Kinetic and Mechanistic Investigation of the Sequential Hydrogenation of Phenylacetylene Catalyzed by OsHCl(CO)(PR3)2

Article abstract of DOI:10.1021/ja00201a024

The reactivities of the hydrido carbonyl complexes OsHCl(CO)(PR3)2 toward hydrogen, alkynes, and oxygen have been studied.The solutions of 1 and 2 are rapidly decolorized upon contact with H2 under ambient conditions; the decolorized solution of 2 shows in benzene-d6 a (1)H NMR spectrum that is consistent with the formation of the dihydrogen compound OsHCl(η²-H2)(CO)(P-i-Pr3)2 (3).The reactivity of 1 and 2 toward alkynes depends on the type of alkyne used.The title complexes react with acetylene, propyne, and phenylacetylene by insertion to give the five-coordinate vinylosmium compounds Os(CH=CHR)Cl(CO)(PR3)2 (7 and 8); the same starting materials in the presence of and are completely inert.Treatment of 2 with leads to the compound ; the trans- and cis-alkyne hydrido intermediates have been observed.The hydridoosmium(II) complexes 1 and 2 also react with O2 to form the dioxygen adducts OsHCl(η²-O2)(CO)(PR3)2 (11 and 12).The complexes 1 and 2 catalyze the sequential hydrogenation of phenylacetylene in 2-propanol solution at 60 degC.Selectivities close to 100percent are achieved for the hydrogenation of the alkyne to the alkene.The kinetic investigation of this reaction provides evidence that indicates that the formation of styryl derivatives is the step that determines the selectivity for the hydrogenation to the alkene.