Abstract
Commercial LiAlH4 can be used in catalytic quantities in the hydrogenation of imines to amines with H2. Combined experimental and theoretical investigations give deeper insight in the mechanism and identifies the most likely catalytic cycle. Activity is lost when Li in LiAlH4 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 MAlH4 (M=Li, Na, K) and different imines. Depending on the imine, double, triple or quadruple imine insertion has been observed. Prolonged reaction of LiAlH4 with PhC(H)=NtBu led to a side‐reaction and gave the double insertion product LiAlH2[N]2 ([N]=N(tBu)CH2Ph) 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)=NtBu with LiAlH4 is LiAlH2[N]2. Insertion of a third imine via a heterobimetallic transition state has a barrier of +23.2 kcal mol−1 (ΔH). The rate‐determining step is hydrogenolysis of LiAlH[N]3 with H2 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 AlH4 − 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.
Highlights
Since its first synthesis LiAlH4 has become one of the most commonly used reducing agents
During the development of early main group metal catalyzed imine hydrogenation,[14] we found that commercially available LiAlH4 can be used under relatively mild conditions in catalytic instead of stoichiometric quantities (2.5 mol % catalyst loading, 1 bar H2 and 85 8C).[13,15]
While NaAlH4 was found to be less active than LiAlH4,[13] we became interested in exploring KAlH4 in imine hydrogenation
Summary
Since its first synthesis LiAlH4 has become one of the most commonly used reducing agents. These applications are based on stoichiometric use of LiAlH4, the last decades have seen some interesting examples of LiAlH4 (or related compounds) in catalysis.[6,7,8,9,10,11,12,13] During the development of early main group metal catalyzed imine hydrogenation,[14] we found that commercially available LiAlH4 can be used under relatively mild conditions in catalytic instead of stoichiometric quantities (2.5 mol % catalyst loading, 1 bar H2 and 85 8C).[13,15] Such a non-stoichiometric route prevents the generally hazardous aqueous work-up and avoids considerable amounts of Li/Al salts as side-products. We report additional experimental proof for such a heterobimetallic mechanism and support our observations with a comprehensive computational study
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