Metallic Barium: A Versatile and Efficient Hydrogenation Catalyst
Philipp Stegner, Christian Färber, Ulrich Zenneck, Christian Knüpfer, Jonathan Eyselein, Michael Wiesinger, Sjoerd Harder
Abstract
Abstract Ba metal was activated by evaporation and cocondensation with heptane. This black powder is a highly active hydrogenation catalyst for the reduction of a variety of unactivated (non‐conjugated) mono‐, di‐ and tri‐substituted alkenes, tetraphenylethylene, benzene, a number of polycyclic aromatic hydrocarbons, aldimines, ketimines and various pyridines. The performance of metallic Ba in hydrogenation catalysis tops that of the hitherto most active molecular group 2 metal catalysts. Depending on the substrate, two different catalytic cycles are proposed. A: a classical metal hydride cycle and B: the Ba metal cycle. The latter is proposed for substrates that are easily reduced by Ba 0 , that is, conjugated alkenes, alkynes, annulated rings, imines and pyridines. In addition, a mechanism in which Ba 0 and BaH 2 are both essential is discussed. DFT calculations on benzene hydrogenation with a simple model system (Ba/BaH 2 ) confirm that the presence of metallic Ba has an accelerating effect.