Selective Catalytic Hydrodimerization of 1,3-Butadiene by Palladium Compounds Dissolved in Ionic Liquids

Palladium(II) compounds dissolved in 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMI·BF₄) ionic liquid are shown to be able to catalyze the hydrodimerization of 1,3-butadiene. In most of the cases, only the 1,3-butadiene dimer 1,3,6-octatriene and the telomer octa-2,7-dien-1-ol have been obtained. The products' selectivity and catalytic activity depend on the reaction conditions. 1,3-Butadiene conversion up to 28%, a turnover frequency (TOF) of 118 h^-1, and a selectivity of 94% on telomer were achieved with (BMI)₂PdCl₄ dissolved in BMI·BF₄. The 1,3-butadiene conversion and TOF were significantly increased to 49% and 204 h^-1, respectively, by a 5-atm pressure of carbon dioxide. The reactions were performed under homogeneous conditions at 70 °C. However, at temperatures below 5 °C, a two-phase system is formed and the products are easily removed from the reaction mixture by simple decanting. The recovered ionic catalyst solution can be reused several times without any significant changes in the catalytic performance. The structure of the new catalyst precursor (BMI)₂PdCl₄ has been determined by X-ray diffraction analysis.