Highly b Selective Cyclopolymerization of 1,6 Heptadiynes and Ring Closing Enyne Metathesis Reaction Using Grubbs Z Selective Catalyst: Unprecedented Regioselectivity for Ru Based Catalysts.
It is well known that Ru based Grubbs catalysts undergo a highly selective a 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 b 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 b 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 b 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 a addition. To enhance the b 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.
Journal of the American Chemical Society