hydrogenated. Kinetic studies and poisoning experiments suggest a homogeneous catalyst. The hydrogenation is initiated by the substitution of arene ligand by two olefins. Consecutive hydrogenation reactions showed that the catalytic system is stable as long as pi-acceptors are present. Consecutive reactions were performed without loss of activity. In chapter 5 a detailed review about reductive cross-coupling reactions between two electrophiles (or reductive cross-electrophile coupling reactions) is included. Compared to oxidative cross-coupling protocols, with two organometallic partners, reductive cross-couplings have a better redox potential economy and sustainability. Most active catalysts for these transformations are 3d-metal complexes (Ni, Co, Fe). As stoichiometric reductants, cheap and non-toxic metals (Zn, Mn, Mg) can be utilized. Further mechanistic understanding of the underlying reductive elemental steps and radical intermediates will certainly fuel wider applications of this general concept. Most mechanistic studies have been done for Ni-catalyzed aryl-alkyl-coupling reactions with manganese as reductant. Promising are especially Fe-based catalysts, due to their superior sustainablility. The need of high catalyst loadings, large amounts of one reactant and reductants and a frequent use of halogen containing substrates are disadvantageous and further optimizations are worthwhile