Combined KH/alkaline-earth metal amide catalysts for hydrogenation of alkenes

Abstract

Breaking saline KH: The combined KH/alkaline-earth metal amide catalysts display much better catalytic activity than their components in the hydrogenation of alkenes, suggesting the degradation and activation of saline KH with the metal amides.

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

“…The increased reactivity resulted again in a small decrease of yield (89–92 % ethylbenzene) alongside minor oligomerization. Contrary to our observations, Guan reported that the hydrogenation of styrene with KH and Mg(HMDS) 2 under 6 bar of H 2 pressure gave only 5 % of ethylbenzene since most of the substrate immediately undergoes polymerization [35] . Assuming that the active catalyst is in both cases a hydride complex, a reasonable explanation is that the protonation of the alkyl intermediate is facilitated with the prior formed H−HMDS (pK a ≈26) [45] than with H 2 (pKa≈49) [46] and thus liberates the product prior to C−C bond formation.…”

“…A low‐cost route to heterobimetallic s ‐block metal hydrides was presented very recently by the Guan group (Scheme 1e). Barely soluble potassium hydride was mixed together with AEM(HMDS) 2 (AEM=Mg, Ca) resulting in formation of an effective hydrogenation catalyst for olefins [35] . Although the active species itself could not be isolated, the group found significant evidence that (in the case of magnesium) the compound is presumably the aforementioned [KMg(H)(HMDS) 2 ] 2 reported by Hill et al [32] …”

“…Barely soluble potassium hydride was mixed together with AEM(HMDS) 2 (AEM=Mg, Ca) resulting in formation of an effective hydro-genation catalyst for olefins. [35] Although the active species itself could not be isolated, the group found significant evidence that (in the case of magnesium) the compound is presumably the aforementioned [KMg(H)(HMDS) 2 ] 2 reported by Hill et al [32] In our continued development of synergistic alkali-metalmediated bimetallic chemistry of main group metals, [36] we recently started to investigate their performance in catalysis. Our initial investigations have found that certain bimetallic systems can operate more efficiently compared to their monometallic counterparts.…”

“…Contrary to our observations, Guan reported that the hydrogenation of styrene with KH and Mg(HMDS) 2 under 6 bar of H 2 pressure gave only 5 % of ethylbenzene since most of the substrate immediately undergoes polymerization. [35] Assuming that the active catalyst is in both cases a hydride complex, a reasonable explanation is that the protonation of the alkyl intermediate is facilitated with the prior formed HÀ HMDS (pK a � 26) [45] than with H 2 (pKa � 49) [46] and thus liberates the product prior to CÀ C bond formation. In all catalytic experiments using potassium, rubidium or caesium magnesiates only small amounts of H 2 were observed as side product, indicating that the hydrogenation (Scheme 2, eqn.…”

Heterometallic Mg−Ba Hydride Clusters in Hydrogenation Catalysis

“…[38][39][40][41] The combination KH/AeN" 2 (Ae=Mg, Ca) was recently put forward as a hydrogenation catalyst for activated (conjugated) alkenes, terminal unactivated alkenes or cyclic internal alkenes. [67] Screening all alkali metals, mixtures of MN" (M=Li, Na, K, Rb, Cs) and MgN" 2 were shown to be catalysts for transferring hydrogen from 1,4-cyclohexadiene to either Ph 2 C=CH 2 or Ph(H) C=CH 2 . [37] It is notable that each component alone was not active in this catalytic transformation.…”