Synthesis of Conjugated Polyenynes with Alternating Six- and Five-Membered Rings via beta-Selective Cascade Metathesis and Metallotropy Polymerization

Cascade metathesis and metallotropy (M&M) polymerization, which involves sequential olefin metathesis and metallotropic 1,3-shift reactions specifically from multiyne monomers, is the only method reported so far to prepare conjugated polyenynes via the chain-growth mechanism. Using this method, various conjugated polyenynes containing cyclopentene units in the backbone could be synthesized via exclusive alpha-addition by using the third-generation Grubbs catalyst. Herein, we demonstrate the complete switch of regioselectivity toward beta-addition using a Ru carbene containing a dithiolate ligand, and thus, synthesized unique conjugated polyenynes having alternating cyclohexene and cyclopentene units in the backbone. Furthermore, detailed in situ NMR studies revealed an interesting phenomenon that the adjacent triple bond strongly chelates to the propagating Ru carbene during the polymerization.