On the Stereoselectivity of Ring-Opening Metathesis
Polymerization (ROMP) of <i>N</i>‑Arylpyrrolidine-Fused
Cyclobutenes with Molybdenum– and Ruthenium–Alkylidene
Catalyst
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Abstract
Ring-opening metathesis polymerization
(ROMP) of cyclobutenes fused
with <i>N</i>-arylpyrrolidene with Schrock–Hoveyda
catalyst containing a racemic biphenolate ligand [Mo(N-2,6-<i>i</i>-Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)(CHCMe<sub>2</sub>Ph)(biphenolate)] gives polycyclobutenes with homogeneous tacticity
and predominantly double bonds in <i>Z</i>-configuration.
Reactions of the same substrates with the first-generation Grubbs
catalyst [(Cy<sub>3</sub>P)<sub>2</sub>Cl<sub>2</sub>RuCHPh]
or Schrock molybdenum carbene with monodendate alkoxy ligands [Mo(N-2,6-<i>i</i>-Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)(CHCMe<sub>2</sub>Ph)(OCMe(CF<sub>3</sub>)<sub>2</sub>] yield the corresponding polycyclobutene
containing a mixture of <i>Z</i>- and <i>E</i>-double bonds. Upon diimide reduction, all these polycyclobutenes
give the same tactic hydrogenated polymers, indicating that the stereochemistry
at the asymmetric carbons remains the same in all these reactions.
The stereoselectivities of ROMP with cyclobutenes and with norbornenes
are compared, and the plausible mechanisms are proposed