Successful Cyclopolymerization of 1,6-Heptadiynes Using First-Generation Grubbs Catalyst Twenty Years after Its Invention: Revealing a Comprehensive Picture of Cyclopolymerization Using Grubbs Catalysts

Cyclopolymerization (CP) of 1,6-heptadiynes using olefin metathesis catalysts is a useful method for producing various conjugated polyenes. Unfortunately, commercially available user-friendly Grubbs catalysts have long been known to be inactive toward CP. However, recent mechanistic studies revealed that the problem did not lie with the intrinsic activities of Grubbs catalysts but the stability of the propagating carbenes, as decomposed carbene species catalyzed a [2 + 2 + 2] cycloaddition side reaction instead of CP. Fortunately, by adding weakly coordinating ligands such as pyridines as stabilizers, a highly active and fast-initiating third-generation Grubbs catalyst could successfully promote living CP. However, there was no report of CP using the much cheaper but less active first-generation Grubbs catalyst (G1), which has been widely used for more than 20 years. Believing that G1 should also be able to catalyze CP, we came up with three strategies to enhance the activity of G1 toward CP. By categorizing numerous additives into three distinct classes and conducting extensive reaction screening, we discovered two new excellent additives to G1: benzoic acid and sodium benzoate, both of which successfully produced various trans-selective conjugated polyenes with molecular weights of up to 23 kDa. Interestingly, additives optimal for CP catalyzed by G1 were not suitable for CP catalyzed by Grubbs catalysts containing N-heterocyclic carbene ligands and vice versa, thus implying that their activation mechanisms were distinctly different. Therefore, we conducted kinetic studies and mechanistic investigation by 1H and 31P NMR spectroscopy to reveal that these two additives, despite having very similar chemical structures, enhance the CP efficiency via very different mechanisms; benzoic acid accelerated phosphine dissociation and stabilized the propagating carbene, while sodium benzoate mediated the exchange of an anionic ligand to afford a more active catalyst. Additionally, these additives also suppressed or retarded the [2 + 2 + 2] cycloaddition side reaction to further enhance the efficiency and selectivity of CP. In brief, this study revealed new insights into the use of Grubbs catalysts to facilitate CP two decades after their invention, and this should greatly broaden the utility of CP because G1 is the cheapest and the most readily available olefin metathesis catalyst.