Light-promoted catalytic hydrogenation of olefins with nickel complexes: the formation of nickel hydrides from the reaction of Ni(I) complexes with hydrogen

The conditions pertaining to triplet excited state ketone-sensitized photoreduction of Ni(acac)2 under hydrogen were shown to serve as a method for light-promoted hydrogenation of olefins with clean chemospecificity. For example, the double bond in the bicyclic system was preferentially hydrogenated over the other double bond in dicyclopentadiene and in 5-methylenebicyclo-[2.2.1]heptene. This photohydrogenation could be run under a wide range of conditions and was apparently a homogeneous catalytic process; in a late stage of photohydrogenation, concurrence of heterogeneous catalytic processes was not ruled out. The Ni(I) complexes of tetrahydrofuran and olefins generated from the photoreduction of Ni(acac)2 were shown to react with hydrogen to give nickel hydride complexes. By analogy with the generally accepted reaction pattern, coordinated olefins in these nickel hydride complexes probably spontaneously undergo intramolecular addition to give Ni–alkyl complexes. It is suggested that these Ni–alkyl complexes are photoexcited to generate a vacant coordination site so that the reaction with hydrogen can proceed to give products and regenerate a nickel hydride catalyst.