Mechanism of hydro-isomerization of methylene-interrupted dienes on nonmetallic palladium-on-resin catalysts III

Abstract

A mixture of conjugated dienes, obtained by alkali-isomerization of methylcis-9,cis-12-octadecadienoate (c9,c12) has been hydrogenated on a nonmetallic palladium-on-resin catalyst at 40C under atmospheric hydrogen pressure in acetone as solvent. The results have been used to quantify the contribution of the conjugation mechanims to the hydrogenation process ofc9,c12. In a 50∶50 mixture conjugated dienes are hydrogenated 4.6 times faster thanc9,c12 at 40 C. This diene is hydrogenated very selectively both by straightforward reduction of one of the double bonds and by reduction of conjugated intermediates. The contribution of the former reaction route is more than 50% under the reaction conditions used. It is assumed that the crucial intermediate in the associative mechanism is formed by hydrogen transfer from the metal ion toc9,c12 resulting in a chelating ring consisting of an ethylene bridge between the π-coordinated double bond and the σ-coordinated bond. This σ, π-complex is responsible for the straightforward, selective hydrogenation ofc9,c12 as well as for the geometric isomerization of this diene.