Homogeneous catalysis by Pt(II)–Sn(II) chloride complex. Part 2.—Mechanism of the isomerization of n-butenes

Abstract

The mechanism of the isomerization of n-butenes catalyzed by a methanolic solution of Pt(II)–Sn(II) chloride complex has been studied by kinetic, mass-spectrometric and microwave-spectroscopic measurements. This isomerization proceeds in the presence of hydrogen. The initial rate of isomerization of 1-butene is of half-order with respect to hydrogen and of first-order with respect to 1-butene in the temperature range 0–20°C and total pressures up to 100 Torr. The deuterium distributions in reactants and products have been measured at 20°C in C₄H₈+ D₂+ CH₃OD systems. It was found that cis-CH₂DCHCHCH₃ and cis-CH₃CDCHCH₃ are formed from 1-C₄H₈ and trans-2-C₄H₈, respectively, and CH₂CHCHDCH₃ is produced from both cis- and trans-2-C₄H₈, and that “simple exchange reaction” of 1-C₄H₈ proceeds faster than the isomerizations, while those of 2-butenes scarcely occur. Double-bond migration and “simple exchange reaction” of propylene in the C₃H₆+ D₂+ CH₃OD system have been studied for comparison with those of n-butene isomerization. Hydrogen atoms in propylene exchanged with different rates depending on their positions.

On the basis of these results, a detailed mechanism is proposed, which involves coordination of olefins to platinum hydride complex, formation of alkyl-platinum, and regeneration of olefins.