The Selective Hydrogenation of 2, 3-Dimethyl-1, -3butadiene with Bis(acetylacetonato)nickel(II)-Triethyldialuminium Trichloride-Triphenylphosphine Catalyst

Abstract

The selective hydrogenation of 2, 3-dimethyl-1, 3-butadiene(DMBD) to monoenes with Ni(acac)₂-Al₂(C₂H₅)₃Cl₃-P(C₆H₅)₃(Ni:Al₂:P=1:10:5) catalyst was investigated. DMBD was hydrogenated in toluene at 40°C under 1 atm of H₂ to give 2, 3-dimethy1-1-butene and 2, 3-dimethy1-2-butene selectively, the ratio being about 2 : 1 in the initial stage (Fig.1). The monoenes formed were isomerized rapidly just before the completion of selective hydrogenation, while, even after its completion they were not practically hydrogenated to 2, 3-dimethylbutane. These results suggest that a π-allylic nickel complex is a reactive intermediate for the selective hydrogenation. The overall rate of hydrogenation, measured by 112 absorption, can be represented by the form : R=k[H₂][Ni], k=10^9.1 exp (-13200/R T) lmol^-1sec^-1, where [H₂] and [Ni] are the concentrations of H₂ and Ni(acac)₂, respectively. The activation parameters are ΔH^≠=12.6 kcal mol^-1 and ΔS^≠=-19 cal deg^-1mol^-1. The kinetic resulsts can be interpreted in terms of the following mechanism and suggest that the latter step is the rate-determining step and the C₆H₁₀ + Ni-H ⇔ Ni(π-C₆H₁₁) Ni(π-C₆H₁₁) + H₂ ⇒ C₆H₁₂ + Ni-H equilibrium prior to the step lies almost entirely in the right under the conditions investigated.