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Syntheses of N-heterocyclic compounds that permit a flexible introduction of various substitution patterns using inexpensive and diversely available starting materials are highly desirable. Easy to handle and reusable catalysts based on earth-abundant metals are especially attractive for these syntheses. We report here on the synthesis of 3,4-dihydro-2H-pyrroles via the hydrogenation and cyclization of nitro ketones. The latter are easily accessible from three components: a ketone, an aldehyde and a nitroalkane. Our reaction has a broad scope and 23 of the 33 products synthesized are compounds which have not yet been reported. The key to the general hydrogenation/cyclization reaction is a highly active, selective and reusable nickel catalyst, which was identified from a library of 24 earth-abundant metal catalysts.
Abstract The design of nanostructured catalysts based on earth-abundant metals that mediate important reactions efficiently, selectively and with a broad scope is highly desirable. Unfortunately, the synthesis of such catalysts is poorly understood. We report here on highly active Ni catalysts for the reductive amination of ketones by ammonia employing hydrogen as a reducing agent. The key functions of the Ni-salen precursor complex during catalyst synthesis have been identified: (1) Ni-salen complexes sublime during catalyst synthesis, which allows molecular dispersion of the metal precursor on the support material. (2) The salen ligand forms a nitrogen-doped carbon shell by decomposition, which embeds and stabilizes the Ni nanoparticles on the γ-Al2O3 support. (3) Parameters, such as flow rate of the pyrolysis gas, determine the carbon supply for the embedding process of Ni nanoparticles.
Electron microscopy of native biological materials is usually hampered by sample preparation procedures such as dehydration and freezing, and by electron beam damage. Liquid phase electron microscopy (LP-EM) allows the observation of biological samples in liquid environments without conventional preparation procedures. However, electron beam damage also occurs in LP-EM, and thresholds for biological samples are not yet fully explored. In this work, the electron dose tolerance of green fluorescent protein (GFP) was analyzed in LP-EM. Protein damage was studied with increasing electron dose, using fluorescence degradation as an indication. Dc < 0.01 e-/Ų and Dc < 0.1 e-/Ų were observed for GFP on silicon nitrite in transmission electron microscopy (TEM) and environmental scanning electron microscopy (ESEM), respectively. In TEM, the dose tolerance was increased by three orders of magnitude when GFP was encapsulated in graphene liquid cells. The dose tolerance of more complex systems was investigated by binding GFP to actin filaments in fixed SKBR3 cells, which showed Dc < 0.1 e-/Ų in TEM and ESEM. In fixed SKBR3 cells, radiation damage was also studied based on the displacement of labeled membrane proteins. At electron doses of D = (7.8 ± 0.4) ∙ 10³ e-/Ų these labels showed a displacement of 0.8%. Procedures for studying biological materials such as proteins and fixed cells in LP-EM are presented in this thesis. Strategies to study and mitigate beam damage are demonstrated.
