Insights into LiAlH₄ Catalyzed Imine Hydrogenation

Abstract

Commercial LiAlH 4 can be used in catalytic quantities in the hydrogenation of iminest oa mines with H 2 .C ombined experimental and theoretical investigations give deeper insighti nt he mechanism and identifies the most likely catalytic cycle. Activity is lost when Li in LiAlH 4 is exchanged for Na or K. Exchanging Al for Bo rG aa lso led to dramatically reduced activities. This indicates ah eterobimetallic mechanism in which cooperation between Li and Al is crucial. Potential intermediates on the catalytic pathway have been isolatedf rom reactions of MAlH 4 (M = Li, Na, K) and differenti mines. Depending on the imine, double, triple or quadruplei mine insertion has been observed. Prolonged reaction of LiAlH 4 with PhC(H) = NtBu led to as ide-reaction and gave the double insertion product LiAlH 2 [N] 2 ([N] = N(tBu)CH 2 Ph) which at higher temperature reacts furtherb y ortho-metallation of the Ph ring. AD FT study led to a number of conclusions. The most likely catalystf or hydrogenation of PhC(H) = NtBu with LiAlH 4 is LiAlH 2 [N] 2 .I nsertion of athird imine via aheterobimetallic transition state has abarrier of + 23.2 kcal mol À1 (DH). The rate-determining step is hydrogenolysis of LiAlH[N] 3 with H 2 with ab arrier of + 29.2 kcal mol À1 .I na greement with experiment,replacingL i for Na (or K) and Al for B(or Ga) led to highercalculated barriers. Also, the AlH 4 À anions howedv ery high barriers. Calculationss upport the experimentally observed effects of the imine substituents at Ca nd N: the lowestb arriers are calculated for iminesw ith aryl-substituents at Ca nd alkyl-sub-stituentsatN .

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

“…They are (mostly) biocompatible, environmentally benign and with their high abundance in the crustal layer and oceans of earth, a low price per mole is guaranteed. Activity in this area has been steadily rising since the turn of the millennium with pioneering contributions by Harder, Hill, Okuda, Cheng and Stock, with several catalytic hydrogenations published [7–27] …”

Alkali‐Metal Mediation: Diversity of Applications in Main‐Group Organometallic Chemistry

“…Thus, the insertion of HBpin into the C=N bond proceeds more readily for the bimetallic compound I [3i] . Harder and co‐workers reinforced this hypothesis by a DFT study in their report on LiAlH 4 ‐catalysed imine hydrogenation, where they showed that the bridging Al‐H‐Li interactions are maintained throughout the catalytic cycle [7c,g] …”

Cationic Aluminium Complexes as Catalysts for Imine Hydrogenation

“…Motivated by the high potential of cationic Al catalysts like AlEt 2 + in CO 2 to methane conversion, [31] we systematically investigated the application of cationic Al catalysts in imine hydrogenation. While it is known that neutral Al compounds like Al i Bu 3 catalyse this reaction under harsh conditions (>100 bar H 2 , 100 °C) [32] and LiAlH 4 is an effective catalyst under relatively mild conditions (1 bar, 80 °C), [33,34] we now report a series of highly Lewis acidic cationic β‐diketiminate (BDI) Al catalysts. Similar as in the recently reported imine hydrogenation with cationic Zr catalysts, [35] the advantage of cationic (BDI)Al + catalysts is the facile control over electronics and sterics by tuning the ligand through variation of substituents.…”