Stereoselective Ring‐Opening Metathesis Polymerization with Tungsten Sulfido Alkylidene N‐Heterocyclic Carbene Complexes

A series of cationic tungsten sulfido alkylidene N‐heterocyclic carbene (NHC) complexes (W01 – W09) of the general formula [W(S)(CHCMe3)(X)(NHC)(CMe3CN)+B(ArF)4−] (NHC = 1,3‐dimesitylimidazol‐2‐ylidene, IMes; 1,3‐dimesityl‐4,5‐dichloroimidazol‐2‐ylidene, IMesCl2; 1,3‐bis(2,6‐xdiisopropyl)phenyl)imidazol‐2‐ylidene, IDipp; X = Cl, C6F5O, 2,6‐Ph2‐C6H3; B(ArF)4− = tetrakis(3,5‐bis(trifluoromethyl)phenyl)borate) are used as initiators in the stereoselective ring‐opening metathesis polymerization (ROMP) of (+) 2,3‐endo, exo‐dicarbomethoxynorborn‐5‐ene ((+)DCMNBE, M1). Trans‐isospecifity up to 84% is achieved along with varying percentages of cis‐syndiospecifity. The different extent of trans‐isospecifity is compared to the one of related benchmark cationic molybdenum and tungsten imido and tungsten oxo alkylidene NHC complexes. Mechanistic investigations suggest that the syn‐isomer of a nitrile‐free initiator reacts with M1 presumably in an eneanti fashion to yield a syn‐first insertion product via turnstile rearrangement, which accounts for the predominant trans‐isospecifity of the polymerization. The cis‐syndiotactic sequences are proposed to stem from the competing enesyn addition of M1 to a nitrile‐containing syn‐isomer of the initiator. Cationic tungsten sulfido alkylidene N‐heterocyclic carbene complexes allow for the stereoselective ring‐opening metathesis polymerization. Investigations suggest that the syn‐isomer of a nitrile‐free initiator reacts with monomer in an eneanti fashion to yield syn‐insertion products via turnstile rearrangement, resulting in a trans‐isotactic structure, while cis‐syndiotactic sequences stem from the competing enesyn addition of monomer to a nitrile‐containing syn‐isomer of the initiator.