Lithium Aluminium Hydrid: Von stöchiometrischer Reduktion zur Katalyse
Zusammenfassung der Projektergebnisse
The project “Lithium Aluminium Hydride: From Stoichiometric Reduction to Catalysis” contributes to a deeper understanding of using LiAlH4, generally known as a stoichiometric reducing agent for C=O and C=N bonds, as a catalyst in hydrogenation reactions. The main achievements and conclusions from the project are following: We synthesized and structurally characterized new metal salts of the alanate anion AlH4¯, for example Ca(AlH4)2 and Sr(AlH4)2. The corresponding Ba alanate complex is fully insoluble. These heavier alkaline-earth metal alanates show significantly higher catalytic activities in imine hydrogenation when compared to LiAlH4. We prepared “naked” aluminate anions in the form of [nBu4N+][AlH4¯] and observed total loss of catalytic activity. This shows that the second metal cation (Li+, Ca2+, Sr2+) is crucial to the activity of alanate catalysts. Exchanging the Al center in LiAlH4 for B or Ga led to poor or no catalyst activity which is due to the lower reactivity of BH4¯ or complete insolubility and instability of GaH4¯ salts. In order to solubilize alanate complexes, we prepared and structurally characterized Mg and Ca β-diketiminate complexes with AlH4¯ anions. They display catalytic activity that is similar to that of Mg(AlH4)2 or Ca(AlH4)2. In an extensive DFT study on imine hydrogenation with lithium alanates we showed that cooperation between Li and Al is an essential aspect of LiAlH4 catalysis. This conclusion was also supported by catalytic inactivity of (BDI)AlH2 complexes (BDI = β-diketiminate ligand). We found, however, that the cation (BDI)AlH+, and especially (BDI)AlMe+, is an active catalyst for imine hydrogenation via an FLP mechanism. In order to extend the scope of using commercially available LiAlH4 as a heterobimetallic catalyst, we discovered that combination of LiAlH4 and Fe0 powder (activated by metal evaporation) is a highly potent catalyst for alkene hydrogenation. The LiAlH4/Fe0 mixture is even able to reduce benzene to cyclohexane. Reduction of highly substituted alkenes like 1-Me-cyclohexene could be achieved overnight at room temperature and only 1 bar of H2 pressure. The actual catalyst in this reaction is the decomposition product of LiAlH4 (LiH, Al0) and Fe0. The Li component is not essential. The current DFG project culminated in a new research field in our group: “Boosting the activity of main group metal hydride catalysts by addition of activated transition metal powders”. The main surprises and unexpected observations in the project were the following: Extensive activities of the main group community in using lithium alanates in catalysis led us to change the focus of the project. - Complete inactivity of a chiral L*AlH2¯Li+ catalyst (e.g. L* = 3,3’-Ph3Si-binaphtol) in imine hydrogenation. - The very high activity of the simple combination LiAlH4/Fe0.
Projektbezogene Publikationen (Auswahl)
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Alkaline Earth Metal Aluminates as Catalysts for Imine Hydrogenation. Organometallics, 39(23), 4238-4246.
Elsen, Holger; Langer, Jens; Wiesinger, Michael & Harder, Sjoerd
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Insights into LiAlH4 Catalyzed Imine Hydrogenation. Chemistry – A European Journal, 27(1), 401-411.
Elsen, Holger; Langer, Jens; Ballmann, Gerd; Wiesinger, Michael & Harder, Sjoerd
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Cationic Aluminium Complexes as Catalysts for Imine Hydrogenation. Chemistry – A European Journal, 27(28), 7756-7763.
Friedrich, Alexander; Eyselein, Jonathan; Elsen, Holger; Langer, Jens; Pahl, Jürgen; Wiesinger, Michael & Harder, Sjoerd
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Lithium Aluminium Hydride and Metallic Iron: A Powerful Team in Alkene and Arene Hydrogenation Catalysis. Angewandte Chemie International Edition, 62(18).
Knüpfer, Christian; Färber, Christian; Langer, Jens & Harder, Sjoerd
