Highly β-Selective Cyclopolymerization of 1,6-Heptadiynes and Ring-Closing Enyne Metathesis Reaction Using Grubbs Z-Selective Catalyst: Unprecedented Regioselectivity for Ru-Based Catalysts

Abstract

It is well-known that Ru-based Grubbs catalysts undergo a highly selective α-addition to alkynes to promote exo-cyclization during ring-closing enyne metathesis (RCEYM) or to produce conjugated polyenes containing five-membered rings during the cyclopolymerization (CP) of 1,6-heptadiynes. There are a few reports of β-selective addition to alkynes using Schrock catalysts based on Mo but none for readily accessible and easy-to-use Ru-based catalysts. We report the first example of β-selective addition to alkynes using Grubbs Z-selective catalyst, which produces only endo products during the RCEYM reaction of terminal enynes and promotes the CP of 1,6-heptadiyne derivatives to give conjugated polyenes containing a six-membered ring as a major repeat unit. This unique preference for β-selectivity originated from the side-bound approach of alkynes to the catalyst, where the steric hindrance between the chelating N-heterocyclic carbene ligand of the catalyst and the alkynes disfavored α-addition. To enhance the β-selectivity for CP further, one could increase the size of the substrates on the monomers and lower the reaction temperature to obtain conjugated polyenes containing up to 95% six-membered rings. Moreover, the physical properties of the resulting polymer were analyzed in detail and compared with those of the conjugated polyenes containing only five-membered rings prepared from the same monomer but with a conventional Grubbs catalyst.

Controlled Cyclopolymerization of 1,5-Hexadiynes to Give Narrow Band Gap Conjugated Polyacetylenes Containing Highly Strained Cyclobutenes

“…Cyclopolymerization (CP) of α,ω -diyne derivatives is a powerful tool for preparing substituted polyacetylenes. − Early studies regarding CP used ill-defined catalysts, including Ziegler–Natta, MoCl 5 , and WCl 6 catalysts, − until the first breakthrough by the Schrock group and later by the Buchmeiser group demonstrating living CP using well-defined Mo-based olefin metathesis catalysts. − Subsequently, the utility of CP broadened when user-friendly Ru-based catalysts were shown to promote living CP. − Extensive studies have been conducted using 1,6-heptadiyne monomers, yielding five- or six-membered rings (in a random or highly regioselective fashion) in the conjugated backbone via α - ,− or β -addition, ,− respectively. Moreoever, recent studies have expanded the scope of CP where 1,7-octadiyne − and 1,8-nonadiyne derivatives were used to successfully produce polyacetylenes containing six- and seven-membered rings via α -addition (Scheme ).…”

Living β-selective cyclopolymerization using Ru dithiolate catalysts

“…The first β-selective CP producing six-membered rings was reported by the Schrock group in 1996, using modified Mo catalysts,37,38 but no follow-up studies have been reported. Recently, our group demonstrated the first β-selective CP using a user-friendly, Ru-based Grubbs Z -selective catalyst39 to generate conjugated polymers containing six-membered rings, with 67–95% β-selectivity 40. Notably, a new catalyst Ru1 containing a dithiolate ligand, developed by the Hoveyda group,41 exerted far higher β-selectivity in CP of 1,6-heptadiyne monomers (85–99% β-selectivity), generating mainly six-membered conjugated polyenes bearing comparable thermal properties to the five-membered counterparts 42.…”

Understanding the Origin of the Regioselectivity in Cyclopolymerizations of Diynes and How to Completely Switch It

“…Recently, we reported a highly efficient living cyclopolymerization employing the third-generation Grubbs catalyst ( Ru1 ) to afford conjugated polyenes consisting of five-membered rings with excellent molecular weight control and narrow dispersity. − Interestingly, Grubbs’ Z -selective catalyst ( Ru2 ), which carries a chelating N -heterocyclic carbene (NHC) ligand, gave β-addition products, forming conjugated polyenes made of six-membered rings with a respectable selectivity of 70–85% at ambient conditions . Thus, for both the Schrock- and Grubbs-type catalysts, regiocontrol is possible.…”