Switched Stereocontrol in Grubbs−Hoveyda Complex Catalyzed ROMP Utilizing Proton-Switched NHC Ligands

Grubbs−Hoveyda and Grubbs III type complexes with ferrocenyl- or −NEt2-substituted NHC ligands were synthesized according to standard procedures. The electron donation of the NHC ligands in the respective ruthenium complexes can be modulated by oxidation of the ferrocenyl moiety or by protonation of the amino group. The neutral and the respective cationic (oxidized or protonated) ruthenium complexes were tested in the ROMP of norbornene. The change in the electron donation of the NHC ligands upon protonation leads to a significant change in the double-bond geometry (from E/Z ratio = 0.78 to E/Z = 1.04) and in the microstructure of the resulting polynorbornene. Consequently, addition of acid and protonation of the living catalyst attached to the polymer chain during the polymerization reaction allows fine-tuning the E/Z ratio of the resulting polynorbornene.