A stereochemical approach of metathesis of cis- and trans-2-pentene has been carried out with several precursor complexes of tungsten. A mechanism of coordination of the internai olefin on the two different metallocarbene moieties is pro-posed. It accounts for the competitive cis-trans isomerization or formai metathesis of the starting olefin. The mechanism is based on kinetic data related to the respective rates of eight elementary steps occurring at low conversion in metathesis of cis-2-pentene. Stereochemistry of metathesis of trans olefins has also been studied: trans olefins do not seem to be able to produce the metallocarbene responsible for the propagation, whereas cis internai olefins or α olefins can do it. The stereoselectivity observed is higher for trans olefins than for cis olefins; they can be explained on the basis of steric repulsion between the R group of the olefin and the bulky ligand coordinated to tungsten or on the basis of stability of various configurations of metallocyclo-butanes. A kinetic model derived from the proposed mechanism accounts for the linear relationship observed between trans/cis C4 or C6, vs. trans/cis C5 at low conversion.
ASJC Scopus subject areas
- Colloid and Surface Chemistry