Hydrogenation of conjugated dienes and isomerization of butenes on CdO, Co3O4 and Cr2O3

Abstract

Deuteration of buta-1,3-diene, 2-methylbuta-1,3-diene and penta-1,3-diene selectively yielded but-2-ene, 2-methylbut-2-ene and 1,4-[²H₂]pent-2-ene on CdO and Co₃O₄, and but-1-ene, 2-methylbut-1-ene and 4,5-[²H₂]pent-2-ene on Cr₂O₃, respectively. These results imply that the selectivity for 1,2- and 1,4-hydrogen addition is maintained in hydrogenation of conjugated dienes, i.e.1,4-addition on CdO and Co₃O₄, and 1,2-addition on Cr₂O₃. The hydrogenation of buta-1,3-diene using a 1:1 mixture of H₂ and D₂ gave [²H₂]but-1-ene in larger amounts than the [²H₀]isomer on Cr₂O₃. An isotope effect smaller than unity was interpreted by preferential adsorption of D₂ on the Cr₂O₃ surface. Formation of [²H₁]butene was dominant in deuteration of buta-1,3-diene, indicating that residual hydrogen participates in the reaction; the amount corresponded to 7 times as much as that of chemisorbed hydrogen at room temperature. Co-isomerization of [²H₀] and [²H₈]but-1-ene yielded, selectively, [²H₀] but-2-ene on CdO. Hydrogenation of buta-1,3-diene with a 1:1:1 mixture of H₂, HD and D₂ also gave [²H₀] butene isomers selectively on CdO. These results infer that one intermediate species for hydrogenation of buta-1,3-diene is closely related to that for isomerization of butenes, and these reactions occur through a π-allylic anion intermediate on CdO.