Catalytic hydrogenations of tolane and stilbene with Ti- and Ce-doped complex metal hydrides

Classical hydrogenation catalysts usually give high activities at mild conditions, but often low selectivities. Hydrogenations with complex hydrides like NaBH4 or LiAlH4 are rather selective, but these hydrides must be employed in stoichiometric amounts. We have investigated the catalytic hydrogenation of tolane (diphenylacetylene) with gas phase hydrogen over Ti- and Ce-doped NaAlH4 and Na3AlH6. These hydrides are reversible; i.e. can be rehydrogenated with pressurized hydrogen after the dehydrogenation, so they can be used in catalytic amounts for the hydrogenation of unsaturated hydrocarbons. We propose a mechanism involving the cycling of the hydride between hydrogenated and dehydrogenated form of the hydride-the hydride hydrogenates the hydrocarbon and is dehydrogenated itself and afterwards takes up hydrogen from the atmosphere to be rehydrogenated.