Regio- and Stereospecific Cyclopolymerization of α,ω-Diynes by Cationic Molybdenum Imido Alkylidene N-Heterocyclic Carbene Complexes.

Both solvent-free and acetonitrile-containing cationic molybdenum imido alkylidene N-heterocyclic carbene (NHC) complexes of the general formula [Mo(NR')(CHCMe ₂ R)(NHC)(X) ⁺ A ^- ] (R' = 2,6-Cl ₂ -C ₆ H ₃ , tBu, 2-CF ₃ -C ₆ H ₄ , 2-tBu-C ₆ H ₄ , 2,6-iPr ₂ -C ₆ H ₃ , 2,6-Me ₂ -C ₆ H ₃ ; R = Me, Ph; NHC = 1,3-dimesitylimidazol-2-ylidene (IMes), 1,3-di-iPr-imidazol-2-ylidene (IPr), 1,3,5-triphenyl-1,3,4-triazol-2-ylidene); X = CF ₃ SO ₃ , C ₆ F ₅ O, OCH(CF ₃ ) ₂ , OC(CF ₃ ) ₃ , pyrrolide, C ₆ F ₅ COO, 2,6-(CF ₃ ) ₂ -C ₆ H ₃ COO; A ^- = B(Ar ^F ) ₄ ^- , Al(OC(CF ₃ ) ₃ ) ₄ ^- ), have been investigated for their propensity to cyclopolymerize 4,4-disubstituted 1,6-heptadiynes. All metal complexes contain a stereogenic (chiral) metal center, which accounts for the high reactivity and high regioselectivity of insertion (>99%) that are observed for all metal complexes, leading to highly conjugated, α-insertion-derived polyenes that are based on a highly regular polymer backbone and that show absorption maxima close to 600 nm. With the chiral monomer 4-(ethoxycarbonyl)-4-(1S,2R,5S)-(-)-menthoxycarbonyl-1,6-heptadiyne, high syndiospecifity (>99% syndiotactic) is observed. A mechanism explaining the high regio- and stereoselectivity is presented. Thus, α-addition of the monomers proceeds chain-end-controlled trans to the NHC and is preferred over β-addition through intramolecular Mo-O chelation. Insertion of the monomers entails double inversion at the stereogenic metal center in the course of one complete monomer insertion.
(© 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)