Simple Alkaline‐Earth Metal Catalysts for Effective Alkene Hydrogenation

Abstract

Alkaline earth metal amides (AeN'' : Ae=Ca, Sr, Ba, N''=N(SiMe ) ) catalyze alkene hydrogenation (80-120 °C, 1-6 bar H , 1-10 mol % cat.), with the activity increasing with metal size. Various activated C=C bonds (styrene, p-MeO-styrene, α-Me-styrene, Ph C=CH , trans-stilbene, cyclohexadiene, 1-Ph-cyclohexene), semi-activated C=C bonds (Me SiCH=CH , norbornadiene), or non-activated (isolated) C=C bonds (norbornene, 4-vinylcyclohexene, 1-hexene) could be reduced. The results show that neutral Ca or Ba catalysts are active in the challenging hydrogenation of isolated double bonds. For activated alkenes (e.g. styrene), polymerization is fully suppressed due to fast protonation of the highly reactive benzyl intermediate by N''H (formed in the catalyst initiation). Using cyclohexadiene as the H source, the first Ae metal catalyzed H-transfer hydrogenation is reported. DFT calculations on styrene hydrogenation using CaN'' show that styrene oligomerization competes with styrene hydrogenation. Calculations also show that protonation of the benzylcalcium intermediate with N''H is a low-energy escape route, thus avoiding oligomerization.

Alkene Transfer Hydrogenation with Alkaline‐Earth Metal Catalysts

“…Apart from this isolated example, the conversion of either imines or alkenes hitherto has not been reported. During our investigation on alkene hydrogenation with H 2 using simple alkaline earth (Ae) metal amide catalysts, the substrate 1,3‐cyclohexadiene (1,3‐CHD) showed dual reactivity and was not only hydrogenated to cyclohexene but also dehydrogenated to benzene. This striking observation implied that this diene may function as a H source offering an alternative route to formation of the metal‐hydride catalyst.…”

“…This striking observation implied that this diene may function as a H source offering an alternative route to formation of the metal‐hydride catalyst. Indeed, a mixture of 1,3‐CHD and styrene could, without H 2 , be converted into ethylbenzene and benzene . Although concomitant reduction of 1,3‐CHD also gave considerable quantities of cyclohexene, this reaction suggested the feasibility of Ae metal catalyzed alkene TH.…”

“…This amine quenches the reactive benzylcalcium intermediate, thus preventing styrene polymerization (Scheme ). Earlier DFT calculations confirm that reaction of N′′CaCH(Me)Ph with N′′H is a low‐energy escape route (Δ G ≠ =+10.2 kcal mol −1 ), which is clearly preferred over styrene polymerization (Δ G ≠ =+14.5 kcal mol −1 ) …”

“…The simple and easily accessible calcium amide catalyst CaN′′ 2 gave selective conversion into ethylbenzene (Table , entry 1). In agreement with our earlier observations, the catalyst CaN′′ 2 has the unique property to inhibit styrene polymerization (<1 % oligomers) . The key to this selectivity is the presence of the relatively acidic amine N′′H ( p K a =25.8) formed during catalyst initiation.…”

Super‐Bulky Penta‐arylcyclopentadienyl Ligands: Isolation of the Full Range of Half‐Sandwich Heavy Alkaline‐Earth Metal Hydrides

“…It is noteworthy that 4‐Ba could also catalyze the hydrogenation of tetra‐substituted conjugated alkene, tetraphenylethylene, although the conversion was low (5 %, 24 h, entry 21). Considering the catalytic conditions and reaction time, 4‐Ba showed significantly higher activities towards alkenes hydrogenation than Ba[N(SiMe 3 ) 2 ] 2 , recently reported by Harder et al.…”