BIAN‐Aluminium‐Catalysed Imine Hydrogenation

Abstract

On the occasion of the 60 th birthday of Matthias BellerHydrogenations have been dominated by transition metal catalysis, while the use of more abundant and inexpensive main group metal catalysts has remained a great challenge. Here, a bimetallic Li/Al dihydride was successfully applied to catalytic hydrogenations of imines. The catalyst [( Dipp BIAN)Al(μ-H) 2 Li-(OEt 2 ) 2 ] was easily prepared from the 2e-reduced BIAN derivative and LiAlH 4 .

Alkali Metal Dihydropyridines in Transfer Hydrogenation Catalysis of Imines: Amide Basicity versus Hydride Surrogacy

“…Examples from the s‐block include Harder's use of Group 2 amides [M{N(SiMe 3 ) 2 } 2 ], where M=Mg, Ca, Sr, Ba, [4b] and aluminates [M(AlH 4 ) 2 ⋅ (THF) n ], where M=Mg, Ca, Sr, [7] and two examples illustrating alkali metal mediation ( AMM ) using pre‐catalyst LiAlH 4 for imine hydrogenation. [ 3c , 4c ] Although direct hydrogenation is effective as an atom economical process, the drawbacks with respect to the handling and storage of H 2 gas makes transfer hydrogenation an appealing alternative. [8] To this end, promising but sporadic reports of both homogeneous and heterogeneous catalysis by alkali metal hydroxides, alkoxides, and phosphates in reduction of polar carbonyl functional groups using iso‐propanol as the transfer hydrogenating reagent have appeared.…”

Alkali Metal Dihydropyridines in Transfer Hydrogenation Catalysis of Imines: Amide Basicity versus Hydride Surrogacy

“…Thus, Stephan pioneered the use of Lewis acidic B(C 6 F 5 ) 3 in metal‐free hydrogenation of imines, exploiting the Frustrated Lewis Pair (FLP) mechanism, [5] while soluble molecular aluminium [6] and zinc [5b] hydrides have been found to accomplish the same, albeit under harsh conditions (about 50–100 °C, 100 atm, mostly over 24 hours). Examples from the s‐block include Harder's use of Group 2 amides [M{N(SiMe 3 ) 2 } 2 ], where M=Mg, Ca, Sr, Ba, [4b] and aluminates [M(AlH 4 ) 2 ⋅ (THF) n ], where M=Mg, Ca, Sr, [7] and two examples illustrating alkali metal mediation ( AMM ) using pre‐catalyst LiAlH 4 for imine hydrogenation [3c, 4c] . Although direct hydrogenation is effective as an atom economical process, the drawbacks with respect to the handling and storage of H 2 gas makes transfer hydrogenation an appealing alternative [8] .…”