Alkene Transfer Hydrogenation with Alkaline‐Earth Metal Catalysts

Abstract

The alkene transfer hydrogenation (TH) of a variety of alkenes has been achieved with simple AeN′′2 catalysts [Ae=Ca, Sr, Ba; N′′=N(SiMe3)2] using 1,4‐cyclohexadiene (1,4‐CHD) as a H source. Reaction of 1,4‐CHD with AeN′′2 gave benzene, N′′H, and the metal hydride species N′′AeH (or aggregates thereof), which is a catalyst for alkene hydrogenation. BaN′′2 is by far the most active catalyst. Hydrogenation of activated C=C bonds (e.g. styrene) proceeded at room temperature without polymer formation. Unactivated (isolated) C=C bonds (e.g. 1‐hexene) needed a higher temperature (120 °C) but proceeded without double‐bond isomerization. The ligands fully control the course of the catalytic reaction, which can be: 1) alkene TH, 2) 1,4‐CHD dehydrogenation, or 3) alkene polymerization. DFT calculations support formation of a metal hydride species by deprotonation of 1,4‐CHD followed by H transfer. Convenient access to larger quantities of BaN′′2, its high activity and selectivity, and the many advantages of TH make this a simple but attractive procedure for alkene hydrogenation.

Reduction of 1,3,5,7-cyclooctatetraene by a molecular calcium hydride: an even electron polarised insertion/deprotonation mechanism

“…20,23 With these observations in mind, our attention was drawn to Harder's recent demonstration of the potency of such hydride reagents toward the transfer hydrogenation of C-C multiple bonds. 24 This chemistry utilised 1,4-cyclohexadiene (1, as the source of dihydrogen during which the cyclic diene is aromatised to benzene. Density functional theory (DFT) calculations demonstrated that this transformation ensues through a sequence of 1,4-CHD deprotonation and hydride elimination from the resultant cyclohexadienyl intermediate.…”

Magnesium hydride alkene insertion and catalytic hydrosilylation

“…3) may be converted to n-alkyl species by their reaction with unactivated alkenes. 12,15,16,55,56 In this joint synthetic and computational study, we demonstrate that terminal alkenes and unhindered alkynes also react directly with compound 4 and that subsequent silane metathesis of the resultant organomagnesium species provides a basis for the hydrosilylation of C-C unsaturated substrates.…”

“…Reminiscent of the reactivity of the calcium hydride (3), 14 compound 4 was found to be completely unreactive towards the internal alkenes, 2,3-dimethyl-2-butene, cyclopentene and cyclohexene but to react smoothly, albeit slowly, with both the strained bicyclic alkene, norbornene (3 days at 80 C) and the internal alkyne, diphenylacetylene (6 days at 80 C), to provide the magnesium norbornyl (15) and (E)-(1,2-diphenylvinyl) (16) derivatives. Although monitoring of the reaction with norbornene revealed tentative evidence that the production of 15 occurs via the formation of a dinuclear hydridonorbornyldimagnesium intermediate (Fig.…”

“…It has previously been shown that the b-diketiminato calcium hydride, [(BDI)CaH] 2 (3; BDI ¼ HC{CMe 2 NDipp} 2 ; Dipp ¼ 2,6-i-Pr 2 C 6 H 3 ), reacts as a dimer via highly polarised pathways, even with unactivated terminal alkenes, to provide exceptionally potent calcium alkyl nucleophiles, [(BDI) CaR] 2 . [12][13][14][15] In related observations, Harder and co-workers have also very recently divulged that similar alkene insertion reactivity may be achieved with b-diketiminato hydride derivatives of calcium's heavier congener, strontium. 16 In contrast, and despite its heavy use to effect the reductive hydroboration or hydrosilylation of an array of polar C]E (E ¼ O, 17,[18][19][20][21][22] ) and C^E (E ¼ O, 23,24 N, 25 NR 26 ) bonded substrates, no analogous reactivity between Jones' lighter, but similarly dimeric, magnesium hydride, [(BDI)MgH] 2 (4), 8,27 with any C]C bonded small molecule appears to have been described.…”

Stabilizing Magnesium Hydride Complexes with Neutral Ligands

“…The formation of these distinct clusters was rationalized via a self‐assembly process that is controlled by the (Me 3 Si) 2 N anion and neutral ligands. It is thought that a variety of similar clusters are the catalytically active species in hydrogenation of alkenes and imines using H 2 and simple alkaline earth metal amide catalysts like Ae[N(SiMe 3 ) 2 ] 2 (Ae = Mg, Ca, Sr, Ba) . Although one example for self‐assembly of a larger magnesium hydride cluster stabilized by a neutral ligand has been mentioned ( IV ), we wondered whether cluster formation may be controlled by variation of the neutral ligand.…”