Ru-based metathesis catalysts employed in cyclopolymerization (CP) of 1,6-heptadiyne derivatives have promoted regioselective α-addition to alkynes, forming various conjugated polyenes containing exclusively five-membered repeat units. Recently, we discovered that a new chelated Ru catalyst could promote regioselective β-addition to produce analogous polyenes containing six-membered rings with moderate to good β-selectivity. Since then, we have focused our research on pursuing more active and β-selective regiocontrol to produce conjugated polymers with excellent βselectivity, with a much broader range of monomers. Herein, we demonstrate highly β-selective CP by combining a new dithiolate-chelated Ru-based catalyst with weakly coordinating pyridine additives, which significantly enhance the conversion and β-selectivity. An in-depth mechanistic investigation by 1 H NMR revealed a prominent role for the additives, which improve the stability of the propagating carbene.
Development of versatile ruthenium olefin-metathesis catalysts with high activity, stability, and selectivity is a continuous challenge. Here we report highly controllable ruthenium catalysts using readily accessible and versatile
N
-vinylsulfonamides as carbene precursors. Catalyst initiation rates were controlled in a straightforward manner, from latent to fast initiating, through the facile modulation of the
N
-vinylsulfonamide ligands. Trifluoromethanesulfonamide-based catalysts initiated ultrarapidly even at temperatures as low as −60 °C and continuously propagated rapidly, enabling the enthalpically and entropically less-favored ring-opening metathesis polymerizations of low-strained functionalized cyclopentene derivatives, some of which are not accessible with previous olefin-metathesis catalysts. To our surprise, the developed catalysts facilitated the polymerization of cyclopentadiene (CPD), a feedstock that is easily and commonly obtainable through the steam cracking of naphtha, which has, to the best of our knowledge, not been previously achieved due to its low ring strain and facile dimerization even at low temperatures (below 0 °C).
Grubbs-type olefin metathesis catalysts are known to cyclopolymerize 1,6-heptadiynes to afford conjugated polyenes containing five- or six-membered carbocycles. Although high levels of regioselectivity up to >99:1 were observed previously for the formation of five-membered rings, it was neither possible to deliberately obtain six-membered rings at similar levels of selectivity nor understood why certain catalysts showed this selectively. Combining experimental and computational methods, a novel and general theory for what controls the regiochemistry of these cyclopolymerizations is presented. The electronic demands of the ruthenium-based Fischer carbenes are found to innately prefer to form five-membered rings. Reducing the electrophilicity of the carbene by enforcing a trigonal-bipyramidal structure for the ruthenium, where stronger π-backdonation increases the electron density on the carbene, is predicted to invert the regioselectivity. Subsequent experiments provide strong support for the new concept, and it is possible to completely switch the regioselectivity to a ratio of <1:99.
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