LiAlH4-catalyzed Imine Hydrogenation with Dihydrogen: New DFT Mechanistic Insights

Abstract

Recently, it was found that the widely used stoichiometric reducing reagent LiAlH4 can also be used as a useful catalyst for imine hydrogenation with H2 under relatively mild conditions. In this work, extensive state-of-the-art DFT calculations are conducted to explore the detailed catalytic mechanism. In sharp contrast to the recent proposal involving heterolytic H2-activation over Al−N amide bonds after multiple imine insertion, our new mechanism highlights the dual role of the Lewis-acidic lithium cation in frustrated-Lewis-pair-like activation of both imine substrate and H2 when combined with the aluminum-bound hydride and amide ligands, respectively. Ionic Li−N amide bonds are about 13 kcal/mol more reactive than Al−N amide bonds for H2 activation, thus providing a useful structural feature for rational design of active hydrogenation catalysts.