Catalysis in Acyclic Diene Metathesis (ADMET) Polymerization - Late Transition Metal Polymerization Catalysis - page 199

6.2.6
Regiochemical and Stereochemical Considerations

Paths F and H in Scheme 6.9 are based on different regiochemical orientations of
the cycloaddition reactions. The olefin and the carbene must be oriented cofacially
prior to the cycloaddition to form the metallacyclobutane ring, so the different
stereochemical pathways possible can be modeled (Scheme 6.10).
The preferences of the various pathways are dependent on the catalyst used,
specifically the electronic and steric factors involved. The electronic contribution is
based on the preference of the metallacycle to have the electron-donating alkyl
groups at either the carbon of the metallacycle [23]. The steric factors

involved in the approach of the olefin to the metal carbene also determine the re-
giochemistry of the metallacyclobutane formed. These factors include both steric
repulsion of the olefin and carbene substituents from each other and from the an-
cillary ligands of the metal complex. Paths (b), (c), and (e) in Scheme 6.10 are im-
portant to productive ADMET. The relative rates of pathways (c) and (e) will deter-
mine the kinetic amount of cis and trans double bonds in the polymer chain.
However, in some cases a more thermodynamic ratio of cis to trans olefin isomers
is attained after long reaction times, presumably by a trans- metathesis olefin equi-
libration mechanism [31] (Scheme 6.11).

or the

Scheme 6.10 Some stereochemical pathways of metathesis.