Stereochemistry in metathesis of n-alkenes using heterogeneous oxide catalysts

Abstract

The selective formation of geometrical product isomers in the metathesis of both propene and pent-2-ene (cis and trans) was studied over several solid catalysts. trans- And cis-but-2-ene were formed in nearly equal amounts from propene, although there were some differences between Re₂O₇/γ-Al₂O₃, WO₃/SiO₂ and MoO₃/SiO₂ catalysts. The initial trans/cis ratio, (t/c)₀, for but-2-ene appeared to be independent of the reaction temperature and the propene pressure. Over Re₂O₇/γ-Al₂O₃, cis-pent-2-ene gave (t/c)₀ values for but-2-ene (B) and hex-3-ene (H) of 0.35–0.40 and 0.95–1.1, respectively. Starting with trans-pent-2-ene a preference was observed for the formation of trans-products: (c/t)₀ B ≈ 0.30 and (c/t)₀ H ≈ 0.12. The weight content of the active phase in the catalyst had no influence on the stereoselectivity, indicating that the active centres have a very distinct character. These results are explained using a stereochemical model based on a metallacyclobutane intermediate. The governing factors in the selectivity of product formation are the repulsive interactions between the alkyl substituents of the intermediate structure and the interactions of the substituents with the catalyst surface.