Mechanistic Investigations on the Competition between the Cyclopolymerization and [2 + 2 + 2] Cycloaddition of 1,6-Heptadiyne Derivatives Using Second-Generation Grubbs Catalysts

Abstract

Until recently, the cyclopolymerization (CP) of 1,6-heptadiyne derivatives using Grubbs catalysts had been unsuccessful, leading to the misbelief that these catalysts were inactive in these circumstances. However, a recent breakthrough has changed this previous perspective of CP, where a successful living CP was reported using a third-generation Grubbs catalyst with the aid of weakly coordinating ligands. Although it became clear that weakly coordinating ligands greatly enhanced the efficiency of CP by suppressing the decomposition of the propagating carbene, it was still unclear as to what was actually occurring during the previous attempts at CP using ligand-free conditions, especially in the case of the Hoveyda–Grubbs catalyst. Here, we have found that second-generation Grubbs or Hoveyda–Grubbs catalysts in dichloromethane (DCM) formed predominantly side products, i.e., dimers and trimers of 1,6-heptadiyne derivatives, instead of producing the desired conjugated polymers. Further mechanistic studies di...