Hydrogenation by Cyanocobaltate. II. Mechanisms of the Hydrogenations of 1,3-Butadiene and Isoprene Catalyzed by Pentacyanocobaltate(II) in Aqueous Solution

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Abstract
Hydrogenations of 1,3-Butadiene and isoprene catalyzed by pentacyanocobaltate(II) have been studied by the measurements of infrared and nuclear magnetic resonance spectra on the reaction solutions and by following the time-courses of the reactions. Infrared spectroscopy has revealed: (1) The aging reaction is promoted by CN- ion and it hardly occurs at a low CN/Co ratio. (2) In the hydrogenation of butadiene the formation of a σ-but-2-enyl complex at a high CN/Co ratio is very rapid, but that of a syn-π-(1-methylallyl) complex at a low CN/Co ratio is generally slow. The rate of the formation of these complexes is affected by the relative concentration of butadiene to Co(CN)5H3−. (3) In the hydrogenation of isoprene a σ-3-methylbut-2-enyl complex is formed regardless of the CN/Co ratio. Nuclear magnetic resonance spectroscopy has confirmed the structures of these complexes and it also showed that the σ-but-2-enyl complex consists of simliar amounts of the cis- and trans-isomers. The reactions are explained by the mechanisms, in which intermediate complexes in the formation of alk-1-enes and alk-2-enes are the cis- and trans σ-but-2-enyl and the syn-π- (1-methylallyl) complexes in the case of butadiene, and the σ-2- and -3-methylbut-2-enyl and the π-(1,2-dimethylallyl) complexes in the case of isoprene. It is described that the reactivity and the selectivity in the hydrogenation of conjugated dienes are strongly controlled by the steric factor in the transition state.