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

Abstract

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.