Lithium Aluminium Hydride and Metallic Iron: A Powerful Team in Alkene and Arene Hydrogenation Catalysis

Alkenes that normally do not react with LiAlH₄ (3-hexene, cyclohexene, 1-Me-cyclohexene), can be reduced to the corresponding alkanes by a mixture of LiAlH₄ and Fe⁰ (the iron was activated by Metal-Vapour-Synthesis). This alkene-to-alkane conversion with a stoichiometric quantity of LiAlH₄/Fe⁰ does not need quenching with water or acids, implying that both H's originate from LiAlH₄. The LiAlH₄/Fe⁰ combination is also a remarkably potent cooperative catalyst for hydrogenation of multi-substituted alkenes and benzene or toluene. An induction period of circa two hours and the minimally required temperature of 120 °C, suggests that the actual catalyst is a combination of Fe⁰ and the decomposition product of LiAlH₄ (LiH and Al⁰). A thermally pre-activated LiAlH₄/Fe⁰ catalyst did not need an induction time and is also active at room temperature and 1 bar H₂. A combination of AliBu₃ and Fe⁰ is an even more active hydrogenation catalyst. Without pre-activation, tetra-substituted alkenes like Me₂C=CMe₂ and toluene could be fully hydrogenated.