Stereoselective ring‐opening metathesis polymerization with tungsten sulfido alkylidene N‐heterocyclic carbene complexes

Abstract

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.