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Related Publication
| Probst, P., Elser, I., Schowner, R., Benedikter, M. J., Buchmeiser, M. R., Regio‐ and Stereospecific Cyclopolymerization of α,ω‐Diynes by Cationic Molybdenum Imido Alkylidene N‐Heterocyclic Carbene Complexes. Macromol. Rapid Commun. 2020, 41, 1900398doi: 10.1002/marc.201900398 |
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Notes
| Abstract: Both solvent‐free and acetonitrile‐containing cationic molybdenum imido alkylidene N‐heterocyclic carbene (NHC) complexes of the general formula [Mo(NR′)(CHCMe2R)(NHC)(X)+ A−] (R′ = 2,6‐Cl2‐C6H3, tBu, 2‐CF3‐C6H4, 2‐tBu‐C6H4, 2,6‐iPr2‐C6H3, 2,6‐Me2‐C6H3; 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 = CF3SO3, C6F5O, OCH(CF3)2, OC(CF3)3, pyrrolide, C6F5COO, 2,6‐(CF3)2‐C6H3COO; A− = B(ArF)4−, Al(OC(CF3)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. |
