Metallic Barium: A Versatile and Efficient Hydrogenation Catalyst

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.

Application of a Stable and Soluble Dibenzylbarium Reagent in the Synthesis of a Barium Imido Cluster

“…17,23 Typical structural features and NMR spectroscopic parameters are a measure for delocalization of the negative charge on the benzylic C α atom over the aryl ring (Table 1). Charge delocalization results in (a) shortening of the C α −C ipso bond, i.e., higher double bond character, (b) a change of sp 3 -to sp 2hybridization at C α resulting in planarization (indicated by the sum of the valence angles: ∑C α ≈ 360°), (c) a squeezed endocyclic C−C−C angle at C ipso (<120°), (d) an increase of the 1 J C−H coupling constant at C α , and (e) a high-field shift of the 1 H and 13 C NMR resonances for the (aryl)CH unit in para-position in respect to the benzylic C α atom (this is due to an increase of the π-electron density). From the data summarized in Table 1, a general order for the extent of charge delocalization in the DMAT anion as a function of the metal was estimated.…”

“…In addition, either elaborate precursor synthesis or poor yields for some of these Ba reagents renders them unsuitable for convenient large-scale synthesis and further application. In light of the steadily increasing popularity of Ba compounds in catalysis, − highly reactive Ba reagents that are also easily accessible on a large scale that can be stored long-term and that are preferably soluble in nonpolar solvents are sought-after synthetic targets.…”

“…1 H NMR (600 13. MHz, C 6 D 6 , 298 K): δ = 0.46 (s, 9H, SiMe 3 ), 1.31−1.34 (m, 4H, OCH 2 CH 2 ), 1.59 (s, 1H, Me 3 SiCH−Ba), 2.41 (s, 6H, N(CH 3 ) 2 ), 3.32−3.36 (m, 4H, OCH 2 CH 2 ), 6.23 (t, 8.0 Hz, 1H, CH−arom), 6.71 (d, 7.6 Hz, 1H, CH−arom), 6.84 (t, 8.0 Hz, 1H, CH−arom), 7.01−7.03 (m, 1H, CH−arom) ppm.…”

Alkali Metal (Li, Na, K, Rb, Cs) Mediation in Magnesium Hexamethyldisilazide [Mg(HMDS)2] Catalysed Transfer Hydrogenation of Alkenes

“…They are (mostly) biocompatible, environmentally benign and with their high abundance in the crustal layer and oceans of earth, a low price per mole is guaranteed. Activity in this area has been steadily rising since the turn of the millennium with pioneering contributions by Harder, Hill, Okuda, Cheng and Stock, with several catalytic hydrogenations published [7–27] …”

Heterometallic Mg−Ba Hydride Clusters in Hydrogenation Catalysis

“…[16] They are also regarded intermediates in hydrogenation catalysis, a field that underwent rapid growth and development. [16][17][18][19][20][21][22][23][24][25][26][27] Their applications also include hydrogenation of challenging arenes, [26,28] Hydrogen-Isotope-Exchange, [29] CÀ F bond activation" [30][31][32] ethylene polymerization, [11] and controlled reactions with olefins to isolate first examples of Ca alkyls and a Sr ethyl complex. [11,33] However, looking back at the beginnings of Ae hydride chemistry, the research field was pioneered by a heterobimetallic system.…”