On the Stereoselectivity of Ring-Opening Metathesis Polymerization (ROMP) of N-Arylpyrrolidine-Fused Cyclobutenes with Molybdenum- and Ruthenium-Alkylidene Catalyst

Ring-opening metathesis polymerization (ROMP) of cyclobutenes fused with N-arylpyrrolidene with Schrock-Hoveyda catalyst containing a racemic biphenolate ligand [Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(biphenolate)] gives polycyclobutenes with homogeneous tacticity and predominantly double bonds in Z-configuration. Reactions of the same substrates with the first-generation Grubbs catalyst [(Cy3P)(2)Cl2Ru=CHPh] or Schrock molybdenum carbene with monodendate alkoxy ligandS [Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(OCMe(CF3)(2)] yield the corresponding polycyclobutene containing a mixture of Z- and E-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.