title: Mn- and Cr-based Complexes for (De-)Hydrogenation Catalysis
Creators: name: Kallmeier, Fabian
Publisher: name: University of Bayreuth
Year: 2021
Subjects: subject: Synthesis
subject: Catalysis
subject: Manganese
subject: Chromium
subject: Borrowing Hydrogen
subject: Acceptorless Dehydrogenative Condensation
subject: Hydrogenation
subject: Pyrroles
subject: Amines
subject: 540

description: The subject of this thesis is the development and application of homogeneous catalysts that are based on cheap and abundantly available transition metals, specifically manganese and chromium. Manganese and chromium have traditionally received no attention in (De-)Hydrogenation and/or Borrowing Hydrogen / Hydrogen Autotransfer (BH/HA) catalysis, due to their propensity for single-electron-transfer steps. To overcome this limitation, bifunctional complexes have been synthesized in this work, allowing – after activation with a strong base – a heterolytic cleavage of dihydrogen under retention of the original oxidation state of the metal. These complexes are based on diamino-s-triazine-derived ligands, which have been established by the groups of Kempe and Kirchner in other (base metal catalyzed) reactions. The seamless synthesis of these ligands, even on multigram scale, makes them ideal candidates for catalyst development. In the first part of this work, manganese (MnI and MnII) complexes have been synthesized by the reaction of P,N,P ligands with the corresponding manganese precursors, [MnBr(CO)5] or MnCl2. These complexes were applied as precatalysts for the chemo-selective hydrogenation of carbonyl compounds. It was established that MnI complexes were catalytically active under mild reaction conditions (typically 0.1 mol% precatalyst loading, 1 mol% KOtBu, 20 bar H2, 80 °C, 4 h) whereas MnII compounds were found to be inactive. It was furthermore demonstrated that manganese catalysts exhibit excellent chemo-selectivity in hydrogenation catalysis. Ketones and aldehydes were selectively hydrogenated even in the presence of functional groups like aryl halides, nitriles, esters, and alkenes. Steric bulk around the carbonyl group in the substrate reduced the yield, which was overcome by longer reaction times and/or increased catalyst loading. Overall, the hydrogenation of thirty examples has been reported with yields ranging between 52 % and &gt; 99 % and, on average, the yield of product was well above 90 %. The following chapter of this work details the application of these newly developed manganese catalysts in the synthesis of 1H-pyrroles from secondary alcohols and 1,2-amino alcohols. The Kempe group recently reported this method for the synthesis of pyrroles based on the acceptorless dehydrogenative condensation (ADC) of secondary alcohols and 1,2 amino alcohols. In this procedure, an alcohol is dehydrogenated to a carbonyl compound (i.e. reverse reaction to hydrogenation) that can then undergo condensation with an amino alcohol, followed by further dehydrogenation and condensation steps to yield the target pyrroles. Using Mn-based catalysts renders the entire process innately more sustainable by avoiding the use of expensive and rare iridium catalysts. The reaction conditions were optimized after catalytic activity was found for MnI complexes in pyrrole synthesis by ADC. The best results were obtained by employing 2 equiv of secondary alcohol, 1 equiv of 1,2 amino alcohol, 1.5 equiv KOtBu when the reaction was refluxed in 2-methyltetrahydrofuran (2-MeTHF; 0.5 M) for 18 hours and 0.5 mol% precatalyst were used. The general applicability was shown by the synthesis and isolation of 29 examples, including products containing sensitive functional groups like cyclopropane, alkene, aryl halides and heterocycles (thiophene and pyridine). Despite its availability and cheap price, chromium has so far been neglected in the development of catalysts aimed at BH/HA catalysis. In the last chapter of this work, however, it was demonstrated that Cr can indeed catalyze such bond forming reactions, using the N-alkylation of amines as an exemplary reaction. First, a library of CrII and CrIII complexes was synthesized. After rigorous optimization of the reaction conditions using the reaction of aniline with benzyl alcohol as a model reaction (1.2 equiv benzyl alcohol, 1 mmol aromatic amine, 0.5 equiv KOtBu, 3 mol% precatalyst, 1,4-dioxane (2 M), 150 °C (oil bath) for 18 h), a total of 35 N-alkylated amines was synthesized and isolated in yields from 46 % to 94 % (average yield is 85 %). Functional groups that were tolerated by the catalyst systems include primary and tertiary amines, nitriles, cyclopropanes, C-C multiple bonds and heteroaromatic amines (e.g. pyrimidine, quinoline, pyrazole).