3d-Metal catalyzed reductive transformations and isomerizations: hydrogenation of unsaturated compounds and trimerization of alkynes

Abstract

After an introductory chapter about catalysis in general and the aim of this work, a review on 3d metal catalyzed semi hydrogenation of alkynes is presented in chapter two. The semi hydrogenation of alkynes offers an efficient synthetically pathway towards cis- and trans-alkenes. Examples for homogeneous as well as heterogeneous catalysts with every 3d metal were reported. An iron catalyzed semi hydrogenation of alkynes was introduced in chapter three. The use of the commercially available iron salt Fe(acac)2 and the reductant DIBAL-H afforded an active catalyst with a high selectivity and activity under mild reaction conditions. Mechanistic studies indicated the operation of small metal nanoclusters and nanoparticles. X-ray absorption spectroscopy was used to monitor the reductive formation of the catalyst with an initial hydride addition to the acac-ligand and the subsequent formation of iron cluster and particles. As a result of the previous investigations from chapter three, a chromium catalyzed semi hydrogenation protocol was developed in chapter four. Similar to the iron system, Cr(acac)3 was used as precursor with the combination of DIBAL-H. Mild reaction conditions combined with high selectivity and activity was achieved, too. Mechanistic experiments revealed that a similar formation of the active catalyst species is operating. However, a smaller amount of metal centered reduction was observed, which could explain a smaller activity compared with the iron system presented in chapter three. Chapter five presents the results of a collaboration, that investigated the formation of cobalt-aluminium nanoparticles in ionic liquids and their properties (Collaboration partners). My task was the application of the provided nanoparticles for catalytic reactions. Best results were achieved in combination with DIBAL-H in the hydrogenation of alkynes and alkenes. Neat particles required elevated temperatures and reaction pressures. A trimerization of internal alkynes to highly substituted arenes is the focus of chapter six. The iron-(II)-salt Fe(hmds)2 with a half equivalent of DIBAL-H afforded an active catalyst for the trimerization of internal alkynes. While the trimerization of terminal alkynes usually operates with ease, the trimerization of internal alkynes is due to steric hindrance a more challenging task, which requires often more forcing conditions. Our ligand free iron catalyst operates under mild conditions. A surprising feature of this system was the unusual ratio of the iron-precursor to the reductant, which was a main focus of mechanistic investigations. Chapter seven provides first results for an iron catalyzed arene hydrogenation, which represents a more challenging task compared with isolated double bonds. Only few examples were reported with iron. The combination of Fe(hmds)2 with DIBAL-H was applied under elevated temperatures and reaction pressure for a variety of substrates.