Chemoselective ring-opening metathesis polymerization of cyclopropenes spirally appended with N-aryl saturated heterocycles

Abstract

The nature of the substituents on cyclopropenes and catalysts could have a profound effect on the reactivity and selectivity of ring-opening metathesis polymerization (ROMP). We have previously reported the Grubbs-I or Grubbs-II catalyzed ROMP of N-aryl-azetidine-appended cyclopropenes to obtain linear or cyclic polymers, respectively. In this work, similar reactions were performed with piperidine-appended spirocyclopropenes (SCPs) and different chemoselectivities were observed. When a Grubbs-I catalyst was used, the monomeric ring-opening cross metathesis product was achieved after quenching with ethyl vinyl ether, whereas the corresponding linear polymers with double bonds in the E-configuration and dimeric cyclohexadienes (CHDs) are obtained in the presence of a Grubbs-II, Hoveyda–Grubbs II or Grubbs-III catalyst. We carefully followed these Ru carbene propagating species during polymerization and clarified their chelation mode. We disclosed that the non-planar and fluxional conformation of the piperidine moiety with higher steric hindrance than the azetidine motif would be the key to the critical discrepancy in both the reactivity and selectivity and the way of polymerization. This work would like to provide insights into the mechanism of the chemoselective ROMP of spirocyclopropenes using Grubbs catalysts and offer clues for designing new ROMP reactions.