Synthetic applications of sensitized photoreduction of BIS (acetylacetonato) nickel (II)

Synthetic Applications of Sensitized Photoreduction of ~i(acac), The photoreduction of bis(acetylacetonato)nickel(II), Ni(acac),, sensitized by triplet excited state acetone in a 2-propanol/benzene mixture or in THF can serve to catalytically hydrogenate olefins i n s i t u under hydrogen. Under these photohydrogenation conditions, the vinyl group in 4-vinylcyclohexene was reduced while the ring double bond remained unaffected. Other olefins such as norbornene, 5-methylene-bicyclo[2.2.1]heptene, 1,7-octadiene, and dicyclopentadiene were re-investigated under modified conditions. In agreement with previous discoveries, bicyclic dienes were hydrogenated at the more strained double bonds. The requirement of light in the catalytic hydrogenation was established by the observation of the cessation of hydrogenation when irradiation was interrupted or by the addition of large excess of Ni(acac),. The hydrogenation of dicyclopentadiene and 1,5-COD was completely stopped in the presence of four equivalents of P(n-Bu),. Intramolecular cycloaddition of dicyclopentadiene can be sensitized by excited state acetone, xanthone, and benzene with different efficiencies to give a caged product. The sensitized photolysis of Ni(acac), under carbon monoxide was applied to the carbonylation of organic halides under mild conditions. Organic substrates such as bromoand iodobenzene, 1-bromonaphthalene, crotyl bromide, and cyclohexyl iodide were carbonylated, probably by N~(cO), generated i n s i t u , either under photolysis at room temperature or under refluxing in the dark. In order to distinguish between homogeneous and heterogeneous reactions, selective poisoning of colloidal and solid nickel was conducted by the addition of mercury(0). The results were unfortunately ambiguous. A mixture of a photolysate from the sensitized reduction of Ni(acac), and an olefin was shaken under a low hydrogen pressure in the dark but gave no hydrogenation products. On the basis of the experimental results the question of homogeneous vs heterogeneous catalytic processes could not be decided.