Insights into LiAlH<sub>4</sub> Catalyzed Imine Hydrogenation
Holger Elsen, Jens Langer, Gerd Ballmann, Michael Wiesinger, Sjoerd Harder
Abstract
Abstract Commercial LiAlH 4 can be used in catalytic quantities in the hydrogenation of imines to amines with H 2 . Combined experimental and theoretical investigations give deeper insight in the 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 B or Ga also led to dramatically reduced activities. This indicates a heterobimetallic mechanism in which cooperation between Li and Al is crucial. Potential intermediates on the catalytic pathway have been isolated from reactions of MAlH 4 (M=Li, Na, K) and different imines. Depending on the imine, double, triple or quadruple imine insertion has been observed. Prolonged reaction of LiAlH 4 with PhC(H)=N t Bu led to a side‐reaction and gave the double insertion product LiAlH 2 [N] 2 ([N]=N( t Bu)CH 2 Ph) which at higher temperature reacts further by ortho ‐metallation of the Ph ring. A DFT study led to a number of conclusions. The most likely catalyst for hydrogenation of PhC(H)=N t Bu with LiAlH 4 is LiAlH 2 [N] 2 . Insertion of a third imine via a heterobimetallic transition state has a barrier of +23.2 kcal mol −1 (Δ H ). The rate‐determining step is hydrogenolysis of LiAlH[N] 3 with H 2 with a barrier of +29.2 kcal mol −1 . In agreement with experiment, replacing Li for Na (or K) and Al for B (or Ga) led to higher calculated barriers. Also, the AlH 4 − anion showed very high barriers. Calculations support the experimentally observed effects of the imine substituents at C and N: the lowest barriers are calculated for imines with aryl‐substituents at C and alkyl‐substituents at N.