6 research outputs found
Intermediates in Iron Group Metal-Catalyzed Hydrogenation Reactions
The present thesis reports on the development of iron group metal-catalyzed hydrogenation reactions. Specifically, mechanistic investigations have been focused to develop better catalysts in future. Chapter 1 advocates kinetic poisoning experiments for the distinction between homogeneous and heterogeneous catalysis with standard laboratory equipment. Chapter 2 reviews dibenzo[a,e]cyclooctatetraene, a typical homogeneous catalyst poison. Simple cobalt catalysts for C=C, C=O, and C=N hydrogenations have been developed in Chapter 3: Olefinic substrates stabilize the reduction of cobalt salts to obtain cobalt nanoparticles. Chapter 4 describes homogeneous hydrogenation catalysts for alkene and imine hydrogenation based on highly-reduced cobaltates. The redoxactive ligand bis(imino)acenaphthene (BIAN) allowed the isolation and detailed mechanistic analyses of olefin cobaltate and hydride intermediates. Related cobaltate catalysts have been developed for amine-borane (de)hydrogenations and transfer hydrogenations to imines, chinolines and alkenes in Chapter 5. Chapter 6 reports on the synthesis, characterization and catalysis of a dimeric iron ate complex with four bridging hydrides. In Chapter 7, olefin-stabilized nickel nanoparticle catalysts have been developed for alkene hydrogenation based on a nickel metalate. These catalysts exhibit a remarkable functional group tolerance. Chapter 8 highlights recent studies by the groups of Bedford and Neidig for ironcatalyzed cross-coupling reactions by triorganoferrates as active species. Chapter 9 summarizes the results of this thesis
Olefin-Stabilized Cobalt Nanoparticles for C=C, C=O, and C=N Hydrogenations
The development of cobalt catalysts that combine easy accessibility and high selectivity constitutes a promising approach to the replacement of noble-metal catalysts in hydrogenation reactions. This report introduces a user-friendly protocol that avoids complex ligands, hazardous reductants, special reaction conditions, and the formation of highly unstable pre-catalysts. Reduction of CoBr2 with LiEt3BH in the presence of alkenes led to the formation of hydrogenation catalysts that effected clean conversions of alkenes, carbonyls, imines, and heteroarenes at mild conditions (3 mol% cat., 2-10 bar H-2, 20-80 degrees C). Poisoning studies and nanoparticle characterization by TEM, EDX, and DLS supported the notion of a heterotopic catalysis mechanism
Heterogeneous Olefin Hydrogenation Enabled by a HighlyâReduced Nickel(âII) Catalyst Precursor
The hydrogenation of olefins, styrenes, enoates, imines, and sterically hindered triâsubstituted olefins was accomplished using the preâcatalyst dilithiumbis(cyclooctaâ1,5âdiene)nickelate(âII) (1). The mild conditions tolerate hydroxyl, halide, ester, and lactone functionalities. Mechanistic studies, including reaction progress analyses, poisoning experiments, and multinuclear NMR monitoring, indicate that a heterotopic (nickel nanoparticle) catalyst is in operation
AmineâBorane Dehydrogenation and Transfer Hydrogenation Catalyzed by αâDiimine Cobaltates
Anionic alpha-diimine cobalt complexes, such as [K(thf)(1.5){((Dipp)BIAN)Co(eta(4)-cod)}] (1; Dipp=2,6-diisopropylphenyl, cod=1,5-cyclooctadiene), catalyze the dehydrogenation of several amine-boranes. Based on the excellent catalytic properties, an especially effective transfer hydrogenation protocol for challenging olefins, imines, and N-heteroarenes was developed. NH3BH3 was used as a dihydrogen surrogate, which transferred up to two equivalents of H-2 per NH3BH3. Detailed spectroscopic and mechanistic studies are presented, which document the rate determination by acidic protons in the amine-borane
Olefin-Stabilized Cobalt Nanoparticles for C=C, C=O, and C=N Hydrogenations
The development of cobalt catalysts that combine easy accessibility and high selectivity constitutes a promising approach to the replacement of noble-metal catalysts in hydrogenation reactions. This report introduces a user-friendly protocol that avoids complex ligands, hazardous reductants, special reaction conditions, and the formation of highly unstable pre-catalysts. Reduction of CoBr2 with LiEt3BH in the presence of alkenes led to the formation of hydrogenation catalysts that effected clean conversions of alkenes, carbonyls, imines, and heteroarenes at mild conditions (3 mol% cat., 2â10 bar H2, 20â 808C). Poisoning studies and nanoparticle characterization by TEM, EDX, and DLS supported the notion of a heterotopic catalysis mechanism
Cobalt-Catalyzed Hydrogenations via Olefin Cobaltate and Hydride Intermediates
Redox noninnocent ligands are a promising tool to
moderate electron transfer processes within base-metal catalysts. This
report introduces bis(imino)acenaphthene (BIAN) cobaltate complexes
as hydrogenation catalysts. Sterically hindered trisubstituted alkenes,
imines, and quinolines underwent clean hydrogenation under mild
conditions (2â10 bar, 20â80 °C) by use of the stable catalyst precursor
[(DippBIAN)CoBr2] and the cocatalyst LiEt3BH. Mechanistic studies
support a homogeneous catalysis pathway involving alkene and hydrido
cobaltates as active catalyst species. Furthermore, considerable reaction
acceleration by alkali cations and Lewis acids was observed. The
dinuclear hydridocobaltate anion with bridging hydride ligands was
isolated and fully characterized