Torker, S.; Müller, A.; Chen, P. “Building Stereoselectivity into a Chemoselective Ring-Opening Metathesis Polymerization Catalyst for Alternating Copolymerization.” Angew. Chem. Int. Ed. 2010, 49, 3762-3766.
A recent report from the Peter Chen group presents a series of elegant investigations that highlight factors that influence the chemoselectivity and stereoselectivity of metathesis polymerizations. Through careful study of the ring-opening metathesis polymerization (ROMP) mechanism, the Chen group has designed a series of catalysts with significant control of polymer structure. Central to their work has been a ruthenium metathesis catalyst bearing a bidentate, asymmetric phosphine ligand (structures 1,2) which is capable of alternating copolymerization of norbornene and cyclooctene1,2.
Many features of metathesis catalysts can be traced to factors affecting the stability of the metallocyclobutane intermediate. For each metathesis event, the alkylidene/metallocyclobutane/alkylidene catalyst cycle alternates the carbene between two different sites on the catalyst. Typically, free rotation of the phosphine ligand (in the case of 1st generation Grubbs catalysts) or ligand symmetry (in the case of 2nd generation Grubbs catalysts) makes these two sites degenerate. The phosphine in Chen’s chemoselective ROMP catalyst cannot rotate, and the catalyst thus alternates between two states with very different steric influences:
Catalyst A will incorporate both norbornene and cyclooctene, as metathesis of either compound will return the carbene to the less-hindered site.
Catalyst B will only incorporate norbornene, as the release of ring strain will drive the metathesis, despite placing the resulting carbene in the more-hindered site.
If the concentration of norbornene is low with respect to cyclooctene, ROMP with this catalyst generates a largely 1:1 alternating copolymer.
Replacing the chorine ligand with a bulky sulfonate adds additional selectivity (2a-2d, see also here). Typically, metathesis favors Z-selectivity, due to steric interaction between the substituents in the metallocyclobutane intermediate. The sulfonate ligand makes E-selectivity more competitive by introducing new steric repulsions which direct the propagating polymer chain away from the sulfonate, while also influencing the directionality of the coordination of the olefin monomer. By increasing the steric bulk of the sulfonate substituents, polymer with up to 51% cis bonds could be formed. Compared to the 90% trans bonds observed with the chloride variant, that’s an impressive shift. The past ten years have produced a wide range of computational and kinetic investigations that clarify many details of the metathesis reaction. Detailed understanding of catalyst mechanisms has allowed Chen and others to push metathesis and ROMP in exciting new directions.
1 Bornand, M.; Torker, S.; Chen, P. “Mechanistically Designed Dual-Site Catalysts for the Alternating ROMP of Norbornene and Cyclooctene.” Organometallics 2007, 26, 3585-3596.
2 Bornand, M.; Chen, P. “Mechanism-Based Design of a ROMP Catalyst for Sequence-Selective Copolymerization.” Angew. Chem. Int. Ed. 2005, 44, 7909-7911.