Lithium aluminum hydride, a homogeneous hydrogenation catalyst

LiAlH4: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

“…[23] Also the last s-bond metathesis step in the cycle seemed questionable.Acomprehensive calculational study, however, demonstrated that the metal hydride mechanism shown in Scheme 1isfeasible. [21] Given the simplicity of these group 2m etal catalysts,w e wondered whether LiAlH 4 ,w hich in imine reduction is normally added stoichiometrically,c ould be catalytically active.A lready in the 1960s,L iAlH 4 was used as ac atalyst for alkyne reduction, [24] however, the very harsh conditions needed (35 mol %c at, 190 8 8C, 80 bar H 2 )d id not encourage follow-up research. Acrucial step is the hydrogenolysis of the metal À Cbond by H 2 ,for which the rate is known to decrease with decreasing bond ionicity:N a À C > Mg À C > Al À C. [25] since it is well-known that RNH 2 and R 2 NH react smoothly with LiAlH 4 to give aluminium amide products and H 2 , [26][27][28][29][30][31] hydrogenolysis of AlÀNbonds to give amines is anticipated to be even more cumbersome.S ubstitution of H 2 for polar borane or silane reductants was therefore al ogical step.…”

“…[31,38] Aluminium-catalyzed hydrogenations using the simple reductant H 2 ,h owever, are limited to highly Lewis-acidic tricoordinate Al catalysts, [43] often using high temperatures and H 2 pressures. [17,24] In contrast, the reversed reaction, namely aluminium-catalyzed dehydrogenation, has been studied in depth:f or example, HC(O)OH!CO 2 + H 2 . [15,41,42] Herein, we introduce LiAlH 4 , which is normally applied as astoichiometric reducing agent, as as imple catalyst for imine reduction using the bulk commodity H 2 at ac onvenient 1bar pressure (Table 1).…”

Alkali Metal Species in the Reversible Activation of H2

“…200 8 8Ca nd H 2 pressure of > 100 bar. This system was subsequently studied in detail by Berkessel et al [5] Similarly,soluble LiAlH 4 [6] or suspensions of NaH, KH, and MgH 2 [7] were reported to effect catalytic hydrogenation of several olefinic and acetylenic substrates, although again rather extreme temperatures (150-225 8 8C) and H 2 pressures (60-100 bar) were required. Under similar forcing conditions (280 8 8C, 150 bar H 2 pressure), Haenel et al reported the reductions of polyaromatics in coal via ac atalytic hydroboration/hydrogenolysis process.…”

LiAlH4: From Stoichiometric Reduction to Imine Hydrogenation Catalysis

“…[19] Since the imine itself is aL ewis base,b ulky imines can be reduced by B(C 6 F 5 ) 3 as ac atalyst. [21] Given the simplicity of these group 2m etal catalysts,w e wondered whether LiAlH 4 ,w hich in imine reduction is normally added stoichiometrically,c ould be catalytically active.A lready in the 1960s,L iAlH 4 was used as ac atalyst for alkyne reduction, [24] however, the very harsh conditions needed (35 mol %c at, 190 8 8C, 80 bar H 2 )d id not encourage follow-up research. [21] This finding was rather unexpected since the catalyst initiation step (Scheme 1) is formally ad eprotonation of H 2 (pK a % 49), [22] thus giving the metal hydride catalyst and amine N''Hw ith am uch lower pK a (25.8).…”

“…Acrucial step is the hydrogenolysis of the metal À Cbond by H 2 ,for which the rate is known to decrease with decreasing bond ionicity:N a À C > Mg À C > Al À C. [25] since it is well-known that RNH 2 and R 2 NH react smoothly with LiAlH 4 to give aluminium amide products and H 2 , [26][27][28][29][30][31] hydrogenolysis of AlÀNbonds to give amines is anticipated to be even more cumbersome.S ubstitution of H 2 for polar borane or silane reductants was therefore al ogical step. [17,24] In contrast, the reversed reaction, namely aluminium-catalyzed dehydrogenation, has been studied in depth:f or example, HC(O)OH!CO 2 + H 2 . [32] Aluminium-driven catalysis using polar activated substrates currently experiences immense interest.…”