In benzene under an atmosphere of hydrogen, triplet excited-state xanthone or acetone sensitizes the photoreduction of bis(acety1acetonato) nickel(ii), Ni(acac),, to transient nickel(i) complexes which can be detected b y ESR spectroscopy in the presence of stabilizing ligands. The kinetics of these ESR signal decays prove that these nickel ( I) complexes are derived from moderately co-ordinating ligands, such as tetrahydrofuran and 1,5-cyclo-octadiene (1,5-COD), and react with hydrogen in the dark t o form nickel hydride complexes. A n excess of 1,5-COD in the photolysate was sequentially and catalytically isomerized and hydrogenated under the irradiation conditions; i.e., 1,5-COD-+I ,4-COD+l,3-COD+COE-+COA.The nickel hydride complexes are proposed as the key species in the initiation of the catalytic cycle in which steps involving the addition and elimination of nickel hydride bonds and the hydrogenolysis of nickel complexes are assumed. As these alkene reactions cease in the dark and are quenched b y a high concentration of 1,3-COD, it can be concluded that one or more steps in the catalytic cycle requires sensitized photoexcitation.We have shown that triplet excited-state ketones with high triplet-state energy (e.g. xanthone and benzophenone) can sensitize the photoreduction of bis(acetylacetonato)nickel(rr), Ni(acac),, to give nickel(r) complexes and acetylacetone (acacH) in an H-atom donating solvent which is itself oxidized.' The Ni' complexes generally disproportionate to give Ni" compounds and Nio as metallic precipitates (or its complexes in the presence of suitable ligands, L). Ni(acac), + RCH,OH-Ni(acac)L, + acacH + RCHO Scheme 1.

The conditions pertaining to triplet excited state ketone-sensitized photoreduction of Ni(acac)., 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. llheptene. 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(1) complexes of tetrahydrofuran and olefins generated from the photoreduction of N i ( a~a c )~ 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.CHOW, Y. L., LI, H., et YANG, M-S. Can. J. Chem. 66,2920Chem. 66, (1988.On a dkmontrt que les conditions permettant d'effectuer la photorkduction du Ni(acac).,, sensibiliske par I'ktat excitt triplet de la cktone et effectuke sous I'influence de l'hydrogtne, peuvent servir de mkthode pour effectuer des hydrogknations d'olkfines amorckes par la lumitre et prtsentant une grande chtmospkcificitk. Par exemple, on a pu effectuer l'hydrogtnation prkfkrentielle de la double liaison du systtme bicyclique par rapport a I'autre double liaison du dicyclopentadikne et du mkthyltne-5 bicyclo[2,2,l]hepttne. Cette photohydrogtnation, qui est apparemment un processus catalytique homogtne, peut &tre rtaliske dans diverses conditions expkrimentales; dans la dernitre portion de la photohydroghation, il est possible que des processus catalytiques hktkrogtnes interviennent d'une faqon concurrente. On a dkmontrk que les complexes du Ni(1) avec le tktrahydrofuranne et les olkfines qui sont produits lors de la photorkduction du Ni(acac)., rkagissent avec I'hydrogtne pour donner des complexes d'hydrure de nickel. Par analogie avec le processus gknkralement accept6 pour cette reaction, on croit que les olkfines coordonnkes aux complexes d'hydrure de nickel subissent spontankment une addition intramolCculaire conduisant a des complexes de Ni-alkyle. On suggtre que ces complexes de Ni-alkyle sont photoexcitts pour gknkrer un site de coordination vacant qui permet a la rkaction avec I'hydrogtne de prockder pour donner les produits et pour rkgknkrer un catalyseur d'hydrure de nickel.[Traduit par la revue]