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 Ba0, that is, conjugated alkenes, alkynes, annulated rings, imines and pyridines. In addition, a mechanism in which Ba0 and BaH2 are both essential is discussed. DFT calculations on benzene hydrogenation with a simple model system (Ba/BaH2) confirm that the presence of metallic Ba has an accelerating effect.
Highlights
Two major milestones stand at the cradle of heterogeneous metal catalysis.[1]
One would expect that the catalyst activity may be increased further using even bulkier ligands, we report that ligandfree metallic Ba is a very simple but highly active hydrogenation catalyst
Small pieces of Ba metal freshly cut under N2 are not catalytically active for alkene hydrogenation and it was Chemie found that commercially available Ba metal is essentially inert to H2 gas
Summary
Two major milestones stand at the cradle of heterogeneous metal catalysis.[1]. Dçbereiner observed in 1823 that hydrogen spontaneously burns in air upon contact with finely divided platinum. It was proposed that the catalytic activity of the latter metals is strongly related to metal hydride formation.[3] later studies by Wright and Weller show that CaH2 and BaH2 are active in ethylene hydrogenation but only after prior thermal treatment under high vacuum, advocating that the presence of the metallic state is of importance.[4] These early reports are limited only to ethylene hydrogenation, they become topical again in light of the recent interest in alkene hydrogenation with heavier group 2 metal catalysts.[5,6,7,8,9] We reported alkene hydrogenation using simple Ae metal amides like Ae[N(SiMe3)2]2 (Ae = Ca, Sr, Ba)[5b] with activities and scope sharply increasing from Ca to. One would expect that the catalyst activity may be increased further using even bulkier ligands, we report that ligandfree metallic Ba is a very simple but highly active hydrogenation catalyst
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