Lithium Aluminium Hydride and Metallic Iron: A Powerful Team in Alkene and Arene Hydrogenation Catalysis
Christian Knüpfer, Christian Färber, Jens Langer, Sjoerd Harder
Abstract
Abstract Alkenes that normally do not react with LiAlH 4 (3‐hexene, cyclohexene, 1‐Me‐cyclohexene), can be reduced to the corresponding alkanes by a mixture of LiAlH 4 and Fe 0 (the iron was activated by Metal‐Vapour‐Synthesis). This alkene‐to‐alkane conversion with a stoichiometric quantity of LiAlH 4 /Fe 0 does not need quenching with water or acids, implying that both H's originate from LiAlH 4 . The LiAlH 4 /Fe 0 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 Fe 0 and the decomposition product of LiAlH 4 (LiH and Al 0 ). A thermally pre‐activated LiAlH 4 /Fe 0 catalyst did not need an induction time and is also active at room temperature and 1 bar H 2 . A combination of Al i Bu 3 and Fe 0 is an even more active hydrogenation catalyst. Without pre‐activation, tetra‐substituted alkenes like Me 2 C=CMe 2 and toluene could be fully hydrogenated.